stabilize new RangeFrom type and iterator

[pitaj]

// in core and std
pub mod range;

// in core::range

pub struct RangeFrom<Idx> {
    pub start: Idx,
}

impl<Idx: fmt::Debug> fmt::Debug for RangeFrom<Idx> { /* ... */ }

impl<Idx: PartialOrd<Idx>> RangeFrom<Idx> {
    pub const fn contains<U>(&self, item: &U) -> bool
    where
        Idx: [const] PartialOrd<U>,
        U: ?Sized + [const] PartialOrd<Idx>;
}

impl<Idx: Step> RangeFrom<Idx> {
    pub fn iter(&self) -> RangeFromIter<Idx>;
}

impl<T> const RangeBounds<T> for RangeFrom<T> { /* ... */ }
impl<T> const RangeBounds<T> for RangeFrom<&T> { /* ... */ }

impl<T> const From<RangeFrom<T>> for legacy::RangeFrom<T> { /* ... */ }
impl<T> const From<legacy::RangeFrom<T>> for RangeFrom<T> { /* ... */ }

pub struct RangeFromIter<A>(/* ... */);

impl<A: Step> RangeFromIter<A> {
    pub fn remainder(self) -> RangeFrom<A>;
}

impl<A: Step> Iterator for RangeFromIter<A> {
    type Item = A;
    /* ... */
}

impl<A: Step> FusedIterator for RangeFromIter<A> { }
impl<A: Step> IntoIterator for RangeFrom<A> {
    type Item = A;
    type IntoIter = RangeFromIter<A>;
    /* ... */
}

unsafe impl<T> const SliceIndex<[T]> for range::RangeFrom<usize> {
    type Output = [T];
    /* ... */
}
unsafe impl const SliceIndex<str> for range::RangeFrom<usize> {
    type Output = str;
    /* ... */
}

impl ops::Index<range::RangeFrom<usize>> for CStr {
    type Output = CStr;
    /* ... */
}

Tracking issue: #125687

r? tgross

[tgross35]

Do we have a test for this?

[pitaj]

Yes: rangefrom-overflow-overflow-checks and friends

[tgross35]

This could probably use an example

[pitaj]

Added examples for RangeInclusive::remainder, RangeFrom::remainder, and Range::remainder

[tgross35]

It doesn't look like RangeFromIter::remainder was listed on the tracking issue and that's new compared to the existing RangeFrom, so nominating for @rust-lang/libs-api. If you'd prefer to drop it here then I'm happy to approve.

@rustbot label +I-libs-api-nominated

The current RangeFrom has Index<RangeFrom<usize>> for ByteString and IndexMut<RangeFrom<usize>> for ByteString which should get added, those are unstable so it can happen here or later.

[tgross35]

Noticing that RangeInclusiveIter::remainder is in the same boat, and is stable on nightly since #150522.

[pitaj]

remainder was in the original RFC: https://github.com/rust-lang/rfcs/blob/master/text/3550-new-range.md#iterator-types
So I don't think this need further discussion by T-libs-api?

[rust-bors]

☔ The latest upstream changes (presumably #153544) made this pull request unmergeable. Please resolve the merge conflicts.