Re: [PATCH] rust: sync: add lazy initialization methods to SetOnce

[Andreas Hindborg]

Andreas Hindborg <a.hindborg@xxxxxxxxxx> writes:

> "Gary Guo" <gary@xxxxxxxxxxx> writes:
>
>> On Sun Feb 15, 2026 at 8:27 PM GMT, Andreas Hindborg wrote:
>>> Add methods to get a reference to the contained value or populate the
>>> SetOnce if empty. The new `as_ref_or_populate` method accepts a value
>>> directly, while `as_ref_or_populate_with` accepts a fallible closure,
>>> allowing for lazy initialization that may fail. Both methods spin-wait
>>> if another thread is concurrently initializing the container.
>>>
>>> Also add `populate_with` which takes a fallible closure and serves as
>>> the implementation basis for the other populate methods.
>>
>> Hi Andreas, in an earlier call I mentioned that I'm working on getting SetOnce
>> to work with pin-init, the capability of which I think is a superset of you have
>> here.
>>
>> The API I have is
>>
>> impl<T> SetOnce<T> {
>>     pub fn init<E>(&self, init: impl Init<T, E>) -> Result<&T, InitError<E>>;
>>     pub fn pin_init<E>(self, Pin<&Self>, init: impl PinInit<T, E>) -> Result<&T, InitError<E>>;
>> }
>>
>> To achieve what you need with a function, you can simply write:
>>
>>     set_once.init(pin_init::init_scope(your_fn))
>>
>> The patch that implement the API is here:
>> https://github.com/nbdd0121/linux/commit/4aabdbcf20b11626c253f203745b1d55c37ab2ee
>> in tree
>> https://github.com/nbdd0121/linux/tree/lazy_revocable_nova_wip/
>>
>> which I haven't submitted to the list as the user side of this API isn't ready.
>
> I probably evicted that from cache.
>
> It looks like that could replace the `populate` in my patch? We would
> still need the synchronization in `as_ref_or_populate`, right? (or
> `as_ref_or_init` if you will)

Looks like this is not ready yet. I think we can move forward with the
current suggestion and then wire up pin-init when it lands.


Best regards,
Andreas Hindborg