Properly evaluate zst enum

Author: oli-obk

src/librustc_mir/interpret/eval_context.rs

@@ -1321,6 +1321,15 @@ impl<'a, 'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> EvalContext<'a, 'mir, 'tcx, M
     pub fn try_read_value(&self, ptr: Pointer, ptr_align: Align, ty: Ty<'tcx>) -> EvalResult<'tcx, Option<Value>> {
         use syntax::ast::FloatTy;
 
+        let layout = self.layout_of(ty)?;
+        // do the strongest layout check of the two
+        let align = layout.align.max(ptr_align);
+        self.memory.check_align(ptr, align)?;
+
+        if layout.size.bytes() == 0 {
+            return Ok(Some(Value::ByVal(PrimVal::Undef)));
+        }
+
         let ptr = ptr.to_ptr()?;
         let val = match ty.sty {
             ty::TyBool => {

src/librustc_mir/interpret/place.rs

@@ -136,6 +136,7 @@ impl<'a, 'mir, 'tcx, M: Machine<'mir, 'tcx>> EvalContext<'a, 'mir, 'tcx, M> {
                 let val = [a, b][field_index];
                 Ok(Some((Value::ByVal(val), field.ty)))
             },
+            // FIXME(oli-obk): figure out whether we should be calling `try_read_value` here
             _ => Ok(None),
         }
     }