if let Some with locked mutex

What it does

This can cause deadlocks. Similar to #5219 but with locks inside the if clause outside of the else clause.

Advantage

No response

Drawbacks

No response

Example

if let Some(value) = mutex.lock().unwrap().do_something() {
  *mutex.lock().unwrap() = new_value();
}

Could be written as:

let value = mutex.lock().unwrap().do_something();
if let Some(value) = value {
  *mutex.lock().unwrap() = new_value();
}

or

let guard = mutex.lock().unwrap();
if let Some(value) = guard.do_something() {
  *guard = new_value();
}

Start by comparing this request with issue #5219 and the two Rust examples showing a mutex lock inside an if-let expression. Trace how an equivalent Clippy lint identifies temporary lock guards, then add coverage for the shown pattern. Done means the problematic form is diagnosed while the explicitly bound alternatives are not.