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...