5 files changed, 44 insertions, 18 deletions

diff --git a/crates/hir_ty/src/infer.rs b/crates/hir_ty/src/infer.rs
index b208d821d..caafeccb8 100644
--- a/crates/hir_ty/src/infer.rs
+++ b/crates/hir_ty/src/infer.rs
@@ -514,10 +514,9 @@ impl<'a> InferenceContext<'a> {
                 }
             }
             TypeNs::TypeAliasId(it) => {
-                let substs = Substitution::build_for_def(self.db, it)
+                let ty = TyBuilder::def_ty(self.db, it.into())
                     .fill(std::iter::repeat_with(|| self.table.new_type_var()))
                     .build();
-                let ty = self.db.ty(it.into()).subst(&substs);
                 let variant = ty_variant(&ty);
                 forbid_unresolved_segments((ty, variant), unresolved)
             }
diff --git a/crates/hir_ty/src/infer/coerce.rs b/crates/hir_ty/src/infer/coerce.rs
index ed8c677a8..028a4d568 100644
--- a/crates/hir_ty/src/infer/coerce.rs
+++ b/crates/hir_ty/src/infer/coerce.rs
@@ -7,10 +7,7 @@
 use chalk_ir::{cast::Cast, Mutability, TyVariableKind};
 use hir_def::lang_item::LangItemTarget;
 
-use crate::{
-    autoderef, traits::Solution, Interner, Ty,
-    TyBuilder, TyKind,
-};
+use crate::{autoderef, traits::Solution, Interner, Ty, TyBuilder, TyKind};
 
 use super::{InEnvironment, InferenceContext};
 
diff --git a/crates/hir_ty/src/infer/path.rs b/crates/hir_ty/src/infer/path.rs
index 282dd499f..c9219776b 100644
--- a/crates/hir_ty/src/infer/path.rs
+++ b/crates/hir_ty/src/infer/path.rs
@@ -243,7 +243,7 @@ impl<'a> InferenceContext<'a> {
                 };
                 let substs = match container {
                     AssocContainerId::ImplId(impl_id) => {
-                        let impl_substs = Substitution::build_for_def(self.db, impl_id)
+                        let impl_substs = TyBuilder::subst_for_def(self.db, impl_id)
                             .fill(iter::repeat_with(|| self.table.new_type_var()))
                             .build();
                         let impl_self_ty = self.db.impl_self_ty(impl_id).subst(&impl_substs);
diff --git a/crates/hir_ty/src/lib.rs b/crates/hir_ty/src/lib.rs
index e586d73d8..75bf8bcd9 100644
--- a/crates/hir_ty/src/lib.rs
+++ b/crates/hir_ty/src/lib.rs
@@ -28,7 +28,10 @@ mod chalk_ext;
 use std::{iter, mem, sync::Arc};
 
 use base_db::salsa;
-use chalk_ir::cast::{CastTo, Caster};
+use chalk_ir::{
+    cast::{CastTo, Caster},
+    interner::HasInterner,
+};
 use hir_def::{
     builtin_type::BuiltinType, expr::ExprId, type_ref::Rawness, AssocContainerId, FunctionId,
     GenericDefId, HasModule, LifetimeParamId, Lookup, TraitId, TypeAliasId, TypeParamId,
@@ -490,13 +493,6 @@ impl Substitution {
         )
     }
 
-    pub fn build_for_def(db: &dyn HirDatabase, def: impl Into<GenericDefId>) -> SubstsBuilder {
-        let def = def.into();
-        let params = generics(db.upcast(), def);
-        let param_count = params.len();
-        Substitution::builder(param_count)
-    }
-
     pub(crate) fn build_for_generics(generic_params: &Generics) -> SubstsBuilder {
         Substitution::builder(generic_params.len())
     }
@@ -894,6 +890,18 @@ impl TyBuilder<()> {
             }
         }
     }
+
+    pub fn subst_for_def(db: &dyn HirDatabase, def: impl Into<GenericDefId>) -> TyBuilder<()> {
+        let def = def.into();
+        let params = generics(db.upcast(), def);
+        let param_count = params.len();
+        TyBuilder::new((), param_count)
+    }
+
+    pub fn build(self) -> Substitution {
+        let ((), subst) = self.build_internal();
+        subst
+    }
 }
 
 impl TyBuilder<hir_def::AdtId> {
@@ -956,6 +964,28 @@ impl TyBuilder<TypeAliasId> {
     }
 }
 
+impl<T: TypeWalk + HasInterner<Interner = Interner>> TyBuilder<Binders<T>> {
+    fn subst_binders(b: Binders<T>) -> Self {
+        let param_count = b.num_binders;
+        TyBuilder::new(b, param_count)
+    }
+
+    pub fn build(self) -> T {
+        let (b, subst) = self.build_internal();
+        b.subst(&subst)
+    }
+}
+
+impl TyBuilder<Binders<Ty>> {
+    pub fn def_ty(db: &dyn HirDatabase, def: TyDefId) -> TyBuilder<Binders<Ty>> {
+        TyBuilder::subst_binders(db.ty(def.into()))
+    }
+
+    pub fn impl_self_ty(db: &dyn HirDatabase, def: hir_def::ImplId) -> TyBuilder<Binders<Ty>> {
+        TyBuilder::subst_binders(db.impl_self_ty(def))
+    }
+}
+
 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 b9cc081e8..a76586f0c 100644
--- a/crates/hir_ty/src/method_resolution.rs
+++ b/crates/hir_ty/src/method_resolution.rs
@@ -709,7 +709,7 @@ pub(crate) fn inherent_impl_substs(
 ) -> Option<Substitution> {
     // we create a var for each type parameter of the impl; we need to keep in
     // mind here that `self_ty` might have vars of its own
-    let vars = Substitution::build_for_def(db, impl_id)
+    let vars = TyBuilder::subst_for_def(db, impl_id)
         .fill_with_bound_vars(DebruijnIndex::INNERMOST, self_ty.binders.len(&Interner))
         .build();
     let self_ty_with_vars = db.impl_self_ty(impl_id).subst(&vars);
@@ -760,13 +760,13 @@ fn transform_receiver_ty(
     self_ty: &Canonical<Ty>,
 ) -> Option<Ty> {
     let substs = match function_id.lookup(db.upcast()).container {
-        AssocContainerId::TraitId(_) => Substitution::build_for_def(db, function_id)
+        AssocContainerId::TraitId(_) => TyBuilder::subst_for_def(db, function_id)
             .push(self_ty.value.clone())
             .fill_with_unknown()
             .build(),
         AssocContainerId::ImplId(impl_id) => {
             let impl_substs = inherent_impl_substs(db, impl_id, &self_ty)?;
-            Substitution::build_for_def(db, function_id)
+            TyBuilder::subst_for_def(db, function_id)
                 .use_parent_substs(&impl_substs)
                 .fill_with_unknown()
                 .build()