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    //!
    //! The **nodrop** crate has the following cargo feature flags:
    //!
    //! - `std`
    //!   - Optional, enabled by default
    //!   - Use libstd
    //! - `use_needs_drop`
    //!   - Optional
    //!   - Requires Rust 1.21
    //!   - Use `needs_drop` to skip overwriting if not necessary
    //! - `use_union`
    //!   - Optional
    //!   - Requires nightly channel
    //!   - Using untagged union, finally we have an implementation of `NoDrop` without hacks,
    //!     for example the fact that `NoDrop<T>` never has a destructor anymore.
    //!

    #![cfg_attr(not(any(test, feature="std")), no_std)]
    #[cfg(not(any(test, feature="std")))]
    extern crate core as std;

    #[cfg(feature = "use_union")]
    extern crate nodrop_union as imp;

    pub use imp::NoDrop;

    #[cfg(not(feature = "use_union"))]
    mod imp {
        use std::ptr;
        use std::mem;
        use std::ops::{Deref, DerefMut};

        /// 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))
            }

            /// 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
            }
        }

        #[cfg(not(feature = "use_needs_drop"))]
        #[inline]
        fn needs_drop<T>() -> bool {
            true
        }

        #[cfg(feature = "use_needs_drop")]
        #[inline]
        fn needs_drop<T>() -> bool {
            unsafe {
                ::std::mem::needs_drop::<T>()
            }
        }

        impl<T> Drop for NoDrop<T> {
            fn drop(&mut self) {
                if needs_drop::<T>() {
                    // 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() {
            // 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);

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

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