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use crate::cell::Cell;
use crate::sync as public;
pub struct Once {
state: Cell<State>,
}
pub struct OnceState {
poisoned: bool,
set_state_to: Cell<State>,
}
#[derive(Clone, Copy, PartialEq, Eq)]
enum State {
Incomplete,
Poisoned,
Running,
Complete,
}
struct CompletionGuard<'a> {
state: &'a Cell<State>,
set_state_on_drop_to: State,
}
impl<'a> Drop for CompletionGuard<'a> {
fn drop(&mut self) {
self.state.set(self.set_state_on_drop_to);
}
}
// Safety: threads are not supported on this platform.
unsafe impl Sync for Once {}
impl Once {
#[inline]
#[rustc_const_stable(feature = "const_once_new", since = "1.32.0")]
pub const fn new() -> Once {
Once { state: Cell::new(State::Incomplete) }
}
#[inline]
pub fn is_completed(&self) -> bool {
self.state.get() == State::Complete
}
#[cold]
#[track_caller]
pub fn call(&self, ignore_poisoning: bool, f: &mut impl FnMut(&public::OnceState)) {
let state = self.state.get();
match state {
State::Poisoned if !ignore_poisoning => {
// Panic to propagate the poison.
panic!("Once instance has previously been poisoned");
}
State::Incomplete | State::Poisoned => {
self.state.set(State::Running);
// `guard` will set the new state on drop.
let mut guard =
CompletionGuard { state: &self.state, set_state_on_drop_to: State::Poisoned };
// Run the function, letting it know if we're poisoned or not.
let f_state = public::OnceState {
inner: OnceState {
poisoned: state == State::Poisoned,
set_state_to: Cell::new(State::Complete),
},
};
f(&f_state);
guard.set_state_on_drop_to = f_state.inner.set_state_to.get();
}
State::Running => {
panic!("one-time initialization may not be performed recursively");
}
State::Complete => {}
}
}
}
impl OnceState {
#[inline]
pub fn is_poisoned(&self) -> bool {
self.poisoned
}
#[inline]
pub fn poison(&self) {
self.set_state_to.set(State::Poisoned)
}
}
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