have better explanation for `relate_types`

1 files changed, 14 insertions, 7 deletions

diff --git a/compiler/rustc_mir_transform/src/add_subtyping_projections.rs b/compiler/rustc_mir_transform/src/add_subtyping_projections.rs
index 594bb9bf142..1cc049d5a22 100644
--- a/compiler/rustc_mir_transform/src/add_subtyping_projections.rs
+++ b/compiler/rustc_mir_transform/src/add_subtyping_projections.rs
@@ -24,24 +24,31 @@ impl<'a, 'tcx> MutVisitor<'tcx> for SubTypeChecker<'a, 'tcx> {
         rvalue: &mut Rvalue<'tcx>,
         location: Location,
     ) {
-        let place_ty = place.ty(self.local_decls, self.tcx);
+        let mut place_ty = place.ty(self.local_decls, self.tcx).ty;
         let mut rval_ty = rvalue.ty(self.local_decls, self.tcx);
-        if place_ty.ty != rval_ty {
-            // Not erasing this causes `Free Regions` errors in validator,
-            // when rval is `ReStatic`.
-            rval_ty = self.tcx.erase_regions_ty(rval_ty);
+        // Not erasing this causes `Free Regions` errors in validator,
+        // when rval is `ReStatic`.
+        rval_ty = self.tcx.erase_regions_ty(rval_ty);
+        place_ty = self.tcx.erase_regions(place_ty);
+        if place_ty != rval_ty {
             let temp = self
                 .patcher
                 .new_temp(rval_ty, self.local_decls[place.as_ref().local].source_info.span);
             let new_place = Place::from(temp);
             self.patcher.add_assign(location, new_place, rvalue.clone());
-            let subtyped =
-                new_place.project_deeper(&[ProjectionElem::Subtype(place_ty.ty)], self.tcx);
+            let subtyped = new_place.project_deeper(&[ProjectionElem::Subtype(place_ty)], self.tcx);
             *rvalue = Rvalue::Use(Operand::Move(subtyped));
         }
     }
 }
 
+// Aim here is to do this kind of transformation:
+//
+// let place: place_ty = rval;
+// // gets transformed to
+// let temp: rval_ty = rval;
+// let place: place_ty = temp as place_ty;
+//
 pub fn subtype_finder<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
     let patch = MirPatch::new(body);
     let mut checker = SubTypeChecker { tcx, patcher: patch, local_decls: &body.local_decls };