libcore: add str::first, str::split_first, str::last and str::split_last methods

[eduardosm]

These methods are equivalent to their slice ([T]) counterparts.

If this PR is accepted, I will create a tracking issue and update the issue value accordingly.

[rust-highfive]

r? @dtolnay

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[camsteffen]

I would add _char to the names. A str is [u8], not [char]. Consider chars is not named iter.

[jfrimmel]

I would add _char to the names. A str is [u8], not [char].

I disagree. Mentally a str is a "list" of characters, that only happens to be stored as bytes. If somebody needs to get the first byte, s.as_bytes().first() (etc.) can be used.

Consider chars is not named iter.

Yes, but there are other iterators (bytes(), char_indices()) as well.

[ChrisDenton]

Mentally a str is a "list" of characters,

What are "characters" in this sense? It could mean "scalar values" (i.e. the atomic unit of Unicode). Or it could mean what the user perceives as characters (e.g. extended grapheme clusters). While the standard library currently only supports the former, it's reasonable for third party crates to add support for the latter. I'm uncertain what these names should be but I think there is at least a reason to prefer unambiguous naming.

I definitely support adding these functions, whatever they are named.

[jfrimmel]

To clarify: I think, that the _char-suffix is not relevant, but others might disagree. But sleeping over it, I have to admit, that of course clear names are always good.

So, maybe my first comment was a little bit to harsh.

[eduardosm]

I like the idea of adding the _char suffix to the function names. I have pushed a new commit adding that

[dtolnay]

Could you provide some more detail on how you're planning to use these methods? Is there maybe an example of real-world open source code you could link to that would benefit from having these?

[crlf0710]

@eduardosm Ping from triage, any updates on this?

[eduardosm]

Could you provide some more detail on how you're planning to use these methods? Is there maybe an example of real-world open source code you could link to that would benefit from having these?

I missed these methods while I was writing a lexer, although its code is not public yet. There is also #48731.

[JohnCSimon]

@eduardosm
Triage: I'm closing this due to inactivity. Please reopen when you are ready to continue with this. Thanks for your contribution
@rustbot label: +S-inactive