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Explain implementation of mem::replace #117243

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Oct 27, 2023
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Explain implementation of mem::replace #117243

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4 changes: 4 additions & 0 deletions library/core/src/mem/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -909,6 +909,10 @@ pub fn take<T: Default>(dest: &mut T) -> T {
#[rustc_const_unstable(feature = "const_replace", issue = "83164")]
#[cfg_attr(not(test), rustc_diagnostic_item = "mem_replace")]
pub const fn replace<T>(dest: &mut T, src: T) -> T {
// It may be tempting to use `swap` to avoid `unsafe` here. Don't!
// The compiler optimizes the implementation below to two `memcpy`s
// while `swap` would require at least three. See PR#83022 for details.

// SAFETY: We read from `dest` but directly write `src` into it afterwards,
// such that the old value is not duplicated. Nothing is dropped and
// nothing here can panic.
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