Auto merge of rust-lang#105587 - tgross35:once-cell-min, r=m-ou-se

Partial stabilization of `once_cell`

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: rust-lang#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~~ rust-lang#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

```rust
// 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>;
```

```rust
// 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:

```rust
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.

compiler/rustc_borrowck/src/lib.rs

@@ -5,7 +5,7 @@
 #![feature(let_chains)]
 #![feature(min_specialization)]
 #![feature(never_type)]
-#![feature(once_cell)]
+#![feature(lazy_cell)]
 #![feature(rustc_attrs)]
 #![feature(stmt_expr_attributes)]
 #![feature(trusted_step)]

compiler/rustc_codegen_llvm/src/lib.rs

@@ -10,7 +10,6 @@
 #![feature(iter_intersperse)]
 #![feature(let_chains)]
 #![feature(never_type)]
-#![feature(once_cell)]
 #![recursion_limit = "256"]
 #![allow(rustc::potential_query_instability)]
 #![deny(rustc::untranslatable_diagnostic)]

compiler/rustc_codegen_ssa/src/lib.rs

@@ -5,7 +5,6 @@
 #![feature(int_roundings)]
 #![feature(let_chains)]
 #![feature(never_type)]
-#![feature(once_cell)]
 #![feature(strict_provenance)]
 #![feature(try_blocks)]
 #![recursion_limit = "256"]

compiler/rustc_data_structures/src/lib.rs

@@ -20,7 +20,7 @@
 #![feature(never_type)]
 #![feature(type_alias_impl_trait)]
 #![feature(new_uninit)]
-#![feature(once_cell)]
+#![feature(lazy_cell)]
 #![feature(rustc_attrs)]
 #![feature(negative_impls)]
 #![feature(test)]

compiler/rustc_driver_impl/src/lib.rs

@@ -6,7 +6,7 @@
 
 #![doc(html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/")]
 #![feature(is_terminal)]
-#![feature(once_cell)]
+#![feature(lazy_cell)]
 #![feature(decl_macro)]
 #![recursion_limit = "256"]
 #![allow(rustc::potential_query_instability)]

compiler/rustc_error_messages/src/lib.rs

@@ -1,5 +1,5 @@
 #![feature(let_chains)]
-#![feature(once_cell)]
+#![feature(lazy_cell)]
 #![feature(rustc_attrs)]
 #![feature(type_alias_impl_trait)]
 #![deny(rustc::untranslatable_diagnostic)]

compiler/rustc_feature/src/lib.rs

@@ -11,7 +11,7 @@
 //! even if it is stabilized or removed, *do not remove it*. Instead, move the
 //! symbol to the `accepted` or `removed` modules respectively.
 
-#![feature(once_cell)]
+#![feature(lazy_cell)]
 #![deny(rustc::untranslatable_diagnostic)]
 #![deny(rustc::diagnostic_outside_of_impl)]
 

compiler/rustc_hir_analysis/src/lib.rs

@@ -67,7 +67,7 @@ This API is completely unstable and subject to change.
 #![feature(let_chains)]
 #![feature(min_specialization)]
 #![feature(never_type)]
-#![feature(once_cell)]
+#![feature(lazy_cell)]
 #![feature(slice_partition_dedup)]
 #![feature(try_blocks)]
 #![feature(is_some_and)]

compiler/rustc_interface/src/lib.rs

@@ -2,7 +2,7 @@
 #![feature(decl_macro)]
 #![feature(internal_output_capture)]
 #![feature(thread_spawn_unchecked)]
-#![feature(once_cell)]
+#![feature(lazy_cell)]
 #![feature(try_blocks)]
 #![recursion_limit = "256"]
 #![allow(rustc::potential_query_instability)]

compiler/rustc_metadata/src/lib.rs

@@ -4,7 +4,6 @@
 #![feature(generators)]
 #![feature(iter_from_generator)]
 #![feature(let_chains)]
-#![feature(once_cell)]
 #![feature(proc_macro_internals)]
 #![feature(macro_metavar_expr)]
 #![feature(min_specialization)]

compiler/rustc_middle/src/lib.rs

@@ -39,7 +39,6 @@
 #![feature(never_type)]
 #![feature(extern_types)]
 #![feature(new_uninit)]
-#![feature(once_cell)]
 #![feature(let_chains)]
 #![feature(min_specialization)]
 #![feature(trusted_len)]

compiler/rustc_mir_build/src/lib.rs

@@ -8,7 +8,6 @@
 #![feature(if_let_guard)]
 #![feature(let_chains)]
 #![feature(min_specialization)]
-#![feature(once_cell)]
 #![feature(try_blocks)]
 #![recursion_limit = "256"]
 

compiler/rustc_mir_dataflow/src/lib.rs

@@ -3,7 +3,6 @@
 #![feature(exact_size_is_empty)]
 #![feature(let_chains)]
 #![feature(min_specialization)]
-#![feature(once_cell)]
 #![feature(stmt_expr_attributes)]
 #![feature(trusted_step)]
 #![recursion_limit = "256"]

compiler/rustc_mir_transform/src/lib.rs

@@ -5,7 +5,6 @@
 #![feature(map_try_insert)]
 #![feature(min_specialization)]
 #![feature(never_type)]
-#![feature(once_cell)]
 #![feature(option_get_or_insert_default)]
 #![feature(trusted_step)]
 #![feature(try_blocks)]

compiler/rustc_query_impl/src/lib.rs

@@ -5,7 +5,6 @@
 #![feature(const_mut_refs)]
 #![feature(min_specialization)]
 #![feature(never_type)]
-#![feature(once_cell)]
 #![feature(rustc_attrs)]
 #![recursion_limit = "256"]
 #![allow(rustc::potential_query_instability)]

compiler/rustc_session/src/lib.rs

@@ -2,7 +2,7 @@
 #![feature(let_chains)]
 #![feature(min_specialization)]
 #![feature(never_type)]
-#![feature(once_cell)]
+#![feature(lazy_cell)]
 #![feature(option_get_or_insert_default)]
 #![feature(rustc_attrs)]
 #![feature(map_many_mut)]

library/alloc/tests/lib.rs

@@ -42,7 +42,6 @@
 #![feature(slice_flatten)]
 #![feature(thin_box)]
 #![feature(strict_provenance)]
-#![feature(once_cell)]
 #![feature(drain_keep_rest)]
 #![deny(fuzzy_provenance_casts)]
 #![deny(unsafe_op_in_unsafe_fn)]

library/core/src/cell.rs

@@ -11,36 +11,77 @@
 //! mutate it.
 //!
 //! Shareable mutable containers exist to permit mutability in a controlled manner, even in the
-//! presence of aliasing. Both [`Cell<T>`] and [`RefCell<T>`] allow doing this in a single-threaded
-//! way. However, neither `Cell<T>` nor `RefCell<T>` are thread safe (they do not implement
-//! [`Sync`]). If you need to do aliasing and mutation between multiple threads it is possible to
-//! use [`Mutex<T>`], [`RwLock<T>`] or [`atomic`] types.
+//! presence of aliasing. [`Cell<T>`], [`RefCell<T>`], and [`OnceCell<T>`] allow doing this in
+//! a single-threaded way—they do not implement [`Sync`]. (If you need to do aliasing and
+//! mutation among multiple threads, [`Mutex<T>`], [`RwLock<T>`], [`OnceLock<T>`] or [`atomic`]
+//! types are the correct data structures to do so).
 //!
-//! Values of the `Cell<T>` and `RefCell<T>` types may be mutated through shared references (i.e.
-//! the common `&T` type), whereas most Rust types can only be mutated through unique (`&mut T`)
-//! references. We say that `Cell<T>` and `RefCell<T>` provide 'interior mutability', in contrast
-//! with typical Rust types that exhibit 'inherited mutability'.
+//! Values of the `Cell<T>`, `RefCell<T>`, and `OnceCell<T>` types may be mutated through shared
+//! references (i.e. the common `&T` type), whereas most Rust types can only be mutated through
+//! unique (`&mut T`) references. We say these cell types provide 'interior mutability'
+//! (mutable via `&T`), in contrast with typical Rust types that exhibit 'inherited mutability'
+//! (mutable only via `&mut T`).
 //!
-//! Cell types come in two flavors: `Cell<T>` and `RefCell<T>`. `Cell<T>` implements interior
-//! mutability by moving values in and out of the `Cell<T>`. To use references instead of values,
-//! one must use the `RefCell<T>` type, acquiring a write lock before mutating. `Cell<T>` provides
-//! methods to retrieve and change the current interior value:
+//! Cell types come in three flavors: `Cell<T>`, `RefCell<T>`, and `OnceCell<T>`. Each provides
+//! a different way of providing safe interior mutability.
+//!
+//! ## `Cell<T>`
+//!
+//! [`Cell<T>`] implements interior mutability by moving values in and out of the cell. That is, an
+//! `&mut T` to the inner value can never be obtained, and the value itself cannot be directly
+//! obtained without replacing it with something else. Both of these rules ensure that there is
+//! never more than one reference pointing to the inner value. This type provides the following
+//! methods:
 //!
 //!  - For types that implement [`Copy`], the [`get`](Cell::get) method retrieves the current
-//!    interior value.
+//!    interior value by duplicating it.
 //!  - For types that implement [`Default`], the [`take`](Cell::take) method replaces the current
 //!    interior value with [`Default::default()`] and returns the replaced value.
-//!  - For all types, the [`replace`](Cell::replace) method replaces the current interior value and
-//!    returns the replaced value and the [`into_inner`](Cell::into_inner) method consumes the
-//!    `Cell<T>` and returns the interior value. Additionally, the [`set`](Cell::set) method
-//!    replaces the interior value, dropping the replaced value.
+//!  - All types have:
+//!    - [`replace`](Cell::replace): replaces the current interior value and returns the replaced
+//!      value.
+//!    - [`into_inner`](Cell::into_inner): this method consumes the `Cell<T>` and returns the
+//!      interior value.
+//!    - [`set`](Cell::set): this method replaces the interior value, dropping the replaced value.
+//!
+//! `Cell<T>` is typically used for more simple types where copying or moving values isn't too
+//! resource intensive (e.g. numbers), and should usually be preferred over other cell types when
+//! possible. For larger and non-copy types, `RefCell` provides some advantages.
 //!
-//! `RefCell<T>` uses Rust's lifetimes to implement 'dynamic borrowing', a process whereby one can
+//! ## `RefCell<T>`
+//!
+//! [`RefCell<T>`] uses Rust's lifetimes to implement "dynamic borrowing", a process whereby one can
 //! claim temporary, exclusive, mutable access to the inner value. Borrows for `RefCell<T>`s are
-//! tracked 'at runtime', unlike Rust's native reference types which are entirely tracked
-//! statically, at compile time. Because `RefCell<T>` borrows are dynamic it is possible to attempt
-//! to borrow a value that is already mutably borrowed; when this happens it results in thread
-//! panic.
+//! tracked at _runtime_, unlike Rust's native reference types which are entirely tracked
+//! statically, at compile time.
+//!
+//! An immutable reference to a `RefCell`'s inner value (`&T`) can be obtained with
+//! [`borrow`](`RefCell::borrow`), and a mutable borrow (`&mut T`) can be obtained with
+//! [`borrow_mut`](`RefCell::borrow_mut`). When these functions are called, they first verify that
+//! Rust's borrow rules will be satisfied: any number of immutable borrows are allowed or a
+//! single immutable borrow is allowed, but never both. If a borrow is attempted that would violate
+//! these rules, the thread will panic.
+//!
+//! The corresponding [`Sync`] version of `RefCell<T>` is [`RwLock<T>`].
+//!
+//! ## `OnceCell<T>`
+//!
+//! [`OnceCell<T>`] is somewhat of a hybrid of `Cell` and `RefCell` that works for values that
+//! typically only need to be set once. This means that a reference `&T` can be obtained without
+//! moving or copying the inner value (unlike `Cell`) but also without runtime checks (unlike
+//! `RefCell`). However, its value can also not be updated once set unless you have a mutable
+//! reference to the `OnceCell`.
+//!
+//! `OnceCell` provides the following methods:
+//!
+//! - [`get`](OnceCell::get): obtain a reference to the inner value
+//! - [`set`](OnceCell::set): set the inner value if it is unset (returns a `Result`)
+//! - [`get_or_init`](OnceCell::get_or_init): return the inner value, initializing it if needed
+//! - [`get_mut`](OnceCell::get_mut): provide a mutable reference to the inner value, only available
+//!   if you have a mutable reference to the cell itself.
+//!
+//! The corresponding [`Sync`] version of `OnceCell<T>` is [`OnceLock<T>`].
+//!
 //!
 //! # When to choose interior mutability
 //!
@@ -188,6 +229,8 @@
 //! [`Rc<T>`]: ../../std/rc/struct.Rc.html
 //! [`RwLock<T>`]: ../../std/sync/struct.RwLock.html
 //! [`Mutex<T>`]: ../../std/sync/struct.Mutex.html
+//! [`OnceLock<T>`]: ../../std/sync/struct.OnceLock.html
+//! [`Sync`]: ../../std/marker/trait.Sync.html
 //! [`atomic`]: crate::sync::atomic
 
 #![stable(feature = "rust1", since = "1.0.0")]
@@ -202,9 +245,9 @@ use crate::ptr::{self, NonNull};
 mod lazy;
 mod once;
 
-#[unstable(feature = "once_cell", issue = "74465")]
+#[unstable(feature = "lazy_cell", issue = "109736")]
 pub use lazy::LazyCell;
-#[unstable(feature = "once_cell", issue = "74465")]
+#[stable(feature = "once_cell", since = "CURRENT_RUSTC_VERSION")]
 pub use once::OnceCell;
 
 /// A mutable memory location.
@@ -419,7 +462,7 @@ impl<T> Cell<T> {
         mem::replace(unsafe { &mut *self.value.get() }, val)
     }
 
-    /// Unwraps the value.
+    /// Unwraps the value, consuming the cell.
     ///
     /// # Examples
     ///
@@ -1969,7 +2012,7 @@ impl<T> UnsafeCell<T> {
         UnsafeCell { value }
     }
 
-    /// Unwraps the value.
+    /// Unwraps the value, consuming the cell.
     ///
     /// # Examples
     ///
@@ -2133,7 +2176,7 @@ impl<T> SyncUnsafeCell<T> {
         Self { value: UnsafeCell { value } }
     }
 
-    /// Unwraps the value.
+    /// Unwraps the value, consuming the cell.
     #[inline]
     pub const fn into_inner(self) -> T {
         self.value.into_inner()

library/core/src/cell/lazy.rs

@@ -11,7 +11,7 @@ use crate::ops::Deref;
 /// # Examples
 ///
 /// ```
-/// #![feature(once_cell)]
+/// #![feature(lazy_cell)]
 ///
 /// use std::cell::LazyCell;
 ///
@@ -29,7 +29,7 @@ use crate::ops::Deref;
 /// //   92
 /// //   92
 /// ```
-#[unstable(feature = "once_cell", issue = "74465")]
+#[unstable(feature = "lazy_cell", issue = "109736")]
 pub struct LazyCell<T, F = fn() -> T> {
     cell: OnceCell<T>,
     init: Cell<Option<F>>,
@@ -41,7 +41,7 @@ impl<T, F: FnOnce() -> T> LazyCell<T, F> {
     /// # Examples
     ///
     /// ```
-    /// #![feature(once_cell)]
+    /// #![feature(lazy_cell)]
     ///
     /// use std::cell::LazyCell;
     ///
@@ -52,7 +52,7 @@ impl<T, F: FnOnce() -> T> LazyCell<T, F> {
     /// assert_eq!(&*lazy, "HELLO, WORLD!");
     /// ```
     #[inline]
-    #[unstable(feature = "once_cell", issue = "74465")]
+    #[unstable(feature = "lazy_cell", issue = "109736")]
     pub const fn new(init: F) -> LazyCell<T, F> {
         LazyCell { cell: OnceCell::new(), init: Cell::new(Some(init)) }
     }
@@ -65,7 +65,7 @@ impl<T, F: FnOnce() -> T> LazyCell<T, F> {
     /// # Examples
     ///
     /// ```
-    /// #![feature(once_cell)]
+    /// #![feature(lazy_cell)]
     ///
     /// use std::cell::LazyCell;
     ///
@@ -75,7 +75,7 @@ impl<T, F: FnOnce() -> T> LazyCell<T, F> {
     /// assert_eq!(&*lazy, &92);
     /// ```
     #[inline]
-    #[unstable(feature = "once_cell", issue = "74465")]
+    #[unstable(feature = "lazy_cell", issue = "109736")]
     pub fn force(this: &LazyCell<T, F>) -> &T {
         this.cell.get_or_init(|| match this.init.take() {
             Some(f) => f(),
@@ -84,7 +84,7 @@ impl<T, F: FnOnce() -> T> LazyCell<T, F> {
     }
 }
 
-#[unstable(feature = "once_cell", issue = "74465")]
+#[unstable(feature = "lazy_cell", issue = "109736")]
 impl<T, F: FnOnce() -> T> Deref for LazyCell<T, F> {
     type Target = T;
     #[inline]
@@ -93,7 +93,7 @@ impl<T, F: FnOnce() -> T> Deref for LazyCell<T, F> {
     }
 }
 
-#[unstable(feature = "once_cell", issue = "74465")]
+#[unstable(feature = "lazy_cell", issue = "109736")]
 impl<T: Default> Default for LazyCell<T> {
     /// Creates a new lazy value using `Default` as the initializing function.
     #[inline]
@@ -102,7 +102,7 @@ impl<T: Default> Default for LazyCell<T> {
     }
 }
 
-#[unstable(feature = "once_cell", issue = "74465")]
+#[unstable(feature = "lazy_cell", issue = "109736")]
 impl<T: fmt::Debug, F> fmt::Debug for LazyCell<T, F> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         f.debug_struct("Lazy").field("cell", &self.cell).field("init", &"..").finish()