cast_mut_to_ref in write_to_shr

Author: Vanille-N

tests/fail/tree_borrows/write_to_shr.rs

@@ -1,5 +1,7 @@
 //@compile-flags: -Zmiri-tree-borrows
 
+#![allow(cast_ref_to_mut)]
+
 // Very simple test that a shared reference without interior mutability cannot
 // be written to.