handle/hack for arbitrary-self dyn receivers

Author: RalfJung

compiler/rustc_const_eval/src/interpret/terminator.rs

@@ -357,14 +357,21 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
                 ty::RawPtr(mt) => mt.ty,
                 // We should only accept `Box` with the default allocator.
                 // It's hard to test for that though so we accept every 1-ZST allocator.
+                // We need to also accept all `DispatchFromDyn` pointer types. (`DispatchFromDyn` is
+                // magic and checks that those satisfy the `repr(transparent)` conditions.)
+                // For now we hard-code `Rc` and `Arc`.
+                // FIXME: find a more systematic way to handle that.
                 ty::Adt(def, args)
-                    if def.is_box()
+                    if (def.is_box()
+                        || self.tcx.is_diagnostic_item(sym::Rc, def.did())
+                        || self.tcx.is_diagnostic_item(sym::Arc, def.did()))
                         && self
                             .layout_of(args[1].as_type().unwrap())
                             .is_ok_and(|l| l.is_1zst()) =>
                 {
                     args[0].as_type().unwrap()
                 }
+
                 _ => return None,
             })
         };

src/tools/miri/tests/pass/dyn-arbitrary-self.rs

@@ -65,6 +65,7 @@ fn pointers_and_wrappers() {
         ops::{CoerceUnsized, Deref, DispatchFromDyn},
     };
 
+    #[repr(transparent)] // FIXME without this, Miri complains about ABI mismatches
     struct Ptr<T: ?Sized>(Box<T>);
 
     impl<T: ?Sized> Deref for Ptr<T> {
@@ -78,6 +79,7 @@ fn pointers_and_wrappers() {
     impl<T: Unsize<U> + ?Sized, U: ?Sized> CoerceUnsized<Ptr<U>> for Ptr<T> {}
     impl<T: Unsize<U> + ?Sized, U: ?Sized> DispatchFromDyn<Ptr<U>> for Ptr<T> {}
 
+    #[repr(transparent)] // FIXME without this, Miri complains about ABI mismatches
     struct Wrapper<T: ?Sized>(T);
 
     impl<T: ?Sized> Deref for Wrapper<T> {