Add more from the std

crates/std/src/sys/sync/once/no_threads.rs

@@ -0,0 +1,114 @@
+use crate::cell::Cell;
+use crate::sync as public;
+use crate::sync::once::OnceExclusiveState;
+
+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]
+    pub const fn new() -> Once {
+        Once { state: Cell::new(State::Incomplete) }
+    }
+
+    #[inline]
+    pub fn is_completed(&self) -> bool {
+        self.state.get() == State::Complete
+    }
+
+    #[inline]
+    pub(crate) fn state(&mut self) -> OnceExclusiveState {
+        match self.state.get() {
+            State::Incomplete => OnceExclusiveState::Incomplete,
+            State::Poisoned => OnceExclusiveState::Poisoned,
+            State::Complete => OnceExclusiveState::Complete,
+            _ => unreachable!("invalid Once state"),
+        }
+    }
+
+    #[inline]
+    pub(crate) fn set_state(&mut self, new_state: OnceExclusiveState) {
+        self.state.set(match new_state {
+            OnceExclusiveState::Incomplete => State::Incomplete,
+            OnceExclusiveState::Poisoned => State::Poisoned,
+            OnceExclusiveState::Complete => State::Complete,
+        });
+    }
+
+    #[cold]
+    #[track_caller]
+    pub fn wait(&self, _ignore_poisoning: bool) {
+        panic!("not implementable on this target");
+    }
+
+    #[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)
+    }
+}