anderskaseorg

    let lock = RwLock::new(Box::new(111));
    let r: &i32 = &**lock.read().unwrap(); // points to 111
    *lock.write().unwrap() = Box::new(222); // allocates a new Box and deallocates 111
    println!("{}", *r); // use after free

    error: calling `Mutex::lock` inside the scope of another `Mutex::lock` causes a deadlock
      --> src/main.rs:5:5
       |
    5  |       if let Some(num) = *map.lock().unwrap() {
       |       ^                   --- this Mutex will remain locked for the entire `if let`-block...
       |  _____|
       | |
    6  | |         eprintln!("There's a number in there: {num}");
    7  | |     } else {
    8  | |         let mut lock2 = map.lock().unwrap();
       | |                         --- ... and is tried to lock again here, which will always deadlock.
    9  | |         *lock2 = Some(5);
    10 | |         eprintln!("There will now be a number {lock2:?}");
    11 | |     }
       | |_____^
       |
       = help: move the lock call outside of the `if let ...` expression
       = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#if_let_mutex
       = note: `#[deny(clippy::if_let_mutex)]` on by default