The `Zip` iterator does not update the underlying iterator correctly in its `TrustedLen` specialization.

I tried this code:

pub fn using_each() {
    let mut it = 0..4;
    it.by_ref().zip(0..2).for_each(|_| {});
    println!("{:?}", it);
}

pub fn using_loop() {
    let mut it = 0..4;
    for _ in it.by_ref().zip(0..2) {}
    println!("{:?}", it);
}

pub fn using_each_dyn() {
    let mut it = 0..4;
    (&mut it as &mut dyn Iterator<Item = i32>).zip(0..2).for_each(|_| {});
    println!("{:?}", it);
}

pub fn using_loop_dyn() {
    let mut it = 0..4;
    for _ in (&mut it as &mut dyn Iterator<Item = i32>).zip(0..2) {}
    println!("{:?}", it);
}

fn main() {
    using_each();
    using_loop();
    using_each_dyn();
    using_loop_dyn();
}

I expected all four functions to behave identically, but instead the above code outputs:

2..4
3..4
3..4
3..4

This occurs because:

• In the general case, the Zip iterator will call .next() on the first iterator to check if the value is None, and then call .next() on the second iterator. This means that if the second iterator is shorter, then the first iterator will have .next() called one past the length of the second iterator.
• In the TrustedLen specialization of Zip's fold() method, if the second iterator is shorter, then both iterators have their .next() called exactly equal to that second iterator's length.
  https://github.com/rust-lang/rust/blob/231257f583c6e782da2e642ad007fca4e76779d6/library/core/src/iter/adapters/zip.rs#L666-L695

This mismatch in behavior seems undesirable.

Discovered in #143966.

Meta

Reproducible on the playground with version 1.90.0-nightly (2025-07-15 3014e79f9c8d5510ea7b)

@rustbot labels +A-iterators +A-specialization

Start with library/core/src/iter/adapters/zip.rs around the TrustedLen-specialized fold implementation and reproduce the four iterator examples from the issue. Compare the specialized and general Zip consumption paths, then verify that the chosen behavior is consistent and covered by an appropriate regression test.