error[E0499]: cannot borrow `a` as mutable more than once at a time
  --> $DIR/feature-gate-if-let-rescope.rs:24:9
   |
LL |     if let None = a.f().as_ref() {
   |                   -----
   |                   |
   |                   first mutable borrow occurs here
   |                   a temporary with access to the first borrow is created here ...
...
LL |         a.f().unwrap();
   |         ^ second mutable borrow occurs here
LL |
LL |     };
   |      - ... and the first borrow might be used here, when that temporary is dropped and runs the destructor for type `Option<B<'_, A>>`

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0499`.