Merge pull request #74 from rtfeldman/pools

Introduce Pools

src/ena/unify/mod.rs

@@ -170,6 +170,10 @@ impl<K: UnifyKey> VarValue<K> {
         self.if_not_self(self.parent, self_key)
     }
 
+    fn raw_parent(&self) -> K {
+        self.parent
+    }
+
     fn if_not_self(&self, key: K, self_key: K) -> Option<K> {
         if key == self_key {
             None
@@ -267,20 +271,29 @@ impl<S: UnificationStore> UnificationTable<S> {
     /// NB. This is a building-block operation and you would probably
     /// prefer to call `probe` below.
     pub fn get_root_key(&mut self, vid: S::Key) -> S::Key {
-        let redirect = {
-            match self.value(vid).parent(vid) {
-                None => return vid,
-                Some(redirect) => redirect,
+        match self.value(vid).parent(vid) {
+            None => vid,
+            Some(redirect) => {
+                let root_key: S::Key = self.get_root_key(redirect);
+                if root_key != redirect {
+                    // Path compression
+                    self.update_value(vid, |value| value.parent = root_key);
+                }
+
+                root_key
             }
-        };
-
-        let root_key: S::Key = self.get_root_key(redirect);
-        if root_key != redirect {
-            // Path compression
-            self.update_value(vid, |value| value.parent = root_key);
         }
+    }
 
-        root_key
+    pub fn get_root_key_without_compacting(&self, vid: S::Key) -> S::Key {
+        match self.value(vid).parent(vid) {
+            None => vid,
+            Some(redirect) => self.get_root_key_without_compacting(redirect),
+        }
+    }
+
+    pub fn is_redirect(&mut self, vid: S::Key) -> bool {
+        self.value(vid).raw_parent() != vid
     }
 
     pub fn update_value<OP>(&mut self, key: S::Key, op: OP)
@@ -400,4 +413,15 @@ where
         let id = self.get_root_key(id);
         self.value(id).value.clone()
     }
+
+    /// This is for a debug_assert! in solve() only. Do not use it elsewhere!
+    pub fn probe_value_without_compacting<K1>(&self, id: K1) -> V
+    where
+        K1: Into<K>,
+    {
+        let id = id.into();
+        let id = self.get_root_key_without_compacting(id);
+
+        self.value(id).value.clone()
+    }
 }

src/infer.rs

@@ -1,5 +1,5 @@
 use crate::collections::ImMap;
-use crate::solve::solve;
+use crate::solve;
 use crate::subs::{Content, Subs, Variable};
 use crate::types::Constraint;
 use crate::unify::Problems;
@@ -10,7 +10,7 @@ pub fn infer_expr(
     constraint: &Constraint,
     expr_var: Variable,
 ) -> Content {
-    solve(&ImMap::default(), problems, subs, constraint);
+    solve::run(&ImMap::default(), problems, subs, constraint);
 
     subs.get(expr_var).content
 }

src/load/mod.rs

@@ -9,7 +9,7 @@ use crate::parse::ast::{self, Attempting, ExposesEntry, ImportsEntry};
 use crate::parse::module::{self, module_defs};
 use crate::parse::parser::{Fail, Parser, State};
 use crate::region::{Located, Region};
-use crate::solve::solve;
+use crate::solve;
 use crate::subs::VarStore;
 use crate::subs::{Subs, Variable};
 use crate::types::Constraint;
@@ -414,8 +414,8 @@ pub fn solve_loaded(
     }
 
     for constraint in constraints {
-        solve(&vars_by_symbol, problems, subs, &constraint);
+        solve::run(&vars_by_symbol, problems, subs, &constraint);
     }
 
-    solve(&vars_by_symbol, problems, subs, &module.constraint);
+    solve::run(&vars_by_symbol, problems, subs, &module.constraint);
 }

src/solve.rs

@@ -2,7 +2,7 @@ use crate::can::ident::Lowercase;
 use crate::can::symbol::Symbol;
 use crate::collections::ImMap;
 use crate::region::Located;
-use crate::subs::{Content, Descriptor, FlatType, Subs, Variable};
+use crate::subs::{Content, Descriptor, FlatType, Mark, Rank, Subs, Variable};
 use crate::types::Constraint::{self, *};
 use crate::types::Problem;
 use crate::types::Type::{self, *};
@@ -10,20 +10,100 @@ use crate::unify::{unify, Problems};
 
 type Env = ImMap<Symbol, Variable>;
 
-pub fn solve(
+const DEFAULT_POOLS: usize = 8;
+
+#[derive(Clone)]
+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 {
+        let mut pools = Vec::with_capacity(num_pools);
+
+        for _ in 0..num_pools {
+            pools.push(Vec::new());
+        }
+
+        Pools(pools)
+    }
+
+    pub fn get_mut(&mut self, rank: Rank) -> &mut Vec<Variable> {
+        self.0
+            .get_mut(rank.into_usize())
+            .unwrap_or_else(|| panic!("Compiler bug: could not find pool at rank {}", rank))
+    }
+
+    pub fn get(&self, rank: Rank) -> &Vec<Variable> {
+        self.0
+            .get(rank.into_usize())
+            .unwrap_or_else(|| panic!("Compiler bug: could not find pool at rank {}", rank))
+    }
+
+    pub fn iter<'a>(&'a self) -> std::slice::Iter<'a, Vec<Variable>> {
+        self.0.iter()
+    }
+
+    pub fn split_last(&self) -> (&Vec<Variable>, &[Vec<Variable>]) {
+        self.0
+            .split_last()
+            .unwrap_or_else(|| panic!("Attempted to split_last() on non-empy Pools"))
+    }
+}
+
+struct State {
+    vars_by_symbol: Env,
+    mark: Mark,
+}
+
+pub fn run(
     vars_by_symbol: &Env,
     problems: &mut Problems,
     subs: &mut Subs,
     constraint: &Constraint,
 ) {
+    let mut pools = Pools::default();
+    let state = State {
+        vars_by_symbol: vars_by_symbol.clone(),
+        mark: Mark::none().next(),
+    };
+    let rank = Rank::outermost();
+
+    solve(
+        vars_by_symbol,
+        state,
+        rank,
+        &mut pools,
+        problems,
+        subs,
+        constraint,
+    );
+}
+
+fn solve(
+    vars_by_symbol: &Env,
+    state: State,
+    rank: Rank,
+    pools: &mut Pools,
+    problems: &mut Problems,
+    subs: &mut Subs,
+    constraint: &Constraint,
+) -> State {
     match constraint {
-        True => (),
+        True => state,
         Eq(typ, expected_type, _region) => {
             // TODO use region?
             let actual = type_to_var(subs, typ.clone());
             let expected = type_to_var(subs, expected_type.clone().get_type());
+            let vars = unify(subs, problems, actual, expected);
 
-            unify(subs, problems, actual, expected);
+            introduce(subs, rank, pools, &vars);
+
+            state
         }
         Lookup(symbol, expected_type, _region) => {
             // TODO use region?
@@ -36,29 +116,80 @@ pub fn solve(
                 )
             }));
             let expected = type_to_var(subs, expected_type.clone().get_type());
+            let vars = unify(subs, problems, actual, expected);
 
-            unify(subs, problems, actual, expected);
+            introduce(subs, rank, pools, &vars);
+
+            state
         }
         And(sub_constraints) => {
+            let mut state = state;
+
             for sub_constraint in sub_constraints.iter() {
-                solve(vars_by_symbol, problems, subs, sub_constraint);
+                state = solve(
+                    vars_by_symbol,
+                    state,
+                    rank,
+                    pools,
+                    problems,
+                    subs,
+                    sub_constraint,
+                );
             }
+
+            state
         }
         Pattern(_region, _category, typ, expected) => {
             // TODO use region?
             let actual = type_to_var(subs, typ.clone());
             let expected = type_to_var(subs, expected.clone().get_type());
+            let vars = unify(subs, problems, actual, expected);
 
-            unify(subs, problems, actual, expected);
+            introduce(subs, rank, pools, &vars);
+
+            state
         }
         Let(let_con) => {
             match &let_con.ret_constraint {
                 True => {
+                    introduce(subs, rank, pools, &let_con.flex_vars);
+
                     // If the return expression is guaranteed to solve,
                     // solve the assignments themselves and move on.
-                    solve(vars_by_symbol, problems, subs, &let_con.defs_constraint)
+                    solve(
+                        vars_by_symbol,
+                        state,
+                        rank,
+                        pools,
+                        problems,
+                        subs,
+                        &let_con.defs_constraint,
+                    )
                 }
                 ret_con => {
+                    let rigid_vars = &let_con.rigid_vars;
+                    let flex_vars = &let_con.flex_vars;
+
+                    // work in the next pool to localize header
+                    let next_rank = rank.next();
+
+                    let mut next_pools = pools.clone();
+
+                    // introduce variables
+                    for &var in rigid_vars.iter() {
+                        subs.set_rank(var, next_rank);
+                    }
+
+                    for &var in flex_vars.iter() {
+                        subs.set_rank(var, next_rank);
+                    }
+
+                    let pool: &mut Vec<Variable> = next_pools.get_mut(next_rank);
+
+                    pool.reserve(rigid_vars.len() + flex_vars.len());
+                    pool.extend(rigid_vars.iter());
+                    pool.extend(flex_vars.iter());
+
                     // Add a variable for each assignment to the vars_by_symbol.
                     let mut locals = ImMap::default();
 
@@ -74,8 +205,31 @@ pub fn solve(
                         );
                     }
 
+                    // run solver in next pool
+
                     // Solve the assignments' constraints first.
-                    solve(vars_by_symbol, problems, subs, &let_con.defs_constraint);
+                    let new_state = solve(
+                        vars_by_symbol,
+                        state,
+                        next_rank,
+                        &mut next_pools,
+                        problems,
+                        subs,
+                        &let_con.defs_constraint,
+                    );
+                    let young_mark = new_state.mark;
+                    let visit_mark = young_mark.next();
+                    let final_mark = visit_mark.next();
+
+                    // pop pool
+                    generalize(subs, young_mark, visit_mark, next_rank, &mut next_pools);
+
+                    next_pools.get_mut(next_rank).clear();
+
+                    // check that things went well
+                    debug_assert!(rigid_vars
+                        .iter()
+                        .all(|&var| subs.get_without_compacting(var).rank == Rank::none()));
 
                     let mut new_vars_by_symbol = vars_by_symbol.clone();
 
@@ -83,13 +237,30 @@ pub fn solve(
                         new_vars_by_symbol.insert(symbol.clone(), loc_var.value);
                     }
 
+                    // Note that this vars_by_symbol is the one returned by the
+                    // previous call to solve()
+                    let temp_state = State {
+                        vars_by_symbol: new_state.vars_by_symbol,
+                        mark: final_mark,
+                    };
+
                     // Now solve the body, using the new vars_by_symbol which includes
                     // the assignments' name-to-variable mappings.
-                    solve(&new_vars_by_symbol, problems, subs, &ret_con);
+                    let new_state = solve(
+                        &new_vars_by_symbol,
+                        temp_state,
+                        rank,
+                        &mut next_pools,
+                        problems,
+                        subs,
+                        &ret_con,
+                    );
 
                     for (symbol, loc_var) in locals {
                         check_for_infinite_type(subs, problems, symbol, loc_var);
                     }
+
+                    new_state
                 }
             }
         }
@@ -193,3 +364,193 @@ fn check_for_infinite_type(
         problems.push(problem);
     }
 }
+
+fn generalize(
+    subs: &mut Subs,
+    young_mark: Mark,
+    visit_mark: Mark,
+    young_rank: Rank,
+    pools: &mut Pools,
+) {
+    let young_vars = pools.get(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 that 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 (last_pool, all_but_last_pool) = rank_table.split_last();
+
+    // For variables that have rank lowerer than youngRank, 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(var).rank;
+
+                pools.get_mut(rank).push(var);
+            }
+        }
+    }
+
+    // For variables with rank youngRank
+    //   If rank < youngRank: register in oldPool
+    //   otherwise generalize
+    for &var in last_pool {
+        if !subs.redundant(var) {
+            let mut desc = subs.get(var);
+
+            if desc.rank < young_rank {
+                pools.get_mut(desc.rank).push(var);
+            } else {
+                desc.rank = Rank::none();
+
+                subs.set(var, desc);
+            }
+        }
+    }
+}
+
+fn pool_to_rank_table(
+    subs: &mut Subs,
+    young_mark: Mark,
+    young_rank: Rank,
+    young_vars: &[Variable],
+) -> Pools {
+    let mut pools = Pools::new(young_rank.into_usize() + 1);
+
+    // Sort the variables into buckets by rank.
+    for &var in young_vars.iter() {
+        let rank = subs.get(var).rank;
+
+        subs.set_mark(var, young_mark);
+
+        pools.get_mut(rank).push(var);
+    }
+
+    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 mut desc = subs.get(var);
+    let mark = desc.mark;
+
+    if mark == young_mark {
+        desc.mark = visit_mark;
+
+        let content = desc.content.clone();
+        let mut marked_desc = desc.clone();
+
+        // Mark the variable as visited before adjusting content, as it may be cyclic.
+        subs.set(var, desc);
+
+        let max_rank = adjust_rank_content(subs, young_mark, visit_mark, group_rank, content);
+        marked_desc.rank = max_rank;
+
+        subs.set(var, marked_desc);
+
+        max_rank
+    } else if mark == visit_mark {
+        desc.rank
+    } else {
+        let min_rank = desc.rank.min(group_rank);
+
+        // TODO from elm-compiler: how can min_rank ever be group_rank?
+        desc.rank = min_rank;
+        desc.mark = visit_mark;
+
+        subs.set(var, desc);
+
+        min_rank
+    }
+}
+
+fn adjust_rank_content(
+    subs: &mut Subs,
+    young_mark: Mark,
+    visit_mark: Mark,
+    group_rank: Rank,
+    content: Content,
+) -> Rank {
+    use crate::subs::Content::*;
+    use crate::subs::FlatType::*;
+
+    match content {
+        FlexVar(_) | RigidVar(_) | Error(_) => group_rank,
+
+        Structure(flat_type) => {
+            match flat_type {
+                Apply { args, .. } => {
+                    let mut rank = Rank::outermost();
+
+                    for var in args {
+                        rank = rank.max(adjust_rank(subs, young_mark, visit_mark, group_rank, var));
+                    }
+
+                    rank
+                }
+
+                Func(arg_vars, ret_var) => {
+                    let mut rank = adjust_rank(subs, young_mark, visit_mark, group_rank, ret_var);
+
+                    for var in arg_vars {
+                        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::outermost()
+                }
+
+                Record(fields, ext_var) => {
+                    let mut rank = adjust_rank(subs, young_mark, visit_mark, group_rank, ext_var);
+
+                    for (_, var) in fields {
+                        rank = rank.max(adjust_rank(subs, young_mark, visit_mark, group_rank, var));
+                    }
+
+                    rank
+                }
+                Erroneous(_) => group_rank,
+            }
+        }
+
+        Alias(_, _, args, _) => {
+            let mut rank = Rank::outermost();
+
+            // from elm-compiler: THEORY: anything in the realVar would be outermostRank
+            for (_, var) in args {
+                rank = rank.max(adjust_rank(subs, young_mark, visit_mark, group_rank, var));
+            }
+
+            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);
+}

src/subs.rs

@@ -11,7 +11,7 @@ pub struct Mark(i32);
 impl Mark {
     #[inline(always)]
     pub fn none() -> Mark {
-        Mark(0)
+        Mark(2)
     }
 
     #[inline(always)]
@@ -21,12 +21,12 @@ impl Mark {
 
     #[inline(always)]
     pub fn get_var_names() -> Mark {
-        Mark(2)
+        Mark(0)
     }
 
     #[inline(always)]
     pub fn next(self) -> Mark {
-        Mark(self.0 - 1)
+        Mark(self.0 + 1)
     }
 }
 
@@ -83,7 +83,7 @@ impl Into<usize> for VarStore {
     }
 }
 
-#[derive(Copy, PartialEq, Eq, Clone, Hash)]
+#[derive(Copy, Clone, PartialEq, Eq, Hash)]
 pub struct Variable(usize);
 
 impl Variable {
@@ -159,6 +159,10 @@ impl Subs {
         self.utable.probe_value(key)
     }
 
+    pub fn get_without_compacting(&self, key: Variable) -> Descriptor {
+        self.utable.probe_value_without_compacting(key)
+    }
+
     pub fn get_root_key(&mut self, key: Variable) -> Variable {
         self.utable.get_root_key(key)
     }
@@ -215,6 +219,10 @@ impl Subs {
         self.utable.unioned(left, right)
     }
 
+    pub fn redundant(&mut self, var: Variable) -> bool {
+        self.utable.is_redirect(var)
+    }
+
     pub fn occurs(&mut self, var: Variable) -> bool {
         occurs(self, &ImSet::default(), var)
     }
@@ -244,7 +252,7 @@ fn unnamed_flex_var() -> Content {
 }
 
 #[derive(Copy, Clone, Default, Debug, PartialEq, Eq, PartialOrd, Ord, Hash)]
-pub struct Rank(u8);
+pub struct Rank(usize);
 
 impl Rank {
     pub fn none() -> Self {
@@ -258,17 +266,27 @@ impl Rank {
     pub fn next(self) -> Self {
         Rank(self.0 + 1)
     }
+
+    pub fn into_usize(self) -> usize {
+        self.0
+    }
 }
 
-impl Into<u8> for Rank {
-    fn into(self) -> u8 {
-        self.0
+impl fmt::Display for Rank {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        self.0.fmt(f)
     }
 }
 
 impl Into<usize> for Rank {
     fn into(self) -> usize {
-        self.0 as usize
+        self.0
+    }
+}
+
+impl From<usize> for Rank {
+    fn from(index: usize) -> Self {
+        Rank(index)
     }
 }
 

src/unify.rs

@@ -18,9 +18,25 @@ struct RecordStructure {
 }
 
 pub type Problems = Vec<Problem>;
+pub type Pool = Vec<Variable>;
 
 #[inline(always)]
-pub fn unify(subs: &mut Subs, problems: &mut Problems, var1: Variable, var2: Variable) {
+pub fn unify(subs: &mut Subs, problems: &mut Problems, var1: Variable, var2: Variable) -> Pool {
+    let mut pool = Vec::new();
+
+    unify_pool(subs, &mut pool, problems, var1, var2);
+
+    pool
+}
+
+#[inline(always)]
+pub fn unify_pool(
+    subs: &mut Subs,
+    pool: &mut Pool,
+    problems: &mut Problems,
+    var1: Variable,
+    var2: Variable,
+) {
     if !subs.equivalent(var1, var2) {
         let ctx = Context {
             first: var1,
@@ -29,19 +45,24 @@ pub fn unify(subs: &mut Subs, problems: &mut Problems, var1: Variable, var2: Var
             second_desc: subs.get(var2),
         };
 
-        unify_context(subs, problems, ctx)
+        unify_context(subs, pool, problems, ctx);
     }
 }
 
-fn unify_context(subs: &mut Subs, problems: &mut Problems, ctx: Context) {
+fn unify_context(subs: &mut Subs, pool: &mut Pool, problems: &mut Problems, ctx: Context) {
     match &ctx.first_desc.content {
         FlexVar(opt_name) => unify_flex(subs, problems, &ctx, opt_name, &ctx.second_desc.content),
         RigidVar(name) => unify_rigid(subs, problems, &ctx, name, &ctx.second_desc.content),
-        Structure(flat_type) => {
-            unify_structure(subs, problems, &ctx, flat_type, &ctx.second_desc.content)
-        }
+        Structure(flat_type) => unify_structure(
+            subs,
+            pool,
+            problems,
+            &ctx,
+            flat_type,
+            &ctx.second_desc.content,
+        ),
         Alias(home, name, args, real_var) => {
-            unify_alias(subs, problems, &ctx, home, name, args, *real_var)
+            unify_alias(subs, pool, problems, &ctx, home, name, args, *real_var)
         }
         Error(problem) => {
             // Error propagates. Whatever we're comparing it to doesn't matter!
@@ -51,9 +72,12 @@ fn unify_context(subs: &mut Subs, problems: &mut Problems, ctx: Context) {
     }
 }
 
+// TODO trim down this arg list
+#[allow(clippy::too_many_arguments)]
 #[inline(always)]
 fn unify_alias(
     subs: &mut Subs,
+    pool: &mut Pool,
     problems: &mut Problems,
     ctx: &Context,
     home: &ModuleName,
@@ -72,7 +96,7 @@ fn unify_alias(
                 Alias(home.clone(), name.clone(), args.to_owned(), real_var),
             );
         }
-        RigidVar(_) => unify(subs, problems, real_var, ctx.second),
+        RigidVar(_) => unify_pool(subs, pool, problems, real_var, ctx.second),
         Alias(other_home, other_name, other_args, other_real_var) => {
             if name == other_name && home == other_home {
                 match args.len().cmp(&other_args.len()) {
@@ -90,16 +114,16 @@ fn unify_alias(
                     }
                     Ordering::Equal => {
                         for ((_, l_var), (_, r_var)) in args.iter().zip(other_args.iter()) {
-                            unify(subs, problems, *l_var, *r_var);
+                            unify_pool(subs, pool, problems, *l_var, *r_var);
                         }
                         merge(subs, &ctx, other_content.clone());
                     }
                 }
             } else {
-                unify(subs, problems, real_var, *other_real_var)
+                unify_pool(subs, pool, problems, real_var, *other_real_var)
             }
         }
-        Structure(_) => unify(subs, problems, real_var, ctx.second),
+        Structure(_) => unify_pool(subs, pool, problems, real_var, ctx.second),
         Error(problem) => {
             merge(subs, ctx, Error(problem.clone()));
             problems.push(problem.clone());
@@ -110,6 +134,7 @@ fn unify_alias(
 #[inline(always)]
 fn unify_structure(
     subs: &mut Subs,
+    pool: &mut Pool,
     problems: &mut Problems,
     ctx: &Context,
     flat_type: &FlatType,
@@ -128,9 +153,9 @@ fn unify_structure(
         }
         Structure(ref other_flat_type) => {
             // Unify the two flat types
-            unify_flat_type(subs, problems, ctx, flat_type, other_flat_type)
+            unify_flat_type(subs, pool, problems, ctx, flat_type, other_flat_type)
         }
-        Alias(_, _, _, real_var) => unify(subs, problems, ctx.first, *real_var),
+        Alias(_, _, _, real_var) => unify_pool(subs, pool, problems, ctx.first, *real_var),
         Error(problem) => {
             // Error propagates.
             merge(subs, ctx, Error(problem.clone()));
@@ -141,6 +166,7 @@ fn unify_structure(
 
 fn unify_record(
     subs: &mut Subs,
+    pool: &mut Pool,
     problems: &mut Problems,
     ctx: &Context,
     rec1: RecordStructure,
@@ -156,9 +182,10 @@ fn unify_record(
 
     if unique_fields1.is_empty() {
         if unique_fields2.is_empty() {
-            unify(subs, problems, rec1.ext, rec2.ext);
+            unify_pool(subs, pool, problems, rec1.ext, rec2.ext);
             unify_shared_fields(
                 subs,
+                pool,
                 problems,
                 ctx,
                 shared_fields,
@@ -167,12 +194,13 @@ fn unify_record(
             )
         } else {
             let flat_type = FlatType::Record(unique_fields2, rec2.ext);
-            let sub_record = subs.fresh(Structure(flat_type).into());
+            let sub_record = fresh(subs, pool, ctx, Structure(flat_type));
 
-            unify(subs, problems, rec1.ext, sub_record);
+            unify_pool(subs, pool, problems, rec1.ext, sub_record);
 
             unify_shared_fields(
                 subs,
+                pool,
                 problems,
                 ctx,
                 shared_fields,
@@ -182,12 +210,13 @@ fn unify_record(
         }
     } else if unique_fields2.is_empty() {
         let flat_type = FlatType::Record(unique_fields1, rec1.ext);
-        let sub_record = subs.fresh(Structure(flat_type).into());
+        let sub_record = fresh(subs, pool, ctx, Structure(flat_type));
 
-        unify(subs, problems, sub_record, rec2.ext);
+        unify_pool(subs, pool, problems, sub_record, rec2.ext);
 
         unify_shared_fields(
             subs,
+            pool,
             problems,
             ctx,
             shared_fields,
@@ -196,21 +225,22 @@ fn unify_record(
         );
     } else {
         let other_fields = unique_fields1.clone().union(unique_fields2.clone());
-        let ext = subs.fresh_unnamed_flex_var();
+        let ext = fresh(subs, pool, ctx, Content::FlexVar(None));
         let flat_type1 = FlatType::Record(unique_fields1, rec1.ext);
-        let sub1 = subs.fresh(Structure(flat_type1).into());
+        let sub1 = fresh(subs, pool, ctx, Structure(flat_type1));
         let flat_type2 = FlatType::Record(unique_fields2, rec2.ext);
-        let sub2 = subs.fresh(Structure(flat_type2).into());
+        let sub2 = fresh(subs, pool, ctx, Structure(flat_type2));
 
-        unify(subs, problems, rec1.ext, sub2);
-        unify(subs, problems, sub1, rec2.ext);
+        unify_pool(subs, pool, problems, rec1.ext, sub2);
+        unify_pool(subs, pool, problems, sub1, rec2.ext);
 
-        unify_shared_fields(subs, problems, ctx, shared_fields, other_fields, ext);
+        unify_shared_fields(subs, pool, problems, ctx, shared_fields, other_fields, ext);
     }
 }
 
 fn unify_shared_fields(
     subs: &mut Subs,
+    pool: &mut Pool,
     problems: &mut Problems,
     ctx: &Context,
     shared_fields: ImMap<Lowercase, (Variable, Variable)>,
@@ -225,7 +255,7 @@ fn unify_shared_fields(
 
         // TODO another way to do this might be to pass around a problems vec
         // and check to see if its length increased after doing this unification.
-        unify(subs, problems, actual, expected);
+        unify_pool(subs, pool, problems, actual, expected);
 
         if problems.len() == prev_problem_count {
             matching_fields.insert(name, actual);
@@ -248,6 +278,7 @@ fn unify_shared_fields(
 #[inline(always)]
 fn unify_flat_type(
     subs: &mut Subs,
+    pool: &mut Pool,
     problems: &mut Problems,
     ctx: &Context,
     left: &FlatType,
@@ -261,18 +292,18 @@ fn unify_flat_type(
         }
 
         (Record(fields, ext), EmptyRecord) if fields.is_empty() => {
-            unify(subs, problems, *ext, ctx.second)
+            unify_pool(subs, pool, problems, *ext, ctx.second)
         }
 
         (EmptyRecord, Record(fields, ext)) if fields.is_empty() => {
-            unify(subs, problems, ctx.first, *ext)
+            unify_pool(subs, pool, problems, ctx.first, *ext)
         }
 
         (Record(fields1, ext1), Record(fields2, ext2)) => {
             let rec1 = gather_fields(subs, problems, fields1.clone(), *ext1);
             let rec2 = gather_fields(subs, problems, fields2.clone(), *ext2);
 
-            unify_record(subs, problems, ctx, rec1, rec2)
+            unify_record(subs, pool, problems, ctx, rec1, rec2)
         }
         (
             Apply {
@@ -286,7 +317,7 @@ fn unify_flat_type(
                 args: r_args,
             },
         ) if l_module_name == r_module_name && l_type_name == r_type_name => {
-            unify_zip(subs, problems, l_args.iter(), r_args.iter());
+            unify_zip(subs, pool, problems, l_args.iter(), r_args.iter());
 
             merge(
                 subs,
@@ -302,8 +333,8 @@ fn unify_flat_type(
             Ordering::Greater => merge(subs, ctx, Error(Problem::ExtraArguments)),
             Ordering::Less => merge(subs, ctx, Error(Problem::MissingArguments)),
             Ordering::Equal => {
-                unify_zip(subs, problems, l_args.iter(), r_args.iter());
-                unify(subs, problems, *l_ret, *r_ret);
+                unify_zip(subs, pool, problems, l_args.iter(), r_args.iter());
+                unify_pool(subs, pool, problems, *l_ret, *r_ret);
                 merge(subs, ctx, Structure(Func((*r_args).clone(), *r_ret)));
             }
         },
@@ -316,12 +347,17 @@ fn unify_flat_type(
     }
 }
 
-fn unify_zip<'a, I>(subs: &mut Subs, problems: &mut Problems, left_iter: I, right_iter: I)
-where
+fn unify_zip<'a, I>(
+    subs: &mut Subs,
+    pool: &mut Pool,
+    problems: &mut Problems,
+    left_iter: I,
+    right_iter: I,
+) where
     I: Iterator<Item = &'a Variable>,
 {
     for (&l_var, &r_var) in left_iter.zip(right_iter) {
-        unify(subs, problems, l_var, r_var);
+        unify_pool(subs, pool, problems, l_var, r_var);
     }
 }
 
@@ -412,3 +448,24 @@ fn merge(subs: &mut Subs, ctx: &Context, content: Content) {
 
     subs.union(ctx.first, ctx.second, desc);
 }
+
+fn register(subs: &mut Subs, desc: Descriptor, pool: &mut Pool) -> Variable {
+    let var = subs.fresh(desc);
+
+    pool.push(var);
+
+    var
+}
+
+fn fresh(subs: &mut Subs, pool: &mut Pool, ctx: &Context, content: Content) -> Variable {
+    register(
+        subs,
+        Descriptor {
+            content,
+            rank: ctx.first_desc.rank.min(ctx.second_desc.rank),
+            mark: Mark::none(),
+            copy: None,
+        },
+        pool,
+    )
+}

tests/test_infer.rs

@@ -509,10 +509,6 @@ mod test_infer {
         );
     }
 
-    // TODO type annotations
-    // TODO BoundTypeVariables
-    // TODO conditionals
-
     #[test]
     fn indirect_always() {
         infer_eq(

tests/test_uniqueness_infer.rs

@@ -527,10 +527,6 @@ mod test_infer_uniq {
         );
     }
 
-    // TODO type annotations
-    // TODO BoundTypeVariables
-    // TODO conditionals
-
     #[test]
     fn indirect_always() {
         infer_eq(