feat: Implement ATPIT

13 files changed, 365 insertions, 55 deletions

diff --git a/crates/hir-ty/src/chalk_db.rs b/crates/hir-ty/src/chalk_db.rs
index e678a2fee13..46612242b09 100644
--- a/crates/hir-ty/src/chalk_db.rs
+++ b/crates/hir-ty/src/chalk_db.rs
@@ -272,6 +272,19 @@ impl chalk_solve::RustIrDatabase<Interner> for ChalkContext<'_> {
                 };
                 chalk_ir::Binders::new(binders, bound)
             }
+            crate::ImplTraitId::AssociatedTypeImplTrait(alias, idx) => {
+                let datas = self
+                    .db
+                    .type_alias_impl_traits(alias)
+                    .expect("impl trait id without impl traits");
+                let (datas, binders) = (*datas).as_ref().into_value_and_skipped_binders();
+                let data = &datas.impl_traits[idx];
+                let bound = OpaqueTyDatumBound {
+                    bounds: make_single_type_binders(data.bounds.skip_binders().to_vec()),
+                    where_clauses: chalk_ir::Binders::empty(Interner, vec![]),
+                };
+                chalk_ir::Binders::new(binders, bound)
+            }
             crate::ImplTraitId::AsyncBlockTypeImplTrait(..) => {
                 if let Some((future_trait, future_output)) = self
                     .db
diff --git a/crates/hir-ty/src/chalk_ext.rs b/crates/hir-ty/src/chalk_ext.rs
index 795a5996912..d1aebeff261 100644
--- a/crates/hir-ty/src/chalk_ext.rs
+++ b/crates/hir-ty/src/chalk_ext.rs
@@ -268,6 +268,13 @@ impl TyExt for Ty {
                             data.substitute(Interner, &subst).into_value_and_skipped_binders().0
                         })
                     }
+                    ImplTraitId::AssociatedTypeImplTrait(alias, idx) => {
+                        db.type_alias_impl_traits(alias).map(|it| {
+                            let data =
+                                (*it).as_ref().map(|rpit| rpit.impl_traits[idx].bounds.clone());
+                            data.substitute(Interner, &subst).into_value_and_skipped_binders().0
+                        })
+                    }
                 }
             }
             TyKind::Alias(AliasTy::Opaque(opaque_ty)) => {
@@ -280,6 +287,13 @@ impl TyExt for Ty {
                             data.substitute(Interner, &opaque_ty.substitution)
                         })
                     }
+                    ImplTraitId::AssociatedTypeImplTrait(alias, idx) => {
+                        db.type_alias_impl_traits(alias).map(|it| {
+                            let data =
+                                (*it).as_ref().map(|rpit| rpit.impl_traits[idx].bounds.clone());
+                            data.substitute(Interner, &opaque_ty.substitution)
+                        })
+                    }
                     // It always has an parameter for Future::Output type.
                     ImplTraitId::AsyncBlockTypeImplTrait(..) => unreachable!(),
                 };
diff --git a/crates/hir-ty/src/db.rs b/crates/hir-ty/src/db.rs
index 28c497989fe..90bf46b5056 100644
--- a/crates/hir-ty/src/db.rs
+++ b/crates/hir-ty/src/db.rs
@@ -11,7 +11,7 @@ use base_db::{
 use hir_def::{
     db::DefDatabase, hir::ExprId, layout::TargetDataLayout, AdtId, BlockId, ConstParamId,
     DefWithBodyId, EnumVariantId, FunctionId, GeneralConstId, GenericDefId, ImplId,
-    LifetimeParamId, LocalFieldId, StaticId, TypeOrConstParamId, VariantId,
+    LifetimeParamId, LocalFieldId, StaticId, TypeAliasId, TypeOrConstParamId, VariantId,
 };
 use la_arena::ArenaMap;
 use smallvec::SmallVec;
@@ -23,9 +23,9 @@ use crate::{
     layout::{Layout, LayoutError},
     method_resolution::{InherentImpls, TraitImpls, TyFingerprint},
     mir::{BorrowckResult, MirBody, MirLowerError},
-    Binders, CallableDefId, ClosureId, Const, FnDefId, GenericArg, ImplTraitId, InferenceResult,
-    Interner, PolyFnSig, QuantifiedWhereClause, ReturnTypeImplTraits, Substitution,
-    TraitEnvironment, TraitRef, Ty, TyDefId, ValueTyDefId,
+    Binders, CallableDefId, ClosureId, Const, FnDefId, GenericArg, ImplTraitId, ImplTraits,
+    InferenceResult, Interner, PolyFnSig, QuantifiedWhereClause, Substitution, TraitEnvironment,
+    TraitRef, Ty, TyDefId, ValueTyDefId,
 };
 use hir_expand::name::Name;
 
@@ -132,10 +132,10 @@ pub trait HirDatabase: DefDatabase + Upcast<dyn DefDatabase> {
     fn callable_item_signature(&self, def: CallableDefId) -> PolyFnSig;
 
     #[salsa::invoke(crate::lower::return_type_impl_traits)]
-    fn return_type_impl_traits(
-        &self,
-        def: FunctionId,
-    ) -> Option<Arc<Binders<ReturnTypeImplTraits>>>;
+    fn return_type_impl_traits(&self, def: FunctionId) -> Option<Arc<Binders<ImplTraits>>>;
+
+    #[salsa::invoke(crate::lower::type_alias_impl_traits)]
+    fn type_alias_impl_traits(&self, def: TypeAliasId) -> Option<Arc<Binders<ImplTraits>>>;
 
     #[salsa::invoke(crate::lower::generic_predicates_for_param_query)]
     #[salsa::cycle(crate::lower::generic_predicates_for_param_recover)]
diff --git a/crates/hir-ty/src/display.rs b/crates/hir-ty/src/display.rs
index 20964f5acbd..269db57bc34 100644
--- a/crates/hir-ty/src/display.rs
+++ b/crates/hir-ty/src/display.rs
@@ -1063,6 +1063,20 @@ impl HirDisplay for Ty {
                         )?;
                         // FIXME: it would maybe be good to distinguish this from the alias type (when debug printing), and to show the substitution
                     }
+                    ImplTraitId::AssociatedTypeImplTrait(alias, idx) => {
+                        let datas =
+                            db.type_alias_impl_traits(alias).expect("impl trait id without data");
+                        let data =
+                            (*datas).as_ref().map(|rpit| rpit.impl_traits[idx].bounds.clone());
+                        let bounds = data.substitute(Interner, &parameters);
+                        let krate = alias.krate(db.upcast());
+                        write_bounds_like_dyn_trait_with_prefix(
+                            f,
+                            "impl",
+                            bounds.skip_binders(),
+                            SizedByDefault::Sized { anchor: krate },
+                        )?;
+                    }
                     ImplTraitId::AsyncBlockTypeImplTrait(body, ..) => {
                         let future_trait = db
                             .lang_item(body.module(db.upcast()).krate(), LangItem::Future)
@@ -1228,6 +1242,20 @@ impl HirDisplay for Ty {
                             SizedByDefault::Sized { anchor: krate },
                         )?;
                     }
+                    ImplTraitId::AssociatedTypeImplTrait(alias, idx) => {
+                        let datas =
+                            db.type_alias_impl_traits(alias).expect("impl trait id without data");
+                        let data =
+                            (*datas).as_ref().map(|rpit| rpit.impl_traits[idx].bounds.clone());
+                        let bounds = data.substitute(Interner, &opaque_ty.substitution);
+                        let krate = alias.krate(db.upcast());
+                        write_bounds_like_dyn_trait_with_prefix(
+                            f,
+                            "impl",
+                            bounds.skip_binders(),
+                            SizedByDefault::Sized { anchor: krate },
+                        )?;
+                    }
                     ImplTraitId::AsyncBlockTypeImplTrait(..) => {
                         write!(f, "{{async block}}")?;
                     }
diff --git a/crates/hir-ty/src/infer.rs b/crates/hir-ty/src/infer.rs
index 34ba17f145e..8847f7e24f6 100644
--- a/crates/hir-ty/src/infer.rs
+++ b/crates/hir-ty/src/infer.rs
@@ -25,8 +25,11 @@ pub(crate) mod unify;
 use std::{convert::identity, iter, ops::Index};
 
 use chalk_ir::{
-    cast::Cast, fold::TypeFoldable, interner::HasInterner, DebruijnIndex, Mutability, Safety,
-    Scalar, TyKind, TypeFlags, Variance,
+    cast::Cast,
+    fold::TypeFoldable,
+    interner::HasInterner,
+    visit::{TypeSuperVisitable, TypeVisitable, TypeVisitor},
+    DebruijnIndex, Mutability, Safety, Scalar, TyKind, TypeFlags, Variance,
 };
 use either::Either;
 use hir_def::{
@@ -53,14 +56,14 @@ use triomphe::Arc;
 use crate::{
     db::HirDatabase,
     fold_tys,
-    infer::coerce::CoerceMany,
+    infer::{coerce::CoerceMany, unify::InferenceTable},
     lower::ImplTraitLoweringMode,
     static_lifetime, to_assoc_type_id,
     traits::FnTrait,
     utils::{InTypeConstIdMetadata, UnevaluatedConstEvaluatorFolder},
     AliasEq, AliasTy, Binders, ClosureId, Const, DomainGoal, GenericArg, Goal, ImplTraitId,
-    InEnvironment, Interner, Lifetime, ProjectionTy, RpitId, Substitution, TraitEnvironment,
-    TraitRef, Ty, TyBuilder, TyExt,
+    ImplTraitIdx, InEnvironment, Interner, Lifetime, OpaqueTyId, ProjectionTy, Substitution,
+    TraitEnvironment, Ty, TyBuilder, TyExt,
 };
 
 // This lint has a false positive here. See the link below for details.
@@ -422,7 +425,7 @@ pub struct InferenceResult {
     /// unresolved or missing subpatterns or subpatterns of mismatched types.
     pub type_of_pat: ArenaMap<PatId, Ty>,
     pub type_of_binding: ArenaMap<BindingId, Ty>,
-    pub type_of_rpit: ArenaMap<RpitId, Ty>,
+    pub type_of_rpit: ArenaMap<ImplTraitIdx, Ty>,
     /// Type of the result of `.into_iter()` on the for. `ExprId` is the one of the whole for loop.
     pub type_of_for_iterator: FxHashMap<ExprId, Ty>,
     type_mismatches: FxHashMap<ExprOrPatId, TypeMismatch>,
@@ -752,7 +755,12 @@ impl<'a> InferenceContext<'a> {
     }
 
     fn collect_const(&mut self, data: &ConstData) {
-        self.return_ty = self.make_ty(&data.type_ref);
+        let return_ty = self.make_ty(&data.type_ref);
+
+        // Constants might be associated items that define ATPITs.
+        self.insert_atpit_coercion_table(iter::once(&return_ty));
+
+        self.return_ty = return_ty;
     }
 
     fn collect_static(&mut self, data: &StaticData) {
@@ -785,11 +793,13 @@ impl<'a> InferenceContext<'a> {
                 self.write_binding_ty(self_param, ty);
             }
         }
+        let mut params_and_ret_tys = Vec::new();
         for (ty, pat) in param_tys.zip(&*self.body.params) {
             let ty = self.insert_type_vars(ty);
             let ty = self.normalize_associated_types_in(ty);
 
             self.infer_top_pat(*pat, &ty);
+            params_and_ret_tys.push(ty);
         }
         let return_ty = &*data.ret_type;
 
@@ -801,8 +811,11 @@ impl<'a> InferenceContext<'a> {
         let return_ty = if let Some(rpits) = self.db.return_type_impl_traits(func) {
             // RPIT opaque types use substitution of their parent function.
             let fn_placeholders = TyBuilder::placeholder_subst(self.db, func);
-            let result =
-                self.insert_inference_vars_for_rpit(return_ty, rpits.clone(), fn_placeholders);
+            let result = self.insert_inference_vars_for_impl_trait(
+                return_ty,
+                rpits.clone(),
+                fn_placeholders,
+            );
             let rpits = rpits.skip_binders();
             for (id, _) in rpits.impl_traits.iter() {
                 if let Entry::Vacant(e) = self.result.type_of_rpit.entry(id) {
@@ -817,13 +830,19 @@ impl<'a> InferenceContext<'a> {
 
         self.return_ty = self.normalize_associated_types_in(return_ty);
         self.return_coercion = Some(CoerceMany::new(self.return_ty.clone()));
+
+        // Functions might be associated items that define ATPITs.
+        // To define an ATPITs, that ATPIT must appear in the function's signitures.
+        // So, it suffices to check for params and return types.
+        params_and_ret_tys.push(self.return_ty.clone());
+        self.insert_atpit_coercion_table(params_and_ret_tys.iter());
     }
 
-    fn insert_inference_vars_for_rpit<T>(
+    fn insert_inference_vars_for_impl_trait<T>(
         &mut self,
         t: T,
-        rpits: Arc<chalk_ir::Binders<crate::ReturnTypeImplTraits>>,
-        fn_placeholders: Substitution,
+        rpits: Arc<chalk_ir::Binders<crate::ImplTraits>>,
+        placeholders: Substitution,
     ) -> T
     where
         T: crate::HasInterner<Interner = Interner> + crate::TypeFoldable<Interner>,
@@ -837,6 +856,7 @@ impl<'a> InferenceContext<'a> {
                 };
                 let idx = match self.db.lookup_intern_impl_trait_id(opaque_ty_id.into()) {
                     ImplTraitId::ReturnTypeImplTrait(_, idx) => idx,
+                    ImplTraitId::AssociatedTypeImplTrait(_, idx) => idx,
                     _ => unreachable!(),
                 };
                 let bounds =
@@ -844,15 +864,14 @@ impl<'a> InferenceContext<'a> {
                 let var = self.table.new_type_var();
                 let var_subst = Substitution::from1(Interner, var.clone());
                 for bound in bounds {
-                    let predicate =
-                        bound.map(|it| it.cloned()).substitute(Interner, &fn_placeholders);
+                    let predicate = bound.map(|it| it.cloned()).substitute(Interner, &placeholders);
                     let (var_predicate, binders) =
                         predicate.substitute(Interner, &var_subst).into_value_and_skipped_binders();
                     always!(binders.is_empty(Interner)); // quantified where clauses not yet handled
-                    let var_predicate = self.insert_inference_vars_for_rpit(
+                    let var_predicate = self.insert_inference_vars_for_impl_trait(
                         var_predicate,
                         rpits.clone(),
-                        fn_placeholders.clone(),
+                        placeholders.clone(),
                     );
                     self.push_obligation(var_predicate.cast(Interner));
                 }
@@ -863,6 +882,106 @@ impl<'a> InferenceContext<'a> {
         )
     }
 
+    /// The coercion of a non-inference var into an opaque type should fail,
+    /// but not in the defining sites of the ATPITs.
+    /// In such cases, we insert an proxy inference var for each ATPIT,
+    /// and coerce into it instead of ATPIT itself.
+    ///
+    /// The inference var stretagy is effective because;
+    ///
+    /// - It can still unify types that coerced into ATPIT
+    /// - We are pushing `impl Trait` bounds into it
+    ///
+    /// This function inserts a map that maps the opaque type to that proxy inference var.
+    fn insert_atpit_coercion_table<'b>(&mut self, tys: impl Iterator<Item = &'b Ty>) {
+        struct OpaqueTyCollector<'a, 'b> {
+            table: &'b mut InferenceTable<'a>,
+            opaque_tys: FxHashMap<OpaqueTyId, Ty>,
+        }
+
+        impl<'a, 'b> TypeVisitor<Interner> for OpaqueTyCollector<'a, 'b> {
+            type BreakTy = ();
+
+            fn as_dyn(&mut self) -> &mut dyn TypeVisitor<Interner, BreakTy = Self::BreakTy> {
+                self
+            }
+
+            fn interner(&self) -> Interner {
+                Interner
+            }
+
+            fn visit_ty(
+                &mut self,
+                ty: &chalk_ir::Ty<Interner>,
+                outer_binder: DebruijnIndex,
+            ) -> std::ops::ControlFlow<Self::BreakTy> {
+                let ty = self.table.resolve_ty_shallow(ty);
+
+                if let TyKind::OpaqueType(id, _) = ty.kind(Interner) {
+                    self.opaque_tys.insert(*id, ty.clone());
+                }
+
+                ty.super_visit_with(self, outer_binder)
+            }
+        }
+
+        // Early return if this is not happening inside the impl block
+        let impl_id = if let Some(impl_id) = self.resolver.impl_def() {
+            impl_id
+        } else {
+            return;
+        };
+
+        let assoc_tys: FxHashSet<_> = self
+            .db
+            .impl_data(impl_id)
+            .items
+            .iter()
+            .filter_map(|item| match item {
+                AssocItemId::TypeAliasId(alias) => Some(*alias),
+                _ => None,
+            })
+            .collect();
+        if assoc_tys.is_empty() {
+            return;
+        }
+
+        let mut collector =
+            OpaqueTyCollector { table: &mut self.table, opaque_tys: FxHashMap::default() };
+        for ty in tys {
+            ty.visit_with(collector.as_dyn(), DebruijnIndex::INNERMOST);
+        }
+        let atpit_coercion_table: FxHashMap<_, _> = collector
+            .opaque_tys
+            .into_iter()
+            .filter_map(|(opaque_ty_id, ty)| {
+                if let ImplTraitId::AssociatedTypeImplTrait(alias_id, _) =
+                    self.db.lookup_intern_impl_trait_id(opaque_ty_id.into())
+                {
+                    if assoc_tys.contains(&alias_id) {
+                        let atpits = self
+                            .db
+                            .type_alias_impl_traits(alias_id)
+                            .expect("Marked as ATPIT but no impl traits!");
+                        let alias_placeholders = TyBuilder::placeholder_subst(self.db, alias_id);
+                        let ty = self.insert_inference_vars_for_impl_trait(
+                            ty,
+                            atpits,
+                            alias_placeholders,
+                        );
+                        return Some((opaque_ty_id, ty));
+                    }
+                }
+
+                None
+            })
+            .collect();
+
+        if !atpit_coercion_table.is_empty() {
+            self.table.atpit_coercion_table = Some(atpit_coercion_table);
+        }
+    }
+
     fn infer_body(&mut self) {
         match self.return_coercion {
             Some(_) => self.infer_return(self.body.body_expr),
diff --git a/crates/hir-ty/src/infer/coerce.rs b/crates/hir-ty/src/infer/coerce.rs
index ff6de61ba64..fba37dbcff4 100644
--- a/crates/hir-ty/src/infer/coerce.rs
+++ b/crates/hir-ty/src/infer/coerce.rs
@@ -276,6 +276,23 @@ impl InferenceTable<'_> {
             return success(simple(Adjust::NeverToAny)(to_ty.clone()), to_ty.clone(), vec![]);
         }
 
+        // If we are coercing into an ATPIT, coerce into its proxy inference var, instead.
+        let mut to_ty = to_ty;
+        let mut _to = None;
+        if let Some(atpit_table) = &self.atpit_coercion_table {
+            if let TyKind::OpaqueType(opaque_ty_id, _) = to_ty.kind(Interner) {
+                if !matches!(
+                    from_ty.kind(Interner),
+                    TyKind::InferenceVar(..) | TyKind::OpaqueType(..)
+                ) {
+                    if let Some(ty) = atpit_table.get(opaque_ty_id) {
+                        _to = Some(ty.clone());
+                        to_ty = _to.as_ref().unwrap();
+                    }
+                }
+            }
+        }
+
         // Consider coercing the subtype to a DST
         if let Ok(ret) = self.try_coerce_unsized(&from_ty, to_ty) {
             return Ok(ret);
diff --git a/crates/hir-ty/src/infer/path.rs b/crates/hir-ty/src/infer/path.rs
index 8f537bb448b..44b35b2ebbc 100644
--- a/crates/hir-ty/src/infer/path.rs
+++ b/crates/hir-ty/src/infer/path.rs
@@ -15,11 +15,11 @@ use crate::{
     method_resolution::{self, VisibleFromModule},
     to_chalk_trait_id,
     utils::generics,
-    InferenceDiagnostic, Interner, Substitution, TraitRefExt, Ty, TyBuilder, TyExt, TyKind,
-    ValueTyDefId,
+    InferenceDiagnostic, Interner, Substitution, TraitRef, TraitRefExt, Ty, TyBuilder, TyExt,
+    TyKind, ValueTyDefId,
 };
 
-use super::{ExprOrPatId, InferenceContext, TraitRef};
+use super::{ExprOrPatId, InferenceContext};
 
 impl InferenceContext<'_> {
     pub(super) fn infer_path(&mut self, path: &Path, id: ExprOrPatId) -> Option<Ty> {
diff --git a/crates/hir-ty/src/infer/unify.rs b/crates/hir-ty/src/infer/unify.rs
index be7547f9bae..93fd54455b9 100644
--- a/crates/hir-ty/src/infer/unify.rs
+++ b/crates/hir-ty/src/infer/unify.rs
@@ -10,6 +10,7 @@ use chalk_solve::infer::ParameterEnaVariableExt;
 use either::Either;
 use ena::unify::UnifyKey;
 use hir_expand::name;
+use rustc_hash::FxHashMap;
 use smallvec::SmallVec;
 use triomphe::Arc;
 
@@ -18,8 +19,9 @@ use crate::{
     consteval::unknown_const, db::HirDatabase, fold_tys_and_consts, static_lifetime,
     to_chalk_trait_id, traits::FnTrait, AliasEq, AliasTy, BoundVar, Canonical, Const, ConstValue,
     DebruijnIndex, DomainGoal, GenericArg, GenericArgData, Goal, GoalData, Guidance, InEnvironment,
-    InferenceVar, Interner, Lifetime, ParamKind, ProjectionTy, ProjectionTyExt, Scalar, Solution,
-    Substitution, TraitEnvironment, Ty, TyBuilder, TyExt, TyKind, VariableKind, WhereClause,
+    InferenceVar, Interner, Lifetime, OpaqueTyId, ParamKind, ProjectionTy, ProjectionTyExt, Scalar,
+    Solution, Substitution, TraitEnvironment, Ty, TyBuilder, TyExt, TyKind, VariableKind,
+    WhereClause,
 };
 
 impl InferenceContext<'_> {
@@ -239,6 +241,7 @@ type ChalkInferenceTable = chalk_solve::infer::InferenceTable<Interner>;
 pub(crate) struct InferenceTable<'a> {
     pub(crate) db: &'a dyn HirDatabase,
     pub(crate) trait_env: Arc<TraitEnvironment>,
+    pub(crate) atpit_coercion_table: Option<FxHashMap<OpaqueTyId, Ty>>,
     var_unification_table: ChalkInferenceTable,
     type_variable_table: SmallVec<[TypeVariableFlags; 16]>,
     pending_obligations: Vec<Canonicalized<InEnvironment<Goal>>>,
@@ -258,6 +261,7 @@ impl<'a> InferenceTable<'a> {
         InferenceTable {
             db,
             trait_env,
+            atpit_coercion_table: None,
             var_unification_table: ChalkInferenceTable::new(),
             type_variable_table: SmallVec::new(),
             pending_obligations: Vec::new(),
diff --git a/crates/hir-ty/src/layout.rs b/crates/hir-ty/src/layout.rs
index 9655981cc9c..dd949e26c2a 100644
--- a/crates/hir-ty/src/layout.rs
+++ b/crates/hir-ty/src/layout.rs
@@ -389,6 +389,9 @@ pub fn layout_of_ty_query(
                     let infer = db.infer(func.into());
                     return db.layout_of_ty(infer.type_of_rpit[idx].clone(), trait_env);
                 }
+                crate::ImplTraitId::AssociatedTypeImplTrait(..) => {
+                    return Err(LayoutError::NotImplemented);
+                }
                 crate::ImplTraitId::AsyncBlockTypeImplTrait(_, _) => {
                     return Err(LayoutError::NotImplemented)
                 }
diff --git a/crates/hir-ty/src/lib.rs b/crates/hir-ty/src/lib.rs
index ec97bdc2c43..a7affc5f924 100644
--- a/crates/hir-ty/src/lib.rs
+++ b/crates/hir-ty/src/lib.rs
@@ -584,24 +584,25 @@ impl TypeFoldable<Interner> for CallableSig {
 
 #[derive(Copy, Clone, PartialEq, Eq, Debug, Hash)]
 pub enum ImplTraitId {
-    ReturnTypeImplTrait(hir_def::FunctionId, RpitId),
+    ReturnTypeImplTrait(hir_def::FunctionId, ImplTraitIdx),
+    AssociatedTypeImplTrait(hir_def::TypeAliasId, ImplTraitIdx),
     AsyncBlockTypeImplTrait(hir_def::DefWithBodyId, ExprId),
 }
 impl_intern_value_trivial!(ImplTraitId);
 
 #[derive(Clone, PartialEq, Eq, Debug, Hash)]
-pub struct ReturnTypeImplTraits {
-    pub(crate) impl_traits: Arena<ReturnTypeImplTrait>,
+pub struct ImplTraits {
+    pub(crate) impl_traits: Arena<ImplTrait>,
 }
 
-has_interner!(ReturnTypeImplTraits);
+has_interner!(ImplTraits);
 
 #[derive(Clone, PartialEq, Eq, Debug, Hash)]
-pub struct ReturnTypeImplTrait {
+pub struct ImplTrait {
     pub(crate) bounds: Binders<Vec<QuantifiedWhereClause>>,
 }
 
-pub type RpitId = Idx<ReturnTypeImplTrait>;
+pub type ImplTraitIdx = Idx<ImplTrait>;
 
 pub fn static_lifetime() -> Lifetime {
     LifetimeData::Static.intern(Interner)
diff --git a/crates/hir-ty/src/lower.rs b/crates/hir-ty/src/lower.rs
index dac20f22597..d65fd4a71cb 100644
--- a/crates/hir-ty/src/lower.rs
+++ b/crates/hir-ty/src/lower.rs
@@ -61,9 +61,9 @@ use crate::{
         InTypeConstIdMetadata,
     },
     AliasEq, AliasTy, Binders, BoundVar, CallableSig, Const, ConstScalar, DebruijnIndex, DynTy,
-    FnAbi, FnPointer, FnSig, FnSubst, ImplTraitId, Interner, ParamKind, PolyFnSig, ProjectionTy,
-    QuantifiedWhereClause, QuantifiedWhereClauses, ReturnTypeImplTrait, ReturnTypeImplTraits,
-    Substitution, TraitEnvironment, TraitRef, TraitRefExt, Ty, TyBuilder, TyKind, WhereClause,
+    FnAbi, FnPointer, FnSig, FnSubst, ImplTrait, ImplTraitId, ImplTraits, Interner, ParamKind,
+    PolyFnSig, ProjectionTy, QuantifiedWhereClause, QuantifiedWhereClauses, Substitution,
+    TraitEnvironment, TraitRef, TraitRefExt, Ty, TyBuilder, TyKind, WhereClause,
 };
 
 #[derive(Debug)]
@@ -76,7 +76,7 @@ enum ImplTraitLoweringState {
     /// we're grouping the mutable data (the counter and this field) together
     /// with the immutable context (the references to the DB and resolver).
     /// Splitting this up would be a possible fix.
-    Opaque(RefCell<Arena<ReturnTypeImplTrait>>),
+    Opaque(RefCell<Arena<ImplTrait>>),
     Param(Cell<u16>),
     Variable(Cell<u16>),
     Disallowed,
@@ -301,15 +301,22 @@ impl<'a> TyLoweringContext<'a> {
             TypeRef::ImplTrait(bounds) => {
                 match &self.impl_trait_mode {
                     ImplTraitLoweringState::Opaque(opaque_type_data) => {
-                        let func = match self.resolver.generic_def() {
-                            Some(GenericDefId::FunctionId(f)) => f,
-                            _ => panic!("opaque impl trait lowering in non-function"),
+                        let (origin, krate) = match self.resolver.generic_def() {
+                            Some(GenericDefId::FunctionId(f)) => {
+                                (Either::Left(f), f.krate(self.db.upcast()))
+                            }
+                            Some(GenericDefId::TypeAliasId(a)) => {
+                                (Either::Right(a), a.krate(self.db.upcast()))
+                            }
+                            _ => panic!(
+                                "opaque impl trait lowering must be in function or type alias"
+                            ),
                         };
 
                         // this dance is to make sure the data is in the right
                         // place even if we encounter more opaque types while
                         // lowering the bounds
-                        let idx = opaque_type_data.borrow_mut().alloc(ReturnTypeImplTrait {
+                        let idx = opaque_type_data.borrow_mut().alloc(ImplTrait {
                             bounds: crate::make_single_type_binders(Vec::new()),
                         });
                         // We don't want to lower the bounds inside the binders
@@ -323,13 +330,17 @@ impl<'a> TyLoweringContext<'a> {
                         // away instead of two.
                         let actual_opaque_type_data = self
                             .with_debruijn(DebruijnIndex::INNERMOST, |ctx| {
-                                ctx.lower_impl_trait(bounds, func)
+                                ctx.lower_impl_trait(bounds, krate)
                             });
                         opaque_type_data.borrow_mut()[idx] = actual_opaque_type_data;
 
-                        let impl_trait_id = ImplTraitId::ReturnTypeImplTrait(func, idx);
+                        let impl_trait_id = origin.either(
+                            |f| ImplTraitId::ReturnTypeImplTrait(f, idx),
+                            |a| ImplTraitId::AssociatedTypeImplTrait(a, idx),
+                        );
                         let opaque_ty_id = self.db.intern_impl_trait_id(impl_trait_id).into();
-                        let generics = generics(self.db.upcast(), func.into());
+                        let generics =
+                            generics(self.db.upcast(), origin.either(|f| f.into(), |a| a.into()));
                         let parameters = generics.bound_vars_subst(self.db, self.in_binders);
                         TyKind::OpaqueType(opaque_ty_id, parameters).intern(Interner)
                     }
@@ -1270,11 +1281,7 @@ impl<'a> TyLoweringContext<'a> {
         }
     }
 
-    fn lower_impl_trait(
-        &self,
-        bounds: &[Interned<TypeBound>],
-        func: FunctionId,
-    ) -> ReturnTypeImplTrait {
+    fn lower_impl_trait(&self, bounds: &[Interned<TypeBound>], krate: CrateId) -> ImplTrait {
         cov_mark::hit!(lower_rpit);
         let self_ty = TyKind::BoundVar(BoundVar::new(DebruijnIndex::INNERMOST, 0)).intern(Interner);
         let predicates = self.with_shifted_in(DebruijnIndex::ONE, |ctx| {
@@ -1284,7 +1291,6 @@ impl<'a> TyLoweringContext<'a> {
                 .collect();
 
             if !ctx.unsized_types.borrow().contains(&self_ty) {
-                let krate = func.krate(ctx.db.upcast());
                 let sized_trait = ctx
                     .db
                     .lang_item(krate, LangItem::Sized)
@@ -1301,7 +1307,7 @@ impl<'a> TyLoweringContext<'a> {
             }
             predicates
         });
-        ReturnTypeImplTrait { bounds: crate::make_single_type_binders(predicates) }
+        ImplTrait { bounds: crate::make_single_type_binders(predicates) }
     }
 }
 
@@ -1869,6 +1875,7 @@ fn type_for_type_alias(db: &dyn HirDatabase, t: TypeAliasId) -> Binders<Ty> {
     let generics = generics(db.upcast(), t.into());
     let resolver = t.resolver(db.upcast());
     let ctx = TyLoweringContext::new(db, &resolver, t.into())
+        .with_impl_trait_mode(ImplTraitLoweringMode::Opaque)
         .with_type_param_mode(ParamLoweringMode::Variable);
     let type_alias_data = db.type_alias_data(t);
     if type_alias_data.is_extern {
@@ -2029,7 +2036,7 @@ pub(crate) fn impl_trait_query(db: &dyn HirDatabase, impl_id: ImplId) -> Option<
 pub(crate) fn return_type_impl_traits(
     db: &dyn HirDatabase,
     def: hir_def::FunctionId,
-) -> Option<Arc<Binders<ReturnTypeImplTraits>>> {
+) -> Option<Arc<Binders<ImplTraits>>> {
     // FIXME unify with fn_sig_for_fn instead of doing lowering twice, maybe
     let data = db.function_data(def);
     let resolver = def.resolver(db.upcast());
@@ -2038,7 +2045,7 @@ pub(crate) fn return_type_impl_traits(
         .with_type_param_mode(ParamLoweringMode::Variable);
     let _ret = ctx_ret.lower_ty(&data.ret_type);
     let generics = generics(db.upcast(), def.into());
-    let return_type_impl_traits = ReturnTypeImplTraits {
+    let return_type_impl_traits = ImplTraits {
         impl_traits: match ctx_ret.impl_trait_mode {
             ImplTraitLoweringState::Opaque(x) => x.into_inner(),
             _ => unreachable!(),
@@ -2051,6 +2058,32 @@ pub(crate) fn return_type_impl_traits(
     }
 }
 
+pub(crate) fn type_alias_impl_traits(
+    db: &dyn HirDatabase,
+    def: hir_def::TypeAliasId,
+) -> Option<Arc<Binders<ImplTraits>>> {
+    let data = db.type_alias_data(def);
+    let resolver = def.resolver(db.upcast());
+    let ctx = TyLoweringContext::new(db, &resolver, def.into())
+        .with_impl_trait_mode(ImplTraitLoweringMode::Opaque)
+        .with_type_param_mode(ParamLoweringMode::Variable);
+    if let Some(type_ref) = &data.type_ref {
+        let _ty = ctx.lower_ty(type_ref);
+    }
+    let generics = generics(db.upcast(), def.into());
+    let type_alias_impl_traits = ImplTraits {
+        impl_traits: match ctx.impl_trait_mode {
+            ImplTraitLoweringState::Opaque(x) => x.into_inner(),
+            _ => unreachable!(),
+        },
+    };
+    if type_alias_impl_traits.impl_traits.is_empty() {
+        None
+    } else {
+        Some(Arc::new(make_binders(db, &generics, type_alias_impl_traits)))
+    }
+}
+
 pub(crate) fn lower_to_chalk_mutability(m: hir_def::type_ref::Mutability) -> Mutability {
     match m {
         hir_def::type_ref::Mutability::Shared => Mutability::Not,
diff --git a/crates/hir-ty/src/mir/monomorphization.rs b/crates/hir-ty/src/mir/monomorphization.rs
index d2e413f0a33..12ad67cdc45 100644
--- a/crates/hir-ty/src/mir/monomorphization.rs
+++ b/crates/hir-ty/src/mir/monomorphization.rs
@@ -82,6 +82,9 @@ impl FallibleTypeFolder<Interner> for Filler<'_> {
                         };
                         filler.try_fold_ty(infer.type_of_rpit[idx].clone(), outer_binder)
                     }
+                    crate::ImplTraitId::AssociatedTypeImplTrait(..) => {
+                        not_supported!("associated type impl trait");
+                    }
                     crate::ImplTraitId::AsyncBlockTypeImplTrait(_, _) => {
                         not_supported!("async block impl trait");
                     }
diff --git a/crates/hir-ty/src/tests/traits.rs b/crates/hir-ty/src/tests/traits.rs
index b80cfe18e4c..83bb90b6178 100644
--- a/crates/hir-ty/src/tests/traits.rs
+++ b/crates/hir-ty/src/tests/traits.rs
@@ -4655,3 +4655,78 @@ fn f<T: Send, U>() {
 "#,
     );
 }
+
+#[test]
+fn associated_type_impl_trait() {
+    check_types(
+        r#"
+trait Foo {}
+struct S1;
+impl Foo for S1 {}
+
+trait Bar {
+    type Item;
+    fn bar(&self) -> Self::Item;
+}
+struct S2;
+impl Bar for S2 {
+    type Item = impl Foo;
+    fn bar(&self) -> Self::Item {
+        S1
+    }
+}
+
+fn test() {
+    let x = S2.bar();
+      //^ impl Foo + ?Sized
+}
+        "#,
+    );
+}
+
+#[test]
+fn associated_type_impl_traits_complex() {
+    check_types(
+        r#"
+struct Unary<T>(T);
+struct Binary<T, U>(T, U);
+
+trait Foo {}
+struct S1;
+impl Foo for S1 {}
+
+trait Bar {
+    type Item;
+    fn bar(&self) -> Unary<Self::Item>;
+}
+struct S2;
+impl Bar for S2 {
+    type Item = Unary<impl Foo>;
+    fn bar(&self) -> Unary<<Self as Bar>::Item> {
+        Unary(Unary(S1))
+    }
+}
+
+trait Baz {
+    type Target1;
+    type Target2;
+    fn baz(&self) -> Binary<Self::Target1, Self::Target2>;
+}
+struct S3;
+impl Baz for S3 {
+    type Target1 = impl Foo;
+    type Target2 = Unary<impl Bar>;
+    fn baz(&self) -> Binary<Self::Target1, Self::Target2> {
+        Binary(S1, Unary(S2))
+    }
+}
+
+fn test() {
+    let x = S3.baz();
+      //^ Binary<impl Foo + ?Sized, Unary<impl Bar + ?Sized>>
+    let y = x.1.0.bar();
+      //^ Unary<Bar::Item<impl Bar + ?Sized>>
+}
+        "#,
+    );
+}