if let Some with locked mutex

[road2react]

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();
}