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    // Copyright 2012 The Rust Project Developers. See the COPYRIGHT
    // file at the top-level directory of this distribution and at
    // http://rust-lang.org/COPYRIGHT.
    //
    // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
    // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
    // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
    // option. This file may not be copied, modified, or distributed
    // except according to those terms.

    //! A structure for holding a set of enum variants.
    //!
    //! This module defines a container which uses an efficient bit mask
    //! representation to hold C-like enum variants.

    #![unstable(feature = "enumset",
                reason = "matches collection reform specification, \
                          waiting for dust to settle",
                issue = "37966")]
    #![rustc_deprecated(since = "1.16.0", reason = "long since replaced")]
    #![allow(deprecated)]

    use core::marker;
    use core::fmt;
    use core::iter::{FromIterator, FusedIterator};
    use core::ops::{Sub, BitOr, BitAnd, BitXor};

    // FIXME(contentions): implement union family of methods? (general design may be
    // wrong here)

    /// A specialized set implementation to use enum types.
    ///
    /// It is a logic error for an item to be modified in such a way that the
    /// transformation of the item to or from a `usize`, as determined by the
    /// `CLike` trait, changes while the item is in the set. This is normally only
    /// possible through `Cell`, `RefCell`, global state, I/O, or unsafe code.
    #[derive(PartialEq, Eq, PartialOrd, Ord, Hash)]
    pub struct EnumSet<E> {
        // We must maintain the invariant that no bits are set
        // for which no variant exists
        bits: usize,
        marker: marker::PhantomData<E>,
    }

    impl<E> Copy for EnumSet<E> {}

    impl<E> Clone for EnumSet<E> {
        fn clone(&self) -> EnumSet<E> {
            *self
        }
    }

    impl<E: CLike + fmt::Debug> fmt::Debug for EnumSet<E> {
        fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
            fmt.debug_set().entries(self).finish()
        }
    }

    /// An interface for casting C-like enum to usize and back.
    /// A typically implementation is as below.
    ///
    /// ```{rust,ignore}
    /// #[repr(usize)]
    /// enum Foo {
    ///     A, B, C
    /// }
    ///
    /// impl CLike for Foo {
    ///     fn to_usize(&self) -> usize {
    ///         *self as usize
    ///     }
    ///
    ///     fn from_usize(v: usize) -> Foo {
    ///         unsafe { mem::transmute(v) }
    ///     }
    /// }
    /// ```
    pub trait CLike {
        /// Converts a C-like enum to a `usize`.
        fn to_usize(&self) -> usize;
        /// Converts a `usize` to a C-like enum.
        fn from_usize(usize) -> Self;
    }

    fn bit<E: CLike>(e: &E) -> usize {
        use core::mem;
        let value = e.to_usize();
        let bits = mem::size_of::<usize>() * 8;
        assert!(value < bits,
                "EnumSet only supports up to {} variants.",
                bits - 1);
        1 << value
    }

    impl<E: CLike> EnumSet<E> {
        /// Returns an empty `EnumSet`.
        pub fn new() -> EnumSet<E> {
            EnumSet {
                bits: 0,
                marker: marker::PhantomData,
            }
        }

        /// Returns the number of elements in the given `EnumSet`.
        pub fn len(&self) -> usize {
            self.bits.count_ones() as usize
        }

        /// Returns true if the `EnumSet` is empty.
        pub fn is_empty(&self) -> bool {
            self.bits == 0
        }

        pub fn clear(&mut self) {
            self.bits = 0;
        }

        /// Returns `false` if the `EnumSet` contains any enum of the given `EnumSet`.
        pub fn is_disjoint(&self, other: &EnumSet<E>) -> bool {
            (self.bits & other.bits) == 0
        }

        /// Returns `true` if a given `EnumSet` is included in this `EnumSet`.
        pub fn is_superset(&self, other: &EnumSet<E>) -> bool {
            (self.bits & other.bits) == other.bits
        }

        /// Returns `true` if this `EnumSet` is included in the given `EnumSet`.
        pub fn is_subset(&self, other: &EnumSet<E>) -> bool {
            other.is_superset(self)
        }

        /// Returns the union of both `EnumSets`.
        pub fn union(&self, e: EnumSet<E>) -> EnumSet<E> {
            EnumSet {
                bits: self.bits | e.bits,
                marker: marker::PhantomData,
            }
        }

        /// Returns the intersection of both `EnumSets`.
        pub fn intersection(&self, e: EnumSet<E>) -> EnumSet<E> {
            EnumSet {
                bits: self.bits & e.bits,
                marker: marker::PhantomData,
            }
        }

        /// Adds an enum to the `EnumSet`, and returns `true` if it wasn't there before
        pub fn insert(&mut self, e: E) -> bool {
            let result = !self.contains(&e);
            self.bits |= bit(&e);
            result
        }

        /// Removes an enum from the EnumSet
        pub fn remove(&mut self, e: &E) -> bool {
            let result = self.contains(e);
            self.bits &= !bit(e);
            result
        }

        /// Returns `true` if an `EnumSet` contains a given enum.
        pub fn contains(&self, e: &E) -> bool {
            (self.bits & bit(e)) != 0
        }

        /// Returns an iterator over an `EnumSet`.
        pub fn iter(&self) -> Iter<E> {
            Iter::new(self.bits)
        }
    }

    impl<E: CLike> Sub for EnumSet<E> {
        type Output = EnumSet<E>;

        fn sub(self, e: EnumSet<E>) -> EnumSet<E> {
            EnumSet {
                bits: self.bits & !e.bits,
                marker: marker::PhantomData,
            }
        }
    }

    impl<E: CLike> BitOr for EnumSet<E> {
        type Output = EnumSet<E>;

        fn bitor(self, e: EnumSet<E>) -> EnumSet<E> {
            EnumSet {
                bits: self.bits | e.bits,
                marker: marker::PhantomData,
            }
        }
    }

    impl<E: CLike> BitAnd for EnumSet<E> {
        type Output = EnumSet<E>;

        fn bitand(self, e: EnumSet<E>) -> EnumSet<E> {
            EnumSet {
                bits: self.bits & e.bits,
                marker: marker::PhantomData,
            }
        }
    }

    impl<E: CLike> BitXor for EnumSet<E> {
        type Output = EnumSet<E>;

        fn bitxor(self, e: EnumSet<E>) -> EnumSet<E> {
            EnumSet {
                bits: self.bits ^ e.bits,
                marker: marker::PhantomData,
            }
        }
    }

    /// An iterator over an EnumSet
    pub struct Iter<E> {
        index: usize,
        bits: usize,
        marker: marker::PhantomData<E>,
    }

    impl<E: fmt::Debug> fmt::Debug for Iter<E> {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            f.debug_tuple("Iter")
             .field(&self.clone())
             .finish()
        }
    }

    // FIXME(#19839) Remove in favor of `#[derive(Clone)]`
    impl<E> Clone for Iter<E> {
        fn clone(&self) -> Iter<E> {
            Iter {
                index: self.index,
                bits: self.bits,
                marker: marker::PhantomData,
            }
        }
    }

    impl<E: CLike> Iter<E> {
        fn new(bits: usize) -> Iter<E> {
            Iter {
                index: 0,
                bits: bits,
                marker: marker::PhantomData,
            }
        }
    }

    impl<E: CLike> Iterator for Iter<E> {
        type Item = E;

        fn next(&mut self) -> Option<E> {
            if self.bits == 0 {
                return None;
            }

            while (self.bits & 1) == 0 {
                self.index += 1;
                self.bits >>= 1;
            }
            let elem = CLike::from_usize(self.index);
            self.index += 1;
            self.bits >>= 1;
            Some(elem)
        }

        fn size_hint(&self) -> (usize, Option<usize>) {
            let exact = self.bits.count_ones() as usize;
            (exact, Some(exact))
        }
    }

    #[unstable(feature = "fused", issue = "35602")]
    impl<E: CLike> FusedIterator for Iter<E> {}

    impl<E: CLike> FromIterator<E> for EnumSet<E> {
        fn from_iter<I: IntoIterator<Item = E>>(iter: I) -> EnumSet<E> {
            let mut ret = EnumSet::new();
            ret.extend(iter);
            ret
        }
    }

    impl<'a, E> IntoIterator for &'a EnumSet<E>
        where E: CLike
    {
        type Item = E;
        type IntoIter = Iter<E>;

        fn into_iter(self) -> Iter<E> {
            self.iter()
        }
    }

    impl<E: CLike> Extend<E> for EnumSet<E> {
        fn extend<I: IntoIterator<Item = E>>(&mut self, iter: I) {
            for element in iter {
                self.insert(element);
            }
        }
    }

    impl<'a, E: 'a + CLike + Copy> Extend<&'a E> for EnumSet<E> {
        fn extend<I: IntoIterator<Item = &'a E>>(&mut self, iter: I) {
            self.extend(iter.into_iter().cloned());
        }
    }

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