"Wren Turkal" <wt@xxxxxxxxxxxxxxxx> writes:
> On 7/2/25 6:18 AM, Andreas Hindborg wrote:
[...]
>> +pub struct OnceLock<T> {
>> + init: Atomic<u32>,
>> + value: Opaque<T>,
>> +}
>
> This type looks very much like the Once type in rust's stdlib. I am
> wondering if the api could be changed to match that api. I know that
> this type is trying to provide a version subset of std::sync::OnceLock
> that doesn't allow resetting the type like these apis:
>
> * https://doc.rust-lang.org/std/sync/struct.OnceLock.html#method.get_mut
> * https://doc.rust-lang.org/std/sync/struct.OnceLock.html#method.take
>
> However, these methods can only be used on mut. See here for failing
> example:
> https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=a78e51203c5b9555e3c151e162f0acab
>
> I think it might make more sense to match the api of the stdlib API and
> maybe only implement the methods you need.
I agree, it would be nice to match the names to std. But I do not like
that they have `OnceLock::set`, `OnceLock::try_init` and
`OnceLock::get_or{_try}_init`. Why is it not `OnceLock::init` or
`OnceLock::try_set`?
>
>> +
>> +impl<T> Default for OnceLock<T> {
>> + fn default() -> Self {
>> + Self::new()
>> + }
>> +}
>
> Any reason not to use #[derive(Default)]?
We don't have `Default` for neither `Atomic` or `Opaque`.
[...]
> Might also be worth implementing get_or_{try,}init, which get the value
> while initializing.
I did not have a user for those, so not adding for now. It would be dead
code. They should be fairly straight forward to add though.
>
>> + // INVARIANT: We obtain exclusive access to the contained allocation and write 1 to
>> + // `init`.
>> + if let Ok(0) = self.init.cmpxchg(0, 1, Acquire) {
>> + // SAFETY: We obtained exclusive access to the contained object.
>> + unsafe { core::ptr::write(self.value.get(), value) };
>> + // INVARIANT: We release our exclusive access and transition the object to shared
>> + // access.
>> + self.init.store(2, Release);
>> + true
>> + } else {
>> + false
>> + }
>> + }
>> +}
>> +
>> +impl<T: Copy> OnceLock<T> {
>> + /// Get a copy of the contained object.
>> + ///
>> + /// Returns [`None`] if the [`OnceLock`] is empty.
>> + pub fn copy(&self) -> Option<T> {
>
> No equivalent in OnceLock. Similar to something like this:
>
> x.get().copied().unwrap(); // x is a OnceLock
>
> Example:
> https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=f21068e55f73722544fb5ad341bce1c5
>
> Maybe not specifically needed?
I don't actually have a user for this, so I think I will drop it.
Best regards,
Andreas Hindborg
