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#[allow(warnings)]
pub mod state {
use core::fmt;
use std::{cell::UnsafeCell, mem};
pub struct State<T> {
// Invariant: written to at most once.
pub inner: UnsafeCell<Option<T>>,
}
impl<T> State<T> {
/// Creates a new empty state.
#[must_use]
pub const fn new() -> State<T> {
State {
inner: UnsafeCell::new(None),
}
}
/// Gets the reference to the underlying value.
///
/// Returns `None` if the state is empty.
pub fn get(&self) -> Option<&T> {
// SAFETY: Safe due to `inner`'s invariant
unsafe { &*self.inner.get() }.as_ref()
}
/// Gets the mutable reference to the underlying value.
///
/// Returns `None` if the state is empty.
pub fn get_mut(&mut self) -> Option<&mut T> {
self.inner.get_mut().as_mut()
}
/// Sets the contents of the state to `value`.
///
/// # Errors
///
/// This method returns `Ok(())` if the state was seted
///
/// # Examples
///
/// ```rust
/// use zoon::State;
/// let state = State::new();
/// assert!(state.get().is_none());
///
/// assert_eq!(state.set(92), Ok(()));
/// assert_eq!(state.set(920), Ok(()));
///
/// assert!(state.get().is_some());
/// ```
pub fn set(&self, value: T) -> Result<(), T> {
// SAFETY: Safe because we cannot have overlapping mutable borrows
let slot = unsafe { &*self.inner.get() };
// if slot.is_some() {
// return Err(value);
// }
// SAFETY: 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.
let slot = unsafe { &mut *self.inner.get() };
*slot = Some(value);
Ok(())
}
/// Gets the contents of the state, initializing it with `f`
/// if the state was empty.
///
/// # Panics
///
/// If `f` panics, the panic is propagated to the caller, and the state
/// remains uninitialized.
///
/// It is an error to reentrantly initialize the state from `f`. Doing
/// so results in a panic.
///
/// # Examples
///
/// ```rust
/// use zoon::State;
/// let state = State::new();
/// let value = state.get_or_init(|| 92);
/// assert_eq!(value, &92);
/// let value = state.get_or_init(|| unreachable!());
/// assert_eq!(value, &92);
/// ```
pub fn get_or_init<F>(&self, f: F) -> &T
where
F: FnOnce() -> T,
{
match self.get_or_try_init(|| Ok::<T, !>(f())) {
Ok(val) => val,
Err(e) => e,
}
}
/// Gets the contents of the state, initializing it with `f` if
/// the state was empty. If the state was empty and `f` failed, an
/// error is returned.
///
/// # Panics
///
/// If `f` panics, the panic is propagated to the caller, and the state
/// remains uninitialized.
///
/// It is an error to reentrantly initialize the state from `f`. Doing
/// so results in a panic.
///
/// # Examples
///
/// ```rust
/// use zoon::State;
/// let state = State::new();
/// assert_eq!(state.get_or_try_init(|| Err(())), Err(()));
/// assert!(state.get().is_none());
/// let value = state.get_or_try_init(|| -> Result<i32, ()> {
/// Ok(92)
/// });
/// assert_eq!(value, Ok(&92));
/// assert_eq!(state.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);
}
/// Avoid inlining the initialization closure into the common path that fetches
/// the already initialized value
#[cold]
fn outlined_call<F, T, E>(f: F) -> Result<T, E>
where
F: FnOnce() -> Result<T, E>,
{
f()
}
let val = outlined_call(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(self.get().unwrap())
}
/// Consumes the state, returning the wrapped value.
///
/// Returns `None` if the state was empty.
///
/// # Examples
///
/// ```rust
/// use zoon::State;
/// let state: State<String> = State::new();
/// assert_eq!(state.into_inner(), None);
///
/// let state = State::new();
/// state.set("hello".to_string()).unwrap();
/// assert_eq!(state.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()
}
/// Takes the value out of this `State`, moving it back to an uninitialized state.
///
/// Has no effect and returns `None` if the `State` hasn't been initialized.
///
/// Safety is guaranteed by requiring a mutable reference.
///
/// # Examples
///
/// ```rust
/// use zoon::State;
/// let mut state: State<String> = State::new();
/// assert_eq!(state.take(), None);
///
/// let mut state = State::new();
/// state.set("hello".to_string()).unwrap();
/// assert_eq!(state.take(), Some("hello".to_string()));
/// assert_eq!(state.get(), None);
/// ```
pub fn take(&mut self) -> Option<T> {
mem::take(self).into_inner()
}
}
impl<T> Default for State<T> {
fn default() -> Self {
Self::new()
}
}
impl<T: fmt::Debug> fmt::Debug for State<T> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
match self.get() {
Some(v) => f.debug_tuple("State").field(v).finish(),
None => f.write_str("State(Uninit)"),
}
}
}
impl<T: Clone> Clone for State<T> {
fn clone(&self) -> State<T> {
let res = State::new();
if let Some(value) = self.get() {
match res.set(value.clone()) {
Ok(()) => (),
Err(_) => unreachable!(),
}
}
res
}
}
impl<T: PartialEq> PartialEq for State<T> {
fn eq(&self, other: &Self) -> bool {
self.get() == other.get()
}
}
impl<T: Eq> Eq for State<T> {}
impl<T> const From<T> for State<T> {
/// Creates a new `State<T>` which already contains the given `value`.
fn from(value: T) -> Self {
State {
inner: UnsafeCell::new(Some(value)),
}
}
}
}