Tracking issue for `step_trait` stabilization

[scottmcm]

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: rust-lang/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)

(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.

[SimonSapin]

I’ve closed that PR since it needs more work that I’m not planning to do soon: #43127 (comment). Hopefully someone else can take over.

[pnkfelix]

I just wanted to leave a note saying that if methods with the behavior of fn replace_one/fn replace_zero stay, I hope they have names that look more like fn replace_with_one/fn replace_with_zero, because the way that my head reads the phrase replace_one is "replace one of these" and replace_zero is as "replace zero of these". (Which are both weird things to say in a method, admittedly, but nonetheless, I'd prefer clearer names.)

Having said that, it sounds like these methods are scheduled to be either removed or replaced with something more generally useful?

Update: (maybe if I care about this, I should spend effort reviving PR #43127...)