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Reading the godbolt it looks like for the push loop llvm is unable to remove the `grow_one` capacity check after every push. Becaue of this the Vec could possibly reallocate after every push meaning it can't auto vectorize.

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It's a little bit surprising to me that LLVM can't eliminate the grow_one check. It looks like there's a test ensure it's not needed in the easier case of vec.push(vec.pop()) [0]. With the iterator the optimization is handled in the standard library using specialization and TrustedLen[1].

[0]: https://github.com/rust-lang/rust/blob/master/tests/codegen/...

[1]: https://github.com/rust-lang/rust/blob/d4025ee454169fbd22f57...

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