arraydeque 0.4.5

A ring buffer with a fixed capacity, which can be stored on the stack.
Documentation 
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//! The **nodrop** crate from arrayvec

use std::ops::{Deref, DerefMut};
use std::ptr;

/// repr(u8) - Make sure the non-nullable pointer optimization does not occur!
#[repr(u8)]
enum Flag<T> {
    Alive(T),
    // Dummy u8 field below, again to beat the enum layout opt
    Dropped(u8),
}

/// A type holding **T** that will not call its destructor on drop
pub struct NoDrop<T>(Flag<T>);

impl<T> NoDrop<T> {
    /// Create a new **NoDrop**.
    #[inline]
    pub fn new(value: T) -> NoDrop<T> {
        NoDrop(Flag::Alive(value))
    }
}

impl<T> Drop for NoDrop<T> {
    fn drop(&mut self) {
        // inhibit drop
        unsafe {
            ptr::write(&mut self.0, Flag::Dropped(0));
        }
    }
}

impl<T> Deref for NoDrop<T> {
    type Target = T;

    // Use type invariant, always Flag::Alive.
    #[inline]
    fn deref(&self) -> &T {
        match self.0 {
            Flag::Alive(ref inner) => inner,
            _ => unsafe { debug_assert_unreachable() },
        }
    }
}

impl<T> DerefMut for NoDrop<T> {
    // Use type invariant, always Flag::Alive.
    #[inline]
    fn deref_mut(&mut self) -> &mut T {
        match self.0 {
            Flag::Alive(ref mut inner) => inner,
            _ => unsafe { debug_assert_unreachable() },
        }
    }
}

#[cfg(test)]
#[test]
fn test_no_nonnullable_opt() {
    use std::mem;

    // Make sure `Flag` does not apply the non-nullable pointer optimization
    // as Option would do.
    assert!(mem::size_of::<Flag<&i32>>() > mem::size_of::<&i32>());
    assert!(mem::size_of::<Flag<Vec<i32>>>() > mem::size_of::<Vec<i32>>());
}

// copying this code saves us microcrate deps
#[inline]
unsafe fn debug_assert_unreachable() -> ! {
    debug_assert!(false, "Reached unreachable section: this is a bug!");
    enum Void {}
    match *(1 as *const Void) {}
}

#[cfg(test)]
mod tests {
    use super::NoDrop;

    #[test]
    fn test_drop() {
        use std::cell::Cell;

        let flag = &Cell::new(0);

        struct Bump<'a>(&'a Cell<i32>);

        impl<'a> Drop for Bump<'a> {
            fn drop(&mut self) {
                let n = self.0.get();
                self.0.set(n + 1);
            }
        }

        {
            let _ = NoDrop::new([Bump(flag), Bump(flag)]);
        }
        assert_eq!(flag.get(), 0);

        // test something with the nullable pointer optimization
        flag.set(0);

        {
            let mut array = NoDrop::new(Vec::new());
            array.push(vec![Bump(flag)]);
            array.push(vec![Bump(flag), Bump(flag)]);
            array.push(vec![]);
            array.push(vec![Bump(flag)]);
            drop(array.pop());
            assert_eq!(flag.get(), 1);
            drop(array.pop());
            assert_eq!(flag.get(), 1);
            drop(array.pop());
            assert_eq!(flag.get(), 3);
        }

        // last one didn't drop.
        assert_eq!(flag.get(), 3);
    }
}