roc/crates/compiler/solve/src/solve.rs

use crate::ability::{
    resolve_ability_specialization, type_implementing_specialization, AbilityImplError,
    DeferredObligations, PendingDerivesTable, RequestedDeriveKey, Resolved, Unfulfilled,
};
use crate::module::Solved;
use bumpalo::Bump;
use roc_can::abilities::{AbilitiesStore, MemberSpecialization};
use roc_can::constraint::Constraint::{self, *};
use roc_can::constraint::{Constraints, Cycle, LetConstraint, OpportunisticResolve};
use roc_can::expected::{Expected, PExpected};
use roc_can::expr::PendingDerives;
use roc_can::module::ExposedByModule;
use roc_collections::all::MutMap;
use roc_debug_flags::dbg_do;
#[cfg(debug_assertions)]
use roc_debug_flags::{ROC_TRACE_COMPACTION, ROC_VERIFY_RIGID_LET_GENERALIZED};
use roc_derive::SharedDerivedModule;
use roc_derive_key::{DeriveError, DeriveKey};
use roc_error_macros::internal_error;
use roc_module::ident::TagName;
use roc_module::symbol::{ModuleId, Symbol};
use roc_problem::can::CycleEntry;
use roc_region::all::{Loc, Region};
use roc_types::subs::{
    self, get_member_lambda_sets_at_region, AliasVariables, Content, Descriptor, FlatType,
    GetSubsSlice, LambdaSet, Mark, OptVariable, Rank, RecordFields, Subs, SubsIndex, SubsSlice,
    UlsOfVar, UnionLabels, UnionLambdas, UnionTags, Variable, VariableSubsSlice,
};
use roc_types::types::Type::{self, *};
use roc_types::types::{
    gather_fields_unsorted_iter, AliasCommon, AliasKind, Category, ErrorType, OptAbleType,
    OptAbleVar, PatternCategory, Reason, TypeExtension, Uls,
};
use roc_unify::unify::{
    unify, unify_introduced_ability_specialization, Mode, MustImplementConstraints, Obligated,
    SpecializationLsetCollector, Unified::*,
};

// Type checking system adapted from Elm by Evan Czaplicki, BSD-3-Clause Licensed
// https://github.com/elm/compiler
// Thank you, Evan!

// A lot of energy was put into making type inference fast. That means it's pretty intimidating.
//
// Fundamentally, type inference assigns very general types based on syntax, and then tries to
// make all the pieces fit together. For instance when writing
//
// > f x
//
// We know that `f` is a function, and thus must have some type `a -> b`.
// `x` is just a variable, that gets the type `c`
//
// Next comes constraint generation. For `f x` to be well-typed,
// it must be the case that `c = a`, So a constraint `Eq(c, a)` is generated.
// But `Eq` is a bit special: `c` does not need to equal `a` exactly, but they need to be equivalent.
// This allows for instance the use of aliases. `c` could be an alias, and so looks different from
// `a`, but they still represent the same type.
//
// Then we get to solving, which happens in this file.
//
// When we hit an `Eq` constraint, then we check whether the two involved types are in fact
// equivalent using unification, and when they are, we can substitute one for the other.
//
// When all constraints are processed, and no unification errors have occurred, then the program
// is type-correct. Otherwise the errors are reported.
//
// Now, coming back to efficiency, this type checker uses *ranks* to optimize
// The rank tracks the number of let-bindings a variable is "under". Top-level definitions
// have rank 1. A let in a top-level definition gets rank 2, and so on.
//
// This has to do with generalization of type variables. This is described here
//
//      http://okmij.org/ftp/ML/generalization.html#levels
//
// The problem is that when doing inference naively, this program would fail to typecheck
//
//  f =
//      id = \x -> x
//
//      { a: id 1, b: id "foo" }
//
// Because `id` is applied to an integer, the type `Int -> Int` is inferred, which then gives a
// type error for `id "foo"`.
//
// Thus instead the inferred type for `id` is generalized (see the `generalize` function) to `a -> a`.
// Ranks are used to limit the number of type variables considered for generalization. Only those inside
// of the let (so those used in inferring the type of `\x -> x`) are considered.

#[derive(Debug, Clone)]
pub enum TypeError {
    BadExpr(Region, Category, ErrorType, Expected<ErrorType>),
    BadPattern(Region, PatternCategory, ErrorType, PExpected<ErrorType>),
    CircularType(Region, Symbol, ErrorType),
    CircularDef(Vec<CycleEntry>),
    BadType(roc_types::types::Problem),
    UnexposedLookup(Symbol),
    UnfulfilledAbility(Unfulfilled),
    BadExprMissingAbility(Region, Category, ErrorType, Vec<Unfulfilled>),
    BadPatternMissingAbility(Region, PatternCategory, ErrorType, Vec<Unfulfilled>),
    Exhaustive(roc_exhaustive::Error),
    StructuralSpecialization {
        region: Region,
        typ: ErrorType,
        ability: Symbol,
        member: Symbol,
    },
    DominatedDerive {
        opaque: Symbol,
        ability: Symbol,
        derive_region: Region,
        impl_region: Region,
    },
}

use roc_types::types::Alias;

#[derive(Debug, Clone, Copy)]
struct DelayedAliasVariables {
    start: u32,
    type_variables_len: u8,
    lambda_set_variables_len: u8,
    recursion_variables_len: u8,
}

impl DelayedAliasVariables {
    fn recursion_variables(self, variables: &mut [OptAbleVar]) -> &mut [OptAbleVar] {
        let start = self.start as usize
            + (self.type_variables_len + self.lambda_set_variables_len) as usize;
        let length = self.recursion_variables_len as usize;

        &mut variables[start..][..length]
    }

    fn lambda_set_variables(self, variables: &mut [OptAbleVar]) -> &mut [OptAbleVar] {
        let start = self.start as usize + self.type_variables_len as usize;
        let length = self.lambda_set_variables_len as usize;

        &mut variables[start..][..length]
    }

    fn type_variables(self, variables: &mut [OptAbleVar]) -> &mut [OptAbleVar] {
        let start = self.start as usize;
        let length = self.type_variables_len as usize;

        &mut variables[start..][..length]
    }
}

#[derive(Debug, Default)]
pub struct Aliases {
    aliases: Vec<(Symbol, Type, DelayedAliasVariables, AliasKind)>,
    variables: Vec<OptAbleVar>,
}

impl Aliases {
    pub fn insert(&mut self, symbol: Symbol, alias: Alias) {
        let alias_variables =
            {
                let start = self.variables.len() as _;

                self.variables.extend(
                    alias
                        .type_variables
                        .iter()
                        .map(|x| OptAbleVar::from(&x.value)),
                );

                self.variables.extend(alias.lambda_set_variables.iter().map(
                    |x| match x.as_inner() {
                        Type::Variable(v) => OptAbleVar::unbound(*v),
                        _ => unreachable!("lambda set type is not a variable"),
                    },
                ));

                let recursion_variables_len = alias.recursion_variables.len() as _;
                self.variables.extend(
                    alias
                        .recursion_variables
                        .iter()
                        .copied()
                        .map(OptAbleVar::unbound),
                );

                DelayedAliasVariables {
                    start,
                    type_variables_len: alias.type_variables.len() as _,
                    lambda_set_variables_len: alias.lambda_set_variables.len() as _,
                    recursion_variables_len,
                }
            };

        self.aliases
            .push((symbol, alias.typ, alias_variables, alias.kind));
    }

    fn instantiate_result_result(
        subs: &mut Subs,
        rank: Rank,
        pools: &mut Pools,
        alias_variables: AliasVariables,
    ) -> Variable {
        let tag_names_slice = Subs::RESULT_TAG_NAMES;

        let err_slice = SubsSlice::new(alias_variables.variables_start + 1, 1);
        let ok_slice = SubsSlice::new(alias_variables.variables_start, 1);

        let variable_slices =
            SubsSlice::extend_new(&mut subs.variable_slices, [err_slice, ok_slice]);

        let union_tags = UnionTags::from_slices(tag_names_slice, variable_slices);
        let ext_var = Variable::EMPTY_TAG_UNION;
        let flat_type = FlatType::TagUnion(union_tags, ext_var);
        let content = Content::Structure(flat_type);

        register(subs, rank, pools, content)
    }

    /// Build an alias of the form `Num range := range`
    fn build_num_opaque(
        subs: &mut Subs,
        rank: Rank,
        pools: &mut Pools,
        symbol: Symbol,
        range_var: Variable,
    ) -> Variable {
        let content = Content::Alias(
            symbol,
            AliasVariables::insert_into_subs(subs, [range_var], []),
            range_var,
            AliasKind::Opaque,
        );

        register(subs, rank, pools, content)
    }

    fn instantiate_builtin_aliases_real_var(
        &mut self,
        subs: &mut Subs,
        rank: Rank,
        pools: &mut Pools,
        symbol: Symbol,
        alias_variables: AliasVariables,
    ) -> Option<(Variable, AliasKind)> {
        match symbol {
            Symbol::RESULT_RESULT => {
                let var = Self::instantiate_result_result(subs, rank, pools, alias_variables);

                Some((var, AliasKind::Structural))
            }
            Symbol::NUM_NUM | Symbol::NUM_INTEGER | Symbol::NUM_FLOATINGPOINT => {
                // Num range := range | Integer range := range | FloatingPoint range := range
                let range_var = subs.variables[alias_variables.variables_start as usize];
                Some((range_var, AliasKind::Opaque))
            }
            Symbol::NUM_INT => {
                // Int range : Num (Integer range)
                //
                // build `Integer range := range`
                let integer_content_var = Self::build_num_opaque(
                    subs,
                    rank,
                    pools,
                    Symbol::NUM_INTEGER,
                    subs.variables[alias_variables.variables_start as usize],
                );

                // build `Num (Integer range) := Integer range`
                let num_content_var =
                    Self::build_num_opaque(subs, rank, pools, Symbol::NUM_NUM, integer_content_var);

                Some((num_content_var, AliasKind::Structural))
            }
            Symbol::NUM_FRAC => {
                // Frac range : Num (FloatingPoint range)
                //
                // build `FloatingPoint range := range`
                let fpoint_content_var = Self::build_num_opaque(
                    subs,
                    rank,
                    pools,
                    Symbol::NUM_FLOATINGPOINT,
                    subs.variables[alias_variables.variables_start as usize],
                );

                // build `Num (FloatingPoint range) := FloatingPoint range`
                let num_content_var =
                    Self::build_num_opaque(subs, rank, pools, Symbol::NUM_NUM, fpoint_content_var);

                Some((num_content_var, AliasKind::Structural))
            }
            _ => None,
        }
    }

    fn instantiate_real_var(
        &mut self,
        subs: &mut Subs,
        rank: Rank,
        pools: &mut Pools,
        arena: &bumpalo::Bump,
        symbol: Symbol,
        alias_variables: AliasVariables,
    ) -> (Variable, AliasKind) {
        // hardcoded instantiations for builtin aliases
        if let Some((var, kind)) = Self::instantiate_builtin_aliases_real_var(
            self,
            subs,
            rank,
            pools,
            symbol,
            alias_variables,
        ) {
            return (var, kind);
        }

        let (typ, delayed_variables, &mut kind) =
            match self.aliases.iter_mut().find(|(s, _, _, _)| *s == symbol) {
                None => internal_error!(
                    "Alias {:?} not registered in delayed aliases! {:?}",
                    symbol,
                    &self.aliases
                ),
                Some((_, typ, delayed_variables, kind)) => (typ, delayed_variables, kind),
            };

        let mut substitutions: MutMap<_, _> = Default::default();

        let old_type_variables = delayed_variables.type_variables(&mut self.variables);
        let new_type_variables = &subs.variables[alias_variables.type_variables().indices()];

        let some_new_vars_are_equivalent = {
            // In practice the number of type variables is tiny, so just do a quadratic check
            // without allocating.
            let mut some_equivalent = false;
            for (i, var) in new_type_variables.iter().enumerate() {
                for other_var in new_type_variables.iter().skip(i + 1) {
                    some_equivalent = some_equivalent || var == other_var;
                }
            }
            some_equivalent
        };

        // If some type variables are equivalent, we have to work over a cloned type variable,
        // otherwise we will leave in place an alias without preserving the property of unique
        // type variables.
        //
        // For example, if a delayed alias `Foo a b` is instantiated with args `t1 t1` without cloning,
        // then the delayed alias would be updated to `Foo t1 t1`, and now the distinction between the
        // two type variables is lost.
        let can_reuse_old_definition = !some_new_vars_are_equivalent;

        for (old, new) in old_type_variables.iter_mut().zip(new_type_variables) {
            // if constraint gen duplicated a type these variables could be the same
            // (happens very often in practice)
            if old.var != *new {
                substitutions.insert(old.var, *new);

                if can_reuse_old_definition {
                    old.var = *new;
                }
            }
        }

        for OptAbleVar {
            var: rec_var,
            opt_ability,
        } in delayed_variables
            .recursion_variables(&mut self.variables)
            .iter_mut()
        {
            debug_assert!(opt_ability.is_none());
            let new_var = subs.fresh_unnamed_flex_var();
            substitutions.insert(*rec_var, new_var);

            if can_reuse_old_definition {
                *rec_var = new_var;
            }
        }

        let old_lambda_set_variables = delayed_variables.lambda_set_variables(&mut self.variables);
        let new_lambda_set_variables =
            &subs.variables[alias_variables.lambda_set_variables().indices()];

        for (old, new) in old_lambda_set_variables
            .iter_mut()
            .zip(new_lambda_set_variables)
        {
            debug_assert!(old.opt_ability.is_none());
            if old.var != *new {
                substitutions.insert(old.var, *new);

                if can_reuse_old_definition {
                    old.var = *new;
                }
            }
        }

        if !can_reuse_old_definition {
            let mut typ = typ.clone();
            typ.substitute_variables(&substitutions);
            let alias_variable = type_to_variable(subs, rank, pools, arena, self, &typ, false);
            (alias_variable, kind)
        } else {
            if !substitutions.is_empty() {
                typ.substitute_variables(&substitutions);
            }

            let mut t = Type::EmptyRec;

            std::mem::swap(typ, &mut t);

            // assumption: an alias does not (transitively) syntactically contain itself
            // (if it did it would have to be a recursive tag union, which we should have fixed up
            // during canonicalization)
            let alias_variable = type_to_variable(subs, rank, pools, arena, self, &t, false);

            {
                match self.aliases.iter_mut().find(|(s, _, _, _)| *s == symbol) {
                    None => unreachable!(),
                    Some((_, typ, _, _)) => {
                        // swap typ back
                        std::mem::swap(typ, &mut t);
                    }
                }
            }

            (alias_variable, kind)
        }
    }
}

#[derive(Clone, Debug, Default)]
pub struct Env {
    symbols: Vec<Symbol>,
    variables: Vec<Variable>,
}

impl Env {
    pub fn vars_by_symbol(&self) -> impl Iterator<Item = (Symbol, Variable)> + '_ {
        let it1 = self.symbols.iter().copied();
        let it2 = self.variables.iter().copied();

        it1.zip(it2)
    }

    #[inline(always)]
    fn get_var_by_symbol(&self, symbol: &Symbol) -> Option<Variable> {
        self.symbols
            .iter()
            .position(|s| s == symbol)
            .map(|index| self.variables[index])
    }

    #[inline(always)]
    fn insert_symbol_var_if_vacant(&mut self, symbol: Symbol, var: Variable) {
        match self.symbols.iter().position(|s| *s == symbol) {
            None => {
                // symbol is not in vars_by_symbol yet; insert it
                self.symbols.push(symbol);
                self.variables.push(var);
            }
            Some(_) => {
                // do nothing
            }
        }
    }
}

const DEFAULT_POOLS: usize = 8;

#[derive(Clone, Debug)]
pub struct Pools(Vec<Vec<Variable>>);

impl Default for Pools {
    fn default() -> Self {
        Pools::new(DEFAULT_POOLS)
    }
}

impl Pools {
    pub fn new(num_pools: usize) -> Self {
        Pools(vec![Vec::new(); num_pools])
    }

    pub fn len(&self) -> usize {
        self.0.len()
    }

    pub fn is_empty(&self) -> bool {
        self.0.is_empty()
    }

    pub fn get_mut(&mut self, rank: Rank) -> &mut Vec<Variable> {
        match self.0.get_mut(rank.into_usize()) {
            Some(reference) => reference,
            None => panic!("Compiler bug: could not find pool at rank {}", rank),
        }
    }

    pub fn get(&self, rank: Rank) -> &Vec<Variable> {
        match self.0.get(rank.into_usize()) {
            Some(reference) => reference,
            None => panic!("Compiler bug: could not find pool at rank {}", rank),
        }
    }

    pub fn iter(&self) -> std::slice::Iter<'_, Vec<Variable>> {
        self.0.iter()
    }

    pub fn split_last(mut self) -> (Vec<Variable>, Vec<Vec<Variable>>) {
        let last = self
            .0
            .pop()
            .unwrap_or_else(|| panic!("Attempted to split_last() on non-empty Pools"));

        (last, self.0)
    }

    pub fn extend_to(&mut self, n: usize) {
        for _ in self.len()..n {
            self.0.push(Vec::new());
        }
    }
}

/// What phase in the compiler is reaching out to solve types.
/// This is important to distinguish subtle differences in the behavior of the solving algorithm.
//
// TODO the APIs of this trait suck, this needs a nice cleanup.
pub trait Phase {
    /// The regular type-solving phase, or during some later phase of compilation.
    /// During the solving phase we must anticipate that some information is still unknown and react to
    /// that; during late phases, we expect that all information is resolved.
    const IS_LATE: bool;

    fn with_module_abilities_store<T, F>(&self, module: ModuleId, f: F) -> T
    where
        F: FnMut(&AbilitiesStore) -> T;

    /// Given a known lambda set's ambient function in an external module, copy that ambient
    /// function into the given subs.
    fn copy_lambda_set_ambient_function_to_home_subs(
        &self,
        external_lambda_set_var: Variable,
        external_module_id: ModuleId,
        home_subs: &mut Subs,
    ) -> Variable;

    /// Find the ambient function var at a given region for an ability member definition (not a
    /// specialization!), and copy that into the given subs.
    fn get_and_copy_ability_member_ambient_function(
        &self,
        ability_member: Symbol,
        region: u8,
        home_subs: &mut Subs,
    ) -> Variable;
}

struct SolvePhase<'a> {
    abilities_store: &'a AbilitiesStore,
}
impl Phase for SolvePhase<'_> {
    const IS_LATE: bool = false;

    fn with_module_abilities_store<T, F>(&self, _module: ModuleId, mut f: F) -> T
    where
        F: FnMut(&AbilitiesStore) -> T,
    {
        // During solving we're only aware of our module's abilities store.
        f(self.abilities_store)
    }

    fn copy_lambda_set_ambient_function_to_home_subs(
        &self,
        external_lambda_set_var: Variable,
        _external_module_id: ModuleId,
        home_subs: &mut Subs,
    ) -> Variable {
        // During solving we're only aware of our module's abilities store, the var must
        // be in our module store. Even if the specialization lambda set comes from another
        // module, we should have taken care to import it before starting solving in this module.
        let LambdaSet {
            ambient_function, ..
        } = home_subs.get_lambda_set(external_lambda_set_var);
        ambient_function
    }

    fn get_and_copy_ability_member_ambient_function(
        &self,
        ability_member: Symbol,
        region: u8,
        home_subs: &mut Subs,
    ) -> Variable {
        // During solving we're only aware of our module's abilities store, the var must
        // be in our module store. Even if the specialization lambda set comes from another
        // module, we should have taken care to import it before starting solving in this module.
        let member_def = self
            .abilities_store
            .member_def(ability_member)
            .unwrap_or_else(|| {
                internal_error!(
                    "{:?} is not resolved, or not an ability member!",
                    ability_member
                )
            });
        let member_var = member_def.signature_var();

        let region_lset = get_member_lambda_sets_at_region(home_subs, member_var, region);

        let LambdaSet {
            ambient_function, ..
        } = home_subs.get_lambda_set(region_lset);

        ambient_function
    }
}

#[derive(Clone)]
struct State {
    env: Env,
    mark: Mark,
}

#[allow(clippy::too_many_arguments)] // TODO: put params in a context/env var
pub fn run(
    home: ModuleId,
    constraints: &Constraints,
    problems: &mut Vec<TypeError>,
    mut subs: Subs,
    aliases: &mut Aliases,
    constraint: &Constraint,
    pending_derives: PendingDerives,
    abilities_store: &mut AbilitiesStore,
    exposed_by_module: &ExposedByModule,
    derived_module: SharedDerivedModule,
) -> (Solved<Subs>, Env) {
    let env = run_in_place(
        home,
        constraints,
        problems,
        &mut subs,
        aliases,
        constraint,
        pending_derives,
        abilities_store,
        exposed_by_module,
        derived_module,
    );

    (Solved(subs), env)
}

/// Modify an existing subs in-place instead
#[allow(clippy::too_many_arguments)] // TODO: put params in a context/env var
fn run_in_place(
    _home: ModuleId, // TODO: remove me?
    constraints: &Constraints,
    problems: &mut Vec<TypeError>,
    subs: &mut Subs,
    aliases: &mut Aliases,
    constraint: &Constraint,
    pending_derives: PendingDerives,
    abilities_store: &mut AbilitiesStore,
    exposed_by_module: &ExposedByModule,
    derived_module: SharedDerivedModule,
) -> Env {
    let mut pools = Pools::default();

    let state = State {
        env: Env::default(),
        mark: Mark::NONE.next(),
    };
    let rank = Rank::toplevel();
    let arena = Bump::new();

    let pending_derives = PendingDerivesTable::new(subs, aliases, pending_derives);
    let mut deferred_obligations = DeferredObligations::new(pending_derives);

    // Because we don't know what ability specializations are available until the entire module is
    // solved, we must wait to solve unspecialized lambda sets then.
    let mut deferred_uls_to_resolve = UlsOfVar::default();

    let state = solve(
        &arena,
        constraints,
        state,
        rank,
        &mut pools,
        problems,
        aliases,
        subs,
        constraint,
        abilities_store,
        &mut deferred_obligations,
        &mut deferred_uls_to_resolve,
    );

    // Now that the module has been solved, we can run through and check all
    // types claimed to implement abilities. This will also tell us what derives
    // are legal, which we need to register.
    let new_must_implement = compact_lambda_sets_of_vars(
        subs,
        &derived_module,
        &arena,
        &mut pools,
        deferred_uls_to_resolve,
        &SolvePhase { abilities_store },
        exposed_by_module,
    );

    deferred_obligations.add(new_must_implement, AbilityImplError::IncompleteAbility);

    let (obligation_problems, _derived) = deferred_obligations.check_all(subs, abilities_store);
    problems.extend(obligation_problems);

    state.env
}

enum Work<'a> {
    Constraint {
        env: &'a Env,
        rank: Rank,
        constraint: &'a Constraint,
    },
    CheckForInfiniteTypes(LocalDefVarsVec<(Symbol, Loc<Variable>)>),
    /// The ret_con part of a let constraint that does NOT introduces rigid and/or flex variables
    LetConNoVariables {
        env: &'a Env,
        rank: Rank,
        let_con: &'a LetConstraint,

        /// The variables used to store imported types in the Subs.
        /// The `Contents` are copied from the source module, but to
        /// mimic `type_to_var`, we must add these variables to `Pools`
        /// at the correct rank
        pool_variables: &'a [Variable],
    },
    /// The ret_con part of a let constraint that introduces rigid and/or flex variables
    ///
    /// These introduced variables must be generalized, hence this variant
    /// is more complex than `LetConNoVariables`.
    LetConIntroducesVariables {
        env: &'a Env,
        rank: Rank,
        let_con: &'a LetConstraint,

        /// The variables used to store imported types in the Subs.
        /// The `Contents` are copied from the source module, but to
        /// mimic `type_to_var`, we must add these variables to `Pools`
        /// at the correct rank
        pool_variables: &'a [Variable],
    },
}

#[allow(clippy::too_many_arguments)]
fn solve(
    arena: &Bump,
    constraints: &Constraints,
    mut state: State,
    rank: Rank,
    pools: &mut Pools,
    problems: &mut Vec<TypeError>,
    aliases: &mut Aliases,
    subs: &mut Subs,
    constraint: &Constraint,
    abilities_store: &mut AbilitiesStore,
    deferred_obligations: &mut DeferredObligations,
    deferred_uls_to_resolve: &mut UlsOfVar,
) -> State {
    let initial = Work::Constraint {
        env: &Env::default(),
        rank,
        constraint,
    };

    let mut stack = vec![initial];

    while let Some(work_item) = stack.pop() {
        let (env, rank, constraint) = match work_item {
            Work::Constraint {
                env,
                rank,
                constraint,
            } => {
                // the default case; actually solve this constraint
                (env, rank, constraint)
            }
            Work::CheckForInfiniteTypes(def_vars) => {
                // after a LetCon, we must check if any of the variables that we introduced
                // loop back to themselves after solving the ret_constraint
                for (symbol, loc_var) in def_vars.iter() {
                    check_for_infinite_type(subs, problems, *symbol, *loc_var);
                }

                continue;
            }
            Work::LetConNoVariables {
                env,
                rank,
                let_con,
                pool_variables,
            } => {
                // NOTE be extremely careful with shadowing here
                let offset = let_con.defs_and_ret_constraint.index();
                let ret_constraint = &constraints.constraints[offset + 1];

                // Add a variable for each def to new_vars_by_env.
                let local_def_vars = LocalDefVarsVec::from_def_types(
                    constraints,
                    rank,
                    pools,
                    aliases,
                    subs,
                    let_con.def_types,
                );

                pools.get_mut(rank).extend(pool_variables);

                let mut new_env = env.clone();
                for (symbol, loc_var) in local_def_vars.iter() {
                    check_ability_specialization(
                        arena,
                        subs,
                        pools,
                        rank,
                        abilities_store,
                        problems,
                        deferred_obligations,
                        deferred_uls_to_resolve,
                        *symbol,
                        *loc_var,
                    );

                    new_env.insert_symbol_var_if_vacant(*symbol, loc_var.value);
                }

                stack.push(Work::CheckForInfiniteTypes(local_def_vars));
                stack.push(Work::Constraint {
                    env: arena.alloc(new_env),
                    rank,
                    constraint: ret_constraint,
                });

                continue;
            }
            Work::LetConIntroducesVariables {
                env,
                rank,
                let_con,
                pool_variables,
            } => {
                // NOTE be extremely careful with shadowing here
                let offset = let_con.defs_and_ret_constraint.index();
                let ret_constraint = &constraints.constraints[offset + 1];

                let next_rank = rank.next();

                let mark = state.mark;
                let saved_env = state.env;

                let young_mark = mark;
                let visit_mark = young_mark.next();
                let final_mark = visit_mark.next();

                // Add a variable for each def to local_def_vars.
                let local_def_vars = LocalDefVarsVec::from_def_types(
                    constraints,
                    next_rank,
                    pools,
                    aliases,
                    subs,
                    let_con.def_types,
                );

                pools.get_mut(next_rank).extend(pool_variables);

                debug_assert_eq!(
                    // Check that no variable ended up in a higher rank than the next rank.. that
                    // would mean we generalized one level more than we need to!
                    {
                        let offenders = pools
                            .get(next_rank)
                            .iter()
                            .filter(|var| {
                                subs.get_rank(**var).into_usize() > next_rank.into_usize()
                            })
                            .collect::<Vec<_>>();

                        let result = offenders.len();

                        if result > 0 {
                            eprintln!("subs = {:?}", &subs);
                            eprintln!("offenders = {:?}", &offenders);
                            eprintln!("let_con.def_types = {:?}", &let_con.def_types);
                        }

                        result
                    },
                    0
                );

                // pop pool
                generalize(subs, young_mark, visit_mark, next_rank, pools);
                debug_assert!(pools.get(next_rank).is_empty());

                // check that things went well
                dbg_do!(ROC_VERIFY_RIGID_LET_GENERALIZED, {
                    let rigid_vars = &constraints.variables[let_con.rigid_vars.indices()];

                    // NOTE the `subs.redundant` check does not come from elm.
                    // It's unclear whether this is a bug with our implementation
                    // (something is redundant that shouldn't be)
                    // or that it just never came up in elm.
                    let mut it = rigid_vars
                        .iter()
                        .filter(|&var| !subs.redundant(*var) && subs.get_rank(*var) != Rank::NONE)
                        .peekable();

                    if it.peek().is_some() {
                        let failing: Vec<_> = it.collect();
                        println!("Rigids {:?}", &rigid_vars);
                        println!("Failing {:?}", failing);
                        debug_assert!(false);
                    }
                });

                let mut new_env = env.clone();
                for (symbol, loc_var) in local_def_vars.iter() {
                    check_ability_specialization(
                        arena,
                        subs,
                        pools,
                        rank,
                        abilities_store,
                        problems,
                        deferred_obligations,
                        deferred_uls_to_resolve,
                        *symbol,
                        *loc_var,
                    );

                    new_env.insert_symbol_var_if_vacant(*symbol, loc_var.value);
                }

                // Note that this vars_by_symbol is the one returned by the
                // previous call to solve()
                let state_for_ret_con = State {
                    env: saved_env,
                    mark: final_mark,
                };

                // Now solve the body, using the new vars_by_symbol which includes
                // the assignments' name-to-variable mappings.
                stack.push(Work::CheckForInfiniteTypes(local_def_vars));
                stack.push(Work::Constraint {
                    env: arena.alloc(new_env),
                    rank,
                    constraint: ret_constraint,
                });

                state = state_for_ret_con;

                continue;
            }
        };

        state = match constraint {
            True => state,
            SaveTheEnvironment => {
                let mut copy = state;

                copy.env = env.clone();

                copy
            }
            Eq(roc_can::constraint::Eq(type_index, expectation_index, category_index, region)) => {
                let category = &constraints.categories[category_index.index()];

                let actual =
                    either_type_index_to_var(constraints, subs, rank, pools, aliases, *type_index);

                let expectation = &constraints.expectations[expectation_index.index()];
                let expected = type_to_var(subs, rank, pools, aliases, expectation.get_type_ref());

                match unify(subs, actual, expected, Mode::EQ) {
                    Success {
                        vars,
                        must_implement_ability,
                        lambda_sets_to_specialize,
                        extra_metadata: _,
                    } => {
                        introduce(subs, rank, pools, &vars);
                        if !must_implement_ability.is_empty() {
                            deferred_obligations.add(
                                must_implement_ability,
                                AbilityImplError::BadExpr(*region, category.clone(), actual),
                            );
                        }
                        deferred_uls_to_resolve.union(lambda_sets_to_specialize);

                        state
                    }
                    Failure(vars, actual_type, expected_type, _bad_impls) => {
                        introduce(subs, rank, pools, &vars);

                        let problem = TypeError::BadExpr(
                            *region,
                            category.clone(),
                            actual_type,
                            expectation.clone().replace(expected_type),
                        );

                        problems.push(problem);

                        state
                    }
                    BadType(vars, problem) => {
                        introduce(subs, rank, pools, &vars);

                        problems.push(TypeError::BadType(problem));

                        state
                    }
                }
            }
            Store(source_index, target, _filename, _linenr) => {
                // a special version of Eq that is used to store types in the AST.
                // IT DOES NOT REPORT ERRORS!
                let actual = either_type_index_to_var(
                    constraints,
                    subs,
                    rank,
                    pools,
                    aliases,
                    *source_index,
                );
                let target = *target;

                match unify(subs, actual, target, Mode::EQ) {
                    Success {
                        vars,
                        // ERROR NOT REPORTED
                        must_implement_ability: _,
                        lambda_sets_to_specialize,
                        extra_metadata: _,
                    } => {
                        introduce(subs, rank, pools, &vars);

                        deferred_uls_to_resolve.union(lambda_sets_to_specialize);

                        state
                    }
                    Failure(vars, _actual_type, _expected_type, _bad_impls) => {
                        introduce(subs, rank, pools, &vars);

                        // ERROR NOT REPORTED

                        state
                    }
                    BadType(vars, _) => {
                        introduce(subs, rank, pools, &vars);

                        // ERROR NOT REPORTED

                        state
                    }
                }
            }
            Lookup(symbol, expectation_index, region) => {
                match env.get_var_by_symbol(symbol) {
                    Some(var) => {
                        // Deep copy the vars associated with this symbol before unifying them.
                        // Otherwise, suppose we have this:
                        //
                        // identity = \a -> a
                        //
                        // x = identity 5
                        //
                        // When we call (identity 5), it's important that we not unify
                        // on identity's original vars. If we do, the type of `identity` will be
                        // mutated to be `Int -> Int` instead of `a -> `, which would be incorrect;
                        // the type of `identity` is more general than that!
                        //
                        // Instead, we want to unify on a *copy* of its vars. If the copy unifies
                        // successfully (in this case, to `Int -> Int`), we can use that to
                        // infer the type of this lookup (in this case, `Int`) without ever
                        // having mutated the original.
                        //
                        // If this Lookup is targeting a value in another module,
                        // then we copy from that module's Subs into our own. If the value
                        // is being looked up in this module, then we use our Subs as both
                        // the source and destination.
                        let actual = deep_copy_var_in(subs, rank, pools, var, arena);
                        let expectation = &constraints.expectations[expectation_index.index()];

                        let expected =
                            type_to_var(subs, rank, pools, aliases, expectation.get_type_ref());

                        match unify(subs, actual, expected, Mode::EQ) {
                            Success {
                                vars,
                                must_implement_ability,
                                lambda_sets_to_specialize,
                                extra_metadata: _,
                            } => {
                                introduce(subs, rank, pools, &vars);
                                if !must_implement_ability.is_empty() {
                                    deferred_obligations.add(
                                        must_implement_ability,
                                        AbilityImplError::BadExpr(
                                            *region,
                                            Category::Lookup(*symbol),
                                            actual,
                                        ),
                                    );
                                }
                                deferred_uls_to_resolve.union(lambda_sets_to_specialize);

                                state
                            }

                            Failure(vars, actual_type, expected_type, _bad_impls) => {
                                introduce(subs, rank, pools, &vars);

                                let problem = TypeError::BadExpr(
                                    *region,
                                    Category::Lookup(*symbol),
                                    actual_type,
                                    expectation.clone().replace(expected_type),
                                );

                                problems.push(problem);

                                state
                            }
                            BadType(vars, problem) => {
                                introduce(subs, rank, pools, &vars);

                                problems.push(TypeError::BadType(problem));

                                state
                            }
                        }
                    }
                    None => {
                        problems.push(TypeError::UnexposedLookup(*symbol));

                        state
                    }
                }
            }
            And(slice) => {
                let it = constraints.constraints[slice.indices()].iter().rev();
                for sub_constraint in it {
                    stack.push(Work::Constraint {
                        env,
                        rank,
                        constraint: sub_constraint,
                    })
                }

                state
            }
            Pattern(type_index, expectation_index, category_index, region)
            | PatternPresence(type_index, expectation_index, category_index, region) => {
                let category = &constraints.pattern_categories[category_index.index()];

                let actual =
                    either_type_index_to_var(constraints, subs, rank, pools, aliases, *type_index);

                let expectation = &constraints.pattern_expectations[expectation_index.index()];
                let expected = type_to_var(subs, rank, pools, aliases, expectation.get_type_ref());

                let mode = match constraint {
                    PatternPresence(..) => Mode::PRESENT,
                    _ => Mode::EQ,
                };

                match unify(subs, actual, expected, mode) {
                    Success {
                        vars,
                        must_implement_ability,
                        lambda_sets_to_specialize,
                        extra_metadata: _,
                    } => {
                        introduce(subs, rank, pools, &vars);
                        if !must_implement_ability.is_empty() {
                            deferred_obligations.add(
                                must_implement_ability,
                                AbilityImplError::BadPattern(*region, category.clone(), actual),
                            );
                        }
                        deferred_uls_to_resolve.union(lambda_sets_to_specialize);

                        state
                    }
                    Failure(vars, actual_type, expected_type, _bad_impls) => {
                        introduce(subs, rank, pools, &vars);

                        let problem = TypeError::BadPattern(
                            *region,
                            category.clone(),
                            actual_type,
                            expectation.clone().replace(expected_type),
                        );

                        problems.push(problem);

                        state
                    }
                    BadType(vars, problem) => {
                        introduce(subs, rank, pools, &vars);

                        problems.push(TypeError::BadType(problem));

                        state
                    }
                }
            }
            Let(index, pool_slice) => {
                let let_con = &constraints.let_constraints[index.index()];

                let offset = let_con.defs_and_ret_constraint.index();
                let defs_constraint = &constraints.constraints[offset];
                let ret_constraint = &constraints.constraints[offset + 1];

                let flex_vars = &constraints.variables[let_con.flex_vars.indices()];
                let rigid_vars = &constraints.variables[let_con.rigid_vars.indices()];

                let pool_variables = &constraints.variables[pool_slice.indices()];

                if matches!(&ret_constraint, True) && let_con.rigid_vars.is_empty() {
                    debug_assert!(pool_variables.is_empty());

                    introduce(subs, rank, pools, flex_vars);

                    // If the return expression is guaranteed to solve,
                    // solve the assignments themselves and move on.
                    stack.push(Work::Constraint {
                        env,
                        rank,
                        constraint: defs_constraint,
                    });

                    state
                } else if let_con.rigid_vars.is_empty() && let_con.flex_vars.is_empty() {
                    // items are popped from the stack in reverse order. That means that we'll
                    // first solve then defs_constraint, and then (eventually) the ret_constraint.
                    //
                    // Note that the LetConSimple gets the current env and rank,
                    // and not the env/rank from after solving the defs_constraint
                    stack.push(Work::LetConNoVariables {
                        env,
                        rank,
                        let_con,
                        pool_variables,
                    });
                    stack.push(Work::Constraint {
                        env,
                        rank,
                        constraint: defs_constraint,
                    });

                    state
                } else {
                    // work in the next pool to localize header
                    let next_rank = rank.next();

                    // introduce variables
                    for &var in rigid_vars.iter().chain(flex_vars.iter()) {
                        subs.set_rank(var, next_rank);
                    }

                    // determine the next pool
                    if next_rank.into_usize() < pools.len() {
                        // Nothing to do, we already accounted for the next rank, no need to
                        // adjust the pools
                    } else {
                        // we should be off by one at this point
                        debug_assert_eq!(next_rank.into_usize(), 1 + pools.len());
                        pools.extend_to(next_rank.into_usize());
                    }

                    let pool: &mut Vec<Variable> = pools.get_mut(next_rank);

                    // Replace the contents of this pool with rigid_vars and flex_vars
                    pool.clear();
                    pool.reserve(rigid_vars.len() + flex_vars.len());
                    pool.extend(rigid_vars.iter());
                    pool.extend(flex_vars.iter());

                    // run solver in next pool

                    // items are popped from the stack in reverse order. That means that we'll
                    // first solve then defs_constraint, and then (eventually) the ret_constraint.
                    //
                    // Note that the LetConSimple gets the current env and rank,
                    // and not the env/rank from after solving the defs_constraint
                    stack.push(Work::LetConIntroducesVariables {
                        env,
                        rank,
                        let_con,
                        pool_variables,
                    });
                    stack.push(Work::Constraint {
                        env,
                        rank: next_rank,
                        constraint: defs_constraint,
                    });

                    state
                }
            }
            IsOpenType(type_index) => {
                let actual =
                    either_type_index_to_var(constraints, subs, rank, pools, aliases, *type_index);

                open_tag_union(subs, actual);

                state
            }
            IncludesTag(index) => {
                let includes_tag = &constraints.includes_tags[index.index()];

                let roc_can::constraint::IncludesTag {
                    type_index,
                    tag_name,
                    types,
                    pattern_category,
                    region,
                } = includes_tag;

                let typ = &constraints.types[type_index.index()];
                let tys = &constraints.types[types.indices()];
                let pattern_category = &constraints.pattern_categories[pattern_category.index()];

                let actual = type_to_var(subs, rank, pools, aliases, typ);
                let tag_ty = Type::TagUnion(
                    vec![(tag_name.clone(), tys.to_vec())],
                    TypeExtension::Closed,
                );
                let includes = type_to_var(subs, rank, pools, aliases, &tag_ty);

                match unify(subs, actual, includes, Mode::PRESENT) {
                    Success {
                        vars,
                        must_implement_ability,
                        lambda_sets_to_specialize,
                        extra_metadata: _,
                    } => {
                        introduce(subs, rank, pools, &vars);
                        if !must_implement_ability.is_empty() {
                            deferred_obligations.add(
                                must_implement_ability,
                                AbilityImplError::BadPattern(
                                    *region,
                                    pattern_category.clone(),
                                    actual,
                                ),
                            );
                        }
                        deferred_uls_to_resolve.union(lambda_sets_to_specialize);

                        state
                    }
                    Failure(vars, actual_type, expected_to_include_type, _bad_impls) => {
                        introduce(subs, rank, pools, &vars);

                        let problem = TypeError::BadPattern(
                            *region,
                            pattern_category.clone(),
                            expected_to_include_type,
                            PExpected::NoExpectation(actual_type),
                        );
                        problems.push(problem);

                        state
                    }
                    BadType(vars, problem) => {
                        introduce(subs, rank, pools, &vars);

                        problems.push(TypeError::BadType(problem));

                        state
                    }
                }
            }
            &Exhaustive(eq, sketched_rows, context, exhaustive_mark) => {
                // A few cases:
                //  1. Either condition or branch types already have a type error. In this case just
                //     propagate it.
                //  2. Types are correct, but there are redundancies. In this case we want
                //     exhaustiveness checking to pull those out.
                //  3. Condition and branch types are "almost equal", that is one or the other is
                //     only missing a few more tags. In this case we want to run
                //     exhaustiveness checking both ways, to see which one is missing tags.
                //  4. Condition and branch types aren't "almost equal", this is just a normal type
                //     error.

                let (real_var, real_region, expected_type, category_and_expected) = match eq {
                    Ok(eq) => {
                        let roc_can::constraint::Eq(real_var, expected, category, real_region) =
                            constraints.eq[eq.index()];
                        let expected = &constraints.expectations[expected.index()];
                        (
                            real_var,
                            real_region,
                            expected.get_type_ref(),
                            Ok((category, expected)),
                        )
                    }
                    Err(peq) => {
                        let roc_can::constraint::PatternEq(
                            real_var,
                            expected,
                            category,
                            real_region,
                        ) = constraints.pattern_eq[peq.index()];
                        let expected = &constraints.pattern_expectations[expected.index()];
                        (
                            real_var,
                            real_region,
                            expected.get_type_ref(),
                            Err((category, expected)),
                        )
                    }
                };

                let real_var =
                    either_type_index_to_var(constraints, subs, rank, pools, aliases, real_var);

                let branches_var = type_to_var(subs, rank, pools, aliases, expected_type);

                let real_content = subs.get_content_without_compacting(real_var);
                let branches_content = subs.get_content_without_compacting(branches_var);
                let already_have_error = matches!(
                    (real_content, branches_content),
                    (
                        Content::Error | Content::Structure(FlatType::Erroneous(_)),
                        _
                    ) | (
                        _,
                        Content::Error | Content::Structure(FlatType::Erroneous(_))
                    )
                );

                let snapshot = subs.snapshot();
                let outcome = unify(subs, real_var, branches_var, Mode::EQ);

                let should_check_exhaustiveness;
                match outcome {
                    Success {
                        vars,
                        must_implement_ability,
                        lambda_sets_to_specialize,
                        extra_metadata: _,
                    } => {
                        subs.commit_snapshot(snapshot);

                        introduce(subs, rank, pools, &vars);

                        deferred_obligations
                            .add(must_implement_ability, AbilityImplError::IncompleteAbility);
                        deferred_uls_to_resolve.union(lambda_sets_to_specialize);

                        // Case 1: unify error types, but don't check exhaustiveness.
                        // Case 2: run exhaustiveness to check for redundant branches.
                        should_check_exhaustiveness = !already_have_error;
                    }
                    Failure(..) => {
                        // Rollback and check for almost-equality.
                        subs.rollback_to(snapshot);

                        let almost_eq_snapshot = subs.snapshot();
                        // TODO: turn this on for bidirectional exhaustiveness checking
                        // open_tag_union(subs, real_var);
                        open_tag_union(subs, branches_var);
                        let almost_eq = matches!(
                            unify(subs, real_var, branches_var, Mode::EQ),
                            Success { .. }
                        );

                        subs.rollback_to(almost_eq_snapshot);

                        if almost_eq {
                            // Case 3: almost equal, check exhaustiveness.
                            should_check_exhaustiveness = true;
                        } else {
                            // Case 4: incompatible types, report type error.
                            // Re-run first failed unification to get the type diff.
                            match unify(subs, real_var, branches_var, Mode::EQ) {
                                Failure(vars, actual_type, expected_type, _bad_impls) => {
                                    introduce(subs, rank, pools, &vars);

                                    // Figure out the problem - it might be pattern or value
                                    // related.
                                    let problem = match category_and_expected {
                                        Ok((category, expected)) => {
                                            let real_category =
                                                constraints.categories[category.index()].clone();
                                            TypeError::BadExpr(
                                                real_region,
                                                real_category,
                                                actual_type,
                                                expected.replace_ref(expected_type),
                                            )
                                        }

                                        Err((category, expected)) => {
                                            let real_category = constraints.pattern_categories
                                                [category.index()]
                                            .clone();
                                            TypeError::BadPattern(
                                                real_region,
                                                real_category,
                                                expected_type,
                                                expected.replace_ref(actual_type),
                                            )
                                        }
                                    };

                                    problems.push(problem);
                                    should_check_exhaustiveness = false;
                                }
                                _ => internal_error!("Must be failure"),
                            }
                        }
                    }
                    BadType(vars, problem) => {
                        subs.commit_snapshot(snapshot);

                        introduce(subs, rank, pools, &vars);

                        problems.push(TypeError::BadType(problem));

                        should_check_exhaustiveness = false;
                    }
                }

                let sketched_rows = constraints.sketched_rows[sketched_rows.index()].clone();

                if should_check_exhaustiveness {
                    use roc_can::exhaustive::{check, ExhaustiveSummary};

                    let ExhaustiveSummary {
                        errors,
                        exhaustive,
                        redundancies,
                    } = check(subs, sketched_rows, context);

                    // Store information about whether the "when" is exhaustive, and
                    // which (if any) of its branches are redundant. Codegen may use
                    // this for branch-fixing and redundant elimination.
                    if !exhaustive {
                        exhaustive_mark.set_non_exhaustive(subs);
                    }
                    for redundant_mark in redundancies {
                        redundant_mark.set_redundant(subs);
                    }

                    // Store the errors.
                    problems.extend(errors.into_iter().map(TypeError::Exhaustive));
                }

                state
            }
            // TODO: turning off opportunistic specialization for now because it doesn't mesh well with
            // the polymorphic lambda resolution algorithm. After
            // https://github.com/rtfeldman/roc/issues/3207 is resolved, this may be redundant
            // anyway.
            &Resolve(OpportunisticResolve {
                specialization_variable,
                specialization_expectation,
                member,
                specialization_id,
            }) => {
                if let Some(Resolved::Specialization(specialization)) =
                    resolve_ability_specialization(
                        subs,
                        abilities_store,
                        member,
                        specialization_variable,
                    )
                {
                    abilities_store.insert_resolved(specialization_id, specialization);

                    // We must now refine the current type state to account for this specialization.
                    let lookup_constr = arena.alloc(Constraint::Lookup(
                        specialization,
                        specialization_expectation,
                        Region::zero(),
                    ));
                    stack.push(Work::Constraint {
                        env,
                        rank,
                        constraint: lookup_constr,
                    });
                }

                state
            }
            CheckCycle(cycle, cycle_mark) => {
                let Cycle {
                    def_names,
                    expr_regions,
                } = &constraints.cycles[cycle.index()];
                let symbols = &constraints.loc_symbols[def_names.indices()];

                // If the type of a symbol is not a function, that's an error.
                // Roc is strict, so only functions can be mutually recursive.
                let any_is_bad = {
                    use Content::*;

                    symbols.iter().any(|(s, _)| {
                        let var = env.get_var_by_symbol(s).expect("Symbol not solved!");
                        let content = subs.get_content_without_compacting(var);
                        !matches!(content, Error | Structure(FlatType::Func(..)))
                    })
                };

                if any_is_bad {
                    // expr regions are stored in loc_symbols (that turned out to be convenient).
                    // The symbol is just a dummy, and should not be used
                    let expr_regions = &constraints.loc_symbols[expr_regions.indices()];

                    let cycle = symbols
                        .iter()
                        .zip(expr_regions.iter())
                        .map(|(&(symbol, symbol_region), &(_, expr_region))| CycleEntry {
                            symbol,
                            symbol_region,
                            expr_region,
                        })
                        .collect();

                    problems.push(TypeError::CircularDef(cycle));

                    cycle_mark.set_illegal(subs);
                }

                state
            }
        };
    }

    state
}

fn open_tag_union(subs: &mut Subs, var: Variable) {
    let mut stack = vec![var];
    while let Some(var) = stack.pop() {
        use {Content::*, FlatType::*};

        let desc = subs.get(var);
        if let Structure(TagUnion(tags, ext)) = desc.content {
            if let Structure(EmptyTagUnion) = subs.get_content_without_compacting(ext) {
                let new_ext = subs.fresh_unnamed_flex_var();
                subs.set_rank(new_ext, desc.rank);
                let new_union = Structure(TagUnion(tags, new_ext));
                subs.set_content(var, new_union);
            }

            // Also open up all nested tag unions.
            let all_vars = tags.variables().into_iter();
            stack.extend(all_vars.flat_map(|slice| subs[slice]).map(|var| subs[var]));
        }

        // Today, an "open" constraint doesn't affect any types
        // other than tag unions. Recursive tag unions are constructed
        // at a later time (during occurs checks after tag unions are
        // resolved), so that's not handled here either.
        // NB: Handle record types here if we add presence constraints
        // to their type inference as well.
    }
}

/// If a symbol claims to specialize an ability member, check that its solved type in fact
/// does specialize the ability, and record the specialization.
#[allow(clippy::too_many_arguments)]
// Aggressive but necessary - there aren't many usages.
#[inline(always)]
fn check_ability_specialization(
    arena: &Bump,
    subs: &mut Subs,
    pools: &mut Pools,
    rank: Rank,
    abilities_store: &mut AbilitiesStore,
    problems: &mut Vec<TypeError>,
    deferred_obligations: &mut DeferredObligations,
    deferred_uls_to_resolve: &mut UlsOfVar,
    symbol: Symbol,
    symbol_loc_var: Loc<Variable>,
) {
    // If the symbol specializes an ability member, we need to make sure that the
    // inferred type for the specialization actually aligns with the expected
    // implementation.
    if let Some((ability_member, root_data)) = abilities_store.root_name_and_def(symbol) {
        let root_signature_var = root_data.signature_var();
        let parent_ability = root_data.parent_ability;

        // Check if they unify - if they don't, then the claimed specialization isn't really one,
        // and that's a type error!
        // This also fixes any latent type variables that need to be specialized to exactly what
        // the ability signature expects.

        // We need to freshly instantiate the root signature so that all unifications are reflected
        // in the specialization type, but not the original signature type.
        let root_signature_var =
            deep_copy_var_in(subs, Rank::toplevel(), pools, root_signature_var, arena);
        let snapshot = subs.snapshot();
        let unified = unify_introduced_ability_specialization(
            subs,
            root_signature_var,
            symbol_loc_var.value,
            Mode::EQ,
        );

        match unified {
            Success {
                vars,
                must_implement_ability,
                lambda_sets_to_specialize: other_lambda_sets_to_specialize,
                extra_metadata: SpecializationLsetCollector(specialization_lambda_sets),
            } => {
                let specialization_type =
                    type_implementing_specialization(&must_implement_ability, parent_ability);

                match specialization_type {
                    Some(Obligated::Opaque(opaque)) => {
                        // This is a specialization for an opaque - that's allowed.

                        subs.commit_snapshot(snapshot);
                        introduce(subs, rank, pools, &vars);

                        let specialization_lambda_sets = specialization_lambda_sets
                            .into_iter()
                            .map(|((symbol, region), var)| {
                                debug_assert_eq!(symbol, ability_member);
                                (region, var)
                            })
                            .collect();

                        deferred_uls_to_resolve.union(other_lambda_sets_to_specialize);

                        let specialization_region = symbol_loc_var.region;
                        let specialization =
                            MemberSpecialization::new(symbol, specialization_lambda_sets);
                        abilities_store.register_specialization_for_type(
                            ability_member,
                            opaque,
                            specialization,
                        );

                        // Make sure we check that the opaque has specialized all members of the
                        // ability, after we finish solving the module.
                        deferred_obligations
                            .add(must_implement_ability, AbilityImplError::IncompleteAbility);
                        // This specialization dominates any derives that might be present.
                        deferred_obligations.dominate(
                            RequestedDeriveKey {
                                opaque,
                                ability: parent_ability,
                            },
                            specialization_region,
                        );
                    }
                    Some(Obligated::Adhoc(var)) => {
                        // This is a specialization of a structural type - never allowed.

                        // Commit so that `var` persists in subs.
                        subs.commit_snapshot(snapshot);

                        let (typ, _problems) = subs.var_to_error_type(var);

                        let problem = TypeError::StructuralSpecialization {
                            region: symbol_loc_var.region,
                            typ,
                            ability: parent_ability,
                            member: ability_member,
                        };

                        problems.push(problem);
                    }
                    None => {
                        // This can happen when every ability constriant on a type variable went
                        // through only another type variable. That means this def is not specialized
                        // for one concrete type - we won't admit this.

                        // Rollback the snapshot so we unlink the root signature with the specialization,
                        // so we can have two separate error types.
                        subs.rollback_to(snapshot);

                        let (expected_type, _problems) = subs.var_to_error_type(root_signature_var);
                        let (actual_type, _problems) = subs.var_to_error_type(symbol_loc_var.value);

                        let reason = Reason::GeneralizedAbilityMemberSpecialization {
                            member_name: ability_member,
                            def_region: root_data.region,
                        };

                        let problem = TypeError::BadExpr(
                            symbol_loc_var.region,
                            Category::AbilityMemberSpecialization(ability_member),
                            actual_type,
                            Expected::ForReason(reason, expected_type, symbol_loc_var.region),
                        );

                        problems.push(problem);
                    }
                }
            }

            Failure(vars, expected_type, actual_type, unimplemented_abilities) => {
                subs.commit_snapshot(snapshot);
                introduce(subs, rank, pools, &vars);

                let reason = Reason::InvalidAbilityMemberSpecialization {
                    member_name: ability_member,
                    def_region: root_data.region,
                    unimplemented_abilities,
                };

                let problem = TypeError::BadExpr(
                    symbol_loc_var.region,
                    Category::AbilityMemberSpecialization(ability_member),
                    actual_type,
                    Expected::ForReason(reason, expected_type, symbol_loc_var.region),
                );

                problems.push(problem);
            }
            BadType(vars, problem) => {
                subs.commit_snapshot(snapshot);
                introduce(subs, rank, pools, &vars);

                problems.push(TypeError::BadType(problem));
            }
        }
    }
}

#[cfg(debug_assertions)]
fn trace_compaction_step_1(subs: &Subs, c_a: Variable, uls_a: &[Variable]) {
    let c_a = roc_types::subs::SubsFmtContent(subs.get_content_without_compacting(c_a), subs);
    let uls_a = uls_a
        .iter()
        .map(|v| {
            format!(
                "{:?}",
                roc_types::subs::SubsFmtContent(subs.get_content_without_compacting(*v), subs)
            )
        })
        .collect::<Vec<_>>()
        .join(",");
    eprintln!("===lambda set compaction===");
    eprintln!("  concrete type: {:?}", c_a);
    eprintln!("  step 1:");
    eprintln!("    uls_a = {{ {} }}", uls_a);
}

#[cfg(debug_assertions)]
fn trace_compaction_step_2(subs: &Subs, uls_a: &[Variable]) {
    let uls_a = uls_a
        .iter()
        .map(|v| {
            format!(
                "{:?}",
                roc_types::subs::SubsFmtContent(subs.get_content_without_compacting(*v), subs)
            )
        })
        .collect::<Vec<_>>()
        .join(",");
    eprintln!("  step 2:");
    eprintln!("    uls_a' = {{ {} }}", uls_a);
}

#[cfg(debug_assertions)]
fn trace_compaction_step_3start() {
    eprintln!("  step 3:");
}

#[cfg(debug_assertions)]
fn trace_compaction_step_3iter_start(
    subs: &Subs,
    iteration_lambda_set: Variable,
    t_f1: Variable,
    t_f2: Variable,
) {
    let iteration_lambda_set = roc_types::subs::SubsFmtContent(
        subs.get_content_without_compacting(iteration_lambda_set),
        subs,
    );
    let t_f1 = roc_types::subs::SubsFmtContent(subs.get_content_without_compacting(t_f1), subs);
    let t_f2 = roc_types::subs::SubsFmtContent(subs.get_content_without_compacting(t_f2), subs);
    eprintln!("    - iteration: {:?}", iteration_lambda_set);
    eprintln!("         {:?}", t_f1);
    eprintln!("      ~  {:?}", t_f2);
}

#[cfg(debug_assertions)]
#[rustfmt::skip]
fn trace_compaction_step_3iter_end(subs: &Subs, t_f_result: Variable, skipped: bool) {
    let t_f_result =
        roc_types::subs::SubsFmtContent(subs.get_content_without_compacting(t_f_result), subs);
    if skipped {
    eprintln!("      SKIP");
    }
    eprintln!("      =  {:?}\n", t_f_result);
}

macro_rules! trace_compact {
    (1. $subs:expr, $c_a:expr, $uls_a:expr) => {{
        dbg_do!(ROC_TRACE_COMPACTION, {
            trace_compaction_step_1($subs, $c_a, $uls_a)
        })
    }};
    (2. $subs:expr, $uls_a:expr) => {{
        dbg_do!(ROC_TRACE_COMPACTION, {
            trace_compaction_step_2($subs, $uls_a)
        })
    }};
    (3start.) => {{
        dbg_do!(ROC_TRACE_COMPACTION, { trace_compaction_step_3start() })
    }};
    (3iter_start. $subs:expr, $iteration_lset:expr, $t_f1:expr, $t_f2:expr) => {{
        dbg_do!(ROC_TRACE_COMPACTION, {
            trace_compaction_step_3iter_start($subs, $iteration_lset, $t_f1, $t_f2)
        })
    }};
    (3iter_end. $subs:expr, $t_f_result:expr) => {{
        dbg_do!(ROC_TRACE_COMPACTION, {
            trace_compaction_step_3iter_end($subs, $t_f_result, false)
        })
    }};
    (3iter_end_skipped. $subs:expr, $t_f_result:expr) => {{
        dbg_do!(ROC_TRACE_COMPACTION, {
            trace_compaction_step_3iter_end($subs, $t_f_result, true)
        })
    }};
}

#[inline(always)]
fn iter_concrete_of_unspecialized<'a>(
    subs: &'a Subs,
    c_a: Variable,
    uls: &'a [Uls],
) -> impl Iterator<Item = &'a Uls> {
    uls.iter()
        .filter(move |Uls(var, _, _)| subs.equivalent_without_compacting(*var, c_a))
}

/// Gets the unique unspecialized lambda resolving to concrete type `c_a` in a list of
/// unspecialized lambda sets.
#[inline(always)]
fn unique_unspecialized_lambda(subs: &Subs, c_a: Variable, uls: &[Uls]) -> Option<Uls> {
    let mut iter_concrete = iter_concrete_of_unspecialized(subs, c_a, uls);
    let uls = iter_concrete.next()?;
    debug_assert!(iter_concrete.next().is_none(), "multiple concrete");
    Some(*uls)
}

#[must_use]
pub fn compact_lambda_sets_of_vars<P: Phase>(
    subs: &mut Subs,
    derived_module: &SharedDerivedModule,
    arena: &Bump,
    pools: &mut Pools,
    uls_of_var: UlsOfVar,
    phase: &P,
    exposed_by_module: &ExposedByModule,
) -> MustImplementConstraints {
    // let mut seen = VecSet::default();
    let mut must_implement = MustImplementConstraints::default();

    let mut uls_of_var_queue = VecDeque::with_capacity(uls_of_var.len());
    uls_of_var_queue.extend(uls_of_var.drain());

    // Suppose a type variable `a` with `uls_of_var` mapping `uls_a = {l1, ... ln}` has been instantiated to a concrete type `C_a`.
    while let Some((c_a, uls_a)) = uls_of_var_queue.pop_front() {
        let c_a = subs.get_root_key_without_compacting(c_a);
        // 1. Let each `l` in `uls_a` be of form `[solved_lambdas + ... + C:f:r + ...]`.
        //    NB: There may be multiple unspecialized lambdas of form `C:f:r, C:f1:r1, ..., C:fn:rn` in `l`.
        //    In this case, let `t1, ... tm` be the other unspecialized lambdas not of form `C:_:_`,
        //    that is, none of which are now specialized to the type `C`. Then, deconstruct
        //    `l` such that `l' = [solved_lambdas + t1 + ... + tm + C:f:r]` and `l1 = [[] + C:f1:r1], ..., ln = [[] + C:fn:rn]`.
        //    Replace `l` with `l', l1, ..., ln` in `uls_a`, flattened.
        //    TODO: the flattening step described above
        let uls_a = uls_a.into_vec();
        trace_compact!(1. subs, c_a, &uls_a);

        // The flattening step - remove lambda sets that don't reference the concrete var, and for
        // flatten lambda sets that reference it more than once.
        let mut uls_a: Vec<_> = uls_a
            .into_iter()
            .flat_map(|lambda_set| {
                let LambdaSet {
                    solved,
                    recursion_var,
                    unspecialized,
                    ambient_function,
                } = subs.get_lambda_set(lambda_set);
                let lambda_set_rank = subs.get_rank(lambda_set);
                let unspecialized = subs.get_subs_slice(unspecialized);
                // TODO: is it faster to traverse once, see if we only have one concrete lambda, and
                // bail in that happy-path, rather than always splitting?
                let (concrete, mut not_concrete): (Vec<_>, Vec<_>) = unspecialized
                    .iter()
                    .copied()
                    .partition(|Uls(var, _, _)| subs.equivalent_without_compacting(*var, c_a));
                if concrete.len() == 1 {
                    // No flattening needs to be done, just return the lambda set as-is
                    return vec![lambda_set];
                }
                // Must flatten
                concrete
                    .into_iter()
                    .enumerate()
                    .map(|(i, concrete_lambda)| {
                        let (var, unspecialized) = if i == 0 {
                            // The first lambda set contains one concrete lambda, plus all solved
                            // lambdas, plus all other unspecialized lambdas.
                            // l' = [solved_lambdas + t1 + ... + tm + C:f:r]
                            let unspecialized = SubsSlice::extend_new(
                                &mut subs.unspecialized_lambda_sets,
                                not_concrete
                                    .drain(..)
                                    .chain(std::iter::once(concrete_lambda)),
                            );
                            (lambda_set, unspecialized)
                        } else {
                            // All the other lambda sets consists only of their respective concrete
                            // lambdas.
                            // ln = [[] + C:fn:rn]
                            let unspecialized = SubsSlice::extend_new(
                                &mut subs.unspecialized_lambda_sets,
                                [concrete_lambda],
                            );
                            let var = subs.fresh(Descriptor {
                                content: Content::Error,
                                rank: lambda_set_rank,
                                mark: Mark::NONE,
                                copy: OptVariable::NONE,
                            });
                            (var, unspecialized)
                        };

                        subs.set_content(
                            var,
                            Content::LambdaSet(LambdaSet {
                                solved,
                                recursion_var,
                                unspecialized,
                                ambient_function,
                            }),
                        );
                        var
                    })
                    .collect()
            })
            .collect();

        // 2. Now, each `l` in `uls_a` has a unique unspecialized lambda of form `C:f:r`.
        //    Sort `uls_a` primarily by `f` (arbitrary order), and secondarily by `r` in descending order.
        uls_a.sort_by(|v1, v2| {
            let unspec_1 = subs.get_subs_slice(subs.get_lambda_set(*v1).unspecialized);
            let unspec_2 = subs.get_subs_slice(subs.get_lambda_set(*v2).unspecialized);

            let Uls(_, f1, r1) = unique_unspecialized_lambda(subs, c_a, unspec_1).unwrap();
            let Uls(_, f2, r2) = unique_unspecialized_lambda(subs, c_a, unspec_2).unwrap();

            match f1.cmp(&f2) {
                std::cmp::Ordering::Equal => {
                    // Order by descending order of region.
                    r2.cmp(&r1)
                }
                ord => ord,
            }
        });

        trace_compact!(2. subs, &uls_a);

        // 3. For each `l` in `uls_a` with unique unspecialized lambda `C:f:r`:
        //    1. Let `t_f1` be the directly ambient function of the lambda set containing `C:f:r`. Remove `C:f:r` from `t_f1`'s lambda set.
        //       - For example, `(b' -[[] + Fo:f:2]-> {})` if `C:f:r=Fo:f:2`. Removing `Fo:f:2`, we get `(b' -[[]]-> {})`.
        //    2. Let `t_f2` be the directly ambient function of the specialization lambda set resolved by `C:f:r`.
        //       - For example, `(b -[[] + b:g:1]-> {})` if `C:f:r=Fo:f:2`, running on example from above.
        //    3. Unify `t_f1 ~ t_f2`.
        trace_compact!(3start.);
        for l in uls_a {
            // let root_lset = subs.get_root_key_without_compacting(l);
            // if seen.contains(&root_lset) {
            //     continue;
            // }

            let (new_must_implement, new_uls_of_var) = compact_lambda_set(
                subs,
                derived_module,
                arena,
                pools,
                c_a,
                l,
                phase,
                exposed_by_module,
            );

            must_implement.extend(new_must_implement);
            uls_of_var_queue.extend(new_uls_of_var.drain());

            // seen.insert(root_lset);
        }
    }

    must_implement
}

#[must_use]
#[allow(clippy::too_many_arguments)]
fn compact_lambda_set<P: Phase>(
    subs: &mut Subs,
    derived_module: &SharedDerivedModule,
    arena: &Bump,
    pools: &mut Pools,
    resolved_concrete: Variable,
    this_lambda_set: Variable,
    phase: &P,
    exposed_by_module: &ExposedByModule,
) -> (MustImplementConstraints, UlsOfVar) {
    // 3. For each `l` in `uls_a` with unique unspecialized lambda `C:f:r`:
    //    1. Let `t_f1` be the directly ambient function of the lambda set containing `C:f:r`. Remove `C:f:r` from `t_f1`'s lambda set.
    //       - For example, `(b' -[[] + Fo:f:2]-> {})` if `C:f:r=Fo:f:2`. Removing `Fo:f:2`, we get `(b' -[[]]-> {})`.
    //    2. Let `t_f2` be the directly ambient function of the specialization lambda set resolved by `C:f:r`.
    //       - For example, `(b -[[] + b:g:1]-> {})` if `C:f:r=Fo:f:2`, from the algorithm's running example.
    //    3. Unify `t_f1 ~ t_f2`.
    let LambdaSet {
        solved,
        recursion_var,
        unspecialized,
        ambient_function: t_f1,
    } = subs.get_lambda_set(this_lambda_set);
    let target_rank = subs.get_rank(this_lambda_set);

    debug_assert!(!unspecialized.is_empty());

    let unspecialized = subs.get_subs_slice(unspecialized);

    // 1. Let `t_f1` be the directly ambient function of the lambda set containing `C:f:r`.
    let Uls(c, f, r) = unique_unspecialized_lambda(subs, resolved_concrete, unspecialized).unwrap();

    debug_assert!(subs.equivalent_without_compacting(c, resolved_concrete));

    // 1b. Remove `C:f:r` from `t_f1`'s lambda set.
    let new_unspecialized: Vec<_> = unspecialized
        .iter()
        .filter(|Uls(v, _, _)| !subs.equivalent_without_compacting(*v, resolved_concrete))
        .copied()
        .collect();
    debug_assert_eq!(new_unspecialized.len(), unspecialized.len() - 1);
    let t_f1_lambda_set_without_concrete = LambdaSet {
        solved,
        recursion_var,
        unspecialized: SubsSlice::extend_new(
            &mut subs.unspecialized_lambda_sets,
            new_unspecialized,
        ),
        ambient_function: t_f1,
    };
    subs.set_content(
        this_lambda_set,
        Content::LambdaSet(t_f1_lambda_set_without_concrete),
    );

    let specialization_decision = make_specialization_decision(subs, c);

    let specialization_key = match specialization_decision {
        SpecializeDecision::Specialize(key) => key,
        SpecializeDecision::Drop => {
            // Do nothing other than to remove the concrete lambda to drop from the lambda set,
            // which we already did in 1b above.
            trace_compact!(3iter_end_skipped. subs, t_f1);
            return (Default::default(), Default::default());
        }
    };

    let specialization_ambient_function_var = get_specialization_lambda_set_ambient_function(
        subs,
        derived_module,
        phase,
        f,
        r,
        specialization_key,
        exposed_by_module,
        target_rank,
    );

    let t_f2 = match specialization_ambient_function_var {
        Ok(lset) => lset,
        Err(()) => {
            // Do nothing other than to remove the concrete lambda to drop from the lambda set,
            // which we already did in 1b above.
            trace_compact!(3iter_end_skipped. subs, t_f1);
            return (Default::default(), Default::default());
        }
    };

    // Ensure the specialized ambient function we'll unify with is not a generalized one, but one
    // at the rank of the lambda set being compacted.
    let t_f2 = deep_copy_var_in(subs, target_rank, pools, t_f2, arena);

    // 3. Unify `t_f1 ~ t_f2`.
    trace_compact!(3iter_start. subs, this_lambda_set, t_f1, t_f2);
    let (vars, new_must_implement_ability, new_lambda_sets_to_specialize, _meta) =
        unify(subs, t_f1, t_f2, Mode::EQ).expect_success("ambient functions don't unify");
    trace_compact!(3iter_end. subs, t_f1);

    introduce(subs, target_rank, pools, &vars);

    (new_must_implement_ability, new_lambda_sets_to_specialize)
}

enum SpecializationTypeKey {
    Opaque(Symbol),
    Derived(DeriveKey),
    Immediate(Symbol),
}

enum SpecializeDecision {
    Specialize(SpecializationTypeKey),
    Drop,
}

fn make_specialization_decision(subs: &Subs, var: Variable) -> SpecializeDecision {
    use Content::*;
    use SpecializationTypeKey::*;
    match subs.get_content_without_compacting(var) {
        Alias(opaque, _, _, AliasKind::Opaque) if opaque.module_id() != ModuleId::NUM => {
            SpecializeDecision::Specialize(Opaque(*opaque))
        }
        Structure(_) | Alias(_, _, _, _) => {
            // This is a structural type, find the name of the derived ability function it
            // should use.
            match roc_derive_key::Derived::encoding(subs, var) {
                Ok(derived) => match derived {
                    roc_derive_key::Derived::Immediate(imm) => {
                        SpecializeDecision::Specialize(Immediate(imm))
                        // todo!("deal with lambda set extraction from immediates")
                    }
                    roc_derive_key::Derived::Key(derive_key) => {
                        SpecializeDecision::Specialize(Derived(derive_key))
                    }
                },
                Err(DeriveError::UnboundVar) => {
                    // not specialized yet, but that also means that it can't possibly be derivable
                    // at this point?
                    // TODO: is this right? Revisit if it causes us problems in the future.
                    SpecializeDecision::Drop
                }
                Err(DeriveError::Underivable) => {
                    // we should have reported an error for this; drop the lambda set.
                    SpecializeDecision::Drop
                }
            }
        }
        Error => SpecializeDecision::Drop,
        FlexAbleVar(_, _)
        | RigidAbleVar(..)
        | FlexVar(..)
        | RigidVar(..)
        | RecursionVar { .. }
        | LambdaSet(..)
        | RangedNumber(..) => {
            internal_error!("unexpected")
        }
    }
}

#[allow(clippy::too_many_arguments)]
fn get_specialization_lambda_set_ambient_function<P: Phase>(
    subs: &mut Subs,
    derived_module: &SharedDerivedModule,
    phase: &P,
    ability_member: Symbol,
    lset_region: u8,
    specialization_key: SpecializationTypeKey,
    exposed_by_module: &ExposedByModule,
    target_rank: Rank,
) -> Result<Variable, ()> {
    match specialization_key {
        SpecializationTypeKey::Opaque(opaque) => {
            let opaque_home = opaque.module_id();
            let external_specialized_lset =
                phase.with_module_abilities_store(opaque_home, |abilities_store| {
                    let opt_specialization =
                        abilities_store.get_specialization(ability_member, opaque);
                    match (P::IS_LATE, opt_specialization) {
                        (false, None) => {
                            // doesn't specialize, we'll have reported an error for this
                            Err(())
                        }
                        (true, None) => {
                            internal_error!(
                            "expected to know a specialization for {:?}#{:?}, but it wasn't found",
                            opaque,
                            ability_member,
                        );
                        }
                        (_, Some(specialization)) => {
                            let specialized_lambda_set = *specialization
                                .specialization_lambda_sets
                                .get(&lset_region)
                                .expect("lambda set region not resolved");
                            Ok(specialized_lambda_set)
                        }
                    }
                })?;

            let specialized_ambient = phase.copy_lambda_set_ambient_function_to_home_subs(
                external_specialized_lset,
                opaque_home,
                subs,
            );

            Ok(specialized_ambient)
        }

        SpecializationTypeKey::Derived(derive_key) => {
            let mut derived_module = derived_module.lock().unwrap();

            let (_, _, specialization_lambda_sets) =
                derived_module.get_or_insert(exposed_by_module, derive_key);

            let specialized_lambda_set = *specialization_lambda_sets
                .get(&lset_region)
                .expect("lambda set region not resolved");

            let specialized_ambient = derived_module.copy_lambda_set_ambient_function_to_subs(
                specialized_lambda_set,
                subs,
                target_rank,
            );

            Ok(specialized_ambient)
        }

        SpecializationTypeKey::Immediate(imm) => {
            // Immediates are like opaques in that we can simply look up their type definition in
            // the ability store, there is nothing new to synthesize.
            //
            // THEORY: if something can become an immediate, it will always be available in the
            // local ability store, because the transformation is local (?)
            let immediate_lambda_set_at_region =
                phase.get_and_copy_ability_member_ambient_function(imm, lset_region, subs);

            Ok(immediate_lambda_set_at_region)
        }
    }
}

#[derive(Debug)]
enum LocalDefVarsVec<T> {
    Stack(arrayvec::ArrayVec<T, 32>),
    Heap(Vec<T>),
}

impl<T> LocalDefVarsVec<T> {
    #[inline(always)]
    fn with_length(length: usize) -> Self {
        if length <= 32 {
            Self::Stack(Default::default())
        } else {
            Self::Heap(Default::default())
        }
    }

    fn push(&mut self, element: T) {
        match self {
            LocalDefVarsVec::Stack(vec) => vec.push(element),
            LocalDefVarsVec::Heap(vec) => vec.push(element),
        }
    }

    fn iter(&self) -> impl Iterator<Item = &T> {
        match self {
            LocalDefVarsVec::Stack(vec) => vec.iter(),
            LocalDefVarsVec::Heap(vec) => vec.iter(),
        }
    }
}

impl LocalDefVarsVec<(Symbol, Loc<Variable>)> {
    fn from_def_types(
        constraints: &Constraints,
        rank: Rank,
        pools: &mut Pools,
        aliases: &mut Aliases,
        subs: &mut Subs,
        def_types_slice: roc_can::constraint::DefTypes,
    ) -> Self {
        let types_slice = &constraints.types[def_types_slice.types.indices()];
        let loc_symbols_slice = &constraints.loc_symbols[def_types_slice.loc_symbols.indices()];

        let mut local_def_vars = Self::with_length(types_slice.len());

        for (&(symbol, region), typ) in (loc_symbols_slice.iter()).zip(types_slice) {
            let var = type_to_var(subs, rank, pools, aliases, typ);

            local_def_vars.push((symbol, Loc { value: var, region }));
        }

        local_def_vars
    }
}

use std::cell::RefCell;
use std::collections::VecDeque;
use std::ops::ControlFlow;
std::thread_local! {
    /// Scratchpad arena so we don't need to allocate a new one all the time
    static SCRATCHPAD: RefCell<Option<bumpalo::Bump>> = RefCell::new(Some(bumpalo::Bump::with_capacity(4 * 1024)));
}

fn take_scratchpad() -> bumpalo::Bump {
    SCRATCHPAD.with(|f| f.take().unwrap())
}

fn put_scratchpad(scratchpad: bumpalo::Bump) {
    SCRATCHPAD.with(|f| {
        f.replace(Some(scratchpad));
    });
}

fn either_type_index_to_var(
    constraints: &Constraints,
    subs: &mut Subs,
    rank: Rank,
    pools: &mut Pools,
    aliases: &mut Aliases,
    either_type_index: roc_collections::soa::EitherIndex<Type, Variable>,
) -> Variable {
    match either_type_index.split() {
        Ok(type_index) => {
            let typ = &constraints.types[type_index.index()];

            type_to_var(subs, rank, pools, aliases, typ)
        }
        Err(var_index) => {
            // we cheat, and  store the variable directly in the index
            unsafe { Variable::from_index(var_index.index() as _) }
        }
    }
}

pub(crate) fn type_to_var(
    subs: &mut Subs,
    rank: Rank,
    pools: &mut Pools,
    aliases: &mut Aliases,
    typ: &Type,
) -> Variable {
    if let Type::Variable(var) = typ {
        *var
    } else {
        let mut arena = take_scratchpad();

        let var = type_to_variable(subs, rank, pools, &arena, aliases, typ, false);

        arena.reset();
        put_scratchpad(arena);

        var
    }
}

enum RegisterVariable {
    /// Based on the Type, we already know what variable this will be
    Direct(Variable),
    /// This Type needs more complicated Content. We reserve a Variable
    /// for it, but put a placeholder Content in subs
    Deferred,
}

impl RegisterVariable {
    fn from_type(
        subs: &mut Subs,
        rank: Rank,
        pools: &mut Pools,
        arena: &'_ bumpalo::Bump,
        typ: &Type,
    ) -> Self {
        use RegisterVariable::*;

        match typ {
            Type::Variable(var) => Direct(*var),
            EmptyRec => Direct(Variable::EMPTY_RECORD),
            EmptyTagUnion => Direct(Variable::EMPTY_TAG_UNION),
            Type::DelayedAlias(AliasCommon { symbol, .. }) => {
                if let Some(reserved) = Variable::get_reserved(*symbol) {
                    if rank.is_none() {
                        // reserved variables are stored with rank NONE
                        return Direct(reserved);
                    } else {
                        // for any other rank, we need to copy; it takes care of adjusting the rank
                        let copied = deep_copy_var_in(subs, rank, pools, reserved, arena);
                        return Direct(copied);
                    }
                }

                Deferred
            }
            Type::Alias { symbol, .. } => {
                if let Some(reserved) = Variable::get_reserved(*symbol) {
                    if rank.is_none() {
                        // reserved variables are stored with rank NONE
                        return Direct(reserved);
                    } else {
                        // for any other rank, we need to copy; it takes care of adjusting the rank
                        let copied = deep_copy_var_in(subs, rank, pools, reserved, arena);
                        return Direct(copied);
                    }
                }

                Deferred
            }
            _ => Deferred,
        }
    }

    #[inline(always)]
    fn with_stack<'a>(
        subs: &mut Subs,
        rank: Rank,
        pools: &mut Pools,
        arena: &'_ bumpalo::Bump,
        typ: &'a Type,
        stack: &mut bumpalo::collections::Vec<'_, TypeToVar<'a>>,
    ) -> Variable {
        match Self::from_type(subs, rank, pools, arena, typ) {
            Self::Direct(var) => var,
            Self::Deferred => {
                let var = subs.fresh_unnamed_flex_var();
                stack.push(TypeToVar::Defer {
                    typ,
                    destination: var,
                    ambient_function: AmbientFunctionPolicy::NoFunction,
                });
                var
            }
        }
    }
}

/// Instantiation of ambient functions in unspecialized lambda sets is somewhat tricky due to other
/// optimizations we have in place. This struct tells us how they should be instantiated.
#[derive(Debug)]
enum AmbientFunctionPolicy {
    /// We're not in a function. This variant may never hold for unspecialized lambda sets.
    NoFunction,
    /// We're in a known function.
    Function(Variable),
}

impl AmbientFunctionPolicy {
    fn link_to_alias_lambda_set_var(&self, subs: &mut Subs, var: Variable) {
        let ambient_function = match self {
            AmbientFunctionPolicy::Function(var) => *var,
            _ => {
                // Might be linked at a deeper point in time, ignore for now
                return;
            }
        };
        let content = subs.get_content_without_compacting(var);
        let new_content = match content {
            Content::LambdaSet(LambdaSet {
                solved,
                recursion_var,
                unspecialized,
                ambient_function: _,
            }) => Content::LambdaSet(LambdaSet {
                solved: *solved,
                recursion_var: *recursion_var,
                unspecialized: *unspecialized,
                ambient_function,
            }),
            Content::FlexVar(_) => {
                // Something like
                //   Encoder fmt <a> : List U8, fmt -a-> List U8 | fmt has EncoderFormatting
                // THEORY: Replace these with empty lambda sets. They will unify the same as a flex
                // var does, but allows us to record the ambient function properly.
                Content::LambdaSet(LambdaSet {
                    solved: UnionLabels::default(),
                    recursion_var: OptVariable::NONE,
                    unspecialized: SubsSlice::default(),
                    ambient_function,
                })
            }
            content => internal_error!("{:?}({:?}) not a lambda set", content, var),
        };
        subs.set_content_unchecked(var, new_content);
    }
}

#[derive(Debug)]
enum TypeToVar<'a> {
    Defer {
        typ: &'a Type,
        destination: Variable,
        ambient_function: AmbientFunctionPolicy,
    },
}

#[allow(clippy::too_many_arguments)]
fn type_to_variable<'a>(
    subs: &mut Subs,
    rank: Rank,
    pools: &mut Pools,
    arena: &'a bumpalo::Bump,
    aliases: &mut Aliases,
    typ: &Type,
    // Helpers for instantiating ambient functions of lambda set variables from type aliases.
    is_alias_lambda_set_arg: bool,
) -> Variable {
    use bumpalo::collections::Vec;

    let mut stack = Vec::with_capacity_in(8, arena);

    macro_rules! helper {
        ($typ:expr, $ambient_function_policy:expr) => {{
            match RegisterVariable::from_type(subs, rank, pools, arena, $typ) {
                RegisterVariable::Direct(var) => {
                    // If the variable is just a type variable but we know we're in a lambda set
                    // context, try to link to the ambient function.
                    $ambient_function_policy.link_to_alias_lambda_set_var(subs, var);

                    var
                }
                RegisterVariable::Deferred => {
                    let var = subs.fresh_unnamed_flex_var();
                    stack.push(TypeToVar::Defer {
                        typ: $typ,
                        destination: var,
                        ambient_function: $ambient_function_policy,
                    });
                    var
                }
            }
        }};
        ($typ:expr) => {{
            helper!($typ, AmbientFunctionPolicy::NoFunction)
        }};
    }

    let result = helper!(typ);

    while let Some(TypeToVar::Defer {
        typ,
        destination,
        ambient_function,
    }) = stack.pop()
    {
        match typ {
            Variable(_) | EmptyRec | EmptyTagUnion => {
                unreachable!("This variant should never be deferred!")
            }
            RangedNumber(range) => {
                let content = Content::RangedNumber(*range);

                register_with_known_var(subs, destination, rank, pools, content)
            }
            Apply(symbol, arguments, _) => {
                let new_arguments = VariableSubsSlice::reserve_into_subs(subs, arguments.len());
                for (target_index, var_index) in (new_arguments.indices()).zip(arguments) {
                    let var = helper!(var_index);
                    subs.variables[target_index] = var;
                }

                let flat_type = FlatType::Apply(*symbol, new_arguments);
                let content = Content::Structure(flat_type);

                register_with_known_var(subs, destination, rank, pools, content)
            }

            ClosureTag {
                name,
                captures,
                ambient_function,
            } => {
                let union_lambdas =
                    create_union_lambda(subs, rank, pools, arena, *name, captures, &mut stack);

                let content = Content::LambdaSet(subs::LambdaSet {
                    solved: union_lambdas,
                    // We may figure out the lambda set is recursive during solving, but it never
                    // is to begin with.
                    recursion_var: OptVariable::NONE,
                    unspecialized: SubsSlice::default(),
                    ambient_function: *ambient_function,
                });

                register_with_known_var(subs, destination, rank, pools, content)
            }
            UnspecializedLambdaSet { unspecialized } => {
                let unspecialized_slice = SubsSlice::extend_new(
                    &mut subs.unspecialized_lambda_sets,
                    std::iter::once(*unspecialized),
                );

                // `ClosureTag` ambient functions are resolved during constraint generation.
                // But `UnspecializedLambdaSet`s can only ever live in a type signature, and don't
                // correspond to a expression, so they are never constrained.
                // Instead, we resolve their ambient functions during type translation, observing
                // the invariant that a lambda set can only ever appear under a function type.
                let ambient_function = match ambient_function {
                    AmbientFunctionPolicy::NoFunction => {
                        debug_assert!(is_alias_lambda_set_arg);
                        // To be filled in during delayed type alias instantiation
                        Variable::NULL
                    }
                    AmbientFunctionPolicy::Function(var) => var,
                };

                let content = Content::LambdaSet(subs::LambdaSet {
                    unspecialized: unspecialized_slice,
                    solved: UnionLabels::default(),
                    recursion_var: OptVariable::NONE,
                    ambient_function,
                });

                register_with_known_var(subs, destination, rank, pools, content)
            }
            // This case is important for the rank of boolean variables
            Function(arguments, closure_type, ret_type) => {
                let new_arguments = VariableSubsSlice::reserve_into_subs(subs, arguments.len());
                for (target_index, var_index) in (new_arguments.indices()).zip(arguments) {
                    let var = helper!(var_index);
                    subs.variables[target_index] = var;
                }

                let ret_var = helper!(ret_type);
                let closure_var =
                    helper!(closure_type, AmbientFunctionPolicy::Function(destination));
                let content =
                    Content::Structure(FlatType::Func(new_arguments, closure_var, ret_var));

                register_with_known_var(subs, destination, rank, pools, content)
            }
            Record(fields, ext) => {
                // An empty fields is inefficient (but would be correct)
                // If hit, try to turn the value into an EmptyRecord in canonicalization
                debug_assert!(!fields.is_empty() || !ext.is_closed());

                let mut field_vars = Vec::with_capacity_in(fields.len(), arena);

                for (field, field_type) in fields {
                    let field_var = {
                        use roc_types::types::RecordField::*;
                        match &field_type {
                            Optional(t) => Optional(helper!(t)),
                            Required(t) => Required(helper!(t)),
                            Demanded(t) => Demanded(helper!(t)),
                        }
                    };

                    field_vars.push((field.clone(), field_var));
                }

                let temp_ext_var = match ext {
                    TypeExtension::Open(ext) => helper!(ext),
                    TypeExtension::Closed => Variable::EMPTY_RECORD,
                };

                let (it, new_ext_var) =
                    gather_fields_unsorted_iter(subs, RecordFields::empty(), temp_ext_var)
                        .expect("Something ended up weird in this record type");

                let it = it
                    .into_iter()
                    .map(|(field, field_type)| (field.clone(), field_type));

                field_vars.extend(it);
                insertion_sort_by(&mut field_vars, RecordFields::compare);

                let record_fields = RecordFields::insert_into_subs(subs, field_vars);

                let content = Content::Structure(FlatType::Record(record_fields, new_ext_var));

                register_with_known_var(subs, destination, rank, pools, content)
            }

            TagUnion(tags, ext) => {
                // An empty tags is inefficient (but would be correct)
                // If hit, try to turn the value into an EmptyTagUnion in canonicalization
                debug_assert!(!tags.is_empty() || !ext.is_closed());

                let (union_tags, ext) =
                    type_to_union_tags(subs, rank, pools, arena, tags, ext, &mut stack);
                let content = Content::Structure(FlatType::TagUnion(union_tags, ext));

                register_with_known_var(subs, destination, rank, pools, content)
            }
            FunctionOrTagUnion(tag_name, symbol, ext) => {
                let temp_ext_var = match ext {
                    TypeExtension::Open(ext) => helper!(ext),
                    TypeExtension::Closed => Variable::EMPTY_TAG_UNION,
                };

                let (it, ext) = roc_types::types::gather_tags_unsorted_iter(
                    subs,
                    UnionTags::default(),
                    temp_ext_var,
                );

                for _ in it {
                    unreachable!("we assert that the ext var is empty; otherwise we'd already know it was a tag union!");
                }

                let slice = SubsIndex::new(subs.tag_names.len() as u32);
                subs.tag_names.push(tag_name.clone());

                let content = Content::Structure(FlatType::FunctionOrTagUnion(slice, *symbol, ext));

                register_with_known_var(subs, destination, rank, pools, content)
            }
            RecursiveTagUnion(rec_var, tags, ext) => {
                // An empty tags is inefficient (but would be correct)
                // If hit, try to turn the value into an EmptyTagUnion in canonicalization
                debug_assert!(!tags.is_empty() || !ext.is_closed());

                let (union_tags, ext) =
                    type_to_union_tags(subs, rank, pools, arena, tags, ext, &mut stack);
                let content =
                    Content::Structure(FlatType::RecursiveTagUnion(*rec_var, union_tags, ext));

                let tag_union_var = destination;
                register_with_known_var(subs, tag_union_var, rank, pools, content);

                register_with_known_var(
                    subs,
                    *rec_var,
                    rank,
                    pools,
                    Content::RecursionVar {
                        opt_name: None,
                        structure: tag_union_var,
                    },
                );

                tag_union_var
            }

            Type::DelayedAlias(AliasCommon {
                symbol,
                type_arguments,
                lambda_set_variables,
            }) => {
                let alias_variables = {
                    let length = type_arguments.len() + lambda_set_variables.len();
                    let new_variables = VariableSubsSlice::reserve_into_subs(subs, length);

                    for (target_index, arg_type) in (new_variables.indices()).zip(type_arguments) {
                        let copy_var = helper!(arg_type);
                        subs.variables[target_index] = copy_var;
                    }

                    let it = (new_variables.indices().skip(type_arguments.len()))
                        .zip(lambda_set_variables);
                    for (target_index, ls) in it {
                        // We MUST do this now, otherwise when linking the ambient function during
                        // instantiation of the real var, there will be nothing to link against.
                        let copy_var =
                            type_to_variable(subs, rank, pools, arena, aliases, &ls.0, true);
                        subs.variables[target_index] = copy_var;
                    }

                    AliasVariables {
                        variables_start: new_variables.start,
                        type_variables_len: type_arguments.len() as _,
                        all_variables_len: length as _,
                    }
                };

                let (alias_variable, kind) = aliases.instantiate_real_var(
                    subs,
                    rank,
                    pools,
                    arena,
                    *symbol,
                    alias_variables,
                );

                let content = Content::Alias(*symbol, alias_variables, alias_variable, kind);

                register_with_known_var(subs, destination, rank, pools, content)
            }

            Type::Alias {
                symbol,
                type_arguments,
                actual,
                lambda_set_variables,
                kind,
            } => {
                debug_assert!(Variable::get_reserved(*symbol).is_none());

                let alias_variables = {
                    let length = type_arguments.len() + lambda_set_variables.len();
                    let new_variables = VariableSubsSlice::reserve_into_subs(subs, length);

                    for (target_index, OptAbleType { typ, opt_ability }) in
                        (new_variables.indices()).zip(type_arguments)
                    {
                        let copy_var = match opt_ability {
                            None => helper!(typ),
                            Some(ability) => {
                                // If this type argument is marked as being bound to an ability, we must
                                // now correctly instantiate it as so.
                                match RegisterVariable::from_type(subs, rank, pools, arena, typ) {
                                    RegisterVariable::Direct(var) => {
                                        use Content::*;
                                        match *subs.get_content_without_compacting(var) {
                                            FlexVar(opt_name) => subs
                                                .set_content(var, FlexAbleVar(opt_name, *ability)),
                                            RigidVar(..) => internal_error!("Rigid var in type arg for {:?} - this is a bug in the solver, or our understanding", actual),
                                            RigidAbleVar(..) | FlexAbleVar(..) => internal_error!("Able var in type arg for {:?} - this is a bug in the solver, or our understanding", actual),
                                            _ => {
                                                // TODO associate the type to the bound ability, and check
                                                // that it correctly implements the ability.
                                            }
                                        }
                                        var
                                    }
                                    RegisterVariable::Deferred => {
                                        // TODO associate the type to the bound ability, and check
                                        // that it correctly implements the ability.
                                        let var = subs.fresh_unnamed_flex_var();
                                        stack.push(TypeToVar::Defer {
                                            typ,
                                            destination: var,
                                            ambient_function: AmbientFunctionPolicy::NoFunction,
                                        });
                                        var
                                    }
                                }
                            }
                        };
                        subs.variables[target_index] = copy_var;
                    }

                    let it = (new_variables.indices().skip(type_arguments.len()))
                        .zip(lambda_set_variables);
                    for (target_index, ls) in it {
                        let copy_var = helper!(&ls.0);
                        subs.variables[target_index] = copy_var;
                    }

                    AliasVariables {
                        variables_start: new_variables.start,
                        type_variables_len: type_arguments.len() as _,
                        all_variables_len: length as _,
                    }
                };

                let alias_variable = if let Symbol::RESULT_RESULT = *symbol {
                    roc_result_to_var(subs, rank, pools, arena, actual, &mut stack)
                } else {
                    helper!(actual)
                };
                let content = Content::Alias(*symbol, alias_variables, alias_variable, *kind);

                register_with_known_var(subs, destination, rank, pools, content)
            }
            HostExposedAlias {
                name: symbol,
                type_arguments,
                actual: alias_type,
                actual_var,
                lambda_set_variables,
                ..
            } => {
                let alias_variables = {
                    let length = type_arguments.len() + lambda_set_variables.len();
                    let new_variables = VariableSubsSlice::reserve_into_subs(subs, length);

                    for (target_index, arg_type) in (new_variables.indices()).zip(type_arguments) {
                        let copy_var = helper!(arg_type);
                        subs.variables[target_index] = copy_var;
                    }
                    let it = (new_variables.indices().skip(type_arguments.len()))
                        .zip(lambda_set_variables);
                    for (target_index, ls) in it {
                        // We MUST do this now, otherwise when linking the ambient function during
                        // instantiation of the real var, there will be nothing to link against.
                        let copy_var =
                            type_to_variable(subs, rank, pools, arena, aliases, &ls.0, true);
                        subs.variables[target_index] = copy_var;
                    }

                    AliasVariables {
                        variables_start: new_variables.start,
                        type_variables_len: type_arguments.len() as _,
                        all_variables_len: length as _,
                    }
                };

                // cannot use helper! here because this variable may be involved in unification below
                let alias_variable =
                    type_to_variable(subs, rank, pools, arena, aliases, alias_type, false);
                // TODO(opaques): I think host-exposed aliases should always be structural
                // (when does it make sense to give a host an opaque type?)
                let content = Content::Alias(
                    *symbol,
                    alias_variables,
                    alias_variable,
                    AliasKind::Structural,
                );
                // let result = register(subs, rank, pools, content);
                let result = register_with_known_var(subs, destination, rank, pools, content);

                // We only want to unify the actual_var with the alias once
                // if it's already redirected (and therefore, redundant)
                // don't do it again
                if !subs.redundant(*actual_var) {
                    let descriptor = subs.get(result);
                    subs.union(result, *actual_var, descriptor);
                }

                result
            }
            Erroneous(problem) => {
                let problem_index = SubsIndex::push_new(&mut subs.problems, problem.clone());
                let content = Content::Structure(FlatType::Erroneous(problem_index));

                register_with_known_var(subs, destination, rank, pools, content)
            }
        };
    }

    result
}

#[inline(always)]
fn roc_result_to_var<'a>(
    subs: &mut Subs,
    rank: Rank,
    pools: &mut Pools,
    arena: &'_ bumpalo::Bump,
    result_type: &'a Type,
    stack: &mut bumpalo::collections::Vec<'_, TypeToVar<'a>>,
) -> Variable {
    match result_type {
        Type::TagUnion(tags, ext) => {
            debug_assert!(ext.is_closed());
            debug_assert!(tags.len() == 2);

            if let [(err, err_args), (ok, ok_args)] = &tags[..] {
                debug_assert_eq!(err, &subs.tag_names[0]);
                debug_assert_eq!(ok, &subs.tag_names[1]);

                if let ([err_type], [ok_type]) = (err_args.as_slice(), ok_args.as_slice()) {
                    let err_var =
                        RegisterVariable::with_stack(subs, rank, pools, arena, err_type, stack);
                    let ok_var =
                        RegisterVariable::with_stack(subs, rank, pools, arena, ok_type, stack);

                    let start = subs.variables.len() as u32;
                    let err_slice = SubsSlice::new(start, 1);
                    let ok_slice = SubsSlice::new(start + 1, 1);

                    subs.variables.push(err_var);
                    subs.variables.push(ok_var);

                    let variables = SubsSlice::new(subs.variable_slices.len() as _, 2);
                    subs.variable_slices.push(err_slice);
                    subs.variable_slices.push(ok_slice);

                    let union_tags = UnionTags::from_slices(Subs::RESULT_TAG_NAMES, variables);
                    let ext = Variable::EMPTY_TAG_UNION;

                    let content = Content::Structure(FlatType::TagUnion(union_tags, ext));

                    return register(subs, rank, pools, content);
                }
            }

            unreachable!("invalid arguments to Result.Result; canonicalization should catch this!")
        }
        _ => unreachable!("not a valid type inside a Result.Result alias"),
    }
}

fn insertion_sort_by<T, F>(arr: &mut [T], mut compare: F)
where
    F: FnMut(&T, &T) -> std::cmp::Ordering,
{
    for i in 1..arr.len() {
        let val = &arr[i];
        let mut j = i;
        let pos = arr[..i]
            .binary_search_by(|x| compare(x, val))
            .unwrap_or_else(|pos| pos);
        // Swap all elements until specific position.
        while j > pos {
            arr.swap(j - 1, j);
            j -= 1;
        }
    }
}

fn sorted_no_duplicates<T>(slice: &[(TagName, T)]) -> bool {
    match slice.split_first() {
        None => true,
        Some(((first, _), rest)) => {
            let mut current = first;

            for (next, _) in rest {
                if current >= next {
                    return false;
                } else {
                    current = next;
                }
            }

            true
        }
    }
}

fn sort_and_deduplicate<T>(tag_vars: &mut bumpalo::collections::Vec<(TagName, T)>) {
    insertion_sort_by(tag_vars, |(a, _), (b, _)| a.cmp(b));

    // deduplicate, keeping the right-most occurrence of a tag name
    let mut i = 0;

    while i < tag_vars.len() {
        match (tag_vars.get(i), tag_vars.get(i + 1)) {
            (Some((t1, _)), Some((t2, _))) => {
                if t1 == t2 {
                    tag_vars.remove(i);
                } else {
                    i += 1;
                }
            }
            _ => break,
        }
    }
}

/// Find whether the current run of tag names is in the subs.tag_names array already. If so,
/// we take a SubsSlice to the existing tag names, so we don't have to add/clone those tag names
/// and keep subs memory consumption low
fn find_tag_name_run<T>(slice: &[(TagName, T)], subs: &mut Subs) -> Option<SubsSlice<TagName>> {
    use std::cmp::Ordering;

    let tag_name = &slice.get(0)?.0;

    let mut result = None;

    // the `SubsSlice<TagName>` that inserting `slice` into subs would give
    let bigger_slice = SubsSlice::new(subs.tag_names.len() as _, slice.len() as _);

    match subs.tag_name_cache.get_mut(tag_name) {
        Some(occupied) => {
            let subs_slice = *occupied;

            let prefix_slice = SubsSlice::new(subs_slice.start, slice.len() as _);

            if slice.len() == 1 {
                return Some(prefix_slice);
            }

            match slice.len().cmp(&subs_slice.len()) {
                Ordering::Less => {
                    // we might have a prefix
                    let tag_names = &subs.tag_names[subs_slice.start as usize..];

                    for (from_subs, (from_slice, _)) in tag_names.iter().zip(slice.iter()) {
                        if from_subs != from_slice {
                            return None;
                        }
                    }

                    result = Some(prefix_slice);
                }
                Ordering::Equal => {
                    let tag_names = &subs.tag_names[subs_slice.indices()];

                    for (from_subs, (from_slice, _)) in tag_names.iter().zip(slice.iter()) {
                        if from_subs != from_slice {
                            return None;
                        }
                    }

                    result = Some(subs_slice);
                }
                Ordering::Greater => {
                    // switch to the bigger slice that is not inserted yet, but will be soon
                    *occupied = bigger_slice;
                }
            }
        }
        None => {
            subs.tag_name_cache.push(tag_name, bigger_slice);
        }
    }

    result
}

#[inline(always)]
fn register_tag_arguments<'a>(
    subs: &mut Subs,
    rank: Rank,
    pools: &mut Pools,
    arena: &'_ bumpalo::Bump,
    stack: &mut bumpalo::collections::Vec<'_, TypeToVar<'a>>,
    arguments: &'a [Type],
) -> VariableSubsSlice {
    if arguments.is_empty() {
        VariableSubsSlice::default()
    } else {
        let new_variables = VariableSubsSlice::reserve_into_subs(subs, arguments.len());
        let it = new_variables.indices().zip(arguments);

        for (target_index, argument) in it {
            let var = RegisterVariable::with_stack(subs, rank, pools, arena, argument, stack);
            subs.variables[target_index] = var;
        }

        new_variables
    }
}

/// Assumes that the tags are sorted and there are no duplicates!
fn insert_tags_fast_path<'a>(
    subs: &mut Subs,
    rank: Rank,
    pools: &mut Pools,
    arena: &'_ bumpalo::Bump,
    tags: &'a [(TagName, Vec<Type>)],
    stack: &mut bumpalo::collections::Vec<'_, TypeToVar<'a>>,
) -> UnionTags {
    if let [(TagName(tag_name), arguments)] = tags {
        let variable_slice =
            register_tag_arguments(subs, rank, pools, arena, stack, arguments.as_slice());
        let new_variable_slices =
            SubsSlice::extend_new(&mut subs.variable_slices, [variable_slice]);

        macro_rules! subs_tag_name {
            ($tag_name_slice:expr) => {
                return UnionTags::from_slices($tag_name_slice, new_variable_slices)
            };
        }

        match tag_name.as_str() {
            "Ok" => subs_tag_name!(Subs::TAG_NAME_OK.as_slice()),
            "Err" => subs_tag_name!(Subs::TAG_NAME_ERR.as_slice()),
            "InvalidNumStr" => subs_tag_name!(Subs::TAG_NAME_INVALID_NUM_STR.as_slice()),
            "BadUtf8" => subs_tag_name!(Subs::TAG_NAME_BAD_UTF_8.as_slice()),
            "OutOfBounds" => subs_tag_name!(Subs::TAG_NAME_OUT_OF_BOUNDS.as_slice()),
            _other => {}
        }
    }

    let new_variable_slices = SubsSlice::reserve_variable_slices(subs, tags.len());
    match find_tag_name_run(tags, subs) {
        Some(new_tag_names) => {
            let it = (new_variable_slices.indices()).zip(tags);

            for (variable_slice_index, (_, arguments)) in it {
                subs.variable_slices[variable_slice_index] =
                    register_tag_arguments(subs, rank, pools, arena, stack, arguments.as_slice());
            }

            UnionTags::from_slices(new_tag_names, new_variable_slices)
        }
        None => {
            let new_tag_names = SubsSlice::reserve_tag_names(subs, tags.len());

            let it = (new_variable_slices.indices())
                .zip(new_tag_names.indices())
                .zip(tags);

            for ((variable_slice_index, tag_name_index), (tag_name, arguments)) in it {
                subs.variable_slices[variable_slice_index] =
                    register_tag_arguments(subs, rank, pools, arena, stack, arguments.as_slice());

                subs.tag_names[tag_name_index] = tag_name.clone();
            }

            UnionTags::from_slices(new_tag_names, new_variable_slices)
        }
    }
}

fn insert_tags_slow_path<'a>(
    subs: &mut Subs,
    rank: Rank,
    pools: &mut Pools,
    arena: &'_ bumpalo::Bump,
    tags: &'a [(TagName, Vec<Type>)],
    mut tag_vars: bumpalo::collections::Vec<(TagName, VariableSubsSlice)>,
    stack: &mut bumpalo::collections::Vec<'_, TypeToVar<'a>>,
) -> UnionTags {
    for (tag, tag_argument_types) in tags {
        let tag_argument_types: &[Type] = tag_argument_types.as_slice();
        let new_slice = VariableSubsSlice::reserve_into_subs(subs, tag_argument_types.len());

        for (i, arg) in (new_slice.indices()).zip(tag_argument_types) {
            let var = RegisterVariable::with_stack(subs, rank, pools, arena, arg, stack);
            subs.variables[i] = var;
        }

        tag_vars.push((tag.clone(), new_slice));
    }

    sort_and_deduplicate(&mut tag_vars);

    UnionTags::insert_slices_into_subs(subs, tag_vars)
}

fn type_to_union_tags<'a>(
    subs: &mut Subs,
    rank: Rank,
    pools: &mut Pools,
    arena: &'_ bumpalo::Bump,
    tags: &'a [(TagName, Vec<Type>)],
    ext: &'a TypeExtension,
    stack: &mut bumpalo::collections::Vec<'_, TypeToVar<'a>>,
) -> (UnionTags, Variable) {
    use bumpalo::collections::Vec;

    let sorted = tags.len() == 1 || sorted_no_duplicates(tags);

    match ext {
        TypeExtension::Closed => {
            let ext = Variable::EMPTY_TAG_UNION;

            let union_tags = if sorted {
                insert_tags_fast_path(subs, rank, pools, arena, tags, stack)
            } else {
                let tag_vars = Vec::with_capacity_in(tags.len(), arena);
                insert_tags_slow_path(subs, rank, pools, arena, tags, tag_vars, stack)
            };

            (union_tags, ext)
        }
        TypeExtension::Open(ext) => {
            let mut tag_vars = Vec::with_capacity_in(tags.len(), arena);

            let temp_ext_var = RegisterVariable::with_stack(subs, rank, pools, arena, ext, stack);
            let (it, ext) = roc_types::types::gather_tags_unsorted_iter(
                subs,
                UnionTags::default(),
                temp_ext_var,
            );

            tag_vars.extend(it.map(|(n, v)| (n.clone(), v)));

            let union_tags = if tag_vars.is_empty() && sorted {
                insert_tags_fast_path(subs, rank, pools, arena, tags, stack)
            } else {
                insert_tags_slow_path(subs, rank, pools, arena, tags, tag_vars, stack)
            };

            (union_tags, ext)
        }
    }
}

fn create_union_lambda<'a>(
    subs: &mut Subs,
    rank: Rank,
    pools: &mut Pools,
    arena: &'_ bumpalo::Bump,
    closure: Symbol,
    capture_types: &'a [Type],
    stack: &mut bumpalo::collections::Vec<'_, TypeToVar<'a>>,
) -> UnionLambdas {
    let variable_slice = register_tag_arguments(subs, rank, pools, arena, stack, capture_types);
    let new_variable_slices = SubsSlice::extend_new(&mut subs.variable_slices, [variable_slice]);

    let lambda_name_slice = SubsSlice::extend_new(&mut subs.closure_names, [closure]);

    UnionLambdas::from_slices(lambda_name_slice, new_variable_slices)
}

fn check_for_infinite_type(
    subs: &mut Subs,
    problems: &mut Vec<TypeError>,
    symbol: Symbol,
    loc_var: Loc<Variable>,
) {
    let var = loc_var.value;

    while let Err((recursive, _chain)) = subs.occurs(var) {
        // try to make a union recursive, see if that helps
        match subs.get_content_without_compacting(recursive) {
            &Content::Structure(FlatType::TagUnion(tags, ext_var)) => {
                subs.mark_tag_union_recursive(recursive, tags, ext_var);
            }
            &Content::LambdaSet(subs::LambdaSet {
                solved,
                recursion_var: _,
                unspecialized,
                ambient_function: ambient_function_var,
            }) => {
                subs.mark_lambda_set_recursive(
                    recursive,
                    solved,
                    unspecialized,
                    ambient_function_var,
                );
            }

            _other => circular_error(subs, problems, symbol, &loc_var),
        }
    }
}

fn circular_error(
    subs: &mut Subs,
    problems: &mut Vec<TypeError>,
    symbol: Symbol,
    loc_var: &Loc<Variable>,
) {
    let var = loc_var.value;
    let (error_type, _) = subs.var_to_error_type(var);
    let problem = TypeError::CircularType(loc_var.region, symbol, error_type);

    subs.set_content(var, Content::Error);

    problems.push(problem);
}

fn generalize(
    subs: &mut Subs,
    young_mark: Mark,
    visit_mark: Mark,
    young_rank: Rank,
    pools: &mut Pools,
) {
    let young_vars = std::mem::take(pools.get_mut(young_rank));
    let rank_table = pool_to_rank_table(subs, young_mark, young_rank, young_vars);

    // Get the ranks right for each entry.
    // Start at low ranks so we only have to pass over the information once.
    for (index, table) in rank_table.iter().enumerate() {
        for &var in table.iter() {
            adjust_rank(subs, young_mark, visit_mark, Rank::from(index), var);
        }
    }

    let (mut last_pool, all_but_last_pool) = rank_table.split_last();

    // For variables that have rank lowerer than young_rank, register them in
    // the appropriate old pool if they are not redundant.
    for vars in all_but_last_pool {
        for var in vars {
            if !subs.redundant(var) {
                let rank = subs.get_rank(var);

                pools.get_mut(rank).push(var);
            }
        }
    }

    // For variables with rank young_rank, if rank < young_rank: register in old pool,
    // otherwise generalize
    for var in last_pool.drain(..) {
        if !subs.redundant(var) {
            let desc_rank = subs.get_rank(var);

            if desc_rank < young_rank {
                pools.get_mut(desc_rank).push(var);
            } else {
                subs.set_rank(var, Rank::NONE);
            }
        }
    }

    // re-use the last_vector (which likely has a good capacity for future runs
    *pools.get_mut(young_rank) = last_pool;
}

/// Sort the variables into buckets by rank.
#[inline]
fn pool_to_rank_table(
    subs: &mut Subs,
    young_mark: Mark,
    young_rank: Rank,
    mut young_vars: Vec<Variable>,
) -> Pools {
    let mut pools = Pools::new(young_rank.into_usize() + 1);

    // the vast majority of young variables have young_rank
    let mut i = 0;
    while i < young_vars.len() {
        let var = subs.get_root_key(young_vars[i]);

        subs.set_mark_unchecked(var, young_mark);
        let rank = subs.get_rank_unchecked(var);

        if rank != young_rank {
            debug_assert!(rank.into_usize() < young_rank.into_usize() + 1);

            pools.get_mut(rank).push(var);

            // swap an element in; don't increment i
            young_vars.swap_remove(i);
        } else {
            i += 1;
        }
    }

    std::mem::swap(pools.get_mut(young_rank), &mut young_vars);

    pools
}

/// Adjust variable ranks such that ranks never increase as you move deeper.
/// This way the outermost rank is representative of the entire structure.
fn adjust_rank(
    subs: &mut Subs,
    young_mark: Mark,
    visit_mark: Mark,
    group_rank: Rank,
    var: Variable,
) -> Rank {
    let var = subs.get_root_key(var);

    let desc_rank = subs.get_rank_unchecked(var);
    let desc_mark = subs.get_mark_unchecked(var);

    if desc_mark == young_mark {
        let content = {
            let ptr = subs.get_content_unchecked(var) as *const _;
            unsafe { &*ptr }
        };

        // Mark the variable as visited before adjusting content, as it may be cyclic.
        subs.set_mark_unchecked(var, visit_mark);

        let max_rank = adjust_rank_content(subs, young_mark, visit_mark, group_rank, content);

        subs.set_rank_unchecked(var, max_rank);
        subs.set_mark_unchecked(var, visit_mark);

        max_rank
    } else if desc_mark == visit_mark {
        // we have already visited this variable
        // (probably two variables had the same root)
        desc_rank
    } else {
        let min_rank = group_rank.min(desc_rank);

        // TODO from elm-compiler: how can min_rank ever be group_rank?
        subs.set_rank_unchecked(var, min_rank);
        subs.set_mark_unchecked(var, visit_mark);

        min_rank
    }
}

fn adjust_rank_content(
    subs: &mut Subs,
    young_mark: Mark,
    visit_mark: Mark,
    group_rank: Rank,
    content: &Content,
) -> Rank {
    use roc_types::subs::Content::*;
    use roc_types::subs::FlatType::*;

    match content {
        FlexVar(_) | RigidVar(_) | FlexAbleVar(_, _) | RigidAbleVar(_, _) | Error => group_rank,

        RecursionVar { .. } => group_rank,

        Structure(flat_type) => {
            match flat_type {
                Apply(_, args) => {
                    let mut rank = Rank::toplevel();

                    for var_index in args.into_iter() {
                        let var = subs[var_index];
                        rank = rank.max(adjust_rank(subs, young_mark, visit_mark, group_rank, var));
                    }

                    rank
                }

                Func(arg_vars, closure_var, ret_var) => {
                    let mut rank = adjust_rank(subs, young_mark, visit_mark, group_rank, *ret_var);

                    // TODO investigate further.
                    //
                    // My theory is that because the closure_var contains variables already
                    // contained in the signature only, it does not need to be part of the rank
                    // calculuation
                    if true {
                        rank = rank.max(adjust_rank(
                            subs,
                            young_mark,
                            visit_mark,
                            group_rank,
                            *closure_var,
                        ));
                    }

                    for index in arg_vars.into_iter() {
                        let var = subs[index];
                        rank = rank.max(adjust_rank(subs, young_mark, visit_mark, group_rank, var));
                    }

                    rank
                }

                EmptyRecord => {
                    // from elm-compiler: THEORY: an empty record never needs to get generalized
                    Rank::toplevel()
                }

                EmptyTagUnion => Rank::toplevel(),

                Record(fields, ext_var) => {
                    let mut rank = adjust_rank(subs, young_mark, visit_mark, group_rank, *ext_var);

                    for index in fields.iter_variables() {
                        let var = subs[index];
                        rank = rank.max(adjust_rank(subs, young_mark, visit_mark, group_rank, var));
                    }

                    rank
                }

                TagUnion(tags, ext_var) => {
                    let mut rank = adjust_rank(subs, young_mark, visit_mark, group_rank, *ext_var);
                    // For performance reasons, we only keep one representation of empty tag unions
                    // in subs. That representation exists at rank 0, which we don't always want to
                    // reflect the whole tag union as, because doing so may over-generalize free
                    // type variables.
                    // Normally this is not a problem because of the loop below that maximizes the
                    // rank from nested types in the union. But suppose we have the simple tag
                    // union
                    //   [Z]{}
                    // there are no nested types in the tags, and the empty tag union is at rank 0,
                    // so we promote the tag union to rank 0. Now if we introduce the presence
                    // constraint
                    //   [Z]{} += [S a]
                    // we'll wind up with [Z, S a]{}, but it will be at rank 0, and "a" will get
                    // over-generalized. Really, the empty tag union should be introduced at
                    // whatever current group rank we're at, and so that's how we encode it here.
                    if *ext_var == Variable::EMPTY_TAG_UNION && rank.is_none() {
                        rank = group_rank;
                    }

                    for (_, index) in tags.iter_all() {
                        let slice = subs[index];
                        for var_index in slice {
                            let var = subs[var_index];
                            rank = rank
                                .max(adjust_rank(subs, young_mark, visit_mark, group_rank, var));
                        }
                    }

                    rank
                }

                FunctionOrTagUnion(_, _, ext_var) => {
                    adjust_rank(subs, young_mark, visit_mark, group_rank, *ext_var)
                }

                RecursiveTagUnion(rec_var, tags, ext_var) => {
                    let mut rank = adjust_rank(subs, young_mark, visit_mark, group_rank, *ext_var);

                    for (_, index) in tags.iter_all() {
                        let slice = subs[index];
                        for var_index in slice {
                            let var = subs[var_index];
                            rank = rank
                                .max(adjust_rank(subs, young_mark, visit_mark, group_rank, var));
                        }
                    }

                    // The recursion var may have a higher rank than the tag union itself, if it is
                    // erroneous and escapes into a region where it is let-generalized before it is
                    // constrained back down to the rank it originated from.
                    //
                    // For example, see the `recursion_var_specialization_error` reporting test -
                    // there, we have
                    //
                    //      Job a : [Job (List (Job a)) a]
                    //
                    //      job : Job Str
                    //
                    //      when job is
                    //          Job lst _ -> lst == ""
                    //
                    // In this case, `lst` is generalized and has a higher rank for the type
                    // `(List (Job a)) as a` - notice that only the recursion var `a` is active
                    // here, not the entire recursive tag union. In the body of this branch, `lst`
                    // becomes a type error, but the nested recursion var `a` is left untouched,
                    // because it is nested under the of `lst`, not the surface type that becomes
                    // an error.
                    //
                    // Had this not become a type error, `lst` would then be constrained against
                    // `job`, and its rank would get pulled back down. So, this can only happen in
                    // the presence of type errors.
                    //
                    // In all other cases, the recursion var has the same rank as the tag union itself
                    // all types it uses are also in the tags already, so it cannot influence the
                    // rank.
                    if cfg!(debug_assertions)
                        && !matches!(
                            subs.get_content_without_compacting(*rec_var),
                            Content::Error | Content::FlexVar(..)
                        )
                    {
                        let rec_var_rank =
                            adjust_rank(subs, young_mark, visit_mark, group_rank, *rec_var);

                        debug_assert!(
                            rank >= rec_var_rank,
                            "rank was {:?} but recursion var <{:?}>{:?} has higher rank {:?}",
                            rank,
                            rec_var,
                            subs.get_content_without_compacting(*rec_var),
                            rec_var_rank
                        );
                    }

                    rank
                }

                Erroneous(_) => group_rank,
            }
        }

        Alias(_, args, real_var, _) => {
            let mut rank = Rank::toplevel();

            for var_index in args.all_variables() {
                let var = subs[var_index];
                rank = rank.max(adjust_rank(subs, young_mark, visit_mark, group_rank, var));
            }

            // from elm-compiler: THEORY: anything in the real_var would be Rank::toplevel()
            // this theory is not true in Roc! aliases of function types capture the closure var
            rank = rank.max(adjust_rank(
                subs, young_mark, visit_mark, group_rank, *real_var,
            ));

            rank
        }

        LambdaSet(subs::LambdaSet {
            solved,
            recursion_var,
            unspecialized,
            ambient_function: ambient_function_var,
        }) => {
            let mut rank = group_rank;

            for (_, index) in solved.iter_all() {
                let slice = subs[index];
                for var_index in slice {
                    let var = subs[var_index];
                    rank = rank.max(adjust_rank(subs, young_mark, visit_mark, group_rank, var));
                }
            }

            for uls_index in *unspecialized {
                let Uls(var, _, _) = subs[uls_index];
                rank = rank.max(adjust_rank(subs, young_mark, visit_mark, group_rank, var));
            }

            if let (true, Some(rec_var)) = (cfg!(debug_assertions), recursion_var.into_variable()) {
                // THEORY: unlike the situation for recursion vars under recursive tag unions,
                // recursive vars inside lambda sets can't escape into higher let-generalized regions
                // because lambda sets aren't user-facing.
                //
                // So the recursion var should be fully accounted by everything else in the lambda set
                // (since it appears in the lambda set), and if the rank is higher, it's either a
                // bug or our theory is wrong and indeed they can escape into higher regions.
                let rec_var_rank = adjust_rank(subs, young_mark, visit_mark, group_rank, rec_var);

                debug_assert!(
                    rank >= rec_var_rank,
                    "rank was {:?} but recursion var <{:?}>{:?} has higher rank {:?}",
                    rank,
                    rec_var,
                    subs.get_content_without_compacting(rec_var),
                    rec_var_rank
                );
            }

            // NEVER TOUCH the ambient function var, it would already have been passed through.
            {
                let _ = ambient_function_var;
            }

            rank
        }

        RangedNumber(_) => group_rank,
    }
}

/// Introduce some variables to Pools at the given rank.
/// Also, set each of their ranks in Subs to be the given rank.
fn introduce(subs: &mut Subs, rank: Rank, pools: &mut Pools, vars: &[Variable]) {
    let pool: &mut Vec<Variable> = pools.get_mut(rank);

    for &var in vars.iter() {
        subs.set_rank(var, rank);
    }

    pool.extend(vars);
}

fn deep_copy_var_in(
    subs: &mut Subs,
    rank: Rank,
    pools: &mut Pools,
    var: Variable,
    arena: &Bump,
) -> Variable {
    let mut visited = bumpalo::collections::Vec::with_capacity_in(256, arena);

    let pool = pools.get_mut(rank);

    let var = subs.get_root_key(var);
    match deep_copy_var_decision(subs, rank, var) {
        ControlFlow::Break(copy) => copy,
        ControlFlow::Continue(copy) => {
            deep_copy_var_help(subs, rank, pool, &mut visited, var, copy);

            // we have tracked all visited variables, and can now traverse them
            // in one go (without looking at the UnificationTable) and clear the copy field
            for var in visited {
                subs.set_copy_unchecked(var, OptVariable::NONE);
            }

            copy
        }
    }
}

#[inline]
fn has_trivial_copy(subs: &Subs, root_var: Variable) -> Option<Variable> {
    let existing_copy = subs.get_copy_unchecked(root_var);

    if let Some(copy) = existing_copy.into_variable() {
        Some(copy)
    } else if subs.get_rank_unchecked(root_var) != Rank::NONE {
        Some(root_var)
    } else {
        None
    }
}

#[inline]
fn deep_copy_var_decision(
    subs: &mut Subs,
    max_rank: Rank,
    var: Variable,
) -> ControlFlow<Variable, Variable> {
    let var = subs.get_root_key(var);
    if let Some(copy) = has_trivial_copy(subs, var) {
        ControlFlow::Break(copy)
    } else {
        let copy_descriptor = Descriptor {
            content: Content::Structure(FlatType::EmptyTagUnion),
            rank: max_rank,
            mark: Mark::NONE,
            copy: OptVariable::NONE,
        };

        let copy = subs.fresh(copy_descriptor);

        // Link the original variable to the new variable. This lets us
        // avoid making multiple copies of the variable we are instantiating.
        //
        // Need to do this before recursively copying to avoid looping.
        subs.set_mark_unchecked(var, Mark::NONE);
        subs.set_copy_unchecked(var, copy.into());

        ControlFlow::Continue(copy)
    }
}

fn deep_copy_var_help(
    subs: &mut Subs,
    max_rank: Rank,
    pool: &mut Vec<Variable>,
    visited: &mut bumpalo::collections::Vec<'_, Variable>,
    initial_source: Variable,
    initial_copy: Variable,
) -> Variable {
    use roc_types::subs::Content::*;
    use roc_types::subs::FlatType::*;

    struct DeepCopyVarWork {
        source: Variable,
        copy: Variable,
    }

    let initial = DeepCopyVarWork {
        source: initial_source,
        copy: initial_copy,
    };
    let mut stack = vec![initial];

    macro_rules! work {
        ($variable:expr) => {{
            let var = subs.get_root_key($variable);
            match deep_copy_var_decision(subs, max_rank, var) {
                ControlFlow::Break(copy) => copy,
                ControlFlow::Continue(copy) => {
                    stack.push(DeepCopyVarWork { source: var, copy });

                    copy
                }
            }
        }};
    }

    macro_rules! copy_sequence {
        ($length:expr, $variables:expr) => {{
            let new_variables = SubsSlice::reserve_into_subs(subs, $length as _);
            for (target_index, var_index) in (new_variables.indices()).zip($variables) {
                let var = subs[var_index];
                let copy_var = work!(var);
                subs.variables[target_index] = copy_var;
            }

            new_variables
        }};
    }

    macro_rules! copy_union {
        ($tags:expr) => {{
            let new_variable_slices = SubsSlice::reserve_variable_slices(subs, $tags.len());

            let it = (new_variable_slices.indices()).zip($tags.variables());
            for (target_index, index) in it {
                let slice = subs[index];

                let new_variables = copy_sequence!(slice.len(), slice);
                subs.variable_slices[target_index] = new_variables;
            }

            UnionLabels::from_slices($tags.labels(), new_variable_slices)
        }};
    }

    while let Some(DeepCopyVarWork { source: var, copy }) = stack.pop() {
        visited.push(var);
        pool.push(copy);

        let content = *subs.get_content_unchecked(var);

        // Now we recursively copy the content of the variable.
        // We have already marked the variable as copied, so we
        // will not repeat this work or crawl this variable again.
        match content {
            Structure(flat_type) => {
                let new_flat_type = match flat_type {
                    Apply(symbol, arguments) => {
                        let new_arguments = copy_sequence!(arguments.len(), arguments);

                        Apply(symbol, new_arguments)
                    }

                    Func(arguments, closure_var, ret_var) => {
                        let new_ret_var = work!(ret_var);
                        let new_closure_var = work!(closure_var);

                        let new_arguments = copy_sequence!(arguments.len(), arguments);

                        Func(new_arguments, new_closure_var, new_ret_var)
                    }

                    same @ EmptyRecord | same @ EmptyTagUnion | same @ Erroneous(_) => same,

                    Record(fields, ext_var) => {
                        let record_fields = {
                            let new_variables =
                                copy_sequence!(fields.len(), fields.iter_variables());

                            RecordFields {
                                length: fields.length,
                                field_names_start: fields.field_names_start,
                                variables_start: new_variables.start,
                                field_types_start: fields.field_types_start,
                            }
                        };

                        Record(record_fields, work!(ext_var))
                    }

                    TagUnion(tags, ext_var) => {
                        let union_tags = copy_union!(tags);

                        TagUnion(union_tags, work!(ext_var))
                    }

                    FunctionOrTagUnion(tag_name, symbol, ext_var) => {
                        FunctionOrTagUnion(tag_name, symbol, work!(ext_var))
                    }

                    RecursiveTagUnion(rec_var, tags, ext_var) => {
                        let union_tags = copy_union!(tags);

                        RecursiveTagUnion(work!(rec_var), union_tags, work!(ext_var))
                    }
                };

                subs.set_content_unchecked(copy, Structure(new_flat_type));
            }

            FlexVar(_) | FlexAbleVar(_, _) | Error => {
                subs.set_content_unchecked(copy, content);
            }

            RecursionVar {
                opt_name,
                structure,
            } => {
                let content = RecursionVar {
                    opt_name,
                    structure: work!(structure),
                };

                subs.set_content_unchecked(copy, content);
            }

            RigidVar(name) => {
                subs.set_content_unchecked(copy, FlexVar(Some(name)));
            }

            RigidAbleVar(name, ability) => {
                subs.set_content_unchecked(copy, FlexAbleVar(Some(name), ability));
            }

            Alias(symbol, arguments, real_type_var, kind) => {
                let new_variables =
                    copy_sequence!(arguments.all_variables_len, arguments.all_variables());

                let new_arguments = AliasVariables {
                    variables_start: new_variables.start,
                    ..arguments
                };

                let new_real_type_var = work!(real_type_var);
                let new_content = Alias(symbol, new_arguments, new_real_type_var, kind);

                subs.set_content_unchecked(copy, new_content);
            }

            LambdaSet(subs::LambdaSet {
                solved,
                recursion_var,
                unspecialized,
                ambient_function: ambient_function_var,
            }) => {
                let lambda_set_var = copy;

                let new_solved = copy_union!(solved);
                let new_rec_var = recursion_var.map(|v| work!(v));
                let new_unspecialized = SubsSlice::reserve_uls_slice(subs, unspecialized.len());

                for (new_uls_index, uls_index) in
                    (new_unspecialized.into_iter()).zip(unspecialized.into_iter())
                {
                    let Uls(var, sym, region) = subs[uls_index];
                    let new_var = work!(var);

                    deep_copy_uls_precondition(subs, var, new_var);

                    subs[new_uls_index] = Uls(new_var, sym, region);

                    subs.uls_of_var.add(new_var, lambda_set_var);
                }

                let new_ambient_function_var = work!(ambient_function_var);
                debug_assert_ne!(
                    ambient_function_var, new_ambient_function_var,
                    "lambda set cloned but its ambient function wasn't?"
                );

                subs.set_content_unchecked(
                    lambda_set_var,
                    LambdaSet(subs::LambdaSet {
                        solved: new_solved,
                        recursion_var: new_rec_var,
                        unspecialized: new_unspecialized,
                        ambient_function: new_ambient_function_var,
                    }),
                );
            }

            RangedNumber(range) => {
                let new_content = RangedNumber(range);

                subs.set_content_unchecked(copy, new_content);
            }
        }
    }

    initial_copy
}

#[inline(always)]
fn deep_copy_uls_precondition(subs: &Subs, original_var: Variable, new_var: Variable) {
    if cfg!(debug_assertions) {
        let content = subs.get_content_without_compacting(original_var);

        debug_assert!(
            matches!(
                content,
                Content::FlexAbleVar(..) | Content::RigidAbleVar(..)
            ),
            "var in unspecialized lamba set is not bound to an ability, it is {:?}",
            roc_types::subs::SubsFmtContent(content, subs)
        );
        debug_assert!(
            original_var != new_var,
            "unspecialized lamba set var was not instantiated"
        );
    }
}

#[inline(always)]
fn register(subs: &mut Subs, rank: Rank, pools: &mut Pools, content: Content) -> Variable {
    let descriptor = Descriptor {
        content,
        rank,
        mark: Mark::NONE,
        copy: OptVariable::NONE,
    };

    let var = subs.fresh(descriptor);

    pools.get_mut(rank).push(var);

    var
}

fn register_with_known_var(
    subs: &mut Subs,
    var: Variable,
    rank: Rank,
    pools: &mut Pools,
    content: Content,
) -> Variable {
    let descriptor = Descriptor {
        content,
        rank,
        mark: Mark::NONE,
        copy: OptVariable::NONE,
    };

    subs.set(var, descriptor);

    pools.get_mut(rank).push(var);

    var
}