illegal_write2 does not work with optimizations

Author: RalfJung

tests/compile-fail/stacked_borrows/illegal_write2.rs

@@ -1,3 +1,7 @@
+// We fail to detect this when neither this nor libstd are optimized/have retagging.
+// FIXME: Investigate that.
+// compile-flags: -Zmir-opt-level=0
+
 #![allow(unused_variables)]
 
 fn main() {