async map functions for tokio::sync::RwLock

[agraven]

Is your feature request related to a problem? Please describe.
It's currently not possible to use async code in tokio::sync::RwLock(Read|Write)Guard::map.

Describe the solution you'd like
Additional .map methods that accept mapping closures that return a future, looking something like

    pub async fn map_async<F, Fut, U: ?Sized>(this: Self, f: F) -> RwLockReadGuard<'a, U>
    where
        F: FnOnce(&T) -> Fut,
        Fut: Future<Output = &U>,
    {
        let data = f(&*this).await as *const U;
        let s = this.s;
        // NB: Forget to avoid drop impl from being called.
        mem::forget(this);
        RwLockReadGuard {
            s,
            data,
            marker: marker::PhantomData,
        }
    }

Describe alternatives you've considered
The owning variants of the guards could be used to solve my particular case, but doing so introduces significant complexity to reasoning about the code, since it means that inner locks can outlive outer locks.

Additional context
I'm trying to solve a problem similar to the one described in this reddit thread, where I have an outer RwLock to an object containing a Vec of inner RwLocks, and I need make a function that returns an object that derefs to data inside one of the inner locks without causing lifetime problems. The Ideal approach in my mind would be to solve the problem with RwLock(Read|Write)Guard::map, but that doesn't work because I need to use async code inside the map invocation. To describe it in pseudocode, I'm trying to do something like

RwLockReadGuard::try_map(self.outer.read.await(), |outer| {
    RwLockReadGuard::try_map(outer.inner.read().await, |inner| inner.returns_option())
}

In other words, I need a a way to get a RwLockReadGuard<RwLockReadGuard<U>>

[Darksonn]

Although this could be useful, it wont help with your use-case. You have to return a reference from the closure, and RwLockReadGuard<U> is not a reference.