4 files changed, 15 insertions, 11 deletions

diff --git a/crates/hir/src/lib.rs b/crates/hir/src/lib.rs
index 003821981..db4ebada4 100644
--- a/crates/hir/src/lib.rs
+++ b/crates/hir/src/lib.rs
@@ -55,6 +55,7 @@ use hir_ty::{
     autoderef, could_unify,
     method_resolution::{self, TyFingerprint},
     primitive::UintTy,
+    subst_prefix,
     traits::FnTrait,
     AliasEq, AliasTy, BoundVar, CallableDefId, CallableSig, Canonical, CanonicalVarKinds, Cast,
     DebruijnIndex, InEnvironment, Interner, QuantifiedWhereClause, Scalar, Solution,
@@ -1503,7 +1504,7 @@ impl TypeParam {
         let krate = self.id.parent.module(db.upcast()).krate();
         let ty = params.get(local_idx)?.clone();
         let subst = TyBuilder::type_params_subst(db, self.id.parent);
-        let ty = ty.substitute(&Interner, &subst.prefix(local_idx));
+        let ty = ty.substitute(&Interner, &subst_prefix(&subst, local_idx));
         Some(Type::new_with_resolver_inner(db, krate, &resolver, ty))
     }
 }
diff --git a/crates/hir_ty/src/display.rs b/crates/hir_ty/src/display.rs
index 91d657ce2..4ef8024d0 100644
--- a/crates/hir_ty/src/display.rs
+++ b/crates/hir_ty/src/display.rs
@@ -19,7 +19,7 @@ use hir_expand::name::Name;
 
 use crate::{
     db::HirDatabase, from_assoc_type_id, from_foreign_def_id, from_placeholder_idx,
-    lt_from_placeholder_idx, primitive, to_assoc_type_id, traits::chalk::from_chalk,
+    lt_from_placeholder_idx, primitive, subst_prefix, to_assoc_type_id, traits::chalk::from_chalk,
     utils::generics, AdtId, AliasEq, AliasTy, CallableDefId, CallableSig, DomainGoal, GenericArg,
     ImplTraitId, Interner, Lifetime, LifetimeData, LifetimeOutlives, Mutability, OpaqueTy,
     ProjectionTy, ProjectionTyExt, QuantifiedWhereClause, Scalar, TraitRef, Ty, TyExt, TyKind,
@@ -484,9 +484,11 @@ impl HirDisplay for Ty {
                                             default_from = i + 1;
                                         }
                                         (_, Some(default_parameter)) => {
-                                            let actual_default = default_parameter
-                                                .clone()
-                                                .substitute(&Interner, &parameters.prefix(i));
+                                            let actual_default =
+                                                default_parameter.clone().substitute(
+                                                    &Interner,
+                                                    &subst_prefix(parameters, i),
+                                                );
                                             if parameter.assert_ty_ref(&Interner) != &actual_default
                                             {
                                                 default_from = i + 1;
diff --git a/crates/hir_ty/src/infer/expr.rs b/crates/hir_ty/src/infer/expr.rs
index 82ab9c5fe..185a2dfc3 100644
--- a/crates/hir_ty/src/infer/expr.rs
+++ b/crates/hir_ty/src/infer/expr.rs
@@ -964,8 +964,10 @@ impl<'a> InferenceContext<'a> {
                     if let AssocContainerId::TraitId(trait_) = f.lookup(self.db.upcast()).container
                     {
                         // construct a TraitRef
-                        let substs =
-                            parameters.prefix(generics(self.db.upcast(), trait_.into()).len());
+                        let substs = crate::subst_prefix(
+                            &*parameters,
+                            generics(self.db.upcast(), trait_.into()).len(),
+                        );
                         self.push_obligation(
                             TraitRef { trait_id: to_chalk_trait_id(trait_), substitution: substs }
                                 .cast(&Interner),
diff --git a/crates/hir_ty/src/lib.rs b/crates/hir_ty/src/lib.rs
index abdcf8829..f74b22b58 100644
--- a/crates/hir_ty/src/lib.rs
+++ b/crates/hir_ty/src/lib.rs
@@ -75,10 +75,9 @@ pub type ChalkTraitId = chalk_ir::TraitId<Interner>;
 
 pub type FnSig = chalk_ir::FnSig<Interner>;
 
-impl Substitution {
-    pub fn prefix(&self, n: usize) -> Substitution {
-        Substitution::intern(self.interned()[..std::cmp::min(self.len(&Interner), n)].into())
-    }
+// FIXME: get rid of this
+pub fn subst_prefix(s: &Substitution, n: usize) -> Substitution {
+    Substitution::intern(s.interned()[..std::cmp::min(s.len(&Interner), n)].into())
 }
 
 /// Return an index of a parameter in the generic type parameter list by it's id.