Step::steps_between does not distinguish overflow and unimplemented (unstable)

[scottmcm]

It returns Option<size>: https://doc.rust-lang.org/std/iter/trait.Step.html#tymethod.steps_between

And the comment says it uses None for overflow: https://doc.rust-lang.org/src/core/iter/range.rs.html#29

But it also uses None for unimplemented https://doc.rust-lang.org/src/core/iter/range.rs.html#155

And as a result, Range needs to return (0, None) when it gets None: https://doc.rust-lang.org/src/core/iter/range.rs.html#235-240

It would be nice to either

• Require that the method is implemented accurately (easy for fundamental integers; harder if Step is expected to support graph walks, but maybe not harder than PartialOrd)
• Change the type to something else, maybe Option<Option<usize>> or be more hint-like with (usize, Option<usize>)
• Something else

cc #42168

[CAD97]

The links appear to have lost their original position.

Does the original concern still stand? (Does it stand with #69659?)

[scottmcm]

@CAD97 Yes, I think there's still essentially the same problem -- the Invariants section still has None for both unreachable (steps_between(1, 0)) and overflow (steps_between(0_u128, 1<< 127)).

So if this is used for size_hint, then getting None back from steps_between means you're stuck returning (0, None), when you'd rather be able to return either (0, Some(0)) or (usize::MAX, None).