Tracking Issue for `OnceCell/Lock::get_mut_or_init`

[tisonkun]

Feature gate: #![feature(once_cell_get_mut)]

This is a tracking issue for

• OnceCell::get_mut_or_init()
• OnceCell::get_mut_or_try_init()
• OnceLock::get_mut_or_init()
• OnceLock::get_mut_or_try_init()

Public API

    pub fn get_mut_or_try_init<F, E>(&mut self, f: F) -> Result<&mut T, E>
    where
        F: FnOnce() -> Result<T, E> { ... }

    pub fn get_mut_or_init<F>(&mut self, f: F) -> &mut T
    where
        F: FnOnce() -> T { ... }

Steps / History

• ACP: Add get_mut_or_init and get_mut_or_try_init for OnceCell libs-team#294
• Implementation: impl get_mut_or_init and get_mut_or_try_init for OnceCell and OnceLock #114788
• Final comment period (FCP)
• Stabilization PR

Unresolved Questions

None yet.

Footnotes

1. https://std-dev-guide.rust-lang.org/feature-lifecycle/stabilization.html ↩

[ChayimFriedman2]

The documentation seems wrong?

Many threads may call get_mut_or_init concurrently with different initializing functions, but it is guaranteed that only one function will be executed.

Since it takes &mut self, this is entirely impossible.

The implementation can likewise be simplified a lot by not using any synchronization.

[tisonkun]

@dtolnay Shall we have an FCP for this feature? I encounter this requriement in stable toolchain today again :P

[dtolnay]

I doubt that get_mut_or_try_init can be stabilized sooner than #109737.

That tracking issue has an unresolved question around core::ops::Try.

[dtolnay]

This and #109737 can be stabilized together if we are going to keep Result in all the signatures (which is my preference).

@rfcbot merge libs-api

[rust-rfcbot]

Team member @dtolnay has proposed to merge this. The next step is review by the rest of the tagged team members:

• @Amanieu
• @BurntSushi
• @dtolnay
• @joshtriplett
• @the8472

No concerns currently listed.

Once a majority of reviewers approve (and at most 2 approvals are outstanding), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

See this document for info about what commands tagged team members can give me.