DeprecatedStepBy<A, RangeInclusive<A>> gives bad size hints

[cuviper]

DeprecatedStepBy being the old unstable core::iter::range::StepBy. cc #27741

Its size_hint over-optimistically adds 1 to Step::steps_between. That works if the step exactly lands on the end condition, a perfect multiple, but otherwise:

    let i = (0...9).step_by(2);
    let v: Vec<_> = i.clone().collect();
    assert_eq!(v.len(), 5); // OK
    assert_eq!(i.size_hint(), (5, Some(5))); // FAILED, gives `(6, Some(6))`

Or if the iterator is already exhausted:

    let mut i = (0...9).step_by(2);
    let v: Vec<_> = i.by_ref().collect();
    assert_eq!(v.len(), 5); // OK
    assert_eq!(i.size_hint(), (0, Some(0))); // FAILED, gives `(1, Some(1))`

At least it doesn't implement TrustedLen.

[scottmcm]

Dup of #41477?

[cuviper]

Yep.