error[E0303]: pattern bindings are not allowed after an `@`
  --> $DIR/pattern-bindings-after-at.rs:8:31
   |
LL |         ref mut z @ &mut Some(ref a) => {
   |                               ^^^^^ not allowed after `@`

warning[E0502]: cannot borrow `_` as immutable because it is also borrowed as mutable
  --> $DIR/pattern-bindings-after-at.rs:8:31
   |
LL |         ref mut z @ &mut Some(ref a) => {
   |         ----------------------^^^^^-
   |         |                     |
   |         |                     immutable borrow occurs here
   |         mutable borrow occurs here
...
LL |             **z = None;
   |             ---------- mutable borrow later used here
   |
   = warning: this error has been downgraded to a warning for backwards compatibility with previous releases
   = warning: this represents potential undefined behavior in your code and this warning will become a hard error in the future

error: aborting due to previous error

Some errors have detailed explanations: E0303, E0502.
For more information about an error, try `rustc --explain E0303`.