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+// Original work Copyright (c) 2014 The Rust Project Developers
+// Modified work Copyright (c) 2016-2018 Nikita Pekin and the lazycell contributors
+// See the README.md file at the top-level directory of this distribution.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#![deny(missing_docs)]
+#![allow(unused)]
+
+//! This crate provides a `LazyCell` struct which acts as a lazily filled
+//! `Cell`.
+//!
+//! With a `RefCell`, the inner contents cannot be borrowed for the lifetime of
+//! the entire object, but only of the borrows returned. A `LazyCell` is a
+//! variation on `RefCell` which allows borrows to be tied to the lifetime of
+//! the outer object.
+//!
+//! `AtomicLazyCell` is a variant that uses an atomic variable to manage
+//! coordination in a thread-safe fashion. The limitation of an `AtomicLazyCell`
+//! is that after it is initialized, it can't be modified.
+
+use std::cell::UnsafeCell;
+use std::mem;
+use std::sync::atomic::{AtomicUsize, Ordering};
+
+/// A lazily filled `Cell`, with mutable contents.
+///
+/// A `LazyCell` is completely frozen once filled, **unless** you have `&mut`
+/// access to it, in which case `LazyCell::borrow_mut` may be used to mutate the
+/// contents.
+#[derive(Debug, Default)]
+pub struct LazyCell<T> {
+ inner: UnsafeCell<Option<T>>,
+}
+
+impl<T> LazyCell<T> {
+ /// Creates a new, empty, `LazyCell`.
+ pub fn new() -> LazyCell<T> {
+ LazyCell { inner: UnsafeCell::new(None) }
+ }
+
+ /// Put a value into this cell.
+ ///
+ /// This function will return `Err(value)` is the cell is already full.
+ pub fn fill(&self, value: T) -> Result<(), T> {
+ let slot = unsafe { &mut *self.inner.get() };
+ if slot.is_some() {
+ return Err(value);
+ }
+ *slot = Some(value);
+
+ Ok(())
+ }
+
+ /// Put a value into this cell.
+ ///
+ /// Note that this function is infallible but requires `&mut self`. By
+ /// requiring `&mut self` we're guaranteed that no active borrows to this
+ /// cell can exist so we can always fill in the value. This may not always
+ /// be usable, however, as `&mut self` may not be possible to borrow.
+ ///
+ /// # Return value
+ ///
+ /// This function returns the previous value, if any.
+ pub fn replace(&mut self, value: T) -> Option<T> {
+ mem::replace(unsafe { &mut *self.inner.get() }, Some(value))
+ }
+
+ /// Test whether this cell has been previously filled.
+ pub fn filled(&self) -> bool {
+ self.borrow().is_some()
+ }
+
+ /// Borrows the contents of this lazy cell for the duration of the cell
+ /// itself.
+ ///
+ /// This function will return `Some` if the cell has been previously
+ /// initialized, and `None` if it has not yet been initialized.
+ pub fn borrow(&self) -> Option<&T> {
+ unsafe { &*self.inner.get() }.as_ref()
+ }
+
+ /// Borrows the contents of this lazy cell mutably for the duration of the cell
+ /// itself.
+ ///
+ /// This function will return `Some` if the cell has been previously
+ /// initialized, and `None` if it has not yet been initialized.
+ pub fn borrow_mut(&mut self) -> Option<&mut T> {
+ unsafe { &mut *self.inner.get() }.as_mut()
+ }
+
+ /// Borrows the contents of this lazy cell for the duration of the cell
+ /// itself.
+ ///
+ /// If the cell has not yet been filled, the cell is first filled using the
+ /// function provided.
+ ///
+ /// # Panics
+ ///
+ /// Panics if the cell becomes filled as a side effect of `f`.
+ pub fn borrow_with<F: FnOnce() -> T>(&self, f: F) -> &T {
+ if let Some(value) = self.borrow() {
+ return value;
+ }
+ let value = f();
+ if self.fill(value).is_err() {
+ panic!("borrow_with: cell was filled by closure")
+ }
+ self.borrow().unwrap()
+ }
+
+ /// Borrows the contents of this `LazyCell` mutably for the duration of the
+ /// cell itself.
+ ///
+ /// If the cell has not yet been filled, the cell is first filled using the
+ /// function provided.
+ ///
+ /// # Panics
+ ///
+ /// Panics if the cell becomes filled as a side effect of `f`.
+ pub fn borrow_mut_with<F: FnOnce() -> T>(&mut self, f: F) -> &mut T {
+ if !self.filled() {
+ let value = f();
+ if self.fill(value).is_err() {
+ panic!("borrow_mut_with: cell was filled by closure")
+ }
+ }
+
+ self.borrow_mut().unwrap()
+ }
+
+ /// Same as `borrow_with`, but allows the initializing function to fail.
+ ///
+ /// # Panics
+ ///
+ /// Panics if the cell becomes filled as a side effect of `f`.
+ pub fn try_borrow_with<E, F>(&self, f: F) -> Result<&T, E>
+ where F: FnOnce() -> Result<T, E>
+ {
+ if let Some(value) = self.borrow() {
+ return Ok(value);
+ }
+ let value = f()?;
+ if self.fill(value).is_err() {
+ panic!("try_borrow_with: cell was filled by closure")
+ }
+ Ok(self.borrow().unwrap())
+ }
+
+ /// Same as `borrow_mut_with`, but allows the initializing function to fail.
+ ///
+ /// # Panics
+ ///
+ /// Panics if the cell becomes filled as a side effect of `f`.
+ pub fn try_borrow_mut_with<E, F>(&mut self, f: F) -> Result<&mut T, E>
+ where F: FnOnce() -> Result<T, E>
+ {
+ if self.filled() {
+ return Ok(self.borrow_mut().unwrap());
+ }
+ let value = f()?;
+ if self.fill(value).is_err() {
+ panic!("try_borrow_mut_with: cell was filled by closure")
+ }
+ Ok(self.borrow_mut().unwrap())
+ }
+
+ /// Consumes this `LazyCell`, returning the underlying value.
+ pub fn into_inner(self) -> Option<T> {
+ // Rust 1.25 changed UnsafeCell::into_inner() from unsafe to safe
+ // function. This unsafe can be removed when supporting Rust older than
+ // 1.25 is not needed.
+ #[allow(unused_unsafe)]
+ unsafe { self.inner.into_inner() }
+ }
+}
+
+impl<T: Copy> LazyCell<T> {
+ /// Returns a copy of the contents of the lazy cell.
+ ///
+ /// This function will return `Some` if the cell has been previously initialized,
+ /// and `None` if it has not yet been initialized.
+ pub fn get(&self) -> Option<T> {
+ unsafe { *self.inner.get() }
+ }
+}
+
+// Tracks the AtomicLazyCell inner state
+const NONE: usize = 0;
+const LOCK: usize = 1;
+const SOME: usize = 2;
+
+/// A lazily filled and thread-safe `Cell`, with frozen contents.
+#[derive(Debug, Default)]
+pub struct AtomicLazyCell<T> {
+ inner: UnsafeCell<Option<T>>,
+ state: AtomicUsize,
+}
+
+impl<T> AtomicLazyCell<T> {
+ /// Creates a new, empty, `AtomicLazyCell`.
+ pub fn new() -> AtomicLazyCell<T> {
+ Self {
+ inner: UnsafeCell::new(None),
+ state: AtomicUsize::new(NONE),
+ }
+ }
+
+ /// Put a value into this cell.
+ ///
+ /// This function will return `Err(value)` is the cell is already full.
+ pub fn fill(&self, t: T) -> Result<(), T> {
+ if NONE != self.state.compare_and_swap(NONE, LOCK, Ordering::Acquire) {
+ return Err(t);
+ }
+
+ unsafe { *self.inner.get() = Some(t) };
+
+ if LOCK != self.state.compare_and_swap(LOCK, SOME, Ordering::Release) {
+ panic!("unable to release lock");
+ }
+
+ Ok(())
+ }
+
+ /// Put a value into this cell.
+ ///
+ /// Note that this function is infallible but requires `&mut self`. By
+ /// requiring `&mut self` we're guaranteed that no active borrows to this
+ /// cell can exist so we can always fill in the value. This may not always
+ /// be usable, however, as `&mut self` may not be possible to borrow.
+ ///
+ /// # Return value
+ ///
+ /// This function returns the previous value, if any.
+ pub fn replace(&mut self, value: T) -> Option<T> {
+ match mem::replace(self.state.get_mut(), SOME) {
+ NONE | SOME => {}
+ _ => panic!("cell in inconsistent state"),
+ }
+ mem::replace(unsafe { &mut *self.inner.get() }, Some(value))
+ }
+
+ /// Test whether this cell has been previously filled.
+ pub fn filled(&self) -> bool {
+ self.state.load(Ordering::Acquire) == SOME
+ }
+
+ /// Borrows the contents of this lazy cell for the duration of the cell
+ /// itself.
+ ///
+ /// This function will return `Some` if the cell has been previously
+ /// initialized, and `None` if it has not yet been initialized.
+ pub fn borrow(&self) -> Option<&T> {
+ match self.state.load(Ordering::Acquire) {
+ SOME => unsafe { &*self.inner.get() }.as_ref(),
+ _ => None,
+ }
+ }
+
+ /// Consumes this `LazyCell`, returning the underlying value.
+ pub fn into_inner(self) -> Option<T> {
+ // Rust 1.25 changed UnsafeCell::into_inner() from unsafe to safe
+ // function. This unsafe can be removed when supporting Rust older than
+ // 1.25 is not needed.
+ #[allow(unused_unsafe)]
+ unsafe { self.inner.into_inner() }
+ }
+}
+
+impl<T: Copy> AtomicLazyCell<T> {
+ /// Returns a copy of the contents of the lazy cell.
+ ///
+ /// This function will return `Some` if the cell has been previously initialized,
+ /// and `None` if it has not yet been initialized.
+ pub fn get(&self) -> Option<T> {
+ match self.state.load(Ordering::Acquire) {
+ SOME => unsafe { *self.inner.get() },
+ _ => None,
+ }
+ }
+}
+
+unsafe impl<T: Sync + Send> Sync for AtomicLazyCell<T> {}
+
+unsafe impl<T: Send> Send for AtomicLazyCell<T> {}
+
+#[cfg(test)]
+mod tests {
+ use super::{AtomicLazyCell, LazyCell};
+
+ #[test]
+ fn test_borrow_from_empty() {
+ let lazycell: LazyCell<usize> = LazyCell::new();
+
+ let value = lazycell.borrow();
+ assert_eq!(value, None);
+
+ let value = lazycell.get();
+ assert_eq!(value, None);
+ }
+
+ #[test]
+ fn test_fill_and_borrow() {
+ let lazycell = LazyCell::new();
+
+ assert!(!lazycell.filled());
+ lazycell.fill(1).unwrap();
+ assert!(lazycell.filled());
+
+ let value = lazycell.borrow();
+ assert_eq!(value, Some(&1));
+
+ let value = lazycell.get();
+ assert_eq!(value, Some(1));
+ }
+
+ #[test]
+ fn test_borrow_mut() {
+ let mut lazycell = LazyCell::new();
+ assert!(lazycell.borrow_mut().is_none());
+
+ lazycell.fill(1).unwrap();
+ assert_eq!(lazycell.borrow_mut(), Some(&mut 1));
+
+ *lazycell.borrow_mut().unwrap() = 2;
+ assert_eq!(lazycell.borrow_mut(), Some(&mut 2));
+
+ // official way to reset the cell
+ lazycell = LazyCell::new();
+ assert!(lazycell.borrow_mut().is_none());
+ }
+
+ #[test]
+ fn test_already_filled_error() {
+ let lazycell = LazyCell::new();
+
+ lazycell.fill(1).unwrap();
+ assert_eq!(lazycell.fill(1), Err(1));
+ }
+
+ #[test]
+ fn test_borrow_with() {
+ let lazycell = LazyCell::new();
+
+ let value = lazycell.borrow_with(|| 1);
+ assert_eq!(&1, value);
+ }
+
+ #[test]
+ fn test_borrow_with_already_filled() {
+ let lazycell = LazyCell::new();
+ lazycell.fill(1).unwrap();
+
+ let value = lazycell.borrow_with(|| 1);
+ assert_eq!(&1, value);
+ }
+
+ #[test]
+ fn test_borrow_with_not_called_when_filled() {
+ let lazycell = LazyCell::new();
+
+ lazycell.fill(1).unwrap();
+
+ let value = lazycell.borrow_with(|| 2);
+ assert_eq!(&1, value);
+ }
+
+ #[test]
+ #[should_panic]
+ fn test_borrow_with_sound_with_reentrancy() {
+ // Kudos to dbaupp for discovering this issue
+ // https://www.reddit.com/r/rust/comments/5vs9rt/lazycell_a_rust_library_providing_a_lazilyfilled/de527xm/
+ let lazycell: LazyCell<Box<i32>> = LazyCell::new();
+
+ let mut reference: Option<&i32> = None;
+
+ lazycell.borrow_with(|| {
+ let _ = lazycell.fill(Box::new(1));
+ reference = lazycell.borrow().map(|r| &**r);
+ Box::new(2)
+ });
+ }
+
+ #[test]
+ fn test_borrow_mut_with() {
+ let mut lazycell = LazyCell::new();
+
+ {
+ let value = lazycell.borrow_mut_with(|| 1);
+ assert_eq!(&mut 1, value);
+ *value = 2;
+ }
+ assert_eq!(&2, lazycell.borrow().unwrap());
+ }
+
+ #[test]
+ fn test_borrow_mut_with_already_filled() {
+ let mut lazycell = LazyCell::new();
+ lazycell.fill(1).unwrap();
+
+ let value = lazycell.borrow_mut_with(|| 1);
+ assert_eq!(&1, value);
+ }
+
+ #[test]
+ fn test_borrow_mut_with_not_called_when_filled() {
+ let mut lazycell = LazyCell::new();
+
+ lazycell.fill(1).unwrap();
+
+ let value = lazycell.borrow_mut_with(|| 2);
+ assert_eq!(&1, value);
+ }
+
+ #[test]
+ fn test_try_borrow_with_ok() {
+ let lazycell = LazyCell::new();
+ let result = lazycell.try_borrow_with::<(), _>(|| Ok(1));
+ assert_eq!(result, Ok(&1));
+ }
+
+ #[test]
+ fn test_try_borrow_with_err() {
+ let lazycell = LazyCell::<()>::new();
+ let result = lazycell.try_borrow_with(|| Err(1));
+ assert_eq!(result, Err(1));
+ }
+
+ #[test]
+ fn test_try_borrow_with_already_filled() {
+ let lazycell = LazyCell::new();
+ lazycell.fill(1).unwrap();
+ let result = lazycell.try_borrow_with::<(), _>(|| unreachable!());
+ assert_eq!(result, Ok(&1));
+ }
+
+ #[test]
+ #[should_panic]
+ fn test_try_borrow_with_sound_with_reentrancy() {
+ let lazycell: LazyCell<Box<i32>> = LazyCell::new();
+
+ let mut reference: Option<&i32> = None;
+
+ let _ = lazycell.try_borrow_with::<(), _>(|| {
+ let _ = lazycell.fill(Box::new(1));
+ reference = lazycell.borrow().map(|r| &**r);
+ Ok(Box::new(2))
+ });
+ }
+
+ #[test]
+ fn test_try_borrow_mut_with_ok() {
+ let mut lazycell = LazyCell::new();
+ {
+ let result = lazycell.try_borrow_mut_with::<(), _>(|| Ok(1));
+ assert_eq!(result, Ok(&mut 1));
+ *result.unwrap() = 2;
+ }
+ assert_eq!(&mut 2, lazycell.borrow().unwrap());
+ }
+
+ #[test]
+ fn test_try_borrow_mut_with_err() {
+ let mut lazycell = LazyCell::<()>::new();
+ let result = lazycell.try_borrow_mut_with(|| Err(1));
+ assert_eq!(result, Err(1));
+ }
+
+ #[test]
+ fn test_try_borrow_mut_with_already_filled() {
+ let mut lazycell = LazyCell::new();
+ lazycell.fill(1).unwrap();
+ let result = lazycell.try_borrow_mut_with::<(), _>(|| unreachable!());
+ assert_eq!(result, Ok(&mut 1));
+ }
+
+ #[test]
+ fn test_into_inner() {
+ let lazycell = LazyCell::new();
+
+ lazycell.fill(1).unwrap();
+ let value = lazycell.into_inner();
+ assert_eq!(value, Some(1));
+ }
+
+ #[test]
+ fn test_atomic_borrow_from_empty() {
+ let lazycell: AtomicLazyCell<usize> = AtomicLazyCell::new();
+
+ let value = lazycell.borrow();
+ assert_eq!(value, None);
+
+ let value = lazycell.get();
+ assert_eq!(value, None);
+ }
+
+ #[test]
+ fn test_atomic_fill_and_borrow() {
+ let lazycell = AtomicLazyCell::new();
+
+ assert!(!lazycell.filled());
+ lazycell.fill(1).unwrap();
+ assert!(lazycell.filled());
+
+ let value = lazycell.borrow();
+ assert_eq!(value, Some(&1));
+
+ let value = lazycell.get();
+ assert_eq!(value, Some(1));
+ }
+
+ #[test]
+ fn test_atomic_already_filled_panic() {
+ let lazycell = AtomicLazyCell::new();
+
+ lazycell.fill(1).unwrap();
+ assert_eq!(1, lazycell.fill(1).unwrap_err());
+ }
+
+ #[test]
+ fn test_atomic_into_inner() {
+ let lazycell = AtomicLazyCell::new();
+
+ lazycell.fill(1).unwrap();
+ let value = lazycell.into_inner();
+ assert_eq!(value, Some(1));
+ }
+
+ #[test]
+ fn normal_replace() {
+ let mut cell = LazyCell::new();
+ assert_eq!(cell.fill(1), Ok(()));
+ assert_eq!(cell.replace(2), Some(1));
+ assert_eq!(cell.replace(3), Some(2));
+ assert_eq!(cell.borrow(), Some(&3));
+
+ let mut cell = LazyCell::new();
+ assert_eq!(cell.replace(2), None);
+ }
+
+ #[test]
+ fn atomic_replace() {
+ let mut cell = AtomicLazyCell::new();
+ assert_eq!(cell.fill(1), Ok(()));
+ assert_eq!(cell.replace(2), Some(1));
+ assert_eq!(cell.replace(3), Some(2));
+ assert_eq!(cell.borrow(), Some(&3));
+ }
+}
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