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
456
457
458
459
//! Composable external iteration.
//!
//! If you've found yourself with a collection of some kind, and needed to
//! perform an operation on the elements of said collection, you'll quickly run
//! into 'iterators'. Iterators are heavily used in idiomatic Rust code, so
//! it's worth becoming familiar with them.
//!
//! Before explaining more, let's talk about how this module is structured:
//!
//! # Organization
//!
//! This module is largely organized by type:
//!
//! * [Traits] are the core portion: these traits define what kind of iterators
//!   exist and what you can do with them. The methods of these traits are worth
//!   putting some extra study time into.
//! * [Functions] provide some helpful ways to create some basic iterators.
//! * [Structs] are often the return types of the various methods on this
//!   module's traits. You'll usually want to look at the method that creates
//!   the `struct`, rather than the `struct` itself. For more detail about why,
//!   see '[Implementing Iterator](#implementing-iterator)'.
//!
//! [Traits]: #traits
//! [Functions]: #functions
//! [Structs]: #structs
//!
//! That's it! Let's dig into iterators.
//!
//! # Iterator
//!
//! The heart and soul of this module is the [`Iterator`] trait. The core of
//! [`Iterator`] looks like this:
//!
//! ```
//! trait Iterator {
//!     type Item;
//!     fn next(&mut self) -> Option<Self::Item>;
//! }
//! ```
//!
//! An iterator has a method, [`next`], which when called, returns an
//! <code>[Option]\<Item></code>. Calling [`next`] will return [`Some(Item)`] as long as there
//! are elements, and once they've all been exhausted, will return `None` to
//! indicate that iteration is finished. Individual iterators may choose to
//! resume iteration, and so calling [`next`] again may or may not eventually
//! start returning [`Some(Item)`] again at some point (for example, see [`TryIter`]).
//!
//! [`Iterator`]'s full definition includes a number of other methods as well,
//! but they are default methods, built on top of [`next`], and so you get
//! them for free.
//!
//! Iterators are also composable, and it's common to chain them together to do
//! more complex forms of processing. See the [Adapters](#adapters) section
//! below for more details.
//!
//! [`Some(Item)`]: Some
//! [`next`]: Iterator::next
//! [`TryIter`]: ../../std/sync/mpsc/struct.TryIter.html
//!
//! # The three forms of iteration
//!
//! There are three common methods which can create iterators from a collection:
//!
//! * `iter()`, which iterates over `&T`.
//! * `iter_mut()`, which iterates over `&mut T`.
//! * `into_iter()`, which iterates over `T`.
//!
//! Various things in the standard library may implement one or more of the
//! three, where appropriate.
//!
//! # Implementing Iterator
//!
//! Creating an iterator of your own involves two steps: creating a `struct` to
//! hold the iterator's state, and then implementing [`Iterator`] for that `struct`.
//! This is why there are so many `struct`s in this module: there is one for
//! each iterator and iterator adapter.
//!
//! Let's make an iterator named `Counter` which counts from `1` to `5`:
//!
//! ```
//! // First, the struct:
//!
//! /// An iterator which counts from one to five
//! struct Counter {
//!     count: usize,
//! }
//!
//! // we want our count to start at one, so let's add a new() method to help.
//! // This isn't strictly necessary, but is convenient. Note that we start
//! // `count` at zero, we'll see why in `next()`'s implementation below.
//! impl Counter {
//!     fn new() -> Counter {
//!         Counter { count: 0 }
//!     }
//! }
//!
//! // Then, we implement `Iterator` for our `Counter`:
//!
//! impl Iterator for Counter {
//!     // we will be counting with usize
//!     type Item = usize;
//!
//!     // next() is the only required method
//!     fn next(&mut self) -> Option<Self::Item> {
//!         // Increment our count. This is why we started at zero.
//!         self.count += 1;
//!
//!         // Check to see if we've finished counting or not.
//!         if self.count < 6 {
//!             Some(self.count)
//!         } else {
//!             None
//!         }
//!     }
//! }
//!
//! // And now we can use it!
//!
//! let mut counter = Counter::new();
//!
//! assert_eq!(counter.next(), Some(1));
//! assert_eq!(counter.next(), Some(2));
//! assert_eq!(counter.next(), Some(3));
//! assert_eq!(counter.next(), Some(4));
//! assert_eq!(counter.next(), Some(5));
//! assert_eq!(counter.next(), None);
//! ```
//!
//! Calling [`next`] this way gets repetitive. Rust has a construct which can
//! call [`next`] on your iterator, until it reaches `None`. Let's go over that
//! next.
//!
//! Also note that `Iterator` provides a default implementation of methods such as `nth` and `fold`
//! which call `next` internally. However, it is also possible to write a custom implementation of
//! methods like `nth` and `fold` if an iterator can compute them more efficiently without calling
//! `next`.
//!
//! # `for` loops and `IntoIterator`
//!
//! Rust's `for` loop syntax is actually sugar for iterators. Here's a basic
//! example of `for`:
//!
//! ```
//! let values = vec![1, 2, 3, 4, 5];
//!
//! for x in values {
//!     println!("{x}");
//! }
//! ```
//!
//! This will print the numbers one through five, each on their own line. But
//! you'll notice something here: we never called anything on our vector to
//! produce an iterator. What gives?
//!
//! There's a trait in the standard library for converting something into an
//! iterator: [`IntoIterator`]. This trait has one method, [`into_iter`],
//! which converts the thing implementing [`IntoIterator`] into an iterator.
//! Let's take a look at that `for` loop again, and what the compiler converts
//! it into:
//!
//! [`into_iter`]: IntoIterator::into_iter
//!
//! ```
//! let values = vec![1, 2, 3, 4, 5];
//!
//! for x in values {
//!     println!("{x}");
//! }
//! ```
//!
//! Rust de-sugars this into:
//!
//! ```
//! let values = vec![1, 2, 3, 4, 5];
//! {
//!     let result = match IntoIterator::into_iter(values) {
//!         mut iter => loop {
//!             let next;
//!             match iter.next() {
//!                 Some(val) => next = val,
//!                 None => break,
//!             };
//!             let x = next;
//!             let () = { println!("{x}"); };
//!         },
//!     };
//!     result
//! }
//! ```
//!
//! First, we call `into_iter()` on the value. Then, we match on the iterator
//! that returns, calling [`next`] over and over until we see a `None`. At
//! that point, we `break` out of the loop, and we're done iterating.
//!
//! There's one more subtle bit here: the standard library contains an
//! interesting implementation of [`IntoIterator`]:
//!
//! ```ignore (only-for-syntax-highlight)
//! impl<I: Iterator> IntoIterator for I
//! ```
//!
//! In other words, all [`Iterator`]s implement [`IntoIterator`], by just
//! returning themselves. This means two things:
//!
//! 1. If you're writing an [`Iterator`], you can use it with a `for` loop.
//! 2. If you're creating a collection, implementing [`IntoIterator`] for it
//!    will allow your collection to be used with the `for` loop.
//!
//! # Iterating by reference
//!
//! Since [`into_iter()`] takes `self` by value, using a `for` loop to iterate
//! over a collection consumes that collection. Often, you may want to iterate
//! over a collection without consuming it. Many collections offer methods that
//! provide iterators over references, conventionally called `iter()` and
//! `iter_mut()` respectively:
//!
//! ```
//! let mut values = vec![41];
//! for x in values.iter_mut() {
//!     *x += 1;
//! }
//! for x in values.iter() {
//!     assert_eq!(*x, 42);
//! }
//! assert_eq!(values.len(), 1); // `values` is still owned by this function.
//! ```
//!
//! If a collection type `C` provides `iter()`, it usually also implements
//! `IntoIterator` for `&C`, with an implementation that just calls `iter()`.
//! Likewise, a collection `C` that provides `iter_mut()` generally implements
//! `IntoIterator` for `&mut C` by delegating to `iter_mut()`. This enables a
//! convenient shorthand:
//!
//! ```
//! let mut values = vec![41];
//! for x in &mut values { // same as `values.iter_mut()`
//!     *x += 1;
//! }
//! for x in &values { // same as `values.iter()`
//!     assert_eq!(*x, 42);
//! }
//! assert_eq!(values.len(), 1);
//! ```
//!
//! While many collections offer `iter()`, not all offer `iter_mut()`. For
//! example, mutating the keys of a [`HashSet<T>`] could put the collection
//! into an inconsistent state if the key hashes change, so this collection
//! only offers `iter()`.
//!
//! [`into_iter()`]: IntoIterator::into_iter
//! [`HashSet<T>`]: ../../std/collections/struct.HashSet.html
//!
//! # Adapters
//!
//! Functions which take an [`Iterator`] and return another [`Iterator`] are
//! often called 'iterator adapters', as they're a form of the 'adapter
//! pattern'.
//!
//! Common iterator adapters include [`map`], [`take`], and [`filter`].
//! For more, see their documentation.
//!
//! If an iterator adapter panics, the iterator will be in an unspecified (but
//! memory safe) state.  This state is also not guaranteed to stay the same
//! across versions of Rust, so you should avoid relying on the exact values
//! returned by an iterator which panicked.
//!
//! [`map`]: Iterator::map
//! [`take`]: Iterator::take
//! [`filter`]: Iterator::filter
//!
//! # Laziness
//!
//! Iterators (and iterator [adapters](#adapters)) are *lazy*. This means that
//! just creating an iterator doesn't _do_ a whole lot. Nothing really happens
//! until you call [`next`]. This is sometimes a source of confusion when
//! creating an iterator solely for its side effects. For example, the [`map`]
//! method calls a closure on each element it iterates over:
//!
//! ```
//! # #![allow(unused_must_use)]
//! # #![allow(map_unit_fn)]
//! let v = vec![1, 2, 3, 4, 5];
//! v.iter().map(|x| println!("{x}"));
//! ```
//!
//! This will not print any values, as we only created an iterator, rather than
//! using it. The compiler will warn us about this kind of behavior:
//!
//! ```text
//! warning: unused result that must be used: iterators are lazy and
//! do nothing unless consumed
//! ```
//!
//! The idiomatic way to write a [`map`] for its side effects is to use a
//! `for` loop or call the [`for_each`] method:
//!
//! ```
//! let v = vec![1, 2, 3, 4, 5];
//!
//! v.iter().for_each(|x| println!("{x}"));
//! // or
//! for x in &v {
//!     println!("{x}");
//! }
//! ```
//!
//! [`map`]: Iterator::map
//! [`for_each`]: Iterator::for_each
//!
//! Another common way to evaluate an iterator is to use the [`collect`]
//! method to produce a new collection.
//!
//! [`collect`]: Iterator::collect
//!
//! # Infinity
//!
//! Iterators do not have to be finite. As an example, an open-ended range is
//! an infinite iterator:
//!
//! ```
//! let numbers = 0..;
//! ```
//!
//! It is common to use the [`take`] iterator adapter to turn an infinite
//! iterator into a finite one:
//!
//! ```
//! let numbers = 0..;
//! let five_numbers = numbers.take(5);
//!
//! for number in five_numbers {
//!     println!("{number}");
//! }
//! ```
//!
//! This will print the numbers `0` through `4`, each on their own line.
//!
//! Bear in mind that methods on infinite iterators, even those for which a
//! result can be determined mathematically in finite time, might not terminate.
//! Specifically, methods such as [`min`], which in the general case require
//! traversing every element in the iterator, are likely not to return
//! successfully for any infinite iterators.
//!
//! ```no_run
//! let ones = std::iter::repeat(1);
//! let least = ones.min().unwrap(); // Oh no! An infinite loop!
//! // `ones.min()` causes an infinite loop, so we won't reach this point!
//! println!("The smallest number one is {least}.");
//! ```
//!
//! [`take`]: Iterator::take
//! [`min`]: Iterator::min

#![stable(feature = "rust1", since = "1.0.0")]

// This needs to be up here in order to be usable in the child modules
macro_rules! impl_fold_via_try_fold {
    (fold -> try_fold) => {
        impl_fold_via_try_fold! { @internal fold -> try_fold }
    };
    (rfold -> try_rfold) => {
        impl_fold_via_try_fold! { @internal rfold -> try_rfold }
    };
    (@internal $fold:ident -> $try_fold:ident) => {
        #[inline]
        fn $fold<AAA, FFF>(mut self, init: AAA, fold: FFF) -> AAA
        where
            FFF: FnMut(AAA, Self::Item) -> AAA,
        {
            use crate::ops::NeverShortCircuit;

            self.$try_fold(init, NeverShortCircuit::wrap_mut_2(fold)).0
        }
    };
}

#[stable(feature = "rust1", since = "1.0.0")]
pub use self::traits::Iterator;

#[unstable(
    feature = "step_trait",
    reason = "likely to be replaced by finer-grained traits",
    issue = "42168"
)]
pub use self::range::Step;

#[unstable(
    feature = "iter_from_generator",
    issue = "43122",
    reason = "generators are unstable"
)]
pub use self::sources::from_generator;
#[stable(feature = "iter_empty", since = "1.2.0")]
pub use self::sources::{empty, Empty};
#[stable(feature = "iter_from_fn", since = "1.34.0")]
pub use self::sources::{from_fn, FromFn};
#[stable(feature = "iter_once", since = "1.2.0")]
pub use self::sources::{once, Once};
#[stable(feature = "iter_once_with", since = "1.43.0")]
pub use self::sources::{once_with, OnceWith};
#[stable(feature = "rust1", since = "1.0.0")]
pub use self::sources::{repeat, Repeat};
#[unstable(feature = "iter_repeat_n", issue = "104434")]
pub use self::sources::{repeat_n, RepeatN};
#[stable(feature = "iterator_repeat_with", since = "1.28.0")]
pub use self::sources::{repeat_with, RepeatWith};
#[stable(feature = "iter_successors", since = "1.34.0")]
pub use self::sources::{successors, Successors};

#[stable(feature = "fused", since = "1.26.0")]
pub use self::traits::FusedIterator;
#[unstable(issue = "none", feature = "inplace_iteration")]
pub use self::traits::InPlaceIterable;
#[unstable(feature = "trusted_len", issue = "37572")]
pub use self::traits::TrustedLen;
#[unstable(feature = "trusted_step", issue = "85731")]
pub use self::traits::TrustedStep;
#[stable(feature = "rust1", since = "1.0.0")]
pub use self::traits::{
    DoubleEndedIterator, ExactSizeIterator, Extend, FromIterator, IntoIterator, Product, Sum,
};

#[stable(feature = "iter_zip", since = "1.59.0")]
pub use self::adapters::zip;
#[unstable(feature = "iter_array_chunks", reason = "recently added", issue = "100450")]
pub use self::adapters::ArrayChunks;
#[unstable(feature = "std_internals", issue = "none")]
pub use self::adapters::ByRefSized;
#[stable(feature = "iter_cloned", since = "1.1.0")]
pub use self::adapters::Cloned;
#[stable(feature = "iter_copied", since = "1.36.0")]
pub use self::adapters::Copied;
#[stable(feature = "iterator_flatten", since = "1.29.0")]
pub use self::adapters::Flatten;
#[stable(feature = "iter_map_while", since = "1.57.0")]
pub use self::adapters::MapWhile;
#[unstable(feature = "inplace_iteration", issue = "none")]
pub use self::adapters::SourceIter;
#[stable(feature = "iterator_step_by", since = "1.28.0")]
pub use self::adapters::StepBy;
#[unstable(feature = "trusted_random_access", issue = "none")]
pub use self::adapters::TrustedRandomAccess;
#[unstable(feature = "trusted_random_access", issue = "none")]
pub use self::adapters::TrustedRandomAccessNoCoerce;
#[stable(feature = "rust1", since = "1.0.0")]
pub use self::adapters::{
    Chain, Cycle, Enumerate, Filter, FilterMap, FlatMap, Fuse, Inspect, Map, Peekable, Rev, Scan,
    Skip, SkipWhile, Take, TakeWhile, Zip,
};
#[unstable(feature = "iter_intersperse", reason = "recently added", issue = "79524")]
pub use self::adapters::{Intersperse, IntersperseWith};

pub(crate) use self::adapters::try_process;
pub(crate) use self::traits::UncheckedIterator;

mod adapters;
mod range;
mod sources;
mod traits;