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nodrop 0.1.0

A wrapper type to inhibit drop (destructor).
Documentation 
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use std::ops::{Deref, DerefMut};
use std::ptr;
use std::mem;

/// repr(u8) - Make sure the non-nullable pointer optimization does not occur!
#[repr(u8)]
enum Flag<T> {
    Alive(T),
    Dropped,
}

/// 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))
    }

    /// Extract the inner value.
    ///
    /// Once extracted, the value can of course drop again.
    #[inline]
    pub fn into_inner(mut self) -> T {
        let inner_ptr = &mut *self;
        unsafe {
            ptr::read(inner_ptr)
        }
    }
}

enum Void { }

/// FIXME: Replace with intrinsic when it's stable
#[inline]
unsafe fn unreachable() -> ! {
    let void: &Void = mem::transmute(&());
    match *void {
        // no cases
    }
}

#[inline]
unsafe fn debug_assert_unreachable() -> ! {
    debug_assert!(false, "Entered unreachable section, this is a bug!");
    unreachable()
}

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

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() }
        }
    }
}

#[test]
fn test_no_nonnullable_opt() {
    // 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>>());
}

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

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

    struct Foo(Rc<Cell<i32>>);

    impl Drop for Foo {
        fn drop(&mut self) {
            let n = self.0.get();
            self.0.set(n + 1);
        }
    }

    {
        let _ = NoDrop::new([Foo(flag.clone()), Foo(flag.clone())]);
    }
    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![Foo(flag.clone())]);
        array.push(vec![Foo(flag.clone()), Foo(flag.clone())]);
        array.push(vec![]);
        array.push(vec![Foo(flag.clone())]);
        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);

    flag.set(0);
    {
        let mut array = NoDrop::new(Foo(flag.clone()));
        array.into_inner();
        assert_eq!(flag.get(), 1);
    }
    assert_eq!(flag.get(), 1);
}