Infallible version of fetch_update

[dev-ardi]

Proposal

Problem statement

fetch_update requires you to return an Option and handle the Err case. Sometimes you don't need to do that

Motivating examples or use cases

        my_atomic
            .fetch_update(Ordering::SeqCst, Ordering::SeqCst, |current| {
                Some(current + 10 % 50)
            })
            .expect("This can't return None");

Solution sketch

An alternative infallible function that doesn't require that:

    pub fn infallible_fetch_update<F>( // Name TBD
        &self,
        set_order: Ordering,
        fetch_order: Ordering,
        mut f: F,
    ) -> u64
    where
        F: FnMut(u64) -> u64,

Links and related work

https://rust-lang.zulipchat.com/#narrow/channel/327149-t-libs-api.2Fapi-changes/topic/Atomics.20fetch_update.20is.20awkward

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.

[GrigorenkoPV]

Bikeshed: fetch_update_infallible

[Amanieu]

We discussed this in the libs-api meeting: we're happy to accept this as fetch_update_infallible. We're not huge fans of the name but couldn't think of anything better (ideallly fetch_update should have been called try_fetch_update, but that ship has sailed).