Implement nth_back for Zip iterator#152652
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The `Zip` iterator was missing a specialized `nth_back` implementation, causing it to fall back to the default `DoubleEndedIterator::nth_back` which calls `next_back()` in a loop. This is O(n) even for iterators that support efficient random access. This adds `nth_back` to all three `ZipImpl` specialization levels: - Default: delegates to `super_nth_back` (calls `next_back` in a loop, same as the trait default but going through the Zip dispatch) - TrustedRandomAccessNoCoerce: uses the default - TrustedRandomAccess: uses direct index-based access with proper side-effect handling and panic safety This completes the last three unchecked items from the tracking list in rust-lang#54054 (Zip, Zip in ZipImpl default fn, Zip in ZipImpl where TrustedRandomAccess). The performance impact is most visible when using patterns like `iter_a.zip(iter_b).rev().step_by(n)`, since `step_by` calls `nth` on the underlying iterator, and `Rev::nth` delegates to `nth_back`.
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Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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Resolves the remaining Zip items from the tracking list in #54054.
The
Zipiterator was missing anth_backspecialization, so calling.rev().step_by(n)(or any other pattern that ends up callingnth_back) on a zipped iterator would fall through to the defaultDoubleEndedIterator::nth_back, which just loops overnext_back(). ForTrustedRandomAccessiterators like slices, this turns what should be O(1) into O(n).This adds
nth_backat all three specialization levels inZipImpl:zip_impl_general_defaultsmacro): delegates to asuper_nth_backhelper that callsnext_back()in a loop, same as the trait default but properly routed through the Zip dispatch.TrustedRandomAccessNoCoerce: inherits the default.TrustedRandomAccess: O(1) via direct__iterator_get_uncheckedindexing, with proper length-equalization on first backward call, side-effect execution for skipped elements (as required by theMAY_HAVE_SIDE_EFFECTcontract), and panic-safeself.lenupdates before each unchecked access.The implementation mirrors the existing
nthspecialization andnext_backbackward-trimming logic.Test plan
test_zip_nth_backcovering basic nth_back, exhaustion, and unequal-length iteratorstest_zip_nth_back_after_nextcovering interleaved forward/backward iterationtest_zip_rev_nth_uses_nth_backcovering the motivating use case (zip(..).rev().step_by(..))