"Alice Ryhl" <[email protected]> writes: > On Mon, Jul 7, 2025 at 3:32 PM Andreas Hindborg <[email protected]> wrote: >> >> Introduce the `SetOnce` type, a container that can only be written once. >> The container uses an internal atomic to synchronize writes to the internal >> value. >> >> Signed-off-by: Andreas Hindborg <[email protected]> > > LGTM: > Reviewed-by: Alice Ryhl <[email protected]> > >> +impl<T> Drop for SetOnce<T> { >> + fn drop(&mut self) { >> + if self.init.load(Acquire) == 2 { >> + // SAFETY: By the type invariants of `Self`, `self.init == 2` >> means that `self.value` >> + // contains a valid value. We have exclusive access, as we hold >> a `mut` reference to >> + // `self`. >> + unsafe { drop_in_place(self.value.get()) }; > > This load does not need to be Acquire. It can be a Relaxed load or > even an unsynchronized one since the access is exclusive.
Right, that is actually very cool. My rationale was that if a reference has been shared to another thread of execution, we would need to synchronize here to see a possible initialization from that other thread. But I guess it is impossible to end the lifetime of a reference without doing a synchronization somewhere else. Best regards, Andreas Hindborg
