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

[Andreas Hindborg <a.hindborg@kernel.org>]

"Gary Guo" <gary@garyguo.net> 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)


Best regards,
Andreas Hindborg