map on homogenous tuples

Proposal

Problem statement

It is impossible to map homogenous tuples as if they were arrays.

Motivating examples or use cases

Often times you see this kind of code: (f(x.0), f(x.1)) (see search).

Solution sketch

impl<T> (T, T…) {
    pub fn map<F, U>(self, f: F) -> (U, U…)
    where
        F: FnMut(T) -> U;
}

Alternatives

One may:

• simply use an array
• <[_; _]>::from(x).map(f).into()

Links and related work

https://github.com/rust-lang/rust/pull/160640

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.

Start by reading the proposal in issue #847 and the linked rust-lang/rust PR #160640, then follow the libs-api feature lifecycle. Done means reaching an agreed decision on whether homogeneous tuple mapping belongs in the standard library and, if approved, defining an implementation plan; no implementation files or tests are named.