miri: lift restriction on extern types being the only field in a struct

2 files changed, 11 insertions, 27 deletions

diff --git a/compiler/rustc_const_eval/src/interpret/eval_context.rs b/compiler/rustc_const_eval/src/interpret/eval_context.rs
index d768f06c4f0..d9faa6777ea 100644
--- a/compiler/rustc_const_eval/src/interpret/eval_context.rs
+++ b/compiler/rustc_const_eval/src/interpret/eval_context.rs
@@ -616,19 +616,9 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
                     match self.size_and_align_of(metadata, &field)? {
                         Some(size_and_align) => size_and_align,
                         None => {
-                            // A field with extern type.  If this field is at offset 0, we behave
-                            // like the underlying extern type.
-                            // FIXME: Once we have made decisions for how to handle size and alignment
-                            // of `extern type`, this should be adapted.  It is just a temporary hack
-                            // to get some code to work that probably ought to work.
-                            if sized_size == Size::ZERO {
-                                return Ok(None);
-                            } else {
-                                span_bug!(
-                                    self.cur_span(),
-                                    "Fields cannot be extern types, unless they are at offset 0"
-                                )
-                            }
+                            // A field with an extern type. We don't know the actual dynamic size
+                            // or the alignment.
+                            return Ok(None);
                         }
                     };
 
diff --git a/compiler/rustc_const_eval/src/interpret/place.rs b/compiler/rustc_const_eval/src/interpret/place.rs
index 851c2a6bb2e..818b95b7fc4 100644
--- a/compiler/rustc_const_eval/src/interpret/place.rs
+++ b/compiler/rustc_const_eval/src/interpret/place.rs
@@ -362,21 +362,15 @@ where
             // Re-use parent metadata to determine dynamic field layout.
             // With custom DSTS, this *will* execute user-defined code, but the same
             // happens at run-time so that's okay.
-            let align = match self.size_and_align_of(&base.meta, &field_layout)? {
-                Some((_, align)) => align,
-                None if offset == Size::ZERO => {
-                    // An extern type at offset 0, we fall back to its static alignment.
-                    // FIXME: Once we have made decisions for how to handle size and alignment
-                    // of `extern type`, this should be adapted.  It is just a temporary hack
-                    // to get some code to work that probably ought to work.
-                    field_layout.align.abi
+            match self.size_and_align_of(&base.meta, &field_layout)? {
+                Some((_, align)) => (base.meta, offset.align_to(align)),
+                None => {
+                    // For unsized types with an extern type tail we perform no adjustments.
+                    // NOTE: keep this in sync with `PlaceRef::project_field` in the codegen backend.
+                    assert!(matches!(base.meta, MemPlaceMeta::None));
+                    (base.meta, offset)
                 }
-                None => span_bug!(
-                    self.cur_span(),
-                    "cannot compute offset for extern type field at non-0 offset"
-                ),
-            };
-            (base.meta, offset.align_to(align))
+            }
         } else {
             // base.meta could be present; we might be accessing a sized field of an unsized
             // struct.