5 files changed, 32 insertions, 25 deletions

diff --git a/crates/hir_ty/src/lib.rs b/crates/hir_ty/src/lib.rs
index 827317fce..daf379ef8 100644
--- a/crates/hir_ty/src/lib.rs
+++ b/crates/hir_ty/src/lib.rs
@@ -124,7 +124,7 @@ impl<T> Binders<T> {
     where
         T: TypeWalk,
     {
-        Binders::empty(&Interner, value.shift_bound_vars(DebruijnIndex::ONE))
+        Binders::empty(&Interner, value.shifted_in_from(DebruijnIndex::ONE))
     }
 }
 
@@ -209,7 +209,8 @@ impl CallableSig {
             params_and_return: fn_ptr
                 .substs
                 .clone()
-                .shift_bound_vars_out(DebruijnIndex::ONE)
+                .shifted_out_to(DebruijnIndex::ONE)
+                .expect("unexpected lifetime vars in fn ptr")
                 .interned()
                 .iter()
                 .map(|arg| arg.assert_ty_ref(&Interner).clone())
diff --git a/crates/hir_ty/src/lower.rs b/crates/hir_ty/src/lower.rs
index 66e72c814..48c26f471 100644
--- a/crates/hir_ty/src/lower.rs
+++ b/crates/hir_ty/src/lower.rs
@@ -483,7 +483,7 @@ impl<'a> TyLoweringContext<'a> {
                         };
                         // We need to shift in the bound vars, since
                         // associated_type_shorthand_candidates does not do that
-                        let substs = substs.shift_bound_vars(self.in_binders);
+                        let substs = substs.shifted_in_from(self.in_binders);
                         // FIXME handle type parameters on the segment
                         return Some(
                             TyKind::Alias(AliasTy::Projection(ProjectionTy {
@@ -831,20 +831,20 @@ pub fn associated_type_shorthand_candidates<R>(
     };
 
     match res {
-        // FIXME: how to correctly handle higher-ranked bounds here?
         TypeNs::SelfType(impl_id) => search(
-            db.impl_trait(impl_id)?
-                .into_value_and_skipped_binders()
-                .0
-                .shift_bound_vars_out(DebruijnIndex::ONE),
+            // we're _in_ the impl -- the binders get added back later. Correct,
+            // but it would be nice to make this more explicit
+            db.impl_trait(impl_id)?.into_value_and_skipped_binders().0,
         ),
         TypeNs::GenericParam(param_id) => {
             let predicates = db.generic_predicates_for_param(param_id);
             let res = predicates.iter().find_map(|pred| match pred.skip_binders().skip_binders() {
                 // FIXME: how to correctly handle higher-ranked bounds here?
-                WhereClause::Implemented(tr) => {
-                    search(tr.clone().shift_bound_vars_out(DebruijnIndex::ONE))
-                }
+                WhereClause::Implemented(tr) => search(
+                    tr.clone()
+                        .shifted_out_to(DebruijnIndex::ONE)
+                        .expect("FIXME unexpected higher-ranked trait bound"),
+                ),
                 _ => None,
             });
             if let res @ Some(_) = res {
diff --git a/crates/hir_ty/src/traits/chalk/mapping.rs b/crates/hir_ty/src/traits/chalk/mapping.rs
index 9db2b0c2f..c3b148cab 100644
--- a/crates/hir_ty/src/traits/chalk/mapping.rs
+++ b/crates/hir_ty/src/traits/chalk/mapping.rs
@@ -531,7 +531,7 @@ pub(super) fn generic_predicate_to_inline_bound(
 ) -> Option<chalk_ir::Binders<rust_ir::InlineBound<Interner>>> {
     // An InlineBound is like a GenericPredicate, except the self type is left out.
     // We don't have a special type for this, but Chalk does.
-    let self_ty_shifted_in = self_ty.clone().shift_bound_vars(DebruijnIndex::ONE);
+    let self_ty_shifted_in = self_ty.clone().shifted_in_from(DebruijnIndex::ONE);
     let (pred, binders) = pred.as_ref().into_value_and_skipped_binders();
     match pred {
         WhereClause::Implemented(trait_ref) => {
diff --git a/crates/hir_ty/src/utils.rs b/crates/hir_ty/src/utils.rs
index c7786a199..d11708299 100644
--- a/crates/hir_ty/src/utils.rs
+++ b/crates/hir_ty/src/utils.rs
@@ -66,9 +66,11 @@ fn direct_super_trait_refs(db: &dyn HirDatabase, trait_ref: &TraitRef) -> Vec<Tr
         .filter_map(|pred| {
             pred.as_ref().filter_map(|pred| match pred.skip_binders() {
                 // FIXME: how to correctly handle higher-ranked bounds here?
-                WhereClause::Implemented(tr) => {
-                    Some(tr.clone().shift_bound_vars_out(DebruijnIndex::ONE))
-                }
+                WhereClause::Implemented(tr) => Some(
+                    tr.clone()
+                        .shifted_out_to(DebruijnIndex::ONE)
+                        .expect("FIXME unexpected higher-ranked trait bound"),
+                ),
                 _ => None,
             })
         })
@@ -103,6 +105,8 @@ pub(super) fn all_super_traits(db: &dyn DefDatabase, trait_: TraitId) -> Vec<Tra
 /// we have `Self: Trait<u32, i32>` and `Trait<T, U>: OtherTrait<U>` we'll get
 /// `Self: OtherTrait<i32>`.
 pub(super) fn all_super_trait_refs(db: &dyn HirDatabase, trait_ref: TraitRef) -> Vec<TraitRef> {
+    // FIXME: replace by Chalk's `super_traits`, maybe make this a query
+
     // we need to take care a bit here to avoid infinite loops in case of cycles
     // (i.e. if we have `trait A: B; trait B: A;`)
     let mut result = vec![trait_ref];
diff --git a/crates/hir_ty/src/walk.rs b/crates/hir_ty/src/walk.rs
index b1e22da0a..b85e6ab4d 100644
--- a/crates/hir_ty/src/walk.rs
+++ b/crates/hir_ty/src/walk.rs
@@ -82,7 +82,7 @@ pub trait TypeWalk {
                         *ty = substs.interned()[bound.index]
                             .assert_ty_ref(&Interner)
                             .clone()
-                            .shift_bound_vars(binders);
+                            .shifted_in_from(binders);
                     }
                 }
             },
@@ -92,7 +92,7 @@ pub trait TypeWalk {
     }
 
     /// Shifts up debruijn indices of `TyKind::Bound` vars by `n`.
-    fn shift_bound_vars(self, n: DebruijnIndex) -> Self
+    fn shifted_in_from(self, n: DebruijnIndex) -> Self
     where
         Self: Sized,
     {
@@ -108,20 +108,22 @@ pub trait TypeWalk {
     }
 
     /// Shifts debruijn indices of `TyKind::Bound` vars out (down) by `n`.
-    fn shift_bound_vars_out(self, n: DebruijnIndex) -> Self
+    fn shifted_out_to(self, n: DebruijnIndex) -> Option<Self>
     where
         Self: Sized + std::fmt::Debug,
     {
-        self.fold_binders(
-            &mut |ty, binders| match ty.kind(&Interner) {
-                TyKind::BoundVar(bound) if bound.debruijn >= binders => {
-                    TyKind::BoundVar(bound.shifted_out_to(n).unwrap_or(bound.clone()))
-                        .intern(&Interner)
+        Some(self.fold_binders(
+            &mut |ty, binders| {
+                match ty.kind(&Interner) {
+                    TyKind::BoundVar(bound) if bound.debruijn >= binders => {
+                        TyKind::BoundVar(bound.shifted_out_to(n).unwrap_or(bound.clone()))
+                            .intern(&Interner)
+                    }
+                    _ => ty,
                 }
-                _ => ty,
             },
             DebruijnIndex::INNERMOST,
-        )
+        ))
     }
 }