Backport v2’s retain implementation to v1 - #596
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alejandro-vaz
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September 9, 2026 13:52
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do not create a tests/retain.rs file for that, include it on src/tests.rs
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Summary
This PR applies the existing v2 compaction approach to v1's
retain: leave the initial retained prefix in place, then move each survivor into the next hole instead of swapping it with a discarded element. Removed elements are dropped during the scan, and a guard restores the untouched tail if the predicate or a destructor panics.I benchmarked this on Linux (x86), and retaining half the input took 40-60% less time for small integer vectors, 16-21% less time for 4,096 integers, and 24-38% less for 4,096 larger elements.
I included some native benchmarks but removed in a follow-up commit; you can see them in the PR history.