note some FIXMEs

src/librustc_mir/interpret/eval_context.rs

@@ -377,6 +377,9 @@ impl<'a, 'mir, 'tcx: 'mir, M: Machine<'a, 'mir, 'tcx>> EvalContext<'a, 'mir, 'tc
                         // 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.
+                        // 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 && sized_align.abi() == 1 {
                             return Ok(None)
                         } else {

src/librustc_mir/interpret/place.rs

@@ -356,6 +356,9 @@ where
             let align = self.size_and_align_of(base.meta, field_layout)?
                 .map(|(_, align)| align)
                 // If this is an extern type, 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.
                 .unwrap_or_else(|| base.layout.align);
             (base.meta, offset.abi_align(align))
         } else {