Auto merge of #147055 - beepster4096:subtype_is_not_a_projection, r=lcnr

Turn ProjectionElem::Subtype into CastKind::Subtype

I noticed that drop elaboration can't, in general, handle `ProjectionElem::SubType`. It creates a disjoint move path that overlaps with other move paths. (`Subslice` does too, and I'm working on a different PR to make that special case less fragile.) If its skipped and treated as the same move path as its parent then `MovePath.place` has multiple possible projections. (It would probably make sense to remove all `Subtype` projections for the canonical place but it doesn't make sense to have this special case for a problem that doesn't actually occur in real MIR.)

The only reason this doesn't break is that `Subtype` is always the sole projection of the local its applied to. For the same reason, it works fine as a `CastKind` so I figured that makes more sense than documenting and validating this hidden invariant.

cc rust-lang/rust#112651, rust-lang/rust#133258

r? Icnr (bc you've been the main person dealing with `Subtype` it looks like)

1 files changed, 1 insertions, 4 deletions

diff --git a/compiler/rustc_mir_dataflow/src/move_paths/builder.rs b/compiler/rustc_mir_dataflow/src/move_paths/builder.rs
index 72d4cd72c2b..434f106302f 100644
--- a/compiler/rustc_mir_dataflow/src/move_paths/builder.rs
+++ b/compiler/rustc_mir_dataflow/src/move_paths/builder.rs
@@ -227,11 +227,8 @@ impl<'a, 'tcx, F: Fn(Ty<'tcx>) -> bool> MoveDataBuilder<'a, 'tcx, F> {
                 ProjectionElem::UnwrapUnsafeBinder(_) => {}
                 // `OpaqueCast`:Only transmutes the type, so no moves there.
                 // `Downcast`  :Only changes information about a `Place` without moving.
-                // `Subtype`   :Only transmutes the type, so moves.
                 // So it's safe to skip these.
-                ProjectionElem::OpaqueCast(_)
-                | ProjectionElem::Subtype(_)
-                | ProjectionElem::Downcast(_, _) => (),
+                ProjectionElem::OpaqueCast(_) | ProjectionElem::Downcast(_, _) => (),
             }
             let elem_ty = PlaceTy::from_ty(place_ty).projection_ty(tcx, elem).ty;
             if !(self.filter)(elem_ty) {