use core::{cell::UnsafeCell, mem::ManuallyDrop};

/// A guard that runs a closure when it is dropped.
pub struct DropGuard<F: FnOnce()>(UnsafeCell<ManuallyDrop<F>>);

impl<F> DropGuard<F>
where
    F: FnOnce(),
{
    pub fn new(f: F) -> DropGuard<F> {
        Self(UnsafeCell::new(ManuallyDrop::new(f)))
    }
}

impl<F> Drop for DropGuard<F>
where
    F: FnOnce(),
{
    fn drop(&mut self) {
        // SAFETY: drop() is called exactly once.
        unsafe {
            ManuallyDrop::take(&mut *self.0.get())();
        }
    }
}

impl<F> From<F> for DropGuard<F>
where
    F: FnOnce(),
{
    fn from(f: F) -> Self {
        DropGuard::new(f)
    }
}