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/*! # Overview `once_cell` provides two new cell-like types, `unsync::OnceCell` and `sync::OnceCell`. `OnceCell` might store arbitrary non-`Copy` types, can be assigned to at most once and provide direct access to the stored contents. In a nutshell, API looks *roughly* like this: ```rust,ignore impl OnceCell<T> { 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 an `&T` instead of `Ref<T>` or `MutexGuard<T>`. # Patterns `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 `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 `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()); } ``` ## 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()) } } ``` ## Building block Naturally, it is possible to build other abstractions on top of `OnceCell`. For example, this is a `regex!` macro which takes a string literal and returns an *expression* that evaluates to `&'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 "compile regex on every loop iteration" problem. # Comparison with std |`!Sync` types | Access Mode | Drawbacks | |----------------------|------------------------|-----------------------------------------------| |`Cell<T>` | `T` | requires `T: Copy` for `get` | |`RefCel<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 `1.31.1`. If optional features are not enabled (`default-features = false` in `Cargo.toml`), MSRV will be updated conservatively. When using specific features or default features, 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 Implementation is based on [`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, which is enabled by default. Performance is the same for both cases, but parking_lot based `OnceCell<T>` is smaller by up to 16 bytes. This crate uses unsafe. # 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) */ #[cfg(feature = "std")] #[cfg(feature = "parking_lot")] #[path = "imp_pl.rs"] mod imp; #[cfg(feature = "std")] #[cfg(not(feature = "parking_lot"))] #[path = "imp_std.rs"] mod imp; pub mod unsync { use core::{fmt, ops::Deref, cell::UnsafeCell, hint::unreachable_unchecked}; #[cfg(feature = "std")] use std::panic::{RefUnwindSafe, UnwindSafe}; /// A cell which can be written to only once. Not thread safe. /// /// Unlike `::std::cell::RefCell`, a `OnceCell` provides simple `&` /// references to the contents. /// /// # 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>>, } #[cfg(feature = "std")] impl<T: RefUnwindSafe + UnwindSafe> RefUnwindSafe for OnceCell<T> {} #[cfg(feature = "std")] 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> { let res = OnceCell::new(); if let Some(value) = self.get() { match res.set(value.clone()) { Ok(()) => (), Err(_) => unreachable!(), } } res } } impl<T: PartialEq> PartialEq for OnceCell<T> { fn eq(&self, other: &Self) -> bool { self.get() == other.get() } } impl<T> From<T> for OnceCell<T> { fn from(value: T) -> Self { OnceCell { inner: UnsafeCell::new(Some(value)) } } } impl<T> OnceCell<T> { /// Creates a new empty cell. pub const fn new() -> OnceCell<T> { OnceCell { inner: UnsafeCell::new(None) } } /// Gets the reference to the underlying value. Returns `None` /// if the cell is empty. pub fn get(&self) -> Option<&T> { // Safe due to `inner`'s invariant unsafe { &*self.inner.get() }.as_ref() } /// 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> { let slot = unsafe { &*self.inner.get() }; if slot.is_some() { return Err(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(()) } /// 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()?; assert!(self.set(val).is_ok(), "reentrant init"); Ok(self.get().unwrap()) } /// 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 /// ``` #[derive(Debug)] pub struct Lazy<T, F = fn() -> T> { cell: OnceCell<T>, init: UnsafeCell<Option<F>>, } impl<T, F> Lazy<T, F> { /// Creates a new lazy value with the given initializing function. /// /// # Example /// ``` /// # extern crate once_cell; /// # 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: UnsafeCell::new(Some(init)) } } } impl<T, F: FnOnce() -> T> Lazy<T, F> { /// Forces the evaluation of this lazy value and /// returns a reference to 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 { // Safe because closure is guaranteed to be called at most once // so we only call `F` once, this also guarantees no race conditions this.cell.get_or_init(|| unsafe { match (*this.init.get()).take() { Some(f) => f(), None => unreachable_unchecked(), } }) } } impl<T, F: FnOnce() -> T> Deref for Lazy<T, F> { type Target = T; fn deref(&self) -> &T { Lazy::force(self) } } } #[cfg(feature = "std")] pub mod sync { use crate::imp::OnceCell as Imp; use std::{fmt, cell::UnsafeCell, hint::unreachable_unchecked}; /// A thread-safe cell which can be written to only once. /// /// Unlike `::std::sync::Mutex`, a `OnceCell` provides simple `&` /// references to the contents. /// /// # 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> { let res = OnceCell::new(); if let Some(value) = self.get() { match res.set(value.clone()) { Ok(()) => (), Err(_) => unreachable!(), } } res } } impl<T> From<T> for OnceCell<T> { fn from(value: T) -> Self { let cell = Self::new(); cell.get_or_init(|| value); cell } } impl<T: PartialEq> PartialEq for OnceCell<T> { fn eq(&self, other: &OnceCell<T>) -> bool { self.get() == other.get() } } impl<T> OnceCell<T> { /// Creates a new empty cell. pub const fn new() -> OnceCell<T> { OnceCell(Imp::new()) } /// Gets the reference to the underlying value. Returns `None` /// if the cell is empty, or being initialized. This method does /// not block. pub fn get(&self) -> Option<&T> { self.0.get() } /// 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> { let mut value = Some(value); self.get_or_init(|| value.take().unwrap()); match value { None => Ok(()), Some(value) => Err(value), } } /// Gets the contents of the cell, initializing it with `f` /// if the cell was empty. May 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>, { self.0.get_or_try_init(f) } /// 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())); /// ``` 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 /// ``` /// extern crate once_cell; /// /// 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: UnsafeCell<Option<F>>, } impl<T: fmt::Debug, F: fmt::Debug> 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 // We do create a `&T` from `&Lazy<T, F>`, so `T` needs `Sync` unsafe impl<T: Sync, F: Send> Sync for Lazy<T, F> {} 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: UnsafeCell::new(Some(f)) } } } impl<T, F: FnOnce() -> T> Lazy<T, F> { /// Forces the evaluation of this lazy value and /// returns a reference to 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 { // Safe because closure is guaranteed to be called at most once // so we only call `F` once, this also guarantees no race conditions this.cell.get_or_init(|| unsafe { match (*this.init.get()).take() { Some(f) => f(), None => unreachable_unchecked(), } }) } } impl<T, F: FnOnce() -> T> ::std::ops::Deref for Lazy<T, F> { type Target = T; fn deref(&self) -> &T { Lazy::force(self) } } }