2 files changed, 15 insertions, 10 deletions

diff --git a/crates/hir_ty/src/infer/unify.rs b/crates/hir_ty/src/infer/unify.rs
index 8370f2e1c..06187df56 100644
--- a/crates/hir_ty/src/infer/unify.rs
+++ b/crates/hir_ty/src/infer/unify.rs
@@ -108,19 +108,22 @@ impl<'a, 'b> Canonicalizer<'a, 'b> {
 }
 
 impl<T> Canonicalized<T> {
-    pub(super) fn decanonicalize_ty(&self, mut ty: Ty) -> Ty {
-        ty.walk_mut_binders(
+    pub(super) fn decanonicalize_ty(&self, ty: Ty) -> Ty {
+        ty.fold_binders(
             &mut |ty, binders| {
-                if let &mut TyKind::BoundVar(bound) = ty.interned_mut() {
+                if let TyKind::BoundVar(bound) = ty.kind(&Interner) {
                     if bound.debruijn >= binders {
                         let (v, k) = self.free_vars[bound.index];
-                        *ty = TyKind::InferenceVar(v, k).intern(&Interner);
+                        TyKind::InferenceVar(v, k).intern(&Interner)
+                    } else {
+                        ty
                     }
+                } else {
+                    ty
                 }
             },
             DebruijnIndex::INNERMOST,
-        );
-        ty
+        )
     }
 
     pub(super) fn apply_solution(
diff --git a/crates/hir_ty/src/lower.rs b/crates/hir_ty/src/lower.rs
index 00838b298..a3c3ef6b2 100644
--- a/crates/hir_ty/src/lower.rs
+++ b/crates/hir_ty/src/lower.rs
@@ -1011,17 +1011,19 @@ pub(crate) fn generic_defaults_query(
                 p.default.as_ref().map_or(TyKind::Error.intern(&Interner), |t| ctx.lower_ty(t));
 
             // Each default can only refer to previous parameters.
-            ty.walk_mut_binders(
-                &mut |ty, binders| match ty.interned_mut() {
+            ty = ty.fold_binders(
+                &mut |ty, binders| match ty.kind(&Interner) {
                     TyKind::BoundVar(BoundVar { debruijn, index }) if *debruijn == binders => {
                         if *index >= idx {
                             // type variable default referring to parameter coming
                             // after it. This is forbidden (FIXME: report
                             // diagnostic)
-                            *ty = TyKind::Error.intern(&Interner);
+                            TyKind::Error.intern(&Interner)
+                        } else {
+                            ty
                         }
                     }
-                    _ => {}
+                    _ => ty,
                 },
                 DebruijnIndex::INNERMOST,
             );