iter.rs.html -- source
Description: Source to the Rust file `src/libcore/array/iter.rs`.
Keywords: rust, rustlang, rust-lang

      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
    176
    177
    178
    179
    180
    181
    182
    183
    184
    185
    186
    187
    188
    189
    190
    191
    192
    193
    194
    195
    196
    197
    198
    199
    200
    201
    202
    203
    204
    205
    206
    207
    208
    209
    210
    211
    212
    213
    214
    215
    216
    217
    218
    219
    220
    221
    222
    223
    224
    225
    226
    227
    228
    229
    230
    231
    232
    233
    234
    235
    236
    237
    238
    239
    240
    241
    242
    243
    244
    245
    246
    247
    248
    249
    250
    251
    252
    253
    254
    255
    256
    257
    258
    259
    260
    261
    262
    263
    264
    265
    266
    267
    268
    269
    270
    271
    272
    273
    274
    275
    276
    277
    278
    279
    280
    281
    282
    283

    //! Defines the `IntoIter` owned iterator for arrays.

    use crate::{
        fmt,
        iter::{ExactSizeIterator, FusedIterator, TrustedLen},
        mem::{self, MaybeUninit},
        ops::Range,
        ptr,
    };
    use super::LengthAtMost32;

    /// A by-value [array] iterator.
    ///
    /// [array]: ../../std/primitive.array.html
    #[unstable(feature = "array_value_iter", issue = "65798")]
    pub struct IntoIter<T, const N: usize>
    where
        [T; N]: LengthAtMost32,
    {
        /// This is the array we are iterating over.
        ///
        /// Elements with index `i` where `alive.start <= i < alive.end` have not
        /// been yielded yet and are valid array entries. Elements with indices `i
        /// < alive.start` or `i >= alive.end` have been yielded already and must
        /// not be accessed anymore! Those dead elements might even be in a
        /// completely uninitialized state!
        ///
        /// So the invariants are:
        /// - `data[alive]` is alive (i.e. contains valid elements)
        /// - `data[..alive.start]` and `data[alive.end..]` are dead (i.e. the
        ///   elements were already read and must not be touched anymore!)
        data: [MaybeUninit<T>; N],

        /// The elements in `data` that have not been yielded yet.
        ///
        /// Invariants:
        /// - `alive.start <= alive.end`
        /// - `alive.end <= N`
        alive: Range<usize>,
    }

    impl<T, const N: usize> IntoIter<T, {N}>
    where
        [T; N]: LengthAtMost32,
    {
        /// Creates a new iterator over the given `array`.
        ///
        /// *Note*: this method might never get stabilized and/or removed in the
        /// future as there will likely be another, preferred way of obtaining this
        /// iterator (either via `IntoIterator` for arrays or via another way).
        #[unstable(feature = "array_value_iter", issue = "65798")]
        pub fn new(array: [T; N]) -> Self {
            // SAFETY: The transmute here is actually safe. The docs of `MaybeUninit`
            // promise:
            //
            // > `MaybeUninit<T>` is guaranteed to have the same size and alignment
            // > as `T`.
            //
            // The docs even show a transmute from an array of `MaybeUninit<T>` to
            // an array of `T`.
            //
            // With that, this initialization satisfies the invariants.

            // FIXME(LukasKalbertodt): actually use `mem::transmute` here, once it
            // works with const generics:
            //     `mem::transmute::<[T; {N}], [MaybeUninit<T>; {N}]>(array)`
            //
            // Until then, we do it manually here. We first create a bitwise copy
            // but cast the pointer so that it is treated as a different type. Then
            // we forget `array` so that it is not dropped.
            let data = unsafe {
                let data = ptr::read(&array as *const [T; N] as *const [MaybeUninit<T>; N]);
                mem::forget(array);
                data
            };

            Self {
                data,
                alive: 0..N,
            }
        }

        /// Returns an immutable slice of all elements that have not been yielded
        /// yet.
        fn as_slice(&self) -> &[T] {
            let slice = &self.data[self.alive.clone()];
            // SAFETY: This transmute is safe. As mentioned in `new`, `MaybeUninit` retains
            // the size and alignment of `T`. Furthermore, we know that all
            // elements within `alive` are properly initialized.
            unsafe {
                mem::transmute::<&[MaybeUninit<T>], &[T]>(slice)
            }
        }

        /// Returns a mutable slice of all elements that have not been yielded yet.
        fn as_mut_slice(&mut self) -> &mut [T] {
            // This transmute is safe, same as in `as_slice` above.
            let slice = &mut self.data[self.alive.clone()];
            // SAFETY: This transmute is safe. As mentioned in `new`, `MaybeUninit` retains
            // the size and alignment of `T`. Furthermore, we know that all
            // elements within `alive` are properly initialized.
            unsafe {
                mem::transmute::<&mut [MaybeUninit<T>], &mut [T]>(slice)
            }
        }
    }

    #[stable(feature = "array_value_iter_impls", since = "1.40.0")]
    impl<T, const N: usize> Iterator for IntoIter<T, {N}>
    where
        [T; N]: LengthAtMost32,
    {
        type Item = T;
        fn next(&mut self) -> Option<Self::Item> {
            if self.alive.start == self.alive.end {
                return None;
            }

            // Bump start index.
            //
            // From the check above we know that `alive.start != alive.end`.
            // Combine this with the invariant `alive.start <= alive.end`, we know
            // that `alive.start < alive.end`. Increasing `alive.start` by 1
            // maintains the invariant regarding `alive`. However, due to this
            // change, for a short time, the alive zone is not `data[alive]`
            // anymore, but `data[idx..alive.end]`.
            let idx = self.alive.start;
            self.alive.start += 1;

            // Read the element from the array.
            // SAFETY: This is safe: `idx` is an index
            // into the "alive" region of the array. Reading this element means
            // that `data[idx]` is regarded as dead now (i.e. do not touch). As
            // `idx` was the start of the alive-zone, the alive zone is now
            // `data[alive]` again, restoring all invariants.
            let out = unsafe { self.data.get_unchecked(idx).read() };

            Some(out)
        }

        fn size_hint(&self) -> (usize, Option<usize>) {
            let len = self.len();
            (len, Some(len))
        }

        fn count(self) -> usize {
            self.len()
        }

        fn last(mut self) -> Option<Self::Item> {
            self.next_back()
        }
    }

    #[stable(feature = "array_value_iter_impls", since = "1.40.0")]
    impl<T, const N: usize> DoubleEndedIterator for IntoIter<T, {N}>
    where
        [T; N]: LengthAtMost32,
    {
        fn next_back(&mut self) -> Option<Self::Item> {
            if self.alive.start == self.alive.end {
                return None;
            }

            // Decrease end index.
            //
            // From the check above we know that `alive.start != alive.end`.
            // Combine this with the invariant `alive.start <= alive.end`, we know
            // that `alive.start < alive.end`. As `alive.start` cannot be negative,
            // `alive.end` is at least 1, meaning that we can safely decrement it
            // by one. This also maintains the invariant `alive.start <=
            // alive.end`. However, due to this change, for a short time, the alive
            // zone is not `data[alive]` anymore, but `data[alive.start..alive.end
            // + 1]`.
            self.alive.end -= 1;

            // Read the element from the array.
            // SAFETY: This is safe: `alive.end` is an
            // index into the "alive" region of the array. Compare the previous
            // comment that states that the alive region is
            // `data[alive.start..alive.end + 1]`. Reading this element means that
            // `data[alive.end]` is regarded as dead now (i.e. do not touch). As
            // `alive.end` was the end of the alive-zone, the alive zone is now
            // `data[alive]` again, restoring all invariants.
            let out = unsafe { self.data.get_unchecked(self.alive.end).read() };

            Some(out)
        }
    }

    #[stable(feature = "array_value_iter_impls", since = "1.40.0")]
    impl<T, const N: usize> Drop for IntoIter<T, {N}>
    where
        [T; N]: LengthAtMost32,
    {
        fn drop(&mut self) {
            // SAFETY: This is safe: `as_mut_slice` returns exactly the sub-slice
            // of elements that have not been moved out yet and that remain
            // to be dropped.
            unsafe {
                ptr::drop_in_place(self.as_mut_slice())
            }
        }
    }

    #[stable(feature = "array_value_iter_impls", since = "1.40.0")]
    impl<T, const N: usize> ExactSizeIterator for IntoIter<T, {N}>
    where
        [T; N]: LengthAtMost32,
    {
        fn len(&self) -> usize {
            // Will never underflow due to the invariant `alive.start <=
            // alive.end`.
            self.alive.end - self.alive.start
        }
        fn is_empty(&self) -> bool {
            self.alive.is_empty()
        }
    }

    #[stable(feature = "array_value_iter_impls", since = "1.40.0")]
    impl<T, const N: usize> FusedIterator for IntoIter<T, {N}>
    where
        [T; N]: LengthAtMost32,
    {}

    // The iterator indeed reports the correct length. The number of "alive"
    // elements (that will still be yielded) is the length of the range `alive`.
    // This range is decremented in length in either `next` or `next_back`. It is
    // always decremented by 1 in those methods, but only if `Some(_)` is returned.
    #[stable(feature = "array_value_iter_impls", since = "1.40.0")]
    unsafe impl<T, const N: usize> TrustedLen for IntoIter<T, {N}>
    where
        [T; N]: LengthAtMost32,
    {}

    #[stable(feature = "array_value_iter_impls", since = "1.40.0")]
    impl<T: Clone, const N: usize> Clone for IntoIter<T, {N}>
    where
        [T; N]: LengthAtMost32,
    {
        fn clone(&self) -> Self {
            // SAFETY: each point of unsafety is documented inside the unsafe block
            unsafe {
                // This creates a new uninitialized array. Note that the `assume_init`
                // refers to the array, not the individual elements. And it is Ok if
                // the array is in an uninitialized state as all elements may be
                // uninitialized (all bit patterns are valid). Compare the
                // `MaybeUninit` docs for more information.
                let mut new_data: [MaybeUninit<T>; N] = MaybeUninit::uninit().assume_init();

                // Clone all alive elements.
                for idx in self.alive.clone() {
                    // The element at `idx` in the old array is alive, so we can
                    // safely call `get_ref()`. We then clone it, and write the
                    // clone into the new array.
                    let clone = self.data.get_unchecked(idx).get_ref().clone();
                    new_data.get_unchecked_mut(idx).write(clone);
                }

                Self {
                    data: new_data,
                    alive: self.alive.clone(),
                }
            }
        }
    }

    #[stable(feature = "array_value_iter_impls", since = "1.40.0")]
    impl<T: fmt::Debug, const N: usize> fmt::Debug for IntoIter<T, {N}>
    where
        [T; N]: LengthAtMost32,
    {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            // Only print the elements that were not yielded yet: we cannot
            // access the yielded elements anymore.
            f.debug_tuple("IntoIter")
                .field(&self.as_slice())
                .finish()
        }
    }