Add `Waker::from_fn_ptr`

[Ddystopia]

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: rust-lang/rust#146057

What happens now?

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Possible responses

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• 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.

[programmerjake]

do we want to assume function pointers will never be larger than data pointers? On some unusual platforms they have different sizes, e.g. 16-bit x86 in the medium memory model has 32-bit function pointers but 16-bit data pointers, meaning that you can't fit fn() into *const () without losing data.

[Ddystopia]

@programmerjake Thanks for the comment. Currently we don't support those platforms, and even in the future I suppose it would be worth it to not provide that method, considering that it is most useful in binary crates which generally are aware of their target, as opposed to libraries in general.