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Rollup merge of rust-lang#128234 - jcsp:retain-empty-case, r=tgross35
Optimize empty case in Vec::retain While profiling some code that happens to call Vec::retain() in a tight loop, I noticed more runtime than expected in retain, even in a bench case where the vector was always empty. When I wrapped my call to retain in `if !myvec.is_empty()` I saw faster execution compared with doing retain on an empty vector. On closer inspection, Vec::retain is doing set_len(0) on itself even when the vector is empty, and then resetting the length again in BackshiftOnDrop::drop. Unscientific screengrab of a flamegraph illustrating how we end up spending time in set_len and drop: ![image](https://github.com/user-attachments/assets/ebc72ace-84a0-4432-9b6f-1b3c96d353ba)
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