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nodrop 0.1.3

A wrapper type to inhibit drop (destructor).
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//!
//! The **nodrop** crate has the following cargo feature flags:
//!
//! - `no_drop_flag`.
//!   - Optional.
//!   - Requires nightly channel.
//!   - Use no drop flag on the **NoDrop** type,
//!     which means less space overhead. Use with care and report a bug if anything
//!     changes behavior with this feature.
//!
//!

#![cfg_attr(feature="no_drop_flag", feature(unsafe_no_drop_flag))]

extern crate odds;

use odds::debug_assert_unreachable;

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
#[cfg_attr(feature="no_drop_flag", unsafe_no_drop_flag)]
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 = unsafe {
            ptr::read(&mut *self)
        };
        // skip Drop, so we don't even have to overwrite
        mem::forget(self);
        inner
    }
}

impl<T> Drop for NoDrop<T> {
    fn drop(&mut self) {
        // no drop flag info: writing repeatedly is idempotent
        // 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>>());
    assert!(mem::size_of::<Option<Flag<&i32>>>() > mem::size_of::<Flag<&i32>>());
}

#[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);

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