Align the changes to the lang decision

1 files changed, 29 insertions, 8 deletions

diff --git a/compiler/rustc_borrowck/src/type_check/mod.rs b/compiler/rustc_borrowck/src/type_check/mod.rs
index 2600c1fbff7..f98ac80a4c9 100644
--- a/compiler/rustc_borrowck/src/type_check/mod.rs
+++ b/compiler/rustc_borrowck/src/type_check/mod.rs
@@ -2323,27 +2323,48 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
                                 let src_tail = tcx.struct_tail_without_normalization(src.ty);
                                 let dst_tail = tcx.struct_tail_without_normalization(dst.ty);
 
-                                if let ty::Dynamic(..) = src_tail.kind()
+                                if let ty::Dynamic(src_tty, ..) = src_tail.kind()
                                     && let ty::Dynamic(dst_tty, ..) = dst_tail.kind()
+                                    && src_tty.principal().is_some()
                                     && dst_tty.principal().is_some()
                                 {
-                                    // Erase trait object lifetimes, to allow casts like `*mut dyn FnOnce()` -> `*mut dyn FnOnce() + 'static`.
-                                    let src_tail =
-                                        erase_single_trait_object_lifetime(tcx, src_tail);
-                                    let dst_tail =
-                                        erase_single_trait_object_lifetime(tcx, dst_tail);
+                                    // Erase trait object lifetimes, to allow casts like `*mut dyn FnOnce()` -> `*mut dyn FnOnce() + 'static`
+                                    // and remove auto traits.
+                                    let src_obj = tcx.mk_ty_from_kind(ty::Dynamic(
+                                        tcx.mk_poly_existential_predicates(
+                                            &src_tty.without_auto_traits().collect::<Vec<_>>(),
+                                        ),
+                                        tcx.lifetimes.re_erased,
+                                        ty::Dyn,
+                                    ));
+                                    let dst_obj = tcx.mk_ty_from_kind(ty::Dynamic(
+                                        tcx.mk_poly_existential_predicates(
+                                            &dst_tty.without_auto_traits().collect::<Vec<_>>(),
+                                        ),
+                                        tcx.lifetimes.re_erased,
+                                        ty::Dyn,
+                                    ));
+
+                                    // FIXME:
+                                    // this currently does nothing, but once we make `ptr_cast_add_auto_to_object`
+                                    // into a hard error, we can remove the above removal of auto traits and only
+                                    // keep this.
+                                    let src_obj = erase_single_trait_object_lifetime(tcx, src_obj);
+                                    let dst_obj = erase_single_trait_object_lifetime(tcx, dst_obj);
 
                                     let trait_ref = ty::TraitRef::new(
                                         tcx,
                                         tcx.require_lang_item(LangItem::Unsize, Some(span)),
-                                        [src_tail, dst_tail],
+                                        [src_obj, dst_obj],
                                     );
 
+                                    debug!(?src_tty, ?dst_tty, ?src_obj, ?dst_obj);
+
                                     self.prove_trait_ref(
                                         trait_ref,
                                         location.to_locations(),
                                         ConstraintCategory::Cast {
-                                            unsize_to: Some(tcx.fold_regions(dst_tail, |r, _| {
+                                            unsize_to: Some(tcx.fold_regions(dst_obj, |r, _| {
                                                 if let ty::ReVar(_) = r.kind() {
                                                     tcx.lifetimes.re_erased
                                                 } else {