Partial stabilization of once_cell

[tgross35]

This PR aims to stabilize a portion of the once_cell feature:

• core::cell::OnceCell
• std::cell::OnceCell (re-export of the above)
• std::sync::OnceLock

This will leave LazyCell and LazyLock unstabilized, which have been moved to the lazy_cell feature flag.

Tracking issue: #74465 (does not fully close, but it may make sense to move to a new issue)

Future steps for separate PRs:

• Add #[inline] to many methods Add #[inline] markers to once_cell methods #105651
• Update cranelift usage of the once_cell crate
• Update rust-analyzer usage of the once_cell crate
• Update error messages discussing once_cell

To be stabilized API summary

// core::cell (in core/cell/once.rs)

pub struct OnceCell<T> { .. }

impl<T> OnceCell<T> {
    pub const fn new() -> OnceCell<T>;
    pub fn get(&self) -> Option<&T>;
    pub fn get_mut(&mut self) -> Option<&mut T>;
    pub fn set(&self, value: T) -> Result<(), T>;
    pub fn get_or_init<F>(&self, f: F) -> &T where F: FnOnce() -> T;
    pub fn into_inner(self) -> Option<T>;
    pub fn take(&mut self) -> Option<T>;
}

impl<T: Clone> Clone for OnceCell<T>;
impl<T: Debug> Debug for OnceCell<T>
impl<T> Default for OnceCell<T>;
impl<T> From<T> for OnceCell<T>;
impl<T: PartialEq> PartialEq for OnceCell<T>;
impl<T: Eq> Eq for OnceCell<T>;

// std::sync (in std/sync/once_lock.rs)

impl<T> OnceLock<T> {
    pub const fn new() -> OnceLock<T>;
    pub fn get(&self) -> Option<&T>;
    pub fn get_mut(&mut self) -> Option<&mut T>;
    pub fn set(&self, value: T) -> Result<(), T>;
    pub fn get_or_init<F>(&self, f: F) -> &T where F: FnOnce() -> T;
    pub fn into_inner(self) -> Option<T>;
    pub fn take(&mut self) -> Option<T>;
}

impl<T: Clone> Clone for OnceLock<T>;
impl<T: Debug> Debug for OnceLock<T>;
impl<T> Default for OnceLock<T>;
impl<#[may_dangle] T> Drop for OnceLock<T>;
impl<T> From<T> for OnceLock<T>;
impl<T: PartialEq> PartialEq for OnceLock<T>
impl<T: Eq> Eq for OnceLock<T>;
impl<T: RefUnwindSafe + UnwindSafe> RefUnwindSafe for OnceLock<T>;
unsafe impl<T: Send> Send for OnceLock<T>;
unsafe impl<T: Sync + Send> Sync for OnceLock<T>;
impl<T: UnwindSafe> UnwindSafe for OnceLock<T>;

No longer planned as part of this PR, and moved to the rust_cell_try feature gate:

impl<T> OnceCell<T> {
    pub fn get_or_try_init<F, E>(&self, f: F) -> Result<&T, E> where F: FnOnce() -> Result<T, E>;
}

impl<T> OnceLock<T> {
    pub fn get_or_try_init<F, E>(&self, f: F) -> Result<&T, E> where F: FnOnce() -> Result<T, E>;
}

I am new to this process so would appreciate mentorship wherever needed.

Post-merge update

With this merge, LazyCell and LazyLock have been moved under the gate lazy_cell (see #109736) and the get_or_try_init functions are under once_cell_try (see #109737)

[rustbot]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @m-ou-se (or someone else) soon.

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[rustbot]

Hey! It looks like you've submitted a new PR for the library teams!

If this PR contains changes to any rust-lang/rust public library APIs then please comment with @rustbot label +T-libs-api -T-libs to tag it appropriately. If this PR contains changes to any unstable APIs please edit the PR description to add a link to the relevant API Change Proposal or create one if you haven't already. If you're unsure where your change falls no worries, just leave it as is and the reviewer will take a look and make a decision to forward on if necessary.

Examples of T-libs-api changes:

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Some changes occurred in src/tools/clippy

cc @rust-lang/clippy

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

[tgross35]

This needs FCP still

@rustbot modify labels: +T-libs-api

[programmerjake]

looks like you're stabilizing OnceLock, not LazyCell...the top comment should be fixed

[tgross35]

Updated, thanks for the catch

[klensy]

If this PR about stabilization, why slapping inline all over methods?

[tgross35]

I just did not know if those were typically added at this time, since these functions are fairly good use cases. I have removed them, matklad suggested we can add them later

Edit: Moved this to #105651

[m-ou-se]

@rfcbot merge

[rfcbot]

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

• @Amanieu
• @BurntSushi
• @dtolnay
• @joshtriplett
• @m-ou-se

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