error[E0597]: `counter` does not live long enough
  --> $DIR/issue-54556-niconii.rs:22:20
   |
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 `std::result::Result<MutexGuard<'_>, ()>`
   |
   = note: The temporary is part of an expression at the end of a block. Consider adding semicolon after the expression so its temporaries are dropped sooner, before the local variables declared by the block are dropped.

error: aborting due to previous error

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