ACP: Make <[[T; N]]>::flatten const

[KamilaBorowska]

Proposal

Problem statement

Allow using <[[T; N]]>::flatten within const fn.

Motivation, use-cases

Could be useful for code like this where you have an array containing arrays (for instance from u32::to_be_bytes) to return an array. For example, Ipv6Addr::new in standard library could make use of it.

[f(a), f(b), f(c), f(d)].flatten().try_into().unwrap()

Solution sketches

Add const keyword. The current implementation is already const-compatible, and I don't see a reason why this would change.

Links and related work

What happens now?

This issue is part of the libs-api team API change proposal process. Once this issue is filed the libs-api team will review open proposals in its weekly meeting. You should receive feedback within a week or two.

[scottmcm]

Thanks for the ACP! For an unstable method, I'm going to consider this something that can be done as a tweak under the existing tracking issue, and not something that needs to wait on a libs-api decision, given that the implementation change is trivial. I've approved the PR.

libs-api folks, please feel free to yell at me if you disagree 🙃

[pitaj]

This can now be closed.