On Tue, Jul 8, 2025 at 10:48 AM Andreas Hindborg <[email protected]> wrote: > > "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.
Yup, a mutable reference generally implies synchronization. Alice
