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    /// Used for indexing operations (`container[index]`) in immutable contexts.
    ///
    /// `container[index]` is actually syntactic sugar for `*container.index(index)`,
    /// but only when used as an immutable value. If a mutable value is requested,
    /// [`IndexMut`] is used instead. This allows nice things such as
    /// `let value = v[index]` if the type of `value` implements [`Copy`].
    ///
    /// [`IndexMut`]: ../../std/ops/trait.IndexMut.html
    /// [`Copy`]: ../../std/marker/trait.Copy.html
    ///
    /// # Examples
    ///
    /// The following example implements `Index` on a read-only `NucleotideCount`
    /// container, enabling individual counts to be retrieved with index syntax.
    ///
    /// ```
    /// use std::ops::Index;
    ///
    /// enum Nucleotide {
    ///     A,
    ///     C,
    ///     G,
    ///     T,
    /// }
    ///
    /// struct NucleotideCount {
    ///     a: usize,
    ///     c: usize,
    ///     g: usize,
    ///     t: usize,
    /// }
    ///
    /// impl Index<Nucleotide> for NucleotideCount {
    ///     type Output = usize;
    ///
    ///     fn index(&self, nucleotide: Nucleotide) -> &usize {
    ///         match nucleotide {
    ///             Nucleotide::A => &self.a,
    ///             Nucleotide::C => &self.c,
    ///             Nucleotide::G => &self.g,
    ///             Nucleotide::T => &self.t,
    ///         }
    ///     }
    /// }
    ///
    /// let nucleotide_count = NucleotideCount {a: 14, c: 9, g: 10, t: 12};
    /// assert_eq!(nucleotide_count[Nucleotide::A], 14);
    /// assert_eq!(nucleotide_count[Nucleotide::C], 9);
    /// assert_eq!(nucleotide_count[Nucleotide::G], 10);
    /// assert_eq!(nucleotide_count[Nucleotide::T], 12);
    /// ```
    #[lang = "index"]
    #[rustc_on_unimplemented(
        message="the type `{Self}` cannot be indexed by `{Idx}`",
        label="`{Self}` cannot be indexed by `{Idx}`",
    )]
    #[stable(feature = "rust1", since = "1.0.0")]
    #[doc(alias = "]")]
    #[doc(alias = "[")]
    #[doc(alias = "[]")]
    pub trait Index<Idx: ?Sized> {
        /// The returned type after indexing.
        #[stable(feature = "rust1", since = "1.0.0")]
        type Output: ?Sized;

        /// Performs the indexing (`container[index]`) operation.
        #[stable(feature = "rust1", since = "1.0.0")]
        fn index(&self, index: Idx) -> &Self::Output;
    }

    /// Used for indexing operations (`container[index]`) in mutable contexts.
    ///
    /// `container[index]` is actually syntactic sugar for
    /// `*container.index_mut(index)`, but only when used as a mutable value. If
    /// an immutable value is requested, the [`Index`] trait is used instead. This
    /// allows nice things such as `v[index] = value`.
    ///
    /// [`Index`]: ../../std/ops/trait.Index.html
    ///
    /// # Examples
    ///
    /// A very simple implementation of a `Balance` struct that has two sides, where
    /// each can be indexed mutably and immutably.
    ///
    /// ```
    /// use std::ops::{Index,IndexMut};
    ///
    /// #[derive(Debug)]
    /// enum Side {
    ///     Left,
    ///     Right,
    /// }
    ///
    /// #[derive(Debug, PartialEq)]
    /// enum Weight {
    ///     Kilogram(f32),
    ///     Pound(f32),
    /// }
    ///
    /// struct Balance {
    ///     pub left: Weight,
    ///     pub right: Weight,
    /// }
    ///
    /// impl Index<Side> for Balance {
    ///     type Output = Weight;
    ///
    ///     fn index<'a>(&'a self, index: Side) -> &'a Weight {
    ///         println!("Accessing {:?}-side of balance immutably", index);
    ///         match index {
    ///             Side::Left => &self.left,
    ///             Side::Right => &self.right,
    ///         }
    ///     }
    /// }
    ///
    /// impl IndexMut<Side> for Balance {
    ///     fn index_mut<'a>(&'a mut self, index: Side) -> &'a mut Weight {
    ///         println!("Accessing {:?}-side of balance mutably", index);
    ///         match index {
    ///             Side::Left => &mut self.left,
    ///             Side::Right => &mut self.right,
    ///         }
    ///     }
    /// }
    ///
    /// let mut balance = Balance {
    ///     right: Weight::Kilogram(2.5),
    ///     left: Weight::Pound(1.5),
    /// };
    ///
    /// // In this case, `balance[Side::Right]` is sugar for
    /// // `*balance.index(Side::Right)`, since we are only *reading*
    /// // `balance[Side::Right]`, not writing it.
    /// assert_eq!(balance[Side::Right], Weight::Kilogram(2.5));
    ///
    /// // However, in this case `balance[Side::Left]` is sugar for
    /// // `*balance.index_mut(Side::Left)`, since we are writing
    /// // `balance[Side::Left]`.
    /// balance[Side::Left] = Weight::Kilogram(3.0);
    /// ```
    #[lang = "index_mut"]
    #[rustc_on_unimplemented(
        message="the type `{Self}` cannot be mutably indexed by `{Idx}`",
        label="`{Self}` cannot be mutably indexed by `{Idx}`",
    )]
    #[stable(feature = "rust1", since = "1.0.0")]
    #[doc(alias = "[")]
    #[doc(alias = "]")]
    #[doc(alias = "[]")]
    pub trait IndexMut<Idx: ?Sized>: Index<Idx> {
        /// Performs the mutable indexing (`container[index]`) operation.
        #[stable(feature = "rust1", since = "1.0.0")]
        fn index_mut(&mut self, index: Idx) -> &mut Self::Output;
    }

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