Clippy already has an error for this pattern with Mutex. It should be trivial to extend it to cover RwLock.
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
There are a lot of alternative lock implementations that are used all the time, with the most common ones probably being tokios RwLock/Mutex and parking_lot.
