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Matthias Krüger
14ef447d12
Weaken guarantee around advancing underlying iterators in zip The current guarantee (introduced in #52279) is too strong as it prevents adapters from exploiting knowledge about the iterator length and using counted loops for example because they would stop calling `next()` before it ever returned `None`. Additionally several nested zip iterators already fail to uphold this. This does not yet remove any of the specialization code that tries (and sometimes fails) to uphold the guarantee for `next()` because removing it would also affect `next_back()` in more surprising ways. The intent is to be able to remove for example this branch https://github.com/rust-lang/rust/blob/36bcf4069717b9dff90270d13b53a3b130329960/library/core/src/iter/adapters/zip.rs#L234-L243 or this test https://github.com/rust-lang/rust/blob/36bcf4069717b9dff90270d13b53a3b130329960/library/core/tests/iter/adapters/zip.rs#L177-L188 Solves #82303 by declaring it a non-issue.