add delay_span_bug to TransmuteSizeDiff, just to be sure

src/librustc_mir/interpret/place.rs

@@ -926,6 +926,10 @@ where
             // most likey we *are* running `typeck` right now. Investigate whether we can bail out
             // on `typeck_tables().has_errors` at all const eval entry points.
             debug!("Size mismatch when transmuting!\nsrc: {:#?}\ndest: {:#?}", src, dest);
+            self.tcx.sess.delay_span_bug(
+                self.tcx.span,
+                "size-changing transmute, should have been caught by transmute checking",
+            );
             throw_inval!(TransmuteSizeDiff(src.layout.ty, dest.layout.ty));
         }
         // Unsized copies rely on interpreting `src.meta` with `dest.layout`, we want