Tracking Issue for `remainder` methods for `str` split iterators

[WaffleLapkin]

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This is a tracking issue for the methods implemented in #75265 and #82570, which allow viewing the remainder of the underlying string in str-split iterators. E.g.:

let mut split = "Mary had a little lamb".split(' '); 
assert_eq!(split.remainder(), Some("Mary had a little lamb"));

_ = split.next();
assert_eq!(split.remainder(), Some("had a little lamb"));

split.by_ref().for_each(drop);
assert_eq!(split.remainder(), None);

The feature gates for the issue are #![feature(str_split_remainder)] (for most split iterators), #![feature(str_split_inclusive_remainder)] (for SplitInclusive) and #![feature(str_split_whitespace_remainder)] (for SplitWhitespace and SplitAsciiWhitespace).

Public API

// mod core::str
impl<'a, P: Pattern<'a>> Split<'a, P> {
    fn remainder(&self) -> Option<&'a str>;
}

// And the same for
// - `RSplit<'a, P>`
// - `SplitTerminator<'a, P>`
// - `RSplitTerminator<'a, P>`
// - `SplitN<'a, P>`
// - `RSplitN<'a, P>`
// - `SplitWhitespace<'a>`
// - `SplitAsciiWhitespace<'a>`
// - `SplitInclusive<'a, P>`

Steps

• Implement the feature
• Adjust documentation (see instructions on rustc-dev-guide)
• Stabilization PR (see instructions on rustc-dev-guide)

Unresolved Questions

• Should empty strings point to the end of the haystack, or they can be arbitrary constants? (see Add str::{Split,RSplit,SplitN,RSplitN,SplitTerminator,RSplitTerminator,SplitInclusive}::as_str methods #75265 (comment)) None is now returned instead of an empty string, making this irrelevant

Implementation history

• Add str::{Split,RSplit,SplitN,RSplitN,SplitTerminator,RSplitTerminator,SplitInclusive}::as_str methods #75265 — implemented as_str methods on core::str::{Split,RSplit,SplitN,RSplitN,SplitTerminator,RSplitTerminator,SplitInclusive}

• Add as_str method for split whitespace str iterators #82570 — implemented as_str methods on core::str::{SplitWhitespace, SplitAsciiWhitespace}

• Split*::as_str refactor #95644 — renamed as_str=>remainder and changed return type to Option<&str>

[SkiFire13]

Looking at its name it isn't immediately clear what it returns. Wouldn't remainder or something along that line be a more self-explanatory name?

[WaffleLapkin]

The name as_str was chosen to be in line with Chars::as_str

[TyPR124]

Can this be added to SplitWhitespace and SplitAsciiWhitespace?

[WaffleLapkin]

@TyPR124 as noted in the implementation PR discussion it is possible, but not trivial.

Both Split*Whitespace are using iterators internally which doesn't allow to access it's inner parts (^1, ^2). It's possible to 'solve' this problem either by making the fields pub(crate::str::iter) (tbh it's a hack) or providing methods on iterator adaptors that allow accessing inner iterators (downside: there are lots of adaptors to change).

I didn't want to complicate the original PR with this, so didn't include anything like this. I can now make a new PR that'll add the same methods to Split*Whitespace, thanks for reminding about it!

[WaffleLapkin]

Update: #82570 was merged, so now SplitWhitespace and SplitAsciiWhitespace has as_str method too!

[soerenmeier]

I just had a case we're i needed to split a str: "tag:value" into a tag: &str and a value: Option<&str>.
I know i can just use Some(iter.as_str()).filter(str::is_empty) to get the Option, but maybe there could be another method named as_opt_str or remainder. That method would return None if calling next would also return None.
What do you think?

[SkiFire13]

Note that split iterators could yield an empty str as their final item, so as_opt_str could return Some("") while your code with always filter that case.

[soerenmeier]

That's true, to be correct the above code should look like this:

let s = iter.as_str();
iter.next().map(|_| s)

Would be nice to have as_opt_str.

[m-ou-se]

I just had a case we're i needed to split a str: "tag:value" into a tag: &str and a value: Option<&str>.

That sounds like a use case for the recently stabilized split_once.