error[E0597]: `counter` does not live long enough
  --> $DIR/issue-54556-niconii.rs:22:20
   |
LL |     let counter = Mutex;
   |         ------- binding `counter` declared here
LL |
LL |     if let Ok(_) = counter.lock() { }
   |                    ^^^^^^^^^^^^^^
   |                    |
   |                    borrowed value does not live long enough
   |                    a temporary with access to the borrow is created here ...
...
LL | }
   | -
   | |
   | `counter` dropped here while still borrowed
   | ... and the borrow might be used here, when that temporary is dropped and runs the destructor for type `Result<MutexGuard<'_>, ()>`
   |
help: consider adding semicolon after the expression so its temporaries are dropped sooner, before the local variables declared by the block are dropped
   |
LL |     if let Ok(_) = counter.lock() { };
   |                                      +

error: aborting due to previous error

For more information about this error, try `rustc --explain E0597`.