librustc: Disallow &mut loans from overlapping with any other loans

src/test/run-pass/alt-implicit-copy-unique.rs

@@ -12,7 +12,7 @@ fn main() {
     let x = ~{mut a: ~10, b: ~20};
     match x {
       ~{a: ref mut a, b: ref b} => {
-        assert **a == 10; (*x).a = ~30; assert **a == 30;
+        assert **a == 10; *a = ~30; assert **a == 30;
       }
     }
 }