4 files changed, 39 insertions, 22 deletions

diff --git a/crates/hir_ty/src/infer.rs b/crates/hir_ty/src/infer.rs
index b871594bd..b208d821d 100644
--- a/crates/hir_ty/src/infer.rs
+++ b/crates/hir_ty/src/infer.rs
@@ -42,7 +42,7 @@ use super::{
 };
 use crate::{
     db::HirDatabase, infer::diagnostics::InferenceDiagnostic, lower::ImplTraitLoweringMode,
-    to_assoc_type_id, to_chalk_trait_id, AliasEq, AliasTy, Interner, TyKind,
+    to_assoc_type_id, AliasEq, AliasTy, Interner, TyBuilder, TyKind,
 };
 
 // This lint has a false positive here. See the link below for details.
@@ -409,16 +409,14 @@ impl<'a> InferenceContext<'a> {
                     _ => panic!("resolve_associated_type called with non-associated type"),
                 };
                 let ty = self.table.new_type_var();
-                let substs = Substitution::build_for_def(self.db, res_assoc_ty)
+                let trait_ref = TyBuilder::trait_ref(self.db, trait_)
                     .push(inner_ty)
                     .fill(params.iter().cloned())
                     .build();
-                let trait_ref =
-                    TraitRef { trait_id: to_chalk_trait_id(trait_), substitution: substs.clone() };
                 let alias_eq = AliasEq {
                     alias: AliasTy::Projection(ProjectionTy {
                         associated_ty_id: to_assoc_type_id(res_assoc_ty),
-                        substitution: substs,
+                        substitution: trait_ref.substitution.clone(),
                     }),
                     ty: ty.clone(),
                 };
diff --git a/crates/hir_ty/src/infer/path.rs b/crates/hir_ty/src/infer/path.rs
index 1ba15f737..282dd499f 100644
--- a/crates/hir_ty/src/infer/path.rs
+++ b/crates/hir_ty/src/infer/path.rs
@@ -10,9 +10,7 @@ use hir_def::{
 };
 use hir_expand::name::Name;
 
-use crate::{
-    method_resolution, to_chalk_trait_id, Interner, Substitution, Ty, TyKind, ValueTyDefId,
-};
+use crate::{method_resolution, Interner, Substitution, Ty, TyBuilder, TyKind, ValueTyDefId};
 
 use super::{ExprOrPatId, InferenceContext, TraitRef};
 
@@ -254,18 +252,12 @@ impl<'a> InferenceContext<'a> {
                     }
                     AssocContainerId::TraitId(trait_) => {
                         // we're picking this method
-                        let trait_substs = Substitution::build_for_def(self.db, trait_)
+                        let trait_ref = TyBuilder::trait_ref(self.db, trait_)
                             .push(ty.clone())
                             .fill(std::iter::repeat_with(|| self.table.new_type_var()))
                             .build();
-                        self.push_obligation(
-                            TraitRef {
-                                trait_id: to_chalk_trait_id(trait_),
-                                substitution: trait_substs.clone(),
-                            }
-                            .cast(&Interner),
-                        );
-                        Some(trait_substs)
+                        self.push_obligation(trait_ref.clone().cast(&Interner));
+                        Some(trait_ref.substitution)
                     }
                     AssocContainerId::ModuleId(_) => None,
                 };
diff --git a/crates/hir_ty/src/lib.rs b/crates/hir_ty/src/lib.rs
index be6fe5016..e586d73d8 100644
--- a/crates/hir_ty/src/lib.rs
+++ b/crates/hir_ty/src/lib.rs
@@ -927,6 +927,35 @@ impl TyBuilder<hir_def::AdtId> {
     }
 }
 
+impl TyBuilder<TraitId> {
+    pub fn trait_ref(db: &dyn HirDatabase, trait_id: TraitId) -> TyBuilder<TraitId> {
+        let generics = generics(db.upcast(), trait_id.into());
+        let param_count = generics.len();
+        TyBuilder::new(trait_id, param_count)
+    }
+
+    pub fn build(self) -> TraitRef {
+        let (trait_id, substitution) = self.build_internal();
+        TraitRef { trait_id: to_chalk_trait_id(trait_id), substitution }
+    }
+}
+
+impl TyBuilder<TypeAliasId> {
+    pub fn assoc_type_projection(
+        db: &dyn HirDatabase,
+        type_alias: TypeAliasId,
+    ) -> TyBuilder<TypeAliasId> {
+        let generics = generics(db.upcast(), type_alias.into());
+        let param_count = generics.len();
+        TyBuilder::new(type_alias, param_count)
+    }
+
+    pub fn build(self) -> ProjectionTy {
+        let (type_alias, substitution) = self.build_internal();
+        ProjectionTy { associated_ty_id: to_assoc_type_id(type_alias), substitution }
+    }
+}
+
 impl Ty {
     pub fn as_reference(&self) -> Option<(&Ty, Mutability)> {
         match self.kind(&Interner) {
diff --git a/crates/hir_ty/src/method_resolution.rs b/crates/hir_ty/src/method_resolution.rs
index c093fce20..b9cc081e8 100644
--- a/crates/hir_ty/src/method_resolution.rs
+++ b/crates/hir_ty/src/method_resolution.rs
@@ -19,10 +19,9 @@ use crate::{
     db::HirDatabase,
     from_foreign_def_id,
     primitive::{self, FloatTy, IntTy, UintTy},
-    to_chalk_trait_id,
     utils::all_super_traits,
     AdtId, Canonical, CanonicalVarKinds, DebruijnIndex, FnPointer, FnSig, ForeignDefId,
-    InEnvironment, Interner, Scalar, Substitution, TraitEnvironment, TraitRef, Ty, TyKind,
+    InEnvironment, Interner, Scalar, Substitution, TraitEnvironment, Ty, TyBuilder, TyKind,
     TypeWalk,
 };
 
@@ -813,7 +812,7 @@ fn generic_implements_goal(
     self_ty: Canonical<Ty>,
 ) -> Canonical<InEnvironment<super::DomainGoal>> {
     let mut kinds = self_ty.binders.interned().to_vec();
-    let substs = super::Substitution::build_for_def(db, trait_)
+    let trait_ref = TyBuilder::trait_ref(db, trait_)
         .push(self_ty.value)
         .fill_with_bound_vars(DebruijnIndex::INNERMOST, kinds.len())
         .build();
@@ -822,9 +821,8 @@ fn generic_implements_goal(
             chalk_ir::VariableKind::Ty(chalk_ir::TyVariableKind::General),
             UniverseIndex::ROOT,
         ))
-        .take(substs.len(&Interner) - 1),
+        .take(trait_ref.substitution.len(&Interner) - 1),
     );
-    let trait_ref = TraitRef { trait_id: to_chalk_trait_id(trait_), substitution: substs };
     let obligation = trait_ref.cast(&Interner);
     Canonical {
         binders: CanonicalVarKinds::from_iter(&Interner, kinds),