Github Don't implement mem::replace with mem::swap. by m-ou-se · Pull Request #8...
source link: https://github.com/rust-lang/rust/pull/83022
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swap is a complicated operation, so this changes the implementation of replace to use read and write instead.
See #83019.
I wrote there:
Implementing the simpler operation (replace) with the much more complicated operation (swap) doesn't make a whole lot of sense.
replaceis just read+write, and the primitive for moving out of a&mut.swapis for doing that to two&mutat the same time, which is both more niche and more complicated (as shown byswap_nonoverlapping_bytes).
This could be especially interesting for Option<VeryLargeStruct>::take(), since swapping such a large structure with swap_nonoverlapping_bytes is going to be much less efficient than ptr::write()'ing a None.
But also for small values where swap just reads/writes using temporary variable, this makes a replace or take operation simpler:
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