Introduce unspecialized lambda sets to Content::LambdaSet

ast/src/solve_type.rs

@@ -1411,6 +1411,8 @@ fn adjust_rank_content(
         LambdaSet(subs::LambdaSet {
             solved,
             recursion_var,
+            // TODO: handle unspecialized
+            unspecialized: _,
         }) => {
             let mut rank = group_rank;
 
@@ -1611,6 +1613,8 @@ fn instantiate_rigids_help(
         LambdaSet(subs::LambdaSet {
             solved,
             recursion_var,
+            // TODO: handle unspecialized
+            unspecialized: _,
         }) => {
             if let Some(rec_var) = recursion_var.into_variable() {
                 instantiate_rigids_help(subs, max_rank, pools, rec_var);
@@ -1892,6 +1896,7 @@ fn deep_copy_var_help(
         LambdaSet(subs::LambdaSet {
             solved,
             recursion_var,
+            unspecialized,
         }) => {
             let mut new_variable_slices = Vec::with_capacity(solved.len());
 
@@ -1924,6 +1929,8 @@ fn deep_copy_var_help(
             let new_content = LambdaSet(subs::LambdaSet {
                 solved: new_solved,
                 recursion_var: new_rec_var,
+                // TODO: actually copy
+                unspecialized,
             });
 
             subs.set(copy, make_descriptor(new_content));

bindgen/src/bindgen.rs

@@ -123,7 +123,15 @@ fn add_type_help<'a>(
         Content::LambdaSet(LambdaSet {
             solved,
             recursion_var: _,
-        }) => add_union(env, opt_name, solved, var, types),
+            unspecialized,
+        }) => {
+            debug_assert!(
+                unspecialized.is_empty(),
+                "unspecialized lambda sets left over"
+            );
+
+            add_union(env, opt_name, solved, var, types)
+        }
         Content::Structure(FlatType::TagUnion(tags, ext_var)) => {
             debug_assert!(ext_var_is_empty_tag_union(subs, *ext_var));
 

compiler/mono/src/copy.rs

@@ -4,9 +4,12 @@ use roc_can::{
     def::Def,
     expr::{AccessorData, ClosureData, Expr, Field, WhenBranch},
 };
-use roc_types::subs::{
+use roc_types::{
+    subs::{
         self, AliasVariables, Descriptor, OptVariable, RecordFields, Subs, SubsSlice, UnionLambdas,
         UnionTags, Variable, VariableSubsSlice,
+    },
+    types::Uls,
 };
 
 /// Deep copies the type variables in the type hosted by [`var`] into [`expr`].
@@ -610,12 +613,17 @@ fn deep_copy_type_vars<'a>(
             LambdaSet(subs::LambdaSet {
                 solved,
                 recursion_var,
+                unspecialized,
             }) => {
                 let new_rec_var = recursion_var.map(|var| descend_var!(var));
                 for variables_slice_index in solved.variables() {
                     let variables_slice = subs[variables_slice_index];
                     descend_slice!(variables_slice);
                 }
+                for uls_index in unspecialized {
+                    let Uls(var, _, _) = subs[uls_index];
+                    descend_var!(var);
+                }
 
                 perform_clone!({
                     let new_variable_slices =
@@ -630,9 +638,19 @@ fn deep_copy_type_vars<'a>(
                     let new_solved =
                         UnionLambdas::from_slices(solved.labels(), new_variable_slices);
 
+                    let new_unspecialized = SubsSlice::reserve_uls_slice(subs, unspecialized.len());
+                    for (target_index, uls_index) in
+                        (new_unspecialized.into_iter()).zip(unspecialized.into_iter())
+                    {
+                        let Uls(var, sym, region) = subs[uls_index];
+                        let copy_var = subs.get_copy(var).into_variable().unwrap_or(var);
+                        subs[target_index] = Uls(copy_var, sym, region);
+                    }
+
                     LambdaSet(subs::LambdaSet {
                         solved: new_solved,
                         recursion_var: new_rec_var,
+                        unspecialized: new_unspecialized,
                     })
                 })
             }

compiler/mono/src/layout.rs

@@ -1698,7 +1698,16 @@ fn layout_from_lambda_set<'a>(
     let subs::LambdaSet {
         solved,
         recursion_var,
+        unspecialized,
     } = lset;
+
+    if !unspecialized.is_empty() {
+        internal_error!(
+            "unspecialized lambda sets remain during layout generation for {:?}",
+            roc_types::subs::SubsFmtContent(&Content::LambdaSet(lset), env.subs)
+        );
+    }
+
     match recursion_var.into_variable() {
         None => {
             let labels = solved.unsorted_lambdas(env.subs);

compiler/mono/src/layout_soa.rs

@@ -276,8 +276,11 @@ impl LambdaSet {
         let subs::LambdaSet {
             solved,
             recursion_var: _,
+            unspecialized: _,
         } = lset;
 
+        // TODO: handle unspecialized
+
         debug_assert!(
             !solved.is_empty(),
             "lambda set must contain atleast the function itself"
@@ -696,8 +699,11 @@ impl Layout {
         let subs::LambdaSet {
             solved,
             recursion_var,
+            unspecialized: _,
         } = lset;
 
+        // TODO: handle unspecialized lambda set
+
         match recursion_var.into_variable() {
             Some(rec_var) => {
                 let rec_var = subs.get_root_key_without_compacting(rec_var);

compiler/solve/src/solve.rs

@@ -19,13 +19,14 @@ use roc_problem::can::CycleEntry;
 use roc_region::all::{Loc, Region};
 use roc_types::solved_types::Solved;
 use roc_types::subs::{
-    self, AliasVariables, Content, Descriptor, FlatType, Mark, OptVariable, Rank, RecordFields,
+    self, AliasVariables, Content, Descriptor, FlatType, GetSubsSlice, Mark, OptVariable, Rank,
-    Subs, SubsIndex, SubsSlice, UnionLabels, UnionLambdas, UnionTags, Variable, VariableSubsSlice,
+    RecordFields, Subs, SubsIndex, SubsSlice, 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,
+    OptAbleVar, PatternCategory, Reason, TypeExtension, Uls,
 };
 use roc_unify::unify::{unify, Mode, Obligated, Unified::*};
 
@@ -1883,17 +1884,20 @@ fn type_to_variable<'a>(
                     // 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(),
                 });
 
                 register_with_known_var(subs, destination, rank, pools, content)
             }
-            UnspecializedLambdaSet(..) => {
+            UnspecializedLambdaSet(uls) => {
-                // TODO: instantiate properly!
+                let unspecialized = SubsSlice::extend_new(
-                let union_lambdas =
+                    &mut subs.unspecialized_lambda_sets,
-                    UnionLambdas::from_slices(SubsSlice::new(0, 0), SubsSlice::new(0, 0));
+                    std::iter::once(*uls),
+                );
 
                 let content = Content::LambdaSet(subs::LambdaSet {
-                    solved: union_lambdas,
+                    unspecialized,
+                    solved: UnionLabels::default(),
                     recursion_var: OptVariable::NONE,
                 });
 
@@ -2555,8 +2559,9 @@ fn check_for_infinite_type(
             &Content::LambdaSet(subs::LambdaSet {
                 solved,
                 recursion_var: _,
+                unspecialized,
             }) => {
-                subs.mark_lambda_set_recursive(recursive, solved);
+                subs.mark_lambda_set_recursive(recursive, solved, unspecialized);
             }
 
             _other => circular_error(subs, problems, symbol, &loc_var),
@@ -2903,6 +2908,7 @@ fn adjust_rank_content(
         LambdaSet(subs::LambdaSet {
             solved,
             recursion_var,
+            unspecialized,
         }) => {
             let mut rank = group_rank;
 
@@ -2914,6 +2920,11 @@ fn adjust_rank_content(
                 }
             }
 
+            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
@@ -3092,6 +3103,7 @@ fn instantiate_rigids_help(subs: &mut Subs, max_rank: Rank, initial: Variable) {
             LambdaSet(subs::LambdaSet {
                 solved,
                 recursion_var,
+                unspecialized,
             }) => {
                 for slice_index in solved.variables() {
                     let slice = subs.variable_slices[slice_index.index as usize];
@@ -3101,6 +3113,10 @@ fn instantiate_rigids_help(subs: &mut Subs, max_rank: Rank, initial: Variable) {
                 if let Some(rec_var) = recursion_var.into_variable() {
                     stack.push(rec_var);
                 }
+
+                for Uls(var, _, _) in subs.get_subs_slice(*unspecialized) {
+                    stack.push(*var);
+                }
             }
             &RangedNumber(typ, _) => {
                 stack.push(typ);
@@ -3364,15 +3380,31 @@ fn deep_copy_var_help(
             LambdaSet(subs::LambdaSet {
                 solved,
                 recursion_var,
+                unspecialized,
             }) => {
                 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);
+
+                    // TODO: bookkeeping of new_var -> lambda set
+                }
 
                 subs.set_content_unchecked(
                     copy,
                     LambdaSet(subs::LambdaSet {
                         solved: new_solved,
                         recursion_var: new_rec_var,
+                        unspecialized: new_unspecialized,
                     }),
                 );
             }
@@ -3388,6 +3420,26 @@ fn deep_copy_var_help(
     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 {

compiler/types/src/pretty_print.rs

@@ -2,7 +2,7 @@ use crate::subs::{
     self, AliasVariables, Content, FlatType, GetSubsSlice, Label, Subs, SubsIndex, UnionLabels,
     UnionTags, Variable,
 };
-use crate::types::{name_type_var, RecordField};
+use crate::types::{name_type_var, RecordField, Uls};
 use roc_collections::all::MutMap;
 use roc_error_macros::internal_error;
 use roc_module::ident::{Lowercase, TagName};
@@ -234,6 +234,7 @@ fn find_names_needed(
         LambdaSet(subs::LambdaSet {
             solved,
             recursion_var,
+            unspecialized,
         }) => {
             for slice_index in solved.variables() {
                 let slice = subs[slice_index];
@@ -243,6 +244,11 @@ fn find_names_needed(
                 }
             }
 
+            for uls_index in unspecialized.into_iter() {
+                let Uls(var, _, _) = subs[uls_index];
+                find_names_needed(var, subs, roots, root_appearances, names_taken);
+            }
+
             if let Some(rec_var) = recursion_var.into_variable() {
                 find_names_needed(rec_var, subs, roots, root_appearances, names_taken);
             }
@@ -965,7 +971,12 @@ pub fn resolve_lambda_set(subs: &Subs, mut var: Variable) -> ResolvedLambdaSet {
             Content::LambdaSet(subs::LambdaSet {
                 solved,
                 recursion_var: _,
+                unspecialized,
             }) => {
+                debug_assert!(
+                    unspecialized.is_empty(),
+                    "unspecialized lambda sets left over during resolution"
+                );
                 push_union(subs, solved, &mut set);
                 return ResolvedLambdaSet::Set(set);
             }

compiler/types/src/subs.rs

@@ -1,6 +1,6 @@
 #![deny(unsafe_op_in_unsafe_fn)]
 use crate::types::{
-    name_type_var, AliasKind, ErrorType, Problem, RecordField, RecordFieldsError, TypeExt,
+    name_type_var, AliasKind, ErrorType, Problem, RecordField, RecordFieldsError, TypeExt, Uls,
 };
 use roc_collections::all::{ImMap, ImSet, MutSet, SendMap};
 use roc_error_macros::internal_error;
@@ -75,6 +75,7 @@ struct SubsHeader {
     field_names: u64,
     record_fields: u64,
     variable_slices: u64,
+    unspecialized_lambda_sets: u64,
     exposed_vars_by_symbol: u64,
 }
 
@@ -92,6 +93,7 @@ impl SubsHeader {
             field_names: subs.field_names.len() as u64,
             record_fields: subs.record_fields.len() as u64,
             variable_slices: subs.variable_slices.len() as u64,
+            unspecialized_lambda_sets: subs.unspecialized_lambda_sets.len() as u64,
             exposed_vars_by_symbol: exposed_vars_by_symbol as u64,
         }
     }
@@ -138,6 +140,7 @@ impl Subs {
         written = Self::serialize_field_names(&self.field_names, writer, written)?;
         written = Self::serialize_slice(&self.record_fields, writer, written)?;
         written = Self::serialize_slice(&self.variable_slices, writer, written)?;
+        written = Self::serialize_slice(&self.unspecialized_lambda_sets, writer, written)?;
         written = Self::serialize_slice(exposed_vars_by_symbol, writer, written)?;
 
         Ok(written)
@@ -221,6 +224,8 @@ impl Subs {
             Self::deserialize_slice(bytes, header.record_fields as usize, offset);
         let (variable_slices, offset) =
             Self::deserialize_slice(bytes, header.variable_slices as usize, offset);
+        let (unspecialized_lambda_sets, offset) =
+            Self::deserialize_slice(bytes, header.unspecialized_lambda_sets as usize, offset);
         let (exposed_vars_by_symbol, _) =
             Self::deserialize_slice(bytes, header.exposed_vars_by_symbol as usize, offset);
 
@@ -233,6 +238,7 @@ impl Subs {
                 field_names,
                 record_fields: record_fields.to_vec(),
                 variable_slices: variable_slices.to_vec(),
+                unspecialized_lambda_sets: unspecialized_lambda_sets.to_vec(),
                 tag_name_cache: Default::default(),
                 problems: Default::default(),
             },
@@ -309,6 +315,7 @@ pub struct Subs {
     pub field_names: Vec<Lowercase>,
     pub record_fields: Vec<RecordField<()>>,
     pub variable_slices: Vec<VariableSubsSlice>,
+    pub unspecialized_lambda_sets: Vec<Uls>,
     pub tag_name_cache: TagNameCache,
     pub problems: Vec<Problem>,
 }
@@ -408,6 +415,20 @@ impl std::ops::IndexMut<SubsIndex<Symbol>> for Subs {
     }
 }
 
+impl std::ops::Index<SubsIndex<Uls>> for Subs {
+    type Output = Uls;
+
+    fn index(&self, index: SubsIndex<Uls>) -> &Self::Output {
+        &self.unspecialized_lambda_sets[index.index as usize]
+    }
+}
+
+impl std::ops::IndexMut<SubsIndex<Uls>> for Subs {
+    fn index_mut(&mut self, index: SubsIndex<Uls>) -> &mut Self::Output {
+        &mut self.unspecialized_lambda_sets[index.index as usize]
+    }
+}
+
 impl std::ops::IndexMut<SubsIndex<Lowercase>> for Subs {
     fn index_mut(&mut self, index: SubsIndex<Lowercase>) -> &mut Self::Output {
         &mut self.field_names[index.index as usize]
@@ -567,6 +588,17 @@ impl SubsSlice<TagName> {
     }
 }
 
+impl SubsSlice<Uls> {
+    pub fn reserve_uls_slice(subs: &mut Subs, length: usize) -> Self {
+        let start = subs.unspecialized_lambda_sets.len() as u32;
+
+        subs.unspecialized_lambda_sets
+            .extend(std::iter::repeat(Uls(Variable::NULL, Symbol::UNDERSCORE, 0)).take(length));
+
+        Self::new(start, length as u16)
+    }
+}
+
 impl<T> SubsIndex<T> {
     pub const fn new(start: u32) -> Self {
         Self {
@@ -640,6 +672,12 @@ impl GetSubsSlice<Symbol> for Subs {
     }
 }
 
+impl GetSubsSlice<Uls> for Subs {
+    fn get_subs_slice(&self, subs_slice: SubsSlice<Uls>) -> &[Uls] {
+        subs_slice.get_slice(&self.unspecialized_lambda_sets)
+    }
+}
+
 impl fmt::Debug for Subs {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         writeln!(f)?;
@@ -711,6 +749,7 @@ fn subs_fmt_content(this: &Content, subs: &Subs, f: &mut fmt::Formatter) -> fmt:
         Content::LambdaSet(LambdaSet {
             solved,
             recursion_var,
+            unspecialized,
         }) => {
             write!(f, "LambdaSet([")?;
 
@@ -727,7 +766,14 @@ fn subs_fmt_content(this: &Content, subs: &Subs, f: &mut fmt::Formatter) -> fmt:
                 write!(f, ", ")?;
             }
 
-            write!(f, "<{:?}>])", recursion_var)
+            write!(f, "]")?;
+            if let Some(rec_var) = recursion_var.into_variable() {
+                write!(f, " as <{:?}>", rec_var)?;
+            }
+            for uls in subs.get_subs_slice(*unspecialized) {
+                write!(f, " + {:?}", uls)?;
+            }
+            write!(f, ")")
         }
         Content::RangedNumber(typ, range) => {
             write!(f, "RangedNumber({:?}, {:?})", typ, range)
@@ -1518,6 +1564,7 @@ impl Subs {
             // store an empty slice at the first position
             // used for "TagOrFunction"
             variable_slices: vec![VariableSubsSlice::default()],
+            unspecialized_lambda_sets: Vec::new(),
             tag_name_cache: Default::default(),
             problems: Vec::new(),
         };
@@ -1765,12 +1812,18 @@ impl Subs {
         self.set_content(recursive, Content::Structure(flat_type));
     }
 
-    pub fn mark_lambda_set_recursive(&mut self, recursive: Variable, solved_lambdas: UnionLambdas) {
+    pub fn mark_lambda_set_recursive(
+        &mut self,
+        recursive: Variable,
+        solved_lambdas: UnionLambdas,
+        unspecialized_lambdas: SubsSlice<Uls>,
+    ) {
         let (rec_var, new_tags) = self.mark_union_recursive_help(recursive, solved_lambdas);
 
         let new_lambda_set = Content::LambdaSet(LambdaSet {
             solved: new_tags,
             recursion_var: OptVariable::from(rec_var),
+            unspecialized: unspecialized_lambdas,
         });
 
         self.set_content(recursive, new_lambda_set);
@@ -2059,6 +2112,8 @@ pub struct LambdaSet {
     ///
     /// However, we don't know if a lambda set is recursive or not until type inference.
     pub recursion_var: OptVariable,
+    /// Lambdas we won't know until an ability specialization is resolved.
+    pub unspecialized: SubsSlice<Uls>,
 }
 
 #[derive(Clone, Copy, Debug, Default)]
@@ -2935,6 +2990,7 @@ fn occurs(
             LambdaSet(self::LambdaSet {
                 solved,
                 recursion_var,
+                unspecialized: _,
             }) => {
                 let mut new_seen = seen.to_owned();
                 new_seen.push(root_var);
@@ -2945,6 +3001,9 @@ fn occurs(
                     }
                 }
 
+                // unspecialized lambda vars excluded because they are not explicitly part of the
+                // type (they only matter after being resolved).
+
                 occurs_union(subs, root_var, &new_seen, include_recursion_var, solved)
             }
             RangedNumber(typ, _range_vars) => {
@@ -3122,15 +3181,21 @@ fn explicit_substitute(
             LambdaSet(self::LambdaSet {
                 solved,
                 recursion_var,
+                unspecialized,
             }) => {
                 // NOTE recursion_var is not substituted, verify that this is correct!
                 let new_solved = explicit_substitute_union(subs, from, to, solved, seen);
 
+                for Uls(v, _, _) in subs.get_subs_slice(unspecialized) {
+                    debug_assert!(*v != from, "unspecialized lambda set vars should never occur in a position where they need to be explicitly substituted.");
+                }
+
                 subs.set_content(
                     in_var,
                     LambdaSet(self::LambdaSet {
                         solved: new_solved,
                         recursion_var,
+                        unspecialized,
                     }),
                 );
 
@@ -3239,12 +3304,18 @@ fn get_var_names(
             LambdaSet(self::LambdaSet {
                 solved,
                 recursion_var,
+                unspecialized,
             }) => {
                 let taken_names = get_var_names_union(subs, solved, taken_names);
-                match recursion_var.into_variable() {
+                let mut taken_names = match recursion_var.into_variable() {
                     Some(v) => get_var_names(subs, v, taken_names),
                     None => taken_names,
+                };
+                for uls_index in unspecialized {
+                    let Uls(v, _, _) = subs[uls_index];
+                    taken_names = get_var_names(subs, v, taken_names);
                 }
+                taken_names
             }
 
             RangedNumber(typ, _) => get_var_names(subs, typ, taken_names),
@@ -3483,10 +3554,7 @@ fn content_to_err_type(
             ErrorType::Alias(symbol, err_args, Box::new(err_type), kind)
         }
 
-        LambdaSet(self::LambdaSet {
+        LambdaSet(self::LambdaSet { .. }) => {
-            solved: _,
-            recursion_var: _r,
-        }) => {
             // Don't print lambda sets since we don't expect them to be exposed to the user
             ErrorType::Error
         }
@@ -3727,6 +3795,7 @@ struct StorageSubsOffsets {
     field_names: u32,
     record_fields: u32,
     variable_slices: u32,
+    unspecialized_lambda_sets: u32,
     problems: u32,
 }
 
@@ -3744,7 +3813,7 @@ impl StorageSubs {
     }
 
     pub fn extend_with_variable(&mut self, source: &mut Subs, variable: Variable) -> Variable {
-        deep_copy_var_to(source, &mut self.subs, variable)
+        storage_copy_var_to(source, &mut self.subs, variable)
     }
 
     pub fn import_variable_from(&mut self, source: &mut Subs, variable: Variable) -> CopiedImport {
@@ -3764,6 +3833,7 @@ impl StorageSubs {
             field_names: self.subs.field_names.len() as u32,
             record_fields: self.subs.record_fields.len() as u32,
             variable_slices: self.subs.variable_slices.len() as u32,
+            unspecialized_lambda_sets: self.subs.unspecialized_lambda_sets.len() as u32,
             problems: self.subs.problems.len() as u32,
         };
 
@@ -3775,6 +3845,7 @@ impl StorageSubs {
             field_names: target.field_names.len() as u32,
             record_fields: target.record_fields.len() as u32,
             variable_slices: target.variable_slices.len() as u32,
+            unspecialized_lambda_sets: target.unspecialized_lambda_sets.len() as u32,
             problems: target.problems.len() as u32,
         };
 
@@ -3821,6 +3892,9 @@ impl StorageSubs {
         target.closure_names.extend(self.subs.closure_names);
         target.field_names.extend(self.subs.field_names);
         target.record_fields.extend(self.subs.record_fields);
+        target
+            .unspecialized_lambda_sets
+            .extend(self.subs.unspecialized_lambda_sets);
         target.problems.extend(self.subs.problems);
 
         debug_assert_eq!(
@@ -3905,9 +3979,11 @@ impl StorageSubs {
             LambdaSet(self::LambdaSet {
                 solved,
                 recursion_var,
+                unspecialized,
             }) => LambdaSet(self::LambdaSet {
                 solved: Self::offset_lambda_set(offsets, *solved),
                 recursion_var: recursion_var.map(|v| Self::offset_variable(offsets, v)),
+                unspecialized: Self::offset_uls_slice(offsets, *unspecialized),
             }),
             RangedNumber(typ, range) => RangedNumber(Self::offset_variable(offsets, *typ), *range),
             Error => Content::Error,
@@ -3978,6 +4054,12 @@ impl StorageSubs {
         slice
     }
 
+    fn offset_uls_slice(offsets: &StorageSubsOffsets, mut slice: SubsSlice<Uls>) -> SubsSlice<Uls> {
+        slice.start += offsets.unspecialized_lambda_sets;
+
+        slice
+    }
+
     fn offset_problem(
         offsets: &StorageSubsOffsets,
         mut problem_index: SubsIndex<Problem>,
@@ -4004,7 +4086,7 @@ fn put_scratchpad(scratchpad: bumpalo::Bump) {
     });
 }
 
-pub fn deep_copy_var_to(
+pub fn storage_copy_var_to(
     source: &mut Subs, // mut to set the copy
     target: &mut Subs,
     var: Variable,
@@ -4016,14 +4098,14 @@ pub fn deep_copy_var_to(
     let copy = {
         let visited = bumpalo::collections::Vec::with_capacity_in(256, &arena);
 
-        let mut env = DeepCopyVarToEnv {
+        let mut env = StorageCopyVarToEnv {
             visited,
             source,
             target,
             max_rank: rank,
         };
 
-        let copy = deep_copy_var_to_help(&mut env, var);
+        let copy = storage_copy_var_to_help(&mut env, var);
 
         // we have tracked all visited variables, and can now traverse them
         // in one go (without looking at the UnificationTable) and clear the copy field
@@ -4046,7 +4128,7 @@ pub fn deep_copy_var_to(
     copy
 }
 
-struct DeepCopyVarToEnv<'a> {
+struct StorageCopyVarToEnv<'a> {
     visited: bumpalo::collections::Vec<'a, Variable>,
     source: &'a mut Subs,
     target: &'a mut Subs,
@@ -4054,8 +4136,8 @@ struct DeepCopyVarToEnv<'a> {
 }
 
 #[inline(always)]
-fn deep_copy_union<L: Label>(
+fn storage_copy_union<L: Label>(
-    env: &mut DeepCopyVarToEnv<'_>,
+    env: &mut StorageCopyVarToEnv<'_>,
     tags: UnionLabels<L>,
 ) -> UnionLabels<L> {
     let new_variable_slices = SubsSlice::reserve_variable_slices(env.target, tags.len());
@@ -4068,7 +4150,7 @@ fn deep_copy_union<L: Label>(
         let it = (new_variables.indices()).zip(slice);
         for (target_index, var_index) in it {
             let var = env.source[var_index];
-            let copy_var = deep_copy_var_to_help(env, var);
+            let copy_var = storage_copy_var_to_help(env, var);
             env.target.variables[target_index] = copy_var;
         }
 
@@ -4085,7 +4167,7 @@ fn deep_copy_union<L: Label>(
     UnionLabels::from_slices(new_tag_names, new_variable_slices)
 }
 
-fn deep_copy_var_to_help(env: &mut DeepCopyVarToEnv<'_>, var: Variable) -> Variable {
+fn storage_copy_var_to_help(env: &mut StorageCopyVarToEnv<'_>, var: Variable) -> Variable {
     use Content::*;
     use FlatType::*;
 
@@ -4138,7 +4220,7 @@ fn deep_copy_var_to_help(env: &mut DeepCopyVarToEnv<'_>, var: Variable) -> Varia
 
                     for (target_index, var_index) in (new_arguments.indices()).zip(arguments) {
                         let var = env.source[var_index];
-                        let copy_var = deep_copy_var_to_help(env, var);
+                        let copy_var = storage_copy_var_to_help(env, var);
                         env.target.variables[target_index] = copy_var;
                     }
 
@@ -4146,15 +4228,15 @@ fn deep_copy_var_to_help(env: &mut DeepCopyVarToEnv<'_>, var: Variable) -> Varia
                 }
 
                 Func(arguments, closure_var, ret_var) => {
-                    let new_ret_var = deep_copy_var_to_help(env, ret_var);
+                    let new_ret_var = storage_copy_var_to_help(env, ret_var);
 
-                    let new_closure_var = deep_copy_var_to_help(env, closure_var);
+                    let new_closure_var = storage_copy_var_to_help(env, closure_var);
 
                     let new_arguments = SubsSlice::reserve_into_subs(env.target, arguments.len());
 
                     for (target_index, var_index) in (new_arguments.indices()).zip(arguments) {
                         let var = env.source[var_index];
-                        let copy_var = deep_copy_var_to_help(env, var);
+                        let copy_var = storage_copy_var_to_help(env, var);
                         env.target.variables[target_index] = copy_var;
                     }
 
@@ -4171,7 +4253,7 @@ fn deep_copy_var_to_help(env: &mut DeepCopyVarToEnv<'_>, var: Variable) -> Varia
                         let it = (new_variables.indices()).zip(fields.iter_variables());
                         for (target_index, var_index) in it {
                             let var = env.source[var_index];
-                            let copy_var = deep_copy_var_to_help(env, var);
+                            let copy_var = storage_copy_var_to_help(env, var);
                             env.target.variables[target_index] = copy_var;
                         }
 
@@ -4195,12 +4277,12 @@ fn deep_copy_var_to_help(env: &mut DeepCopyVarToEnv<'_>, var: Variable) -> Varia
                         }
                     };
 
-                    Record(record_fields, deep_copy_var_to_help(env, ext_var))
+                    Record(record_fields, storage_copy_var_to_help(env, ext_var))
                 }
 
                 TagUnion(tags, ext_var) => {
-                    let new_ext = deep_copy_var_to_help(env, ext_var);
+                    let new_ext = storage_copy_var_to_help(env, ext_var);
-                    let union_tags = deep_copy_union(env, tags);
+                    let union_tags = storage_copy_union(env, tags);
 
                     TagUnion(union_tags, new_ext)
                 }
@@ -4210,14 +4292,14 @@ fn deep_copy_var_to_help(env: &mut DeepCopyVarToEnv<'_>, var: Variable) -> Varia
 
                     env.target.tag_names.push(env.source[tag_name].clone());
 
-                    FunctionOrTagUnion(new_tag_name, symbol, deep_copy_var_to_help(env, ext_var))
+                    FunctionOrTagUnion(new_tag_name, symbol, storage_copy_var_to_help(env, ext_var))
                 }
 
                 RecursiveTagUnion(rec_var, tags, ext_var) => {
-                    let union_tags = deep_copy_union(env, tags);
+                    let union_tags = storage_copy_union(env, tags);
 
-                    let new_ext = deep_copy_var_to_help(env, ext_var);
+                    let new_ext = storage_copy_var_to_help(env, ext_var);
-                    let new_rec_var = deep_copy_var_to_help(env, rec_var);
+                    let new_rec_var = storage_copy_var_to_help(env, rec_var);
 
                     RecursiveTagUnion(new_rec_var, union_tags, new_ext)
                 }
@@ -4245,7 +4327,7 @@ fn deep_copy_var_to_help(env: &mut DeepCopyVarToEnv<'_>, var: Variable) -> Varia
             opt_name,
             structure,
         } => {
-            let new_structure = deep_copy_var_to_help(env, structure);
+            let new_structure = storage_copy_var_to_help(env, structure);
 
             debug_assert!((new_structure.index() as usize) < env.target.len());
 
@@ -4299,7 +4381,7 @@ fn deep_copy_var_to_help(env: &mut DeepCopyVarToEnv<'_>, var: Variable) -> Varia
                 (new_variables.indices()).zip(arguments.all_variables())
             {
                 let var = env.source[var_index];
-                let copy_var = deep_copy_var_to_help(env, var);
+                let copy_var = storage_copy_var_to_help(env, var);
                 env.target.variables[target_index] = copy_var;
             }
 
@@ -4308,7 +4390,7 @@ fn deep_copy_var_to_help(env: &mut DeepCopyVarToEnv<'_>, var: Variable) -> Varia
                 ..arguments
             };
 
-            let new_real_type_var = deep_copy_var_to_help(env, real_type_var);
+            let new_real_type_var = storage_copy_var_to_help(env, real_type_var);
             let new_content = Alias(symbol, new_arguments, new_real_type_var, kind);
 
             env.target.set(copy, make_descriptor(new_content));
@@ -4319,20 +4401,33 @@ fn deep_copy_var_to_help(env: &mut DeepCopyVarToEnv<'_>, var: Variable) -> Varia
         LambdaSet(self::LambdaSet {
             solved,
             recursion_var,
+            unspecialized,
         }) => {
-            let new_solved = deep_copy_union(env, solved);
+            let new_solved = storage_copy_union(env, solved);
-            let new_rec_var = recursion_var.map(|v| deep_copy_var_to_help(env, v));
+            let new_rec_var = recursion_var.map(|v| storage_copy_var_to_help(env, v));
+
+            // NB: we are only copying into storage here, not instantiating like in solve::deep_copy_var.
+            // So no bookkeeping should be done for the new unspecialized lambda sets.
+            let new_unspecialized = SubsSlice::reserve_uls_slice(env.target, unspecialized.len());
+            for (target_index, source_index) in
+                (new_unspecialized.into_iter()).zip(unspecialized.into_iter())
+            {
+                let Uls(var, sym, region) = env.source[source_index];
+                let new_var = storage_copy_var_to_help(env, var);
+                env.target[target_index] = Uls(new_var, sym, region);
+            }
 
             let new_content = LambdaSet(self::LambdaSet {
                 solved: new_solved,
                 recursion_var: new_rec_var,
+                unspecialized: new_unspecialized,
             });
             env.target.set(copy, make_descriptor(new_content));
             copy
         }
 
         RangedNumber(typ, range) => {
-            let new_typ = deep_copy_var_to_help(env, typ);
+            let new_typ = storage_copy_var_to_help(env, typ);
 
             let new_content = RangedNumber(new_typ, range);
 
@@ -4769,14 +4864,27 @@ fn copy_import_to_help(env: &mut CopyImportEnv<'_>, max_rank: Rank, var: Variabl
         LambdaSet(self::LambdaSet {
             solved,
             recursion_var,
+            unspecialized,
         }) => {
             let new_solved = copy_union(env, max_rank, solved);
             let new_rec_var =
                 recursion_var.map(|rec_var| copy_import_to_help(env, max_rank, rec_var));
 
+            // NB: we are only copying across subs here, not instantiating like in deep_copy_var.
+            // So no bookkeeping should be done for the new unspecialized lambda sets.
+            let new_unspecialized = SubsSlice::reserve_uls_slice(env.target, unspecialized.len());
+            for (target_index, source_index) in
+                (new_unspecialized.into_iter()).zip(unspecialized.into_iter())
+            {
+                let Uls(var, sym, region) = env.source[source_index];
+                let new_var = copy_import_to_help(env, max_rank, var);
+                env.target[target_index] = Uls(new_var, sym, region);
+            }
+
             let new_content = LambdaSet(self::LambdaSet {
                 solved: new_solved,
                 recursion_var: new_rec_var,
+                unspecialized: new_unspecialized,
             });
 
             env.target.set(copy, make_descriptor(new_content));

compiler/types/src/types.rs

@@ -273,8 +273,14 @@ pub enum Type {
 /// usage site. Unspecialized lambda sets aid us in recovering those lambda sets; when we
 /// instantiate `a` with a proper type `T`, we'll know to resolve the lambda set by extracting
 /// it at region "1" from the specialization of "default" for `T`.
-#[derive(PartialEq, Eq, Clone, Copy)]
+#[derive(PartialEq, Eq, Clone, Copy, PartialOrd, Ord)]
-pub struct Uls(Variable, Symbol, u8);
+pub struct Uls(pub Variable, pub Symbol, pub u8);
+
+impl std::fmt::Debug for Uls {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "Uls({:?}:{:?}:{:?})", self.0, self.1, self.2)
+    }
+}
 
 static mut TYPE_CLONE_COUNT: std::sync::atomic::AtomicUsize =
     std::sync::atomic::AtomicUsize::new(0);
@@ -648,8 +654,8 @@ impl fmt::Debug for Type {
             Type::RangedNumber(typ, range_vars) => {
                 write!(f, "Ranged({:?}, {:?})", typ, range_vars)
             }
-            Type::UnspecializedLambdaSet(Uls(a, mem, region)) => {
+            Type::UnspecializedLambdaSet(uls) => {
-                write!(f, "ULS({:?}:{:?}:{:?})", a, mem, region)
+                write!(f, "{:?}", uls)
             }
         }
     }

compiler/unify/src/unify.rs

@@ -815,10 +815,12 @@ fn unify_lambda_set_help(
     let LambdaSet {
         solved: solved1,
         recursion_var: rec1,
+        unspecialized: uls1,
     } = lset1;
     let LambdaSet {
         solved: solved2,
         recursion_var: rec2,
+        unspecialized: uls2,
     } = lset2;
 
     debug_assert!(
@@ -893,10 +895,30 @@ fn unify_lambda_set_help(
             (None, None) => OptVariable::NONE,
         };
 
+        // Combine the unspecialized lambda sets as needed. Note that we don't need to update the
+        // bookkeeping of variable -> lambda set to be resolved, because if we had v1 -> lset1, and
+        // now lset1 ~ lset2, then afterward either lset1 still resolves to itself or re-points to
+        // lset2. In either case the merged unspecialized lambda sets will be there.
+        let merged_unspecialized = match (uls1.is_empty(), uls2.is_empty()) {
+            (true, true) => SubsSlice::default(),
+            (false, true) => uls1,
+            (true, false) => uls2,
+            (false, false) => {
+                let mut all_uls = (subs.get_subs_slice(uls1).iter())
+                    .chain(subs.get_subs_slice(uls2))
+                    .copied()
+                    .collect::<Vec<_>>();
+                all_uls.sort();
+                all_uls.dedup();
+                SubsSlice::extend_new(&mut subs.unspecialized_lambda_sets, all_uls)
+            }
+        };
+
         let new_solved = UnionLabels::insert_into_subs(subs, all_lambdas);
         let new_lambda_set = Content::LambdaSet(LambdaSet {
             solved: new_solved,
             recursion_var,
+            unspecialized: merged_unspecialized,
         });
 
         merge(subs, ctx, new_lambda_set)
@@ -1523,8 +1545,9 @@ fn maybe_mark_union_recursive(subs: &mut Subs, union_var: Variable) {
                 LambdaSet(self::LambdaSet {
                     solved,
                     recursion_var: OptVariable::NONE,
+                    unspecialized,
                 }) => {
-                    subs.mark_lambda_set_recursive(v, solved);
+                    subs.mark_lambda_set_recursive(v, solved, unspecialized);
                     continue 'outer;
                 }
                 _ => { /* fall through */ }
@@ -2246,6 +2269,7 @@ fn unify_function_or_tag_union_and_func(
         let lambda_set_content = LambdaSet(self::LambdaSet {
             solved: union_tags,
             recursion_var: OptVariable::NONE,
+            unspecialized: SubsSlice::default(),
         });
 
         let tag_lambda_set = register(