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
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
/// Conversion from an [`Iterator`].
///
/// By implementing `FromIterator` for a type, you define how it will be
/// created from an iterator. This is common for types which describe a
/// collection of some kind.
///
/// If you want to create a collection from the contents of an iterator, the
/// [`Iterator::collect()`] method is preferred. However, when you need to
/// specify the container type, [`FromIterator::from_iter()`] can be more
/// readable than using a turbofish (e.g. `::<Vec<_>>()`). See the
/// [`Iterator::collect()`] documentation for more examples of its use.
///
/// See also: [`IntoIterator`].
///
/// # Examples
///
/// Basic usage:
///
/// ```
/// let five_fives = std::iter::repeat(5).take(5);
///
/// let v = Vec::from_iter(five_fives);
///
/// assert_eq!(v, vec![5, 5, 5, 5, 5]);
/// ```
///
/// Using [`Iterator::collect()`] to implicitly use `FromIterator`:
///
/// ```
/// let five_fives = std::iter::repeat(5).take(5);
///
/// let v: Vec<i32> = five_fives.collect();
///
/// assert_eq!(v, vec![5, 5, 5, 5, 5]);
/// ```
///
/// Using [`FromIterator::from_iter()`] as a more readable alternative to
/// [`Iterator::collect()`]:
///
/// ```
/// use std::collections::VecDeque;
/// let first = (0..10).collect::<VecDeque<i32>>();
/// let second = VecDeque::from_iter(0..10);
///
/// assert_eq!(first, second);
/// ```
///
/// Implementing `FromIterator` for your type:
///
/// ```
/// // A sample collection, that's just a wrapper over Vec<T>
/// #[derive(Debug)]
/// struct MyCollection(Vec<i32>);
///
/// // Let's give it some methods so we can create one and add things
/// // to it.
/// impl MyCollection {
///     fn new() -> MyCollection {
///         MyCollection(Vec::new())
///     }
///
///     fn add(&mut self, elem: i32) {
///         self.0.push(elem);
///     }
/// }
///
/// // and we'll implement FromIterator
/// impl FromIterator<i32> for MyCollection {
///     fn from_iter<I: IntoIterator<Item=i32>>(iter: I) -> Self {
///         let mut c = MyCollection::new();
///
///         for i in iter {
///             c.add(i);
///         }
///
///         c
///     }
/// }
///
/// // Now we can make a new iterator...
/// let iter = (0..5).into_iter();
///
/// // ... and make a MyCollection out of it
/// let c = MyCollection::from_iter(iter);
///
/// assert_eq!(c.0, vec![0, 1, 2, 3, 4]);
///
/// // collect works too!
///
/// let iter = (0..5).into_iter();
/// let c: MyCollection = iter.collect();
///
/// assert_eq!(c.0, vec![0, 1, 2, 3, 4]);
/// ```
#[stable(feature = "rust1", since = "1.0.0")]
#[rustc_on_unimplemented(
    on(
        _Self = "&[{A}]",
        message = "a slice of type `{Self}` cannot be built since we need to store the elements somewhere",
        label = "try explicitly collecting into a `Vec<{A}>`",
    ),
    on(
        all(A = "{integer}", any(_Self = "&[{integral}]",)),
        message = "a slice of type `{Self}` cannot be built since we need to store the elements somewhere",
        label = "try explicitly collecting into a `Vec<{A}>`",
    ),
    on(
        _Self = "[{A}]",
        message = "a slice of type `{Self}` cannot be built since `{Self}` has no definite size",
        label = "try explicitly collecting into a `Vec<{A}>`",
    ),
    on(
        all(A = "{integer}", any(_Self = "[{integral}]",)),
        message = "a slice of type `{Self}` cannot be built since `{Self}` has no definite size",
        label = "try explicitly collecting into a `Vec<{A}>`",
    ),
    on(
        _Self = "[{A}; _]",
        message = "an array of type `{Self}` cannot be built directly from an iterator",
        label = "try collecting into a `Vec<{A}>`, then using `.try_into()`",
    ),
    on(
        all(A = "{integer}", any(_Self = "[{integral}; _]",)),
        message = "an array of type `{Self}` cannot be built directly from an iterator",
        label = "try collecting into a `Vec<{A}>`, then using `.try_into()`",
    ),
    message = "a value of type `{Self}` cannot be built from an iterator \
               over elements of type `{A}`",
    label = "value of type `{Self}` cannot be built from `std::iter::Iterator<Item={A}>`"
)]
#[rustc_diagnostic_item = "FromIterator"]
pub trait FromIterator<A>: Sized {
    /// Creates a value from an iterator.
    ///
    /// See the [module-level documentation] for more.
    ///
    /// [module-level documentation]: crate::iter
    ///
    /// # Examples
    ///
    /// ```
    /// let five_fives = std::iter::repeat(5).take(5);
    ///
    /// let v = Vec::from_iter(five_fives);
    ///
    /// assert_eq!(v, vec![5, 5, 5, 5, 5]);
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_diagnostic_item = "from_iter_fn"]
    fn from_iter<T: IntoIterator<Item = A>>(iter: T) -> Self;
}

/// Conversion into an [`Iterator`].
///
/// By implementing `IntoIterator` for a type, you define how it will be
/// converted to an iterator. This is common for types which describe a
/// collection of some kind.
///
/// One benefit of implementing `IntoIterator` is that your type will [work
/// with Rust's `for` loop syntax](crate::iter#for-loops-and-intoiterator).
///
/// See also: [`FromIterator`].
///
/// # Examples
///
/// Basic usage:
///
/// ```
/// let v = [1, 2, 3];
/// let mut iter = v.into_iter();
///
/// assert_eq!(Some(1), iter.next());
/// assert_eq!(Some(2), iter.next());
/// assert_eq!(Some(3), iter.next());
/// assert_eq!(None, iter.next());
/// ```
/// Implementing `IntoIterator` for your type:
///
/// ```
/// // A sample collection, that's just a wrapper over Vec<T>
/// #[derive(Debug)]
/// struct MyCollection(Vec<i32>);
///
/// // Let's give it some methods so we can create one and add things
/// // to it.
/// impl MyCollection {
///     fn new() -> MyCollection {
///         MyCollection(Vec::new())
///     }
///
///     fn add(&mut self, elem: i32) {
///         self.0.push(elem);
///     }
/// }
///
/// // and we'll implement IntoIterator
/// impl IntoIterator for MyCollection {
///     type Item = i32;
///     type IntoIter = std::vec::IntoIter<Self::Item>;
///
///     fn into_iter(self) -> Self::IntoIter {
///         self.0.into_iter()
///     }
/// }
///
/// // Now we can make a new collection...
/// let mut c = MyCollection::new();
///
/// // ... add some stuff to it ...
/// c.add(0);
/// c.add(1);
/// c.add(2);
///
/// // ... and then turn it into an Iterator:
/// for (i, n) in c.into_iter().enumerate() {
///     assert_eq!(i as i32, n);
/// }
/// ```
///
/// It is common to use `IntoIterator` as a trait bound. This allows
/// the input collection type to change, so long as it is still an
/// iterator. Additional bounds can be specified by restricting on
/// `Item`:
///
/// ```rust
/// fn collect_as_strings<T>(collection: T) -> Vec<String>
/// where
///     T: IntoIterator,
///     T::Item: std::fmt::Debug,
/// {
///     collection
///         .into_iter()
///         .map(|item| format!("{item:?}"))
///         .collect()
/// }
/// ```
#[rustc_diagnostic_item = "IntoIterator"]
#[rustc_skip_array_during_method_dispatch]
#[stable(feature = "rust1", since = "1.0.0")]
pub trait IntoIterator {
    /// The type of the elements being iterated over.
    #[stable(feature = "rust1", since = "1.0.0")]
    type Item;

    /// Which kind of iterator are we turning this into?
    #[stable(feature = "rust1", since = "1.0.0")]
    type IntoIter: Iterator<Item = Self::Item>;

    /// Creates an iterator from a value.
    ///
    /// See the [module-level documentation] for more.
    ///
    /// [module-level documentation]: crate::iter
    ///
    /// # Examples
    ///
    /// ```
    /// let v = [1, 2, 3];
    /// let mut iter = v.into_iter();
    ///
    /// assert_eq!(Some(1), iter.next());
    /// assert_eq!(Some(2), iter.next());
    /// assert_eq!(Some(3), iter.next());
    /// assert_eq!(None, iter.next());
    /// ```
    #[lang = "into_iter"]
    #[stable(feature = "rust1", since = "1.0.0")]
    fn into_iter(self) -> Self::IntoIter;
}

#[rustc_const_unstable(feature = "const_intoiterator_identity", issue = "90603")]
#[stable(feature = "rust1", since = "1.0.0")]
impl<I: Iterator> IntoIterator for I {
    type Item = I::Item;
    type IntoIter = I;

    #[inline]
    fn into_iter(self) -> I {
        self
    }
}

/// Extend a collection with the contents of an iterator.
///
/// Iterators produce a series of values, and collections can also be thought
/// of as a series of values. The `Extend` trait bridges this gap, allowing you
/// to extend a collection by including the contents of that iterator. When
/// extending a collection with an already existing key, that entry is updated
/// or, in the case of collections that permit multiple entries with equal
/// keys, that entry is inserted.
///
/// # Examples
///
/// Basic usage:
///
/// ```
/// // You can extend a String with some chars:
/// let mut message = String::from("The first three letters are: ");
///
/// message.extend(&['a', 'b', 'c']);
///
/// assert_eq!("abc", &message[29..32]);
/// ```
///
/// Implementing `Extend`:
///
/// ```
/// // A sample collection, that's just a wrapper over Vec<T>
/// #[derive(Debug)]
/// struct MyCollection(Vec<i32>);
///
/// // Let's give it some methods so we can create one and add things
/// // to it.
/// impl MyCollection {
///     fn new() -> MyCollection {
///         MyCollection(Vec::new())
///     }
///
///     fn add(&mut self, elem: i32) {
///         self.0.push(elem);
///     }
/// }
///
/// // since MyCollection has a list of i32s, we implement Extend for i32
/// impl Extend<i32> for MyCollection {
///
///     // This is a bit simpler with the concrete type signature: we can call
///     // extend on anything which can be turned into an Iterator which gives
///     // us i32s. Because we need i32s to put into MyCollection.
///     fn extend<T: IntoIterator<Item=i32>>(&mut self, iter: T) {
///
///         // The implementation is very straightforward: loop through the
///         // iterator, and add() each element to ourselves.
///         for elem in iter {
///             self.add(elem);
///         }
///     }
/// }
///
/// let mut c = MyCollection::new();
///
/// c.add(5);
/// c.add(6);
/// c.add(7);
///
/// // let's extend our collection with three more numbers
/// c.extend(vec![1, 2, 3]);
///
/// // we've added these elements onto the end
/// assert_eq!("MyCollection([5, 6, 7, 1, 2, 3])", format!("{c:?}"));
/// ```
#[stable(feature = "rust1", since = "1.0.0")]
pub trait Extend<A> {
    /// Extends a collection with the contents of an iterator.
    ///
    /// As this is the only required method for this trait, the [trait-level] docs
    /// contain more details.
    ///
    /// [trait-level]: Extend
    ///
    /// # Examples
    ///
    /// ```
    /// // You can extend a String with some chars:
    /// let mut message = String::from("abc");
    ///
    /// message.extend(['d', 'e', 'f'].iter());
    ///
    /// assert_eq!("abcdef", &message);
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    fn extend<T: IntoIterator<Item = A>>(&mut self, iter: T);

    /// Extends a collection with exactly one element.
    #[unstable(feature = "extend_one", issue = "72631")]
    fn extend_one(&mut self, item: A) {
        self.extend(Some(item));
    }

    /// Reserves capacity in a collection for the given number of additional elements.
    ///
    /// The default implementation does nothing.
    #[unstable(feature = "extend_one", issue = "72631")]
    fn extend_reserve(&mut self, additional: usize) {
        let _ = additional;
    }
}

#[stable(feature = "extend_for_unit", since = "1.28.0")]
impl Extend<()> for () {
    fn extend<T: IntoIterator<Item = ()>>(&mut self, iter: T) {
        iter.into_iter().for_each(drop)
    }
    fn extend_one(&mut self, _item: ()) {}
}

#[stable(feature = "extend_for_tuple", since = "1.56.0")]
impl<A, B, ExtendA, ExtendB> Extend<(A, B)> for (ExtendA, ExtendB)
where
    ExtendA: Extend<A>,
    ExtendB: Extend<B>,
{
    /// Allows to `extend` a tuple of collections that also implement `Extend`.
    ///
    /// See also: [`Iterator::unzip`]
    ///
    /// # Examples
    /// ```
    /// let mut tuple = (vec![0], vec![1]);
    /// tuple.extend([(2, 3), (4, 5), (6, 7)]);
    /// assert_eq!(tuple.0, [0, 2, 4, 6]);
    /// assert_eq!(tuple.1, [1, 3, 5, 7]);
    ///
    /// // also allows for arbitrarily nested tuples as elements
    /// let mut nested_tuple = (vec![1], (vec![2], vec![3]));
    /// nested_tuple.extend([(4, (5, 6)), (7, (8, 9))]);
    ///
    /// let (a, (b, c)) = nested_tuple;
    /// assert_eq!(a, [1, 4, 7]);
    /// assert_eq!(b, [2, 5, 8]);
    /// assert_eq!(c, [3, 6, 9]);
    /// ```
    fn extend<T: IntoIterator<Item = (A, B)>>(&mut self, into_iter: T) {
        let (a, b) = self;
        let iter = into_iter.into_iter();

        fn extend<'a, A, B>(
            a: &'a mut impl Extend<A>,
            b: &'a mut impl Extend<B>,
        ) -> impl FnMut((), (A, B)) + 'a {
            move |(), (t, u)| {
                a.extend_one(t);
                b.extend_one(u);
            }
        }

        let (lower_bound, _) = iter.size_hint();
        if lower_bound > 0 {
            a.extend_reserve(lower_bound);
            b.extend_reserve(lower_bound);
        }

        iter.fold((), extend(a, b));
    }

    fn extend_one(&mut self, item: (A, B)) {
        self.0.extend_one(item.0);
        self.1.extend_one(item.1);
    }

    fn extend_reserve(&mut self, additional: usize) {
        self.0.extend_reserve(additional);
        self.1.extend_reserve(additional);
    }
}