Adding upstream version 115.7.0esr.upstream/115.7.0esrupstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 1387 insertions, 0 deletions

diff --git a/third_party/rust/once_cell/src/lib.rs b/third_party/rust/once_cell/src/lib.rs
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+//! # Overview
+//!
+//! `once_cell` provides two new cell-like types, [`unsync::OnceCell`] and [`sync::OnceCell`]. A `OnceCell`
+//! might store arbitrary non-`Copy` types, can be assigned to at most once and provides direct access
+//! to the stored contents. The core API looks *roughly* like this (and there's much more inside, read on!):
+//!
+//! ```rust,ignore
+//! impl<T> OnceCell<T> {
+//!     const fn new() -> OnceCell<T> { ... }
+//!     fn set(&self, value: T) -> Result<(), T> { ... }
+//!     fn get(&self) -> Option<&T> { ... }
+//! }
+//! ```
+//!
+//! Note that, like with [`RefCell`] and [`Mutex`], the `set` method requires only a shared reference.
+//! Because of the single assignment restriction `get` can return a `&T` instead of `Ref<T>`
+//! or `MutexGuard<T>`.
+//!
+//! The `sync` flavor is thread-safe (that is, implements the [`Sync`] trait), while the `unsync` one is not.
+//!
+//! [`unsync::OnceCell`]: unsync/struct.OnceCell.html
+//! [`sync::OnceCell`]: sync/struct.OnceCell.html
+//! [`RefCell`]: https://doc.rust-lang.org/std/cell/struct.RefCell.html
+//! [`Mutex`]: https://doc.rust-lang.org/std/sync/struct.Mutex.html
+//! [`Sync`]: https://doc.rust-lang.org/std/marker/trait.Sync.html
+//!
+//! # Recipes
+//!
+//! `OnceCell` might be useful for a variety of patterns.
+//!
+//! ## Safe Initialization of Global Data
+//!
+//! ```rust
+//! use std::{env, io};
+//!
+//! use once_cell::sync::OnceCell;
+//!
+//! #[derive(Debug)]
+//! pub struct Logger {
+//!     // ...
+//! }
+//! static INSTANCE: OnceCell<Logger> = OnceCell::new();
+//!
+//! impl Logger {
+//!     pub fn global() -> &'static Logger {
+//!         INSTANCE.get().expect("logger is not initialized")
+//!     }
+//!
+//!     fn from_cli(args: env::Args) -> Result<Logger, std::io::Error> {
+//!        // ...
+//! #      Ok(Logger {})
+//!     }
+//! }
+//!
+//! fn main() {
+//!     let logger = Logger::from_cli(env::args()).unwrap();
+//!     INSTANCE.set(logger).unwrap();
+//!     // use `Logger::global()` from now on
+//! }
+//! ```
+//!
+//! ## Lazy Initialized Global Data
+//!
+//! This is essentially the `lazy_static!` macro, but without a macro.
+//!
+//! ```rust
+//! use std::{sync::Mutex, collections::HashMap};
+//!
+//! use once_cell::sync::OnceCell;
+//!
+//! fn global_data() -> &'static Mutex<HashMap<i32, String>> {
+//!     static INSTANCE: OnceCell<Mutex<HashMap<i32, String>>> = OnceCell::new();
+//!     INSTANCE.get_or_init(|| {
+//!         let mut m = HashMap::new();
+//!         m.insert(13, "Spica".to_string());
+//!         m.insert(74, "Hoyten".to_string());
+//!         Mutex::new(m)
+//!     })
+//! }
+//! ```
+//!
+//! There are also the [`sync::Lazy`] and [`unsync::Lazy`] convenience types to streamline this pattern:
+//!
+//! ```rust
+//! use std::{sync::Mutex, collections::HashMap};
+//! use once_cell::sync::Lazy;
+//!
+//! static GLOBAL_DATA: Lazy<Mutex<HashMap<i32, String>>> = Lazy::new(|| {
+//!     let mut m = HashMap::new();
+//!     m.insert(13, "Spica".to_string());
+//!     m.insert(74, "Hoyten".to_string());
+//!     Mutex::new(m)
+//! });
+//!
+//! fn main() {
+//!     println!("{:?}", GLOBAL_DATA.lock().unwrap());
+//! }
+//! ```
+//!
+//! Note that the variable that holds `Lazy` is declared as `static`, *not*
+//! `const`. This is important: using `const` instead compiles, but works wrong.
+//!
+//! [`sync::Lazy`]: sync/struct.Lazy.html
+//! [`unsync::Lazy`]: unsync/struct.Lazy.html
+//!
+//! ## General purpose lazy evaluation
+//!
+//! Unlike `lazy_static!`, `Lazy` works with local variables.
+//!
+//! ```rust
+//! use once_cell::unsync::Lazy;
+//!
+//! fn main() {
+//!     let ctx = vec![1, 2, 3];
+//!     let thunk = Lazy::new(|| {
+//!         ctx.iter().sum::<i32>()
+//!     });
+//!     assert_eq!(*thunk, 6);
+//! }
+//! ```
+//!
+//! If you need a lazy field in a struct, you probably should use `OnceCell`
+//! directly, because that will allow you to access `self` during initialization.
+//!
+//! ```rust
+//! use std::{fs, path::PathBuf};
+//!
+//! use once_cell::unsync::OnceCell;
+//!
+//! struct Ctx {
+//!     config_path: PathBuf,
+//!     config: OnceCell<String>,
+//! }
+//!
+//! impl Ctx {
+//!     pub fn get_config(&self) -> Result<&str, std::io::Error> {
+//!         let cfg = self.config.get_or_try_init(|| {
+//!             fs::read_to_string(&self.config_path)
+//!         })?;
+//!         Ok(cfg.as_str())
+//!     }
+//! }
+//! ```
+//!
+//! ## Lazily Compiled Regex
+//!
+//! This is a `regex!` macro which takes a string literal and returns an
+//! *expression* that evaluates to a `&'static Regex`:
+//!
+//! ```
+//! macro_rules! regex {
+//!     ($re:literal $(,)?) => {{
+//!         static RE: once_cell::sync::OnceCell<regex::Regex> = once_cell::sync::OnceCell::new();
+//!         RE.get_or_init(|| regex::Regex::new($re).unwrap())
+//!     }};
+//! }
+//! ```
+//!
+//! This macro can be useful to avoid the "compile regex on every loop iteration" problem.
+//!
+//! ## Runtime `include_bytes!`
+//!
+//! The `include_bytes` macro is useful to include test resources, but it slows
+//! down test compilation a lot. An alternative is to load the resources at
+//! runtime:
+//!
+//! ```
+//! use std::path::Path;
+//!
+//! use once_cell::sync::OnceCell;
+//!
+//! pub struct TestResource {
+//!     path: &'static str,
+//!     cell: OnceCell<Vec<u8>>,
+//! }
+//!
+//! impl TestResource {
+//!     pub const fn new(path: &'static str) -> TestResource {
+//!         TestResource { path, cell: OnceCell::new() }
+//!     }
+//!     pub fn bytes(&self) -> &[u8] {
+//!         self.cell.get_or_init(|| {
+//!             let dir = std::env::var("CARGO_MANIFEST_DIR").unwrap();
+//!             let path = Path::new(dir.as_str()).join(self.path);
+//!             std::fs::read(&path).unwrap_or_else(|_err| {
+//!                 panic!("failed to load test resource: {}", path.display())
+//!             })
+//!         }).as_slice()
+//!     }
+//! }
+//!
+//! static TEST_IMAGE: TestResource = TestResource::new("test_data/lena.png");
+//!
+//! #[test]
+//! fn test_sobel_filter() {
+//!     let rgb: &[u8] = TEST_IMAGE.bytes();
+//!     // ...
+//! # drop(rgb);
+//! }
+//! ```
+//!
+//! ## `lateinit`
+//!
+//! `LateInit` type for delayed initialization. It is reminiscent of Kotlin's
+//! `lateinit` keyword and allows construction of cyclic data structures:
+//!
+//!
+//! ```
+//! use once_cell::sync::OnceCell;
+//!
+//! pub struct LateInit<T> { cell: OnceCell<T> }
+//!
+//! impl<T> LateInit<T> {
+//!     pub fn init(&self, value: T) {
+//!         assert!(self.cell.set(value).is_ok())
+//!     }
+//! }
+//!
+//! impl<T> Default for LateInit<T> {
+//!     fn default() -> Self { LateInit { cell: OnceCell::default() } }
+//! }
+//!
+//! impl<T> std::ops::Deref for LateInit<T> {
+//!     type Target = T;
+//!     fn deref(&self) -> &T {
+//!         self.cell.get().unwrap()
+//!     }
+//! }
+//!
+//! #[derive(Default)]
+//! struct A<'a> {
+//!     b: LateInit<&'a B<'a>>,
+//! }
+//!
+//! #[derive(Default)]
+//! struct B<'a> {
+//!     a: LateInit<&'a A<'a>>
+//! }
+//!
+//!
+//! fn build_cycle() {
+//!     let a = A::default();
+//!     let b = B::default();
+//!     a.b.init(&b);
+//!     b.a.init(&a);
+//!     
+//!     let _a = &a.b.a.b.a;
+//! }
+//! ```
+//!
+//! # Comparison with std
+//!
+//! |`!Sync` types         | Access Mode            | Drawbacks                                     |
+//! |----------------------|------------------------|-----------------------------------------------|
+//! |`Cell<T>`             | `T`                    | requires `T: Copy` for `get`                  |
+//! |`RefCell<T>`          | `RefMut<T>` / `Ref<T>` | may panic at runtime                          |
+//! |`unsync::OnceCell<T>` | `&T`                   | assignable only once                          |
+//!
+//! |`Sync` types          | Access Mode            | Drawbacks                                     |
+//! |----------------------|------------------------|-----------------------------------------------|
+//! |`AtomicT`             | `T`                    | works only with certain `Copy` types          |
+//! |`Mutex<T>`            | `MutexGuard<T>`        | may deadlock at runtime, may block the thread |
+//! |`sync::OnceCell<T>`   | `&T`                   | assignable only once, may block the thread    |
+//!
+//! Technically, calling `get_or_init` will also cause a panic or a deadlock if it recursively calls
+//! itself. However, because the assignment can happen only once, such cases should be more rare than
+//! equivalents with `RefCell` and `Mutex`.
+//!
+//! # Minimum Supported `rustc` Version
+//!
+//! This crate's minimum supported `rustc` version is `1.56.0`.
+//!
+//! If only the `std` feature is enabled, MSRV will be updated conservatively, supporting at least latest 8 versions of the compiler.
+//! When using other features, like `parking_lot`, MSRV might be updated more frequently, up to the latest stable.
+//! In both cases, increasing MSRV is *not* considered a semver-breaking change.
+//!
+//! # Implementation details
+//!
+//! The implementation is based on the [`lazy_static`](https://github.com/rust-lang-nursery/lazy-static.rs/)
+//! and [`lazy_cell`](https://github.com/indiv0/lazycell/) crates and [`std::sync::Once`]. In some sense,
+//! `once_cell` just streamlines and unifies those APIs.
+//!
+//! To implement a sync flavor of `OnceCell`, this crates uses either a custom
+//! re-implementation of `std::sync::Once` or `parking_lot::Mutex`. This is
+//! controlled by the `parking_lot` feature (disabled by default). Performance
+//! is the same for both cases, but the `parking_lot` based `OnceCell<T>` is
+//! smaller by up to 16 bytes.
+//!
+//! This crate uses `unsafe`.
+//!
+//! [`std::sync::Once`]: https://doc.rust-lang.org/std/sync/struct.Once.html
+//!
+//! # F.A.Q.
+//!
+//! **Should I use lazy_static or once_cell?**
+//!
+//! To the first approximation, `once_cell` is both more flexible and more convenient than `lazy_static`
+//! and should be preferred.
+//!
+//! Unlike `once_cell`, `lazy_static` supports spinlock-based implementation of blocking which works with
+//! `#![no_std]`.
+//!
+//! `lazy_static` has received significantly more real world testing, but `once_cell` is also a widely
+//! used crate.
+//!
+//! **Should I use the sync or unsync flavor?**
+//!
+//! Because Rust compiler checks thread safety for you, it's impossible to accidentally use `unsync` where
+//! `sync` is required. So, use `unsync` in single-threaded code and `sync` in multi-threaded. It's easy
+//! to switch between the two if code becomes multi-threaded later.
+//!
+//! At the moment, `unsync` has an additional benefit that reentrant initialization causes a panic, which
+//! might be easier to debug than a deadlock.
+//!
+//! **Does this crate support async?**
+//!
+//! No, but you can use [`async_once_cell`](https://crates.io/crates/async_once_cell) instead.
+//!
+//! **Can I bring my own mutex?**
+//!
+//! There is [generic_once_cell](https://crates.io/crates/generic_once_cell) to allow just that.
+//!
+//! # Related crates
+//!
+//! * [double-checked-cell](https://github.com/niklasf/double-checked-cell)
+//! * [lazy-init](https://crates.io/crates/lazy-init)
+//! * [lazycell](https://crates.io/crates/lazycell)
+//! * [mitochondria](https://crates.io/crates/mitochondria)
+//! * [lazy_static](https://crates.io/crates/lazy_static)
+//! * [async_once_cell](https://crates.io/crates/async_once_cell)
+//! * [generic_once_cell](https://crates.io/crates/generic_once_cell) (bring your own mutex)
+//!
+//! Most of this crate's functionality is available in `std` in nightly Rust.
+//! See the [tracking issue](https://github.com/rust-lang/rust/issues/74465).
+
+#![cfg_attr(not(feature = "std"), no_std)]
+
+#[cfg(feature = "alloc")]
+extern crate alloc;
+
+#[cfg(all(feature = "critical-section", not(feature = "std")))]
+#[path = "imp_cs.rs"]
+mod imp;
+
+#[cfg(all(feature = "std", feature = "parking_lot"))]
+#[path = "imp_pl.rs"]
+mod imp;
+
+#[cfg(all(feature = "std", not(feature = "parking_lot")))]
+#[path = "imp_std.rs"]
+mod imp;
+
+/// Single-threaded version of `OnceCell`.
+pub mod unsync {
+    use core::{
+        cell::{Cell, UnsafeCell},
+        fmt, mem,
+        ops::{Deref, DerefMut},
+        panic::{RefUnwindSafe, UnwindSafe},
+    };
+
+    use super::unwrap_unchecked;
+
+    /// A cell which can be written to only once. It is not thread safe.
+    ///
+    /// Unlike [`std::cell::RefCell`], a `OnceCell` provides simple `&`
+    /// references to the contents.
+    ///
+    /// [`std::cell::RefCell`]: https://doc.rust-lang.org/std/cell/struct.RefCell.html
+    ///
+    /// # Example
+    /// ```
+    /// use once_cell::unsync::OnceCell;
+    ///
+    /// let cell = OnceCell::new();
+    /// assert!(cell.get().is_none());
+    ///
+    /// let value: &String = cell.get_or_init(|| {
+    ///     "Hello, World!".to_string()
+    /// });
+    /// assert_eq!(value, "Hello, World!");
+    /// assert!(cell.get().is_some());
+    /// ```
+    pub struct OnceCell<T> {
+        // Invariant: written to at most once.
+        inner: UnsafeCell<Option<T>>,
+    }
+
+    // Similarly to a `Sync` bound on `sync::OnceCell`, we can use
+    // `&unsync::OnceCell` to sneak a `T` through `catch_unwind`,
+    // by initializing the cell in closure and extracting the value in the
+    // `Drop`.
+    impl<T: RefUnwindSafe + UnwindSafe> RefUnwindSafe for OnceCell<T> {}
+    impl<T: UnwindSafe> UnwindSafe for OnceCell<T> {}
+
+    impl<T> Default for OnceCell<T> {
+        fn default() -> Self {
+            Self::new()
+        }
+    }
+
+    impl<T: fmt::Debug> fmt::Debug for OnceCell<T> {
+        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+            match self.get() {
+                Some(v) => f.debug_tuple("OnceCell").field(v).finish(),
+                None => f.write_str("OnceCell(Uninit)"),
+            }
+        }
+    }
+
+    impl<T: Clone> Clone for OnceCell<T> {
+        fn clone(&self) -> OnceCell<T> {
+            match self.get() {
+                Some(value) => OnceCell::with_value(value.clone()),
+                None => OnceCell::new(),
+            }
+        }
+
+        fn clone_from(&mut self, source: &Self) {
+            match (self.get_mut(), source.get()) {
+                (Some(this), Some(source)) => this.clone_from(source),
+                _ => *self = source.clone(),
+            }
+        }
+    }
+
+    impl<T: PartialEq> PartialEq for OnceCell<T> {
+        fn eq(&self, other: &Self) -> bool {
+            self.get() == other.get()
+        }
+    }
+
+    impl<T: Eq> Eq for OnceCell<T> {}
+
+    impl<T> From<T> for OnceCell<T> {
+        fn from(value: T) -> Self {
+            OnceCell::with_value(value)
+        }
+    }
+
+    impl<T> OnceCell<T> {
+        /// Creates a new empty cell.
+        pub const fn new() -> OnceCell<T> {
+            OnceCell { inner: UnsafeCell::new(None) }
+        }
+
+        /// Creates a new initialized cell.
+        pub const fn with_value(value: T) -> OnceCell<T> {
+            OnceCell { inner: UnsafeCell::new(Some(value)) }
+        }
+
+        /// Gets a reference to the underlying value.
+        ///
+        /// Returns `None` if the cell is empty.
+        #[inline]
+        pub fn get(&self) -> Option<&T> {
+            // Safe due to `inner`'s invariant of being written to at most once.
+            // Had multiple writes to `inner` been allowed, a reference to the
+            // value we return now would become dangling by a write of a
+            // different value later.
+            unsafe { &*self.inner.get() }.as_ref()
+        }
+
+        /// Gets a mutable reference to the underlying value.
+        ///
+        /// Returns `None` if the cell is empty.
+        ///
+        /// This method is allowed to violate the invariant of writing to a `OnceCell`
+        /// at most once because it requires `&mut` access to `self`. As with all
+        /// interior mutability, `&mut` access permits arbitrary modification:
+        ///
+        /// ```
+        /// use once_cell::unsync::OnceCell;
+        ///
+        /// let mut cell: OnceCell<u32> = OnceCell::new();
+        /// cell.set(92).unwrap();
+        /// *cell.get_mut().unwrap() = 93;
+        /// assert_eq!(cell.get(), Some(&93));
+        /// ```
+        #[inline]
+        pub fn get_mut(&mut self) -> Option<&mut T> {
+            // Safe because we have unique access
+            unsafe { &mut *self.inner.get() }.as_mut()
+        }
+
+        /// Sets the contents of this cell to `value`.
+        ///
+        /// Returns `Ok(())` if the cell was empty and `Err(value)` if it was
+        /// full.
+        ///
+        /// # Example
+        /// ```
+        /// use once_cell::unsync::OnceCell;
+        ///
+        /// let cell = OnceCell::new();
+        /// assert!(cell.get().is_none());
+        ///
+        /// assert_eq!(cell.set(92), Ok(()));
+        /// assert_eq!(cell.set(62), Err(62));
+        ///
+        /// assert!(cell.get().is_some());
+        /// ```
+        pub fn set(&self, value: T) -> Result<(), T> {
+            match self.try_insert(value) {
+                Ok(_) => Ok(()),
+                Err((_, value)) => Err(value),
+            }
+        }
+
+        /// Like [`set`](Self::set), but also returns a reference to the final cell value.
+        ///
+        /// # Example
+        /// ```
+        /// use once_cell::unsync::OnceCell;
+        ///
+        /// let cell = OnceCell::new();
+        /// assert!(cell.get().is_none());
+        ///
+        /// assert_eq!(cell.try_insert(92), Ok(&92));
+        /// assert_eq!(cell.try_insert(62), Err((&92, 62)));
+        ///
+        /// assert!(cell.get().is_some());
+        /// ```
+        pub fn try_insert(&self, value: T) -> Result<&T, (&T, T)> {
+            if let Some(old) = self.get() {
+                return Err((old, value));
+            }
+
+            let slot = unsafe { &mut *self.inner.get() };
+            // This is the only place where we set the slot, no races
+            // due to reentrancy/concurrency are possible, and we've
+            // checked that slot is currently `None`, so this write
+            // maintains the `inner`'s invariant.
+            *slot = Some(value);
+            Ok(unsafe { unwrap_unchecked(slot.as_ref()) })
+        }
+
+        /// Gets the contents of the cell, initializing it with `f`
+        /// if the cell was empty.
+        ///
+        /// # Panics
+        ///
+        /// If `f` panics, the panic is propagated to the caller, and the cell
+        /// remains uninitialized.
+        ///
+        /// It is an error to reentrantly initialize the cell from `f`. Doing
+        /// so results in a panic.
+        ///
+        /// # Example
+        /// ```
+        /// use once_cell::unsync::OnceCell;
+        ///
+        /// let cell = OnceCell::new();
+        /// let value = cell.get_or_init(|| 92);
+        /// assert_eq!(value, &92);
+        /// let value = cell.get_or_init(|| unreachable!());
+        /// assert_eq!(value, &92);
+        /// ```
+        pub fn get_or_init<F>(&self, f: F) -> &T
+        where
+            F: FnOnce() -> T,
+        {
+            enum Void {}
+            match self.get_or_try_init(|| Ok::<T, Void>(f())) {
+                Ok(val) => val,
+                Err(void) => match void {},
+            }
+        }
+
+        /// Gets the contents of the cell, initializing it with `f` if
+        /// the cell was empty. If the cell was empty and `f` failed, an
+        /// error is returned.
+        ///
+        /// # Panics
+        ///
+        /// If `f` panics, the panic is propagated to the caller, and the cell
+        /// remains uninitialized.
+        ///
+        /// It is an error to reentrantly initialize the cell from `f`. Doing
+        /// so results in a panic.
+        ///
+        /// # Example
+        /// ```
+        /// use once_cell::unsync::OnceCell;
+        ///
+        /// let cell = OnceCell::new();
+        /// assert_eq!(cell.get_or_try_init(|| Err(())), Err(()));
+        /// assert!(cell.get().is_none());
+        /// let value = cell.get_or_try_init(|| -> Result<i32, ()> {
+        ///     Ok(92)
+        /// });
+        /// assert_eq!(value, Ok(&92));
+        /// assert_eq!(cell.get(), Some(&92))
+        /// ```
+        pub fn get_or_try_init<F, E>(&self, f: F) -> Result<&T, E>
+        where
+            F: FnOnce() -> Result<T, E>,
+        {
+            if let Some(val) = self.get() {
+                return Ok(val);
+            }
+            let val = f()?;
+            // Note that *some* forms of reentrant initialization might lead to
+            // UB (see `reentrant_init` test). I believe that just removing this
+            // `assert`, while keeping `set/get` would be sound, but it seems
+            // better to panic, rather than to silently use an old value.
+            assert!(self.set(val).is_ok(), "reentrant init");
+            Ok(unsafe { unwrap_unchecked(self.get()) })
+        }
+
+        /// Takes the value out of this `OnceCell`, moving it back to an uninitialized state.
+        ///
+        /// Has no effect and returns `None` if the `OnceCell` hasn't been initialized.
+        ///
+        /// # Examples
+        ///
+        /// ```
+        /// use once_cell::unsync::OnceCell;
+        ///
+        /// let mut cell: OnceCell<String> = OnceCell::new();
+        /// assert_eq!(cell.take(), None);
+        ///
+        /// let mut cell = OnceCell::new();
+        /// cell.set("hello".to_string()).unwrap();
+        /// assert_eq!(cell.take(), Some("hello".to_string()));
+        /// assert_eq!(cell.get(), None);
+        /// ```
+        ///
+        /// This method is allowed to violate the invariant of writing to a `OnceCell`
+        /// at most once because it requires `&mut` access to `self`. As with all
+        /// interior mutability, `&mut` access permits arbitrary modification:
+        ///
+        /// ```
+        /// use once_cell::unsync::OnceCell;
+        ///
+        /// let mut cell: OnceCell<u32> = OnceCell::new();
+        /// cell.set(92).unwrap();
+        /// cell = OnceCell::new();
+        /// ```
+        pub fn take(&mut self) -> Option<T> {
+            mem::replace(self, Self::default()).into_inner()
+        }
+
+        /// Consumes the `OnceCell`, returning the wrapped value.
+        ///
+        /// Returns `None` if the cell was empty.
+        ///
+        /// # Examples
+        ///
+        /// ```
+        /// use once_cell::unsync::OnceCell;
+        ///
+        /// let cell: OnceCell<String> = OnceCell::new();
+        /// assert_eq!(cell.into_inner(), None);
+        ///
+        /// let cell = OnceCell::new();
+        /// cell.set("hello".to_string()).unwrap();
+        /// assert_eq!(cell.into_inner(), Some("hello".to_string()));
+        /// ```
+        pub fn into_inner(self) -> Option<T> {
+            // Because `into_inner` takes `self` by value, the compiler statically verifies
+            // that it is not currently borrowed. So it is safe to move out `Option<T>`.
+            self.inner.into_inner()
+        }
+    }
+
+    /// A value which is initialized on the first access.
+    ///
+    /// # Example
+    /// ```
+    /// use once_cell::unsync::Lazy;
+    ///
+    /// let lazy: Lazy<i32> = Lazy::new(|| {
+    ///     println!("initializing");
+    ///     92
+    /// });
+    /// println!("ready");
+    /// println!("{}", *lazy);
+    /// println!("{}", *lazy);
+    ///
+    /// // Prints:
+    /// //   ready
+    /// //   initializing
+    /// //   92
+    /// //   92
+    /// ```
+    pub struct Lazy<T, F = fn() -> T> {
+        cell: OnceCell<T>,
+        init: Cell<Option<F>>,
+    }
+
+    impl<T, F: RefUnwindSafe> RefUnwindSafe for Lazy<T, F> where OnceCell<T>: RefUnwindSafe {}
+
+    impl<T: fmt::Debug, F> fmt::Debug for Lazy<T, F> {
+        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+            f.debug_struct("Lazy").field("cell", &self.cell).field("init", &"..").finish()
+        }
+    }
+
+    impl<T, F> Lazy<T, F> {
+        /// Creates a new lazy value with the given initializing function.
+        ///
+        /// # Example
+        /// ```
+        /// # fn main() {
+        /// use once_cell::unsync::Lazy;
+        ///
+        /// let hello = "Hello, World!".to_string();
+        ///
+        /// let lazy = Lazy::new(|| hello.to_uppercase());
+        ///
+        /// assert_eq!(&*lazy, "HELLO, WORLD!");
+        /// # }
+        /// ```
+        pub const fn new(init: F) -> Lazy<T, F> {
+            Lazy { cell: OnceCell::new(), init: Cell::new(Some(init)) }
+        }
+
+        /// Consumes this `Lazy` returning the stored value.
+        ///
+        /// Returns `Ok(value)` if `Lazy` is initialized and `Err(f)` otherwise.
+        pub fn into_value(this: Lazy<T, F>) -> Result<T, F> {
+            let cell = this.cell;
+            let init = this.init;
+            cell.into_inner().ok_or_else(|| {
+                init.take().unwrap_or_else(|| panic!("Lazy instance has previously been poisoned"))
+            })
+        }
+    }
+
+    impl<T, F: FnOnce() -> T> Lazy<T, F> {
+        /// Forces the evaluation of this lazy value and returns a reference to
+        /// the result.
+        ///
+        /// This is equivalent to the `Deref` impl, but is explicit.
+        ///
+        /// # Example
+        /// ```
+        /// use once_cell::unsync::Lazy;
+        ///
+        /// let lazy = Lazy::new(|| 92);
+        ///
+        /// assert_eq!(Lazy::force(&lazy), &92);
+        /// assert_eq!(&*lazy, &92);
+        /// ```
+        pub fn force(this: &Lazy<T, F>) -> &T {
+            this.cell.get_or_init(|| match this.init.take() {
+                Some(f) => f(),
+                None => panic!("Lazy instance has previously been poisoned"),
+            })
+        }
+
+        /// Forces the evaluation of this lazy value and returns a mutable reference to
+        /// the result.
+        ///
+        /// This is equivalent to the `DerefMut` impl, but is explicit.
+        ///
+        /// # Example
+        /// ```
+        /// use once_cell::unsync::Lazy;
+        ///
+        /// let mut lazy = Lazy::new(|| 92);
+        ///
+        /// assert_eq!(Lazy::force_mut(&mut lazy), &92);
+        /// assert_eq!(*lazy, 92);
+        /// ```
+        pub fn force_mut(this: &mut Lazy<T, F>) -> &mut T {
+            Self::force(this);
+            Self::get_mut(this).unwrap_or_else(|| unreachable!())
+        }
+
+        /// Gets the reference to the result of this lazy value if
+        /// it was initialized, otherwise returns `None`.
+        ///
+        /// # Example
+        /// ```
+        /// use once_cell::unsync::Lazy;
+        ///
+        /// let lazy = Lazy::new(|| 92);
+        ///
+        /// assert_eq!(Lazy::get(&lazy), None);
+        /// assert_eq!(&*lazy, &92);
+        /// assert_eq!(Lazy::get(&lazy), Some(&92));
+        /// ```
+        pub fn get(this: &Lazy<T, F>) -> Option<&T> {
+            this.cell.get()
+        }
+
+        /// Gets the mutable reference to the result of this lazy value if
+        /// it was initialized, otherwise returns `None`.
+        ///
+        /// # Example
+        /// ```
+        /// use once_cell::unsync::Lazy;
+        ///
+        /// let mut lazy = Lazy::new(|| 92);
+        ///
+        /// assert_eq!(Lazy::get_mut(&mut lazy), None);
+        /// assert_eq!(*lazy, 92);
+        /// assert_eq!(Lazy::get_mut(&mut lazy), Some(&mut 92));
+        /// ```
+        pub fn get_mut(this: &mut Lazy<T, F>) -> Option<&mut T> {
+            this.cell.get_mut()
+        }
+    }
+
+    impl<T, F: FnOnce() -> T> Deref for Lazy<T, F> {
+        type Target = T;
+        fn deref(&self) -> &T {
+            Lazy::force(self)
+        }
+    }
+
+    impl<T, F: FnOnce() -> T> DerefMut for Lazy<T, F> {
+        fn deref_mut(&mut self) -> &mut T {
+            Lazy::force(self);
+            self.cell.get_mut().unwrap_or_else(|| unreachable!())
+        }
+    }
+
+    impl<T: Default> Default for Lazy<T> {
+        /// Creates a new lazy value using `Default` as the initializing function.
+        fn default() -> Lazy<T> {
+            Lazy::new(T::default)
+        }
+    }
+}
+
+/// Thread-safe, blocking version of `OnceCell`.
+#[cfg(any(feature = "std", feature = "critical-section"))]
+pub mod sync {
+    use core::{
+        cell::Cell,
+        fmt, mem,
+        ops::{Deref, DerefMut},
+        panic::RefUnwindSafe,
+    };
+
+    use super::{imp::OnceCell as Imp, unwrap_unchecked};
+
+    /// A thread-safe cell which can be written to only once.
+    ///
+    /// `OnceCell` provides `&` references to the contents without RAII guards.
+    ///
+    /// Reading a non-`None` value out of `OnceCell` establishes a
+    /// happens-before relationship with a corresponding write. For example, if
+    /// thread A initializes the cell with `get_or_init(f)`, and thread B
+    /// subsequently reads the result of this call, B also observes all the side
+    /// effects of `f`.
+    ///
+    /// # Example
+    /// ```
+    /// use once_cell::sync::OnceCell;
+    ///
+    /// static CELL: OnceCell<String> = OnceCell::new();
+    /// assert!(CELL.get().is_none());
+    ///
+    /// std::thread::spawn(|| {
+    ///     let value: &String = CELL.get_or_init(|| {
+    ///         "Hello, World!".to_string()
+    ///     });
+    ///     assert_eq!(value, "Hello, World!");
+    /// }).join().unwrap();
+    ///
+    /// let value: Option<&String> = CELL.get();
+    /// assert!(value.is_some());
+    /// assert_eq!(value.unwrap().as_str(), "Hello, World!");
+    /// ```
+    pub struct OnceCell<T>(Imp<T>);
+
+    impl<T> Default for OnceCell<T> {
+        fn default() -> OnceCell<T> {
+            OnceCell::new()
+        }
+    }
+
+    impl<T: fmt::Debug> fmt::Debug for OnceCell<T> {
+        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+            match self.get() {
+                Some(v) => f.debug_tuple("OnceCell").field(v).finish(),
+                None => f.write_str("OnceCell(Uninit)"),
+            }
+        }
+    }
+
+    impl<T: Clone> Clone for OnceCell<T> {
+        fn clone(&self) -> OnceCell<T> {
+            match self.get() {
+                Some(value) => Self::with_value(value.clone()),
+                None => Self::new(),
+            }
+        }
+
+        fn clone_from(&mut self, source: &Self) {
+            match (self.get_mut(), source.get()) {
+                (Some(this), Some(source)) => this.clone_from(source),
+                _ => *self = source.clone(),
+            }
+        }
+    }
+
+    impl<T> From<T> for OnceCell<T> {
+        fn from(value: T) -> Self {
+            Self::with_value(value)
+        }
+    }
+
+    impl<T: PartialEq> PartialEq for OnceCell<T> {
+        fn eq(&self, other: &OnceCell<T>) -> bool {
+            self.get() == other.get()
+        }
+    }
+
+    impl<T: Eq> Eq for OnceCell<T> {}
+
+    impl<T> OnceCell<T> {
+        /// Creates a new empty cell.
+        pub const fn new() -> OnceCell<T> {
+            OnceCell(Imp::new())
+        }
+
+        /// Creates a new initialized cell.
+        pub const fn with_value(value: T) -> OnceCell<T> {
+            OnceCell(Imp::with_value(value))
+        }
+
+        /// Gets the reference to the underlying value.
+        ///
+        /// Returns `None` if the cell is empty, or being initialized. This
+        /// method never blocks.
+        pub fn get(&self) -> Option<&T> {
+            if self.0.is_initialized() {
+                // Safe b/c value is initialized.
+                Some(unsafe { self.get_unchecked() })
+            } else {
+                None
+            }
+        }
+
+        /// Gets the reference to the underlying value, blocking the current
+        /// thread until it is set.
+        ///
+        /// ```
+        /// use once_cell::sync::OnceCell;
+        ///
+        /// let mut cell = std::sync::Arc::new(OnceCell::new());
+        /// let t = std::thread::spawn({
+        ///     let cell = std::sync::Arc::clone(&cell);
+        ///     move || cell.set(92).unwrap()
+        /// });
+        ///
+        /// // Returns immediately, but might return None.
+        /// let _value_or_none = cell.get();
+        ///
+        /// // Will return 92, but might block until the other thread does `.set`.
+        /// let value: &u32 = cell.wait();
+        /// assert_eq!(*value, 92);
+        /// t.join().unwrap();
+        /// ```
+        #[cfg(feature = "std")]
+        pub fn wait(&self) -> &T {
+            if !self.0.is_initialized() {
+                self.0.wait()
+            }
+            debug_assert!(self.0.is_initialized());
+            // Safe b/c of the wait call above and the fact that we didn't
+            // relinquish our borrow.
+            unsafe { self.get_unchecked() }
+        }
+
+        /// Gets the mutable reference to the underlying value.
+        ///
+        /// Returns `None` if the cell is empty.
+        ///
+        /// This method is allowed to violate the invariant of writing to a `OnceCell`
+        /// at most once because it requires `&mut` access to `self`. As with all
+        /// interior mutability, `&mut` access permits arbitrary modification:
+        ///
+        /// ```
+        /// use once_cell::sync::OnceCell;
+        ///
+        /// let mut cell: OnceCell<u32> = OnceCell::new();
+        /// cell.set(92).unwrap();
+        /// cell = OnceCell::new();
+        /// ```
+        #[inline]
+        pub fn get_mut(&mut self) -> Option<&mut T> {
+            self.0.get_mut()
+        }
+
+        /// Get the reference to the underlying value, without checking if the
+        /// cell is initialized.
+        ///
+        /// # Safety
+        ///
+        /// Caller must ensure that the cell is in initialized state, and that
+        /// the contents are acquired by (synchronized to) this thread.
+        #[inline]
+        pub unsafe fn get_unchecked(&self) -> &T {
+            self.0.get_unchecked()
+        }
+
+        /// Sets the contents of this cell to `value`.
+        ///
+        /// Returns `Ok(())` if the cell was empty and `Err(value)` if it was
+        /// full.
+        ///
+        /// # Example
+        ///
+        /// ```
+        /// use once_cell::sync::OnceCell;
+        ///
+        /// static CELL: OnceCell<i32> = OnceCell::new();
+        ///
+        /// fn main() {
+        ///     assert!(CELL.get().is_none());
+        ///
+        ///     std::thread::spawn(|| {
+        ///         assert_eq!(CELL.set(92), Ok(()));
+        ///     }).join().unwrap();
+        ///
+        ///     assert_eq!(CELL.set(62), Err(62));
+        ///     assert_eq!(CELL.get(), Some(&92));
+        /// }
+        /// ```
+        pub fn set(&self, value: T) -> Result<(), T> {
+            match self.try_insert(value) {
+                Ok(_) => Ok(()),
+                Err((_, value)) => Err(value),
+            }
+        }
+
+        /// Like [`set`](Self::set), but also returns a reference to the final cell value.
+        ///
+        /// # Example
+        ///
+        /// ```
+        /// use once_cell::unsync::OnceCell;
+        ///
+        /// let cell = OnceCell::new();
+        /// assert!(cell.get().is_none());
+        ///
+        /// assert_eq!(cell.try_insert(92), Ok(&92));
+        /// assert_eq!(cell.try_insert(62), Err((&92, 62)));
+        ///
+        /// assert!(cell.get().is_some());
+        /// ```
+        pub fn try_insert(&self, value: T) -> Result<&T, (&T, T)> {
+            let mut value = Some(value);
+            let res = self.get_or_init(|| unsafe { unwrap_unchecked(value.take()) });
+            match value {
+                None => Ok(res),
+                Some(value) => Err((res, value)),
+            }
+        }
+
+        /// Gets the contents of the cell, initializing it with `f` if the cell
+        /// was empty.
+        ///
+        /// Many threads may call `get_or_init` concurrently with different
+        /// initializing functions, but it is guaranteed that only one function
+        /// will be executed.
+        ///
+        /// # Panics
+        ///
+        /// If `f` panics, the panic is propagated to the caller, and the cell
+        /// remains uninitialized.
+        ///
+        /// It is an error to reentrantly initialize the cell from `f`. The
+        /// exact outcome is unspecified. Current implementation deadlocks, but
+        /// this may be changed to a panic in the future.
+        ///
+        /// # Example
+        /// ```
+        /// use once_cell::sync::OnceCell;
+        ///
+        /// let cell = OnceCell::new();
+        /// let value = cell.get_or_init(|| 92);
+        /// assert_eq!(value, &92);
+        /// let value = cell.get_or_init(|| unreachable!());
+        /// assert_eq!(value, &92);
+        /// ```
+        pub fn get_or_init<F>(&self, f: F) -> &T
+        where
+            F: FnOnce() -> T,
+        {
+            enum Void {}
+            match self.get_or_try_init(|| Ok::<T, Void>(f())) {
+                Ok(val) => val,
+                Err(void) => match void {},
+            }
+        }
+
+        /// Gets the contents of the cell, initializing it with `f` if
+        /// the cell was empty. If the cell was empty and `f` failed, an
+        /// error is returned.
+        ///
+        /// # Panics
+        ///
+        /// If `f` panics, the panic is propagated to the caller, and
+        /// the cell remains uninitialized.
+        ///
+        /// It is an error to reentrantly initialize the cell from `f`.
+        /// The exact outcome is unspecified. Current implementation
+        /// deadlocks, but this may be changed to a panic in the future.
+        ///
+        /// # Example
+        /// ```
+        /// use once_cell::sync::OnceCell;
+        ///
+        /// let cell = OnceCell::new();
+        /// assert_eq!(cell.get_or_try_init(|| Err(())), Err(()));
+        /// assert!(cell.get().is_none());
+        /// let value = cell.get_or_try_init(|| -> Result<i32, ()> {
+        ///     Ok(92)
+        /// });
+        /// assert_eq!(value, Ok(&92));
+        /// assert_eq!(cell.get(), Some(&92))
+        /// ```
+        pub fn get_or_try_init<F, E>(&self, f: F) -> Result<&T, E>
+        where
+            F: FnOnce() -> Result<T, E>,
+        {
+            // Fast path check
+            if let Some(value) = self.get() {
+                return Ok(value);
+            }
+
+            self.0.initialize(f)?;
+
+            // Safe b/c value is initialized.
+            debug_assert!(self.0.is_initialized());
+            Ok(unsafe { self.get_unchecked() })
+        }
+
+        /// Takes the value out of this `OnceCell`, moving it back to an uninitialized state.
+        ///
+        /// Has no effect and returns `None` if the `OnceCell` hasn't been initialized.
+        ///
+        /// # Examples
+        ///
+        /// ```
+        /// use once_cell::sync::OnceCell;
+        ///
+        /// let mut cell: OnceCell<String> = OnceCell::new();
+        /// assert_eq!(cell.take(), None);
+        ///
+        /// let mut cell = OnceCell::new();
+        /// cell.set("hello".to_string()).unwrap();
+        /// assert_eq!(cell.take(), Some("hello".to_string()));
+        /// assert_eq!(cell.get(), None);
+        /// ```
+        ///
+        /// This method is allowed to violate the invariant of writing to a `OnceCell`
+        /// at most once because it requires `&mut` access to `self`. As with all
+        /// interior mutability, `&mut` access permits arbitrary modification:
+        ///
+        /// ```
+        /// use once_cell::sync::OnceCell;
+        ///
+        /// let mut cell: OnceCell<u32> = OnceCell::new();
+        /// cell.set(92).unwrap();
+        /// cell = OnceCell::new();
+        /// ```
+        pub fn take(&mut self) -> Option<T> {
+            mem::replace(self, Self::default()).into_inner()
+        }
+
+        /// Consumes the `OnceCell`, returning the wrapped value. Returns
+        /// `None` if the cell was empty.
+        ///
+        /// # Examples
+        ///
+        /// ```
+        /// use once_cell::sync::OnceCell;
+        ///
+        /// let cell: OnceCell<String> = OnceCell::new();
+        /// assert_eq!(cell.into_inner(), None);
+        ///
+        /// let cell = OnceCell::new();
+        /// cell.set("hello".to_string()).unwrap();
+        /// assert_eq!(cell.into_inner(), Some("hello".to_string()));
+        /// ```
+        #[inline]
+        pub fn into_inner(self) -> Option<T> {
+            self.0.into_inner()
+        }
+    }
+
+    /// A value which is initialized on the first access.
+    ///
+    /// This type is thread-safe and can be used in statics.
+    ///
+    /// # Example
+    ///
+    /// ```
+    /// use std::collections::HashMap;
+    ///
+    /// use once_cell::sync::Lazy;
+    ///
+    /// static HASHMAP: Lazy<HashMap<i32, String>> = Lazy::new(|| {
+    ///     println!("initializing");
+    ///     let mut m = HashMap::new();
+    ///     m.insert(13, "Spica".to_string());
+    ///     m.insert(74, "Hoyten".to_string());
+    ///     m
+    /// });
+    ///
+    /// fn main() {
+    ///     println!("ready");
+    ///     std::thread::spawn(|| {
+    ///         println!("{:?}", HASHMAP.get(&13));
+    ///     }).join().unwrap();
+    ///     println!("{:?}", HASHMAP.get(&74));
+    ///
+    ///     // Prints:
+    ///     //   ready
+    ///     //   initializing
+    ///     //   Some("Spica")
+    ///     //   Some("Hoyten")
+    /// }
+    /// ```
+    pub struct Lazy<T, F = fn() -> T> {
+        cell: OnceCell<T>,
+        init: Cell<Option<F>>,
+    }
+
+    impl<T: fmt::Debug, F> fmt::Debug for Lazy<T, F> {
+        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+            f.debug_struct("Lazy").field("cell", &self.cell).field("init", &"..").finish()
+        }
+    }
+
+    // We never create a `&F` from a `&Lazy<T, F>` so it is fine to not impl
+    // `Sync` for `F`. We do create a `&mut Option<F>` in `force`, but this is
+    // properly synchronized, so it only happens once so it also does not
+    // contribute to this impl.
+    unsafe impl<T, F: Send> Sync for Lazy<T, F> where OnceCell<T>: Sync {}
+    // auto-derived `Send` impl is OK.
+
+    impl<T, F: RefUnwindSafe> RefUnwindSafe for Lazy<T, F> where OnceCell<T>: RefUnwindSafe {}
+
+    impl<T, F> Lazy<T, F> {
+        /// Creates a new lazy value with the given initializing
+        /// function.
+        pub const fn new(f: F) -> Lazy<T, F> {
+            Lazy { cell: OnceCell::new(), init: Cell::new(Some(f)) }
+        }
+
+        /// Consumes this `Lazy` returning the stored value.
+        ///
+        /// Returns `Ok(value)` if `Lazy` is initialized and `Err(f)` otherwise.
+        pub fn into_value(this: Lazy<T, F>) -> Result<T, F> {
+            let cell = this.cell;
+            let init = this.init;
+            cell.into_inner().ok_or_else(|| {
+                init.take().unwrap_or_else(|| panic!("Lazy instance has previously been poisoned"))
+            })
+        }
+    }
+
+    impl<T, F: FnOnce() -> T> Lazy<T, F> {
+        /// Forces the evaluation of this lazy value and
+        /// returns a reference to the result. This is equivalent
+        /// to the `Deref` impl, but is explicit.
+        ///
+        /// # Example
+        /// ```
+        /// use once_cell::sync::Lazy;
+        ///
+        /// let lazy = Lazy::new(|| 92);
+        ///
+        /// assert_eq!(Lazy::force(&lazy), &92);
+        /// assert_eq!(&*lazy, &92);
+        /// ```
+        pub fn force(this: &Lazy<T, F>) -> &T {
+            this.cell.get_or_init(|| match this.init.take() {
+                Some(f) => f(),
+                None => panic!("Lazy instance has previously been poisoned"),
+            })
+        }
+
+        /// Forces the evaluation of this lazy value and
+        /// returns a mutable reference to the result. This is equivalent
+        /// to the `Deref` impl, but is explicit.
+        ///
+        /// # Example
+        /// ```
+        /// use once_cell::sync::Lazy;
+        ///
+        /// let mut lazy = Lazy::new(|| 92);
+        ///
+        /// assert_eq!(Lazy::force_mut(&mut lazy), &mut 92);
+        /// ```
+        pub fn force_mut(this: &mut Lazy<T, F>) -> &mut T {
+            Self::force(this);
+            Self::get_mut(this).unwrap_or_else(|| unreachable!())
+        }
+
+        /// Gets the reference to the result of this lazy value if
+        /// it was initialized, otherwise returns `None`.
+        ///
+        /// # Example
+        /// ```
+        /// use once_cell::sync::Lazy;
+        ///
+        /// let lazy = Lazy::new(|| 92);
+        ///
+        /// assert_eq!(Lazy::get(&lazy), None);
+        /// assert_eq!(&*lazy, &92);
+        /// assert_eq!(Lazy::get(&lazy), Some(&92));
+        /// ```
+        pub fn get(this: &Lazy<T, F>) -> Option<&T> {
+            this.cell.get()
+        }
+
+        /// Gets the reference to the result of this lazy value if
+        /// it was initialized, otherwise returns `None`.
+        ///
+        /// # Example
+        /// ```
+        /// use once_cell::sync::Lazy;
+        ///
+        /// let mut lazy = Lazy::new(|| 92);
+        ///
+        /// assert_eq!(Lazy::get_mut(&mut lazy), None);
+        /// assert_eq!(&*lazy, &92);
+        /// assert_eq!(Lazy::get_mut(&mut lazy), Some(&mut 92));
+        /// ```
+        pub fn get_mut(this: &mut Lazy<T, F>) -> Option<&mut T> {
+            this.cell.get_mut()
+        }
+    }
+
+    impl<T, F: FnOnce() -> T> Deref for Lazy<T, F> {
+        type Target = T;
+        fn deref(&self) -> &T {
+            Lazy::force(self)
+        }
+    }
+
+    impl<T, F: FnOnce() -> T> DerefMut for Lazy<T, F> {
+        fn deref_mut(&mut self) -> &mut T {
+            Lazy::force(self);
+            self.cell.get_mut().unwrap_or_else(|| unreachable!())
+        }
+    }
+
+    impl<T: Default> Default for Lazy<T> {
+        /// Creates a new lazy value using `Default` as the initializing function.
+        fn default() -> Lazy<T> {
+            Lazy::new(T::default)
+        }
+    }
+
+    /// ```compile_fail
+    /// struct S(*mut ());
+    /// unsafe impl Sync for S {}
+    ///
+    /// fn share<T: Sync>(_: &T) {}
+    /// share(&once_cell::sync::OnceCell::<S>::new());
+    /// ```
+    ///
+    /// ```compile_fail
+    /// struct S(*mut ());
+    /// unsafe impl Sync for S {}
+    ///
+    /// fn share<T: Sync>(_: &T) {}
+    /// share(&once_cell::sync::Lazy::<S>::new(|| unimplemented!()));
+    /// ```
+    fn _dummy() {}
+}
+
+#[cfg(feature = "race")]
+pub mod race;
+
+// Remove once MSRV is at least 1.58.
+#[inline]
+unsafe fn unwrap_unchecked<T>(val: Option<T>) -> T {
+    match val {
+        Some(value) => value,
+        None => {
+            debug_assert!(false);
+            core::hint::unreachable_unchecked()
+        }
+    }
+}