Tracking issue for `step_trait` stabilization

[scottmcm]

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Split off from #27741 because the stabilization path for step_by has moved to being on iterators (#41439), and thus not using the Step trait.

• Remove step, steps_between, and is_negative once Range::step_by is deleted
• Replace replace_zero and replace_one with something more useful (some options: Make RangeInclusive just a two-field struct (amend 1192) rfcs#1980 (comment))
• Change steps_between_by_one so that Range<u128> can be TrustedLen (rather than it only working well with types that fit in usize)
• Make a decision on how steps_between should work Step::steps_between does not distinguish overflow and unimplemented (unstable) #48117
• Should this live in core::iter or core::range?

(and probably more)

[scottmcm]

Some progress on this in #42534

[SimonSapin]

PR #43077 does items 1 and 3 in the original message of this issue.

[SimonSapin]

I’ve removed the i128/u128 stuff from my PR because I suspect my mixed-signdeness mixed-width interger arithmetic was buggy. I also massaged the Step trait some more and came up with this:

/// Supporting trait for allowing ranges of various different types to be iterators.
#[unstable(feature = "step_trait",
           reason = "recently redesigned",
           issue = "42168")]
pub trait Step: Clone + PartialOrd + Sized {
    /// Returns the number of steps between two step objects. The count is
    /// inclusive of `start` and exclusive of `end`.
    ///
    /// Returns `None` if it is not possible to calculate `steps_between`
    /// without overflow.
    fn steps_between(start: &Self, end: &Self) -> Option<usize>;

    /// “Go forward” (for integers, add) the given number of steps, returning None on overflow.
    fn forward(&self, step_count: usize) -> Option<Self>;

    /// “Go backward” (for integers, subtract) the given number of steps, returning None on overflow.
    fn backward(&self, step_count: usize) -> Option<Self>;

    /// Modify the given inclusive range so that it becomes empty,
    /// for example by setting it to `1...0`.
    fn make_inclusive_range_empty(range: &mut ops::RangeInclusive<Self>);
}

How does it look?

I’m a bit uncertain of the exact impls for integers, there is up to 6 cases to consider: {smaller, same width, larger} than usize/isize × {signed, unsigned}.

[SimonSapin]

I’ve tweaked this design some more and opened #43127. I believe that PR fixes every issue I know of around the Step trait.