test for offset and alignment of the sized part, instead of field count

Author: RalfJung

src/librustc_mir/interpret/eval_context.rs

@@ -374,13 +374,13 @@ impl<'a, 'mir, 'tcx: 'mir, M: Machine<'a, 'mir, 'tcx>> EvalContext<'a, 'mir, 'tc
                 let (unsized_size, unsized_align) = match self.size_and_align_of(metadata, field)? {
                     Some(size_and_align) => size_and_align,
                     None => {
-                        // A field with extern type.  If this is the only field,
-                        // we treat this struct just the same.  Else, this is an error
-                        // (for now).
-                        if layout.fields.count() == 1 {
+                        // A field with extern type.  If this field is at offset 0 and the sized
+                        // part makes no alignment constraints, we behave like the underlying
+                        // extern type.
+                        if sized_size == Size::ZERO && sized_align.abi() == 1 {
                             return Ok(None)
                         } else {
-                            bug!("Fields cannot be extern types, unless they are the only field")
+                            bug!("Fields cannot be extern types, unless they are at offset 0")
                         }
                     }
                 };