step_by + skip produce incorrect results

The following snippet demonstrates the problem:

println!("first");
for i in (0..640).step_by(128) {
    println!("{}", i)
}
println!("second");
for i in (0..640).step_by(128).skip(1) {
    println!("{}", i)
}
println!("third");
for i in (0..640).step_by(128).map(|i| i).skip(1) {
    println!("{}", i)
}

It prints:

first
0
128
256
384
512
second
128
129
257
385
513
third
128
256
384
512

I expect second iterator to produce the same result as for the third one.

Regression from 1.28.0 to 1.29.0. Introduced in #51601

The problem is interaction between StepBy::nth and StepBySpecIterator::spec_next. spec_next ignores the inner working of StepBy.

Test-case reduced to using StepBy without Skip.

fn main() {
    let mut it = (0_usize..640).step_by(128);
    it.nth(0);
    assert_eq!(it.next(), Some(128));
}

From a cursory look, it seems to me that the problem is that nth and next are out of sync. StepBy specifically allows two different kind of semantics for the underlying iterator, either next(), nth(step-1), nth(step-1), … or advance_n_and_return_first. If both next and nth follow the same one, the bug shouldn't occur.

In other words, if next is specialized and nth overrides the default, then nth needs to be specialized as well.

Auto merge of #56049 - newpavlov:revert_51601, r=sfackler

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step_by + skip produce incorrect results #55985

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step_by + skip produce incorrect results #55985

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jethrogb commented on Nov 15, 2018

m1el commented on Nov 15, 2018

Emerentius commented on Nov 19, 2018

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