Lint against RefCell::borrow/borrow_mut on &mut RefCell

What it does

On this code:

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.

Start by reviewing the proposed unnecessary_runtime_borrow lint and its Rust example, then inspect existing rust-clippy lint patterns for comparable APIs. Done means the lint identifies borrow() and borrow_mut() on &mut RefCell and suggests get_mut(), with the Mutex and RwLock cases resolved explicitly.