bors dba8e2d2c2 Auto merge of #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)

2024-07-30 00:55:52 +00:00

..

benches

Reformat use declarations.

2024-07-29 08:26:52 +10:00

src

Auto merge of #128234 - jcsp:retain-empty-case, r=tgross35

2024-07-30 00:55:52 +00:00

tests

Reformat use declarations.

2024-07-29 08:26:52 +10:00

Cargo.toml

Auto merge of #125016 - nicholasbishop:bishop-cb-112, r=tgross35

2024-07-29 07:41:33 +00:00