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//! This crate provides the [`take_static`] macro to create statics that provide mutable access only once:
//!
//! ```
//! use take_static::take_static;
//!
//! take_static! {
//! static NUMBER: usize = 5;
//! }
//!
//! assert_eq!(NUMBER.take(), Some(&mut 5));
//! assert_eq!(NUMBER.take(), None);
//! ```
//!
//! The resulting statics are [`TakeStatic`].
#![no_std]
use core::cell::UnsafeCell;
use core::fmt;
use call_once::CallOnce;
/// A synchronization primitive which can be accessed only once.
///
/// This type is a thread-safe cell that can only be used in statics.
/// [`TakeStatic::take`] provides a mutable reference to the contents without RAII guards, but only on the first try.
///
/// # Similarities with `takecell::TakeCell`
///
/// `TakeStatic` is similar to [`takecell::TakeCell`] in that both can be used to create singletons.
/// Additionally, `TakeCell`s can be created at runtime, making them suitable for other use-cases such as [doing work only once].
/// `TakeStatic`, on the other hand, is specialized to creating statics through [`take_static`], allowing even `!Send` types to be used and not requiring explicitly naming the wrapper type.
///
/// [`takecell::TakeCell`]: https://docs.rs/takecell/0.1.1/takecell/struct.TakeCell.html
/// [doing work only once]: https://docs.rs/takecell/0.1.1/takecell/index.html#doing-work-only-once
///
/// # Similarities with `cortex_m::singleton`
///
/// `TakeStatic` can be used similarly to [`cortex_m::singleton`] to create a mutable reference to a statically allocated value.
/// In contrast to `cortex_m::singleton`, `TakeStatic` is thread safe.
///
/// [`cortex_m::singleton`]: https://docs.rs/cortex-m/0.7.7/cortex_m/macro.singleton.html
///
/// # Examples
///
/// ```
/// use take_static::take_static;
///
/// take_static! {
/// static NUMBER: usize = 5;
/// }
///
/// assert_eq!(NUMBER.take(), Some(&mut 5));
/// assert_eq!(NUMBER.take(), None);
/// ```
pub struct TakeStatic<T: ?Sized> {
taken: CallOnce,
data: UnsafeCell<T>,
}
impl<T: ?Sized> fmt::Debug for TakeStatic<T> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
f.debug_struct("TakeStatic")
.field("taken", &self.taken)
.field("data", &&self.data)
.finish()
}
}
// SAFETY: This is safe even without `Send` bound,
// as `TakeStatics` can only ever be constructed at compile-time for statics.
// The consteval “thread” is not considered part of the `Send` contract.
unsafe impl<T: ?Sized> Sync for TakeStatic<T> {}
/// Declare a new static that provides mutable access—but only once.
///
/// # Syntax
///
/// The macro wraps any number of static declarations and makes them [`TakeStatic`].
/// Publicity and attributes for each static are allowed. Example:
///
/// ```
/// use take_static::take_static;
///
/// take_static! {
/// pub static FOO: u32 = 1;
///
/// static BAR: f32 = 1.0;
/// }
///
/// assert_eq!(FOO.take(), Some(&mut 1));
/// assert_eq!(BAR.take(), Some(&mut 1.0));
/// ```
///
/// See [`TakeStatic`] documentation for more information.
// Modeled after `thread_local` and `lazy_static`.
#[macro_export]
macro_rules! take_static {
// empty (base case for the recursion)
() => {};
// process multiple declarations
($(#[$attr:meta])* $vis:vis static $name:ident: $t:ty = $init:expr; $($rest:tt)*) => (
$crate::take_static!($(#[$attr])* $vis static $name: $t = $init);
$crate::take_static!($($rest)*);
);
// handle a single declaration
($(#[$attr:meta])* $vis:vis static $name:ident: $t:ty = $init:expr) => (
$(#[$attr])* $vis static $name: $crate::TakeStatic<$t> = {
const INIT_EXPR: $t = $init;
// SAFETY: this initializes a static.
unsafe { $crate::TakeStatic::new(INIT_EXPR) }
};
);
}
impl<T> TakeStatic<T> {
/// Creates a new `TakeStatic` containing the given value.
///
/// # Examples
///
/// ```
/// use take_static::TakeStatic;
///
/// static TAKE_STATIC: TakeStatic<usize> = unsafe { TakeStatic::new(5) };
/// ```
///
/// # Safety
///
/// This function may only be used to initialize a static item.
/// This is due to the unconditional `Sync` impl of `TakeStatic`.
#[doc(hidden)]
#[inline]
pub const unsafe fn new(val: T) -> Self {
Self {
taken: CallOnce::new(),
data: UnsafeCell::new(val),
}
}
}
impl<T: ?Sized> TakeStatic<T> {
/// Takes the mutable reference to the wrapped value.
///
/// Only the first call returns `Some`.
/// All subsequent calls return `None`.
///
/// # Examples
///
/// ```
/// use take_static::take_static;
///
/// take_static! {
/// static TAKE_STATIC: usize = 5;
/// }
///
/// let number = TAKE_STATIC.take().unwrap();
/// assert_eq!(number, &mut 5);
/// assert!(TAKE_STATIC.take().is_none());
/// ```
#[inline]
#[must_use]
pub fn take(&self) -> Option<&mut T> {
self.taken
.call_once()
.ok()
.map(|_| unsafe { &mut *self.data.get() })
}
/// Returns `true` if the mutable reference has been taken.
///
/// # Examples
///
/// ```
/// use take_static::take_static;
///
/// take_static! {
/// static TAKE_STATIC: usize = 5;
/// }
///
/// assert!(!TAKE_STATIC.is_taken());
/// let number = TAKE_STATIC.take().unwrap();
/// assert!(TAKE_STATIC.is_taken());
/// ```
#[inline]
pub fn is_taken(&self) -> bool {
self.taken.was_called()
}
}
#[cfg(test)]
mod tests {
use super::*;
#[test]
fn visibility() {
take_static! {
static SELF: u8 = 3;
}
mod module {
use super::*;
take_static! {
static _SELF: u8 = 3;
pub(super) static SUPER: u8 = 3;
pub(crate) static CRATE: u8 = 3;
pub static PUB: u8 = 3;
}
}
assert_eq!(SELF.take(), Some(&mut 3));
assert_eq!(module::SUPER.take(), Some(&mut 3));
assert_eq!(module::CRATE.take(), Some(&mut 3));
assert_eq!(module::PUB.take(), Some(&mut 3));
}
#[test]
fn always_sync() {
use core::ptr;
take_static! {
static FOO: *mut u8 = ptr::null_mut();
}
assert_eq!(FOO.take(), Some(&mut ptr::null_mut()));
}
#[test]
fn init_block() {
take_static! {
static FOO: u8 = {
let value = 1 + 2;
value
};
}
assert_eq!(FOO.take(), Some(&mut 3));
}
#[test]
fn single_declaration() {
take_static!(static FOO: u8 = 3);
assert_eq!(FOO.take(), Some(&mut 3));
}
}