await_holding_borrow - like await_holding_lock, but for RefCell

What it does

This lint should warn users attempting to hold a RefCell's Ref or RefMut guards across an await point.

Advantage

Even in a single-threaded environment, external code can run during awaits. This code may attempt to borrow the same RefCell, causing a panic.

Drawbacks

Holding a borrow may be intentional, although it likely isn't.

Example

async {
    let my_cell: RefCell<_> = /* divine it from somewhere */;
    let my_borrow = my_cell.borrow();
    some_future().await;
    println!("The value is {}!", *my_borrow);
}

There is no trivial improvement to this code. Some examples of a fix might be

• moving the println! before the await;
• wrapping the value in an Rc, cloning it, and dropping the borrow before the await;
• or simply removing the borrow (and println!).

Start by reading the existing await_holding_lock lint and its tests, then compare how RefCell Ref and RefMut guards are identified across an await point. Done means the lint warns for the described borrow patterns, covers the example cases, and avoids unrelated borrows.