fix: Off-by-one in lifetime binders when lowering dyn Trait<'a>

crates/hir-ty/src/lower.rs

@@ -932,7 +932,7 @@ impl<'db, 'a> TyLoweringContext<'db, 'a> {
         bound: &'b TypeBound,
         self_ty: Ty<'db>,
         ignore_bindings: bool,
-    ) -> impl Iterator<Item = (Clause<'db>, GenericPredicateSource)> + use<'b, 'a, 'db> {
+    ) -> impl Iterator<Item = (Clause<'db>, GenericPredicateSource)> + use<'db> {
         let interner = self.interner;
         let meta_sized = self.lang_items.MetaSized;
         let pointee_sized = self.lang_items.PointeeSized;
@@ -1030,7 +1030,17 @@ impl<'db, 'a> TyLoweringContext<'db, 'a> {
 
             for b in bounds {
                 let db = self.db;
-                self.lower_type_bound(b, dummy_self_ty, false).for_each(|(b, _)| {
+                match b {
+                    TypeBound::Path(_, TraitBoundModifier::None) => {
+                        // `dyn Trait<'a>` is an existential predicate that introduces a binder.
+                        self.with_shifted_in(&[], |ctx| {
+                            ctx.lower_type_bound(b, dummy_self_ty, false)
+                        })
+                        .0
+                    }
+                    _ => self.lower_type_bound(b, dummy_self_ty, false),
+                }
+                .for_each(|(b, _)| {
                     match b.kind().skip_binder() {
                         rustc_type_ir::ClauseKind::Trait(t) => {
                             let id = t.def_id();

crates/hir-ty/src/tests/regression.rs

@@ -3138,3 +3138,13 @@ fn main() {
     "#,
     );
 }
+
+#[test]
+fn dyn_trait_binder_inside_fn_ptr() {
+    check_no_mismatches(
+        r#"
+trait Trait<'a> {}
+fn f<'a>(_: fn() -> &'a dyn Trait<'a>) {}
+    "#,
+    );
+}