// Original work Copyright (c) 2014 The Rust Project Developers
// Modified work Copyright (c) 2016-2020 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.
#![cfg_attr(not(test), no_std)]
#![deny(missing_docs)]
#![cfg_attr(feature = "nightly", feature(plugin))]
#![cfg_attr(feature = "clippy", plugin(clippy))]
//! 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.
//!
//! # Example
//!
//! The following example shows a quick example of the basic functionality of
//! `LazyCell`.
//!
//! ```
//! use lazycell::LazyCell;
//!
//! let lazycell = LazyCell::new();
//!
//! assert_eq!(lazycell.borrow(), None);
//! assert!(!lazycell.filled());
//! lazycell.fill(1).ok();
//! assert!(lazycell.filled());
//! assert_eq!(lazycell.borrow(), Some(&1));
//! assert_eq!(lazycell.into_inner(), Some(1));
//! ```
//!
//! `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.
#[cfg(not(test))]
#[macro_use]
extern crate core as std;
#[cfg(feature = "serde")]
extern crate serde;
#[cfg(feature = "serde")]
mod serde_impl;
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)]
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)` if the cell is already full.
pub fn fill(&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() };
*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() }
}
}
impl<T> Default for LazyCell<T> {
fn default() -> Self {
Self::new()
}
}
impl <T: Clone> Clone for LazyCell<T> {
/// Create a clone of this `LazyCell`
///
/// If self has not been initialized, returns an uninitialized `LazyCell`
/// otherwise returns a `LazyCell` already initialized with a clone of the
/// contents of self.
fn clone(&self) -> LazyCell<T> {
LazyCell { inner: UnsafeCell::new(self.borrow().map(Clone::clone) ) }
}
}
// 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)]
pub struct AtomicLazyCell<T> {
inner: UnsafeCell<Option<T>>,
state: AtomicUsize,
}
impl<T> AtomicLazyCell<T> {
/// An empty `AtomicLazyCell`.
pub const NONE: Self = Self {
inner: UnsafeCell::new(None),
state: AtomicUsize::new(NONE),
};
/// Creates a new, empty, `AtomicLazyCell`.
pub fn new() -> AtomicLazyCell<T> {
Self::NONE
}
/// Put a value into this cell.
///
/// This function will return `Err(value)` if 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,
}
}
}
impl<T> Default for AtomicLazyCell<T> {
fn default() -> Self {
Self::new()
}
}
impl<T: Clone> Clone for AtomicLazyCell<T> {
/// Create a clone of this `AtomicLazyCell`
///
/// If self has not been initialized, returns an uninitialized `AtomicLazyCell`
/// otherwise returns an `AtomicLazyCell` already initialized with a clone of the
/// contents of self.
fn clone(&self) -> AtomicLazyCell<T> {
self.borrow().map_or(
Self::NONE,
|v| AtomicLazyCell {
inner: UnsafeCell::new(Some(v.clone())),
state: AtomicUsize::new(SOME),
}
)
}
}
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));
}
#[test]
fn clone() {
let mut cell = LazyCell::new();
let clone1 = cell.clone();
assert_eq!(clone1.borrow(), None);
assert_eq!(cell.fill(1), Ok(()));
let mut clone2 = cell.clone();
assert_eq!(clone1.borrow(), None);
assert_eq!(clone2.borrow(), Some(&1));
assert_eq!(cell.replace(2), Some(1));
assert_eq!(clone1.borrow(), None);
assert_eq!(clone2.borrow(), Some(&1));
assert_eq!(clone1.fill(3), Ok(()));
assert_eq!(clone2.replace(4), Some(1));
assert_eq!(clone1.borrow(), Some(&3));
assert_eq!(clone2.borrow(), Some(&4));
assert_eq!(cell.borrow(), Some(&2));
}
#[test]
fn clone_atomic() {
let mut cell = AtomicLazyCell::new();
let clone1 = cell.clone();
assert_eq!(clone1.borrow(), None);
assert_eq!(cell.fill(1), Ok(()));
let mut clone2 = cell.clone();
assert_eq!(clone1.borrow(), None);
assert_eq!(clone2.borrow(), Some(&1));
assert_eq!(cell.replace(2), Some(1));
assert_eq!(clone1.borrow(), None);
assert_eq!(clone2.borrow(), Some(&1));
assert_eq!(clone1.fill(3), Ok(()));
assert_eq!(clone2.replace(4), Some(1));
assert_eq!(clone1.borrow(), Some(&3));
assert_eq!(clone2.borrow(), Some(&4));
assert_eq!(cell.borrow(), Some(&2));
}
#[test]
fn default() {
#[derive(Default)]
struct Defaultable;
struct NonDefaultable;
let _: LazyCell<Defaultable> = LazyCell::default();
let _: LazyCell<NonDefaultable> = LazyCell::default();
let _: AtomicLazyCell<Defaultable> = AtomicLazyCell::default();
let _: AtomicLazyCell<NonDefaultable> = AtomicLazyCell::default();
}
}
