Add `Waker::from_fn_ptr`

Proposal

Add Waker::from_fn_ptr to core.

Problem statement

We already have a way to safely create wakers in std: https://doc.rust-lang.org/beta/std/task/trait.Wake.html. The problem is, it is using Arc which is both a memory and performance problem, as well as it is not available for no_std and no alloc crates.

Considering that std already committed to provide safe methods of creating wakers, an extremely limited and scoped but more performant and memory efficient way seems only natural.

Motivating examples or use cases

Often end users of binary crates are able to have fn() that will schedule the poll of the future on some executor (in contrast to libraries that need to be generic). Most often it may be used in embedded development, as async is used extensively and tasks are usually static, so a simple function pointer would be enough.

Solution sketch

Extremely simple addition to Waker (and LocalWaker). Data pointer is a function pointer and a vtable that will cast data pointer back to fn(), with drop being no-op. unsafe only used to transmute *const () back to fn() and to call Waker::from_raw:

pub const fn from_fn_ptr(f: fn()) -> Self {
    // SAFETY: Unsafe is used for transmutes, pointer came from `fn()` so it
    //         is sound to transmute it back to `fn()`.
    static VTABLE: RawWakerVTable = unsafe {
        RawWakerVTable::new(
            |this| RawWaker::new(this, &VTABLE),
            |this| transmute::<*const (), fn()>(this)(),
            |this| transmute::<*const (), fn()>(this)(),
            |_| {},
        )
    };
    let raw = RawWaker::new(f as *const (), &VTABLE);

    // SAFETY: `clone` is just a copy, `drop` is a no-op while `wake` and
    //         `wake_by_ref` just call the function pointer.
    unsafe { Self::from_raw(raw) }
}

Alternatives

Alternatively you could use Wake trait or waker_fn crate from crates.io, but both require Arc.

It makes more sense to have it in core due to it's simplicity and extremely small scope and volume.

It includes some unsafe code which we would not want to push on users, especially in such trivial case.

Links and related work

https://doc.rust-lang.org/beta/std/task/trait.Wake.html
https://docs.rs/waker-fn/latest/waker_fn/
PR: https://github.com/rust-lang/rust/pull/146057

What happens now?

This issue contains an API change proposal (or ACP) and is part of the libs-api team feature lifecycle. Once this issue is filed, the libs-api team will review open proposals as capability becomes available. Current response times do not have a clear estimate, but may be up to several months.

Possible responses

The libs team may respond in various different ways. First, the team will consider the problem (this doesn't require any concrete solution or alternatives to have been proposed):

• We think this problem seems worth solving, and the standard library might be the right place to solve it.
• We think that this probably doesn't belong in the standard library.

Second, if there's a concrete solution:

• We think this specific solution looks roughly right, approved, you or someone else should implement this. (Further review will still happen on the subsequent implementation PR.)
• We're not sure this is the right solution, and the alternatives or other materials don't give us enough information to be sure about that. Here are some questions we have that aren't answered, or rough ideas about alternatives we'd want to see discussed.

Review the proposed core Waker and LocalWaker API alongside RawWakerVTable and the linked related PR. First determine whether the libs-api team accepts the problem and solution; done means an approved implementation path exists and the API proposal has progressed beyond team review.