o3 0.23.0

shared-nothing primitives
Documentation 
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use std::{mem::MaybeUninit, ops, pin};

use crate::base::marker::ThreadBound;

/// Owns one initialized value while exclusively borrowing its backing storage.
///
/// Observations borrow this owner. Forgetting it leaks the value but leaves the
/// backing uninitialized from its owner's perspective, as with `MaybeUninit`.
#[derive(Debug)]
#[repr(transparent)]
pub struct Occupied<'a, T> {
    pub(super) value: &'a mut MaybeUninit<T>,
    pub(super) bound: ThreadBound,
}

impl<'a, T> Occupied<'a, T> {
    /// Moves out the initialized value, releasing its borrowed backing.
    pub fn into_inner(self) -> T {
        let this = std::mem::ManuallyDrop::new(self);
        // SAFETY: this consumed owner retains its initialized slot; suppressing
        // its destructor transfers the value exactly once without a second drop.
        unsafe { this.value.assume_init_read() }
    }

    /// Pins this initialized value until the callback returns, then drops it in place.
    /// The consumed owner retains destruction even if the callback forgets its pin.
    pub fn with_pin<R>(self, run: impl for<'p> FnOnce(pin::Pin<&'p mut T>) -> R) -> R {
        // SAFETY: this owner retains the initialized slot and its exclusive loan.
        // The callback cannot return its borrow, and this frame drops the value at
        // the same address before releasing the backing storage.
        run(unsafe { pin::Pin::new_unchecked(self.value.assume_init_mut()) })
    }
}

impl<T> ops::Deref for Occupied<'_, T> {
    type Target = T;

    fn deref(&self) -> &T {
        // SAFETY: construction initializes this exact slot. Its exclusive loan
        // remains retained, and the observation cannot outlive this owner.
        unsafe { self.value.assume_init_ref() }
    }
}

impl<T> Drop for Occupied<'_, T> {
    fn drop(&mut self) {
        // SAFETY: this affine owner consumes its exact initialized value once.
        // The borrowed MaybeUninit backing has no independent T destructor.
        unsafe { self.value.assume_init_drop() };
    }
}