Make some region folders a little stricter.

Because certain regions cannot occur in them.

2 files changed, 9 insertions, 8 deletions

diff --git a/compiler/rustc_trait_selection/src/solve/assembly/structural_traits.rs b/compiler/rustc_trait_selection/src/solve/assembly/structural_traits.rs
index 1a566e87dc8..996dc329dcb 100644
--- a/compiler/rustc_trait_selection/src/solve/assembly/structural_traits.rs
+++ b/compiler/rustc_trait_selection/src/solve/assembly/structural_traits.rs
@@ -91,14 +91,15 @@ pub(in crate::solve) fn replace_erased_lifetimes_with_bound_vars<'tcx>(
 ) -> ty::Binder<'tcx, Ty<'tcx>> {
     debug_assert!(!ty.has_late_bound_regions());
     let mut counter = 0;
-    let ty = tcx.fold_regions(ty, |mut r, current_depth| {
-        if let ty::ReErased = r.kind() {
+    let ty = tcx.fold_regions(ty, |r, current_depth| match r.kind() {
+        ty::ReErased => {
             let br =
                 ty::BoundRegion { var: ty::BoundVar::from_u32(counter), kind: ty::BrAnon(None) };
             counter += 1;
-            r = tcx.mk_re_late_bound(current_depth, br);
+            tcx.mk_re_late_bound(current_depth, br)
         }
-        r
+        // All free regions should be erased here.
+        r => bug!("unexpected region: {r:?}"),
     });
     let bound_vars = tcx.mk_bound_variable_kinds_from_iter(
         (0..counter).map(|_| ty::BoundVariableKind::Region(ty::BrAnon(None))),
diff --git a/compiler/rustc_trait_selection/src/traits/select/mod.rs b/compiler/rustc_trait_selection/src/traits/select/mod.rs
index 081e4d7cfa4..af61ca0c29f 100644
--- a/compiler/rustc_trait_selection/src/traits/select/mod.rs
+++ b/compiler/rustc_trait_selection/src/traits/select/mod.rs
@@ -3006,16 +3006,16 @@ fn bind_generator_hidden_types_above<'tcx>(
 
             // Only remap erased regions if we use them.
             if considering_regions {
-                ty = tcx.fold_regions(ty, |mut r, current_depth| {
-                    if let ty::ReErased = r.kind() {
+                ty = tcx.fold_regions(ty, |r, current_depth| match r.kind() {
+                    ty::ReErased => {
                         let br = ty::BoundRegion {
                             var: ty::BoundVar::from_u32(counter),
                             kind: ty::BrAnon(None),
                         };
                         counter += 1;
-                        r = tcx.mk_re_late_bound(current_depth, br);
+                        tcx.mk_re_late_bound(current_depth, br)
                     }
-                    r
+                    r => bug!("unexpected region: {r:?}"),
                 })
             }