runtime: remove misleading use of UnsafeCell::with_mut

[e00E]

The code that we're removing calls UnsafeCell::with_mut with the argument std::mem::drop. This is misleading because the use of drop has no effect. with_mut takes an argument of type impl FnOnce(*mut T) -> R. The argument to the argument function is a pointer. Dropping a pointer has no effect.

The comment above the first instance of this pattern claims that this releases some resource. This is false because the call has no effect. The intention might have been to drop the value behind the pointer. If this did happen, it would be a bug because the resource (waker) would be dropped again at the end of the function when the containing object is dropped.

I looked through the history of this code. This code originally called with_mut with the argument |_| (). Calling with_mut with an argument function that does nothing has a side effect when testing with loom. When testing with loom, the code uses loom's UnsafeCell type instead of std's. The intention of the code was likely to make use of that side effect because we expect to have exclusive access here as we are going to drop the containing object. The side effect is that loom checks that Rust's reference uniqueness properties are upheld.

Instead of removing the whole code, I considered changing drop back to |_|() and documenting what I wrote above. I decided on removing the code because nowhere else do we use with_mut in this way and the purpose of this check would be better served in loom directly as part of UnsafeCell's Drop implementation. I created an issue about this in loom [1].

[1] tokio-rs/loom#349

[Darksonn]

Thank you for the PR. This suggestion makes a lot of sense.

I would like to have this fixed in loom before we merge this. In the meantime, we can merge a PR that fixes the incorrect comments if you would like.

[e00E]

I've changed the PR to only remove the use of drop while keeping with_mut. The last paragraph of the commit message is now:

To continue to check this, I have only removed the use of drop while
keeping with_mut. It would be even better to have loom check this
implicitly when UnsafeCell is dropped. I created an issue about this in
loom [1].