Disallow access to free BoundVars outside TyLoweringContext

Author: lowr

crates/hir-ty/src/lower.rs

@@ -1190,9 +1190,9 @@ pub fn associated_type_shorthand_candidates<R>(
     db: &dyn HirDatabase,
     def: GenericDefId,
     res: TypeNs,
-    cb: impl FnMut(&Name, &TraitRef, TypeAliasId) -> Option<R>,
+    mut cb: impl FnMut(&Name, TypeAliasId) -> Option<R>,
 ) -> Option<R> {
-    named_associated_type_shorthand_candidates(db, def, res, None, cb)
+    named_associated_type_shorthand_candidates(db, def, res, None, |name, _, id| cb(name, id))
 }
 
 fn named_associated_type_shorthand_candidates<R>(
@@ -1202,6 +1202,9 @@ fn named_associated_type_shorthand_candidates<R>(
     def: GenericDefId,
     res: TypeNs,
     assoc_name: Option<Name>,
+    // Do NOT let `cb` touch `TraitRef` outside of `TyLoweringContext`. Its substitution contains
+    // free `BoundVar`s that need to be shifted and only `TyLoweringContext` knows how to do that
+    // properly (see `TyLoweringContext::select_associated_type()`).
     mut cb: impl FnMut(&Name, &TraitRef, TypeAliasId) -> Option<R>,
 ) -> Option<R> {
     let mut search = |t| {

crates/hir/src/semantics.rs

@@ -1600,7 +1600,7 @@ impl<'a> SemanticsScope<'a> {
             self.db,
             def,
             resolution.in_type_ns()?,
-            |name, _, id| cb(name, id.into()),
+            |name, id| cb(name, id.into()),
         )
     }
 }

crates/hir/src/source_analyzer.rs

@@ -967,7 +967,7 @@ fn resolve_hir_path_(
                         db,
                         def,
                         res.in_type_ns()?,
-                        |name, _, id| (name == unresolved.name).then(|| id),
+                        |name, id| (name == unresolved.name).then(|| id),
                     )
                 })
                 .map(TypeAlias::from)