### What it does On this code: ```rust use std::cell::RefCell; pub fn foo(x: &mut RefCell<i32>) { *x.borrow_mut() += 1; } ``` the lint would suggest to use `get_mut()` rather than `borrow_mut()`. (Same for `borrow()`, of course.) ### Lint Name unnecessary_runtime_borrow ### Category perf ### Advantage - Removes unnecessary run-time borrow tracking of `RefCell` ### Drawbacks None that I know if ### Example See above. `Mutex` and `RwLock` also have `get_mut` functions, so the same kind of lint could be added for them.