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### What it does
Checks for calls to await while holding a `RefCell` `Ref` or `RefMut`.
### Why is this bad?
`RefCell` refs only check for exclusive mutable access
at runtime. Holding onto a `RefCell` ref across an `await` suspension point
risks panics from a mutable ref shared while other refs are outstanding.
### Known problems
Will report false positive for explicitly dropped refs
([#6353](https://github.com/rust-lang/rust-clippy/issues/6353)). A workaround for this is
to wrap the `.borrow[_mut]()` call in a block instead of explicitly dropping the ref.
### Example
```
async fn foo(x: &RefCell<u32>) {
let mut y = x.borrow_mut();
*y += 1;
baz().await;
}
async fn bar(x: &RefCell<u32>) {
let mut y = x.borrow_mut();
*y += 1;
drop(y); // explicit drop
baz().await;
}
```
Use instead:
```
async fn foo(x: &RefCell<u32>) {
{
let mut y = x.borrow_mut();
*y += 1;
}
baz().await;
}
async fn bar(x: &RefCell<u32>) {
{
let mut y = x.borrow_mut();
*y += 1;
} // y dropped here at end of scope
baz().await;
}
```
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