once_cell/src/lib.rs

//! # 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
            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()
        }
    }
}