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>