Add `[u8]::split_ascii_whitespace`

[jdahlstrom]

Proposal

Problem statement

str and [u8] share a bunch of *ascii* methods. There is (at least?) one exception: str::split_ascii_whitespace was stabilized in 1.34, but does not exist in [u8], which seems like an oversight. It (and split_whitespace) is similarly missing from ByteStr, and could be added there as well.

Motivating examples or use cases

This method is useful for ad-hoc parsers of the multitude of textual file formats made of fields separated by ASCII whitespace.

Solution sketch

Add to core::slice:

impl [u8] {
    pub fn split_ascii_whitespace(&self) -> SplitAsciiWhitespace<'_> {
        let inner = self
            .split(core::str::IsAsciiWhitespace)
            .filter(core::str::BytesIsNotEmpty)
        SplitAsciiWhitespace { inner }
    }
}

pub struct SplitAsciiWhitespace<'a> {
    pub(super) inner:
        Filter<SliceSplit<'a, u8, IsAsciiWhitespace>, BytesIsNotEmpty>,
}

The existing str::split_ascii_whitespace already works on bytes internally. Change it to:

    pub fn split_ascii_whitespace(&self) -> core::str::SplitAsciiWhitespace<'_> {
        let inner = self.as_bytes()
            .split_ascii_whitespace()
            .inner
            .map(UnsafeBytesToStr)
        SplitAsciiWhitespace { inner }
    }

Alternatives

It's not difficult to work around with split(is_ascii_whitespace), but there's the gotcha that you have to remember to filter out empty subslices to get the same semantics.

Links and related work

i.r-l.o thread

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.

[Amanieu]

We discussed this @rust-lang/libs-api meeting and we're happy to add split_ascii_whitespace to [u8]. It does seem like an oversight.

Please open a tracking issue and open a PR to rust-lang/rust to add it as an unstable feature. You can close this ACP once the tracking issue has been created.

[jdahlstrom]

Tracking issue.