1
  2
  3
  4
  5
  6
  7
  8
  9
 10
 11
 12
 13
 14
 15
 16
 17
 18
 19
 20
 21
 22
 23
 24
 25
 26
 27
 28
 29
 30
 31
 32
 33
 34
 35
 36
 37
 38
 39
 40
 41
 42
 43
 44
 45
 46
 47
 48
 49
 50
 51
 52
 53
 54
 55
 56
 57
 58
 59
 60
 61
 62
 63
 64
 65
 66
 67
 68
 69
 70
 71
 72
 73
 74
 75
 76
 77
 78
 79
 80
 81
 82
 83
 84
 85
 86
 87
 88
 89
 90
 91
 92
 93
 94
 95
 96
 97
 98
 99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175

/// 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`].
///
/// # 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) -> &Self::Output {
///         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.
    ///
    /// # Panics
    ///
    /// May panic if the index is out of bounds.
    #[stable(feature = "rust1", since = "1.0.0")]
    #[track_caller]
    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`.
///
/// # 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(&self, index: Side) -> &Self::Output {
///         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(&mut self, index: Side) -> &mut Self::Output {
///         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(
    on(
        _Self = "&str",
        note = "you can use `.chars().nth()` or `.bytes().nth()`
see chapter in The Book <https://doc.rust-lang.org/book/ch08-02-strings.html#indexing-into-strings>"
    ),
    on(
        _Self = "str",
        note = "you can use `.chars().nth()` or `.bytes().nth()`
see chapter in The Book <https://doc.rust-lang.org/book/ch08-02-strings.html#indexing-into-strings>"
    ),
    on(
        _Self = "std::string::String",
        note = "you can use `.chars().nth()` or `.bytes().nth()`
see chapter in The Book <https://doc.rust-lang.org/book/ch08-02-strings.html#indexing-into-strings>"
    ),
    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.
    ///
    /// # Panics
    ///
    /// May panic if the index is out of bounds.
    #[stable(feature = "rust1", since = "1.0.0")]
    #[track_caller]
    fn index_mut(&mut self, index: Idx) -> &mut Self::Output;
}