library/core/src/iter/traits/collect.rs

   1 /// Conversion from an [`Iterator`].
   2 ///
   3 /// By implementing `FromIterator` for a type, you define how it will be
   4 /// created from an iterator. This is common for types which describe a
   5 /// collection of some kind.
   6 ///
   7 /// If you want to create a collection from the contents of an iterator, the
   8 /// [`Iterator::collect()`] method is preferred. However, when you need to
   9 /// specify the container type, [`FromIterator::from_iter()`] can be more
  10 /// readable than using a turbofish (e.g. `::<Vec<_>>()`). See the
  11 /// [`Iterator::collect()`] documentation for more examples of its use.
  12 ///
  13 /// See also: [`IntoIterator`].
  14 ///
  15 /// # Examples
  16 ///
  17 /// Basic usage:
  18 ///
  19 /// ```
  20 /// let five_fives = std::iter::repeat(5).take(5);
  21 ///
  22 /// let v = Vec::from_iter(five_fives);
  23 ///
  24 /// assert_eq!(v, vec![5, 5, 5, 5, 5]);
  25 /// ```
  26 ///
  27 /// Using [`Iterator::collect()`] to implicitly use `FromIterator`:
  28 ///
  29 /// ```
  30 /// let five_fives = std::iter::repeat(5).take(5);
  31 ///
  32 /// let v: Vec<i32> = five_fives.collect();
  33 ///
  34 /// assert_eq!(v, vec![5, 5, 5, 5, 5]);
  35 /// ```
  36 ///
  37 /// Using [`FromIterator::from_iter()`] as a more readable alternative to
  38 /// [`Iterator::collect()`]:
  39 ///
  40 /// ```
  41 /// use std::collections::VecDeque;
  42 /// let first = (0..10).collect::<VecDeque<i32>>();
  43 /// let second = VecDeque::from_iter(0..10);
  44 ///
  45 /// assert_eq!(first, second);
  46 /// ```
  47 ///
  48 /// Implementing `FromIterator` for your type:
  49 ///
  50 /// ```
  51 /// // A sample collection, that's just a wrapper over Vec<T>
  52 /// #[derive(Debug)]
  53 /// struct MyCollection(Vec<i32>);
  54 ///
  55 /// // Let's give it some methods so we can create one and add things
  56 /// // to it.
  57 /// impl MyCollection {
  58 ///     fn new() -> MyCollection {
  59 ///         MyCollection(Vec::new())
  60 ///     }
  61 ///
  62 ///     fn add(&mut self, elem: i32) {
  63 ///         self.0.push(elem);
  64 ///     }
  65 /// }
  66 ///
  67 /// // and we'll implement FromIterator
  68 /// impl FromIterator<i32> for MyCollection {
  69 ///     fn from_iter<I: IntoIterator<Item=i32>>(iter: I) -> Self {
  70 ///         let mut c = MyCollection::new();
  71 ///
  72 ///         for i in iter {
  73 ///             c.add(i);
  74 ///         }
  75 ///
  76 ///         c
  77 ///     }
  78 /// }
  79 ///
  80 /// // Now we can make a new iterator...
  81 /// let iter = (0..5).into_iter();
  82 ///
  83 /// // ... and make a MyCollection out of it
  84 /// let c = MyCollection::from_iter(iter);
  85 ///
  86 /// assert_eq!(c.0, vec![0, 1, 2, 3, 4]);
  87 ///
  88 /// // collect works too!
  89 ///
  90 /// let iter = (0..5).into_iter();
  91 /// let c: MyCollection = iter.collect();
  92 ///
  93 /// assert_eq!(c.0, vec![0, 1, 2, 3, 4]);
  94 /// ```
  95 #[stable(feature = "rust1", since = "1.0.0")]
  96 #[rustc_on_unimplemented(
  97     on(
  98         _Self = "[{A}]",
  99         message = "a value of type `{Self}` cannot be built since `{Self}` has no definite size",
 100         label = "try explicitly collecting into a `Vec<{A}>`",
 101     ),
 102     on(
 103         all(
 104             A = "{integer}",
 105             any(
 106                 _Self = "[i8]",
 107                 _Self = "[i16]",
 108                 _Self = "[i32]",
 109                 _Self = "[i64]",
 110                 _Self = "[i128]",
 111                 _Self = "[isize]",
 112                 _Self = "[u8]",
 113                 _Self = "[u16]",
 114                 _Self = "[u32]",
 115                 _Self = "[u64]",
 116                 _Self = "[u128]",
 117                 _Self = "[usize]"
 118             )
 119         ),
 120         message = "a value of type `{Self}` cannot be built since `{Self}` has no definite size",
 121         label = "try explicitly collecting into a `Vec<{A}>`",
 122     ),
 123     message = "a value of type `{Self}` cannot be built from an iterator \
 124                over elements of type `{A}`",
 125     label = "value of type `{Self}` cannot be built from `std::iter::Iterator<Item={A}>`"
 126 )]
 127 #[rustc_diagnostic_item = "FromIterator"]
 128 pub trait FromIterator<A>: Sized {
 129     /// Creates a value from an iterator.
 130     ///
 131     /// See the [module-level documentation] for more.
 132     ///
 133     /// [module-level documentation]: crate::iter
 134     ///
 135     /// # Examples
 136     ///
 137     /// Basic usage:
 138     ///
 139     /// ```
 140     /// let five_fives = std::iter::repeat(5).take(5);
 141     ///
 142     /// let v = Vec::from_iter(five_fives);
 143     ///
 144     /// assert_eq!(v, vec![5, 5, 5, 5, 5]);
 145     /// ```
 146     #[stable(feature = "rust1", since = "1.0.0")]
 147     fn from_iter<T: IntoIterator<Item = A>>(iter: T) -> Self;
 148 }
 149
 150 /// Conversion into an [`Iterator`].
 151 ///
 152 /// By implementing `IntoIterator` for a type, you define how it will be
 153 /// converted to an iterator. This is common for types which describe a
 154 /// collection of some kind.
 155 ///
 156 /// One benefit of implementing `IntoIterator` is that your type will [work
 157 /// with Rust's `for` loop syntax](crate::iter#for-loops-and-intoiterator).
 158 ///
 159 /// See also: [`FromIterator`].
 160 ///
 161 /// # Examples
 162 ///
 163 /// Basic usage:
 164 ///
 165 /// ```
 166 /// let v = [1, 2, 3];
 167 /// let mut iter = v.into_iter();
 168 ///
 169 /// assert_eq!(Some(1), iter.next());
 170 /// assert_eq!(Some(2), iter.next());
 171 /// assert_eq!(Some(3), iter.next());
 172 /// assert_eq!(None, iter.next());
 173 /// ```
 174 /// Implementing `IntoIterator` for your type:
 175 ///
 176 /// ```
 177 /// // A sample collection, that's just a wrapper over Vec<T>
 178 /// #[derive(Debug)]
 179 /// struct MyCollection(Vec<i32>);
 180 ///
 181 /// // Let's give it some methods so we can create one and add things
 182 /// // to it.
 183 /// impl MyCollection {
 184 ///     fn new() -> MyCollection {
 185 ///         MyCollection(Vec::new())
 186 ///     }
 187 ///
 188 ///     fn add(&mut self, elem: i32) {
 189 ///         self.0.push(elem);
 190 ///     }
 191 /// }
 192 ///
 193 /// // and we'll implement IntoIterator
 194 /// impl IntoIterator for MyCollection {
 195 ///     type Item = i32;
 196 ///     type IntoIter = std::vec::IntoIter<Self::Item>;
 197 ///
 198 ///     fn into_iter(self) -> Self::IntoIter {
 199 ///         self.0.into_iter()
 200 ///     }
 201 /// }
 202 ///
 203 /// // Now we can make a new collection...
 204 /// let mut c = MyCollection::new();
 205 ///
 206 /// // ... add some stuff to it ...
 207 /// c.add(0);
 208 /// c.add(1);
 209 /// c.add(2);
 210 ///
 211 /// // ... and then turn it into an Iterator:
 212 /// for (i, n) in c.into_iter().enumerate() {
 213 ///     assert_eq!(i as i32, n);
 214 /// }
 215 /// ```
 216 ///
 217 /// It is common to use `IntoIterator` as a trait bound. This allows
 218 /// the input collection type to change, so long as it is still an
 219 /// iterator. Additional bounds can be specified by restricting on
 220 /// `Item`:
 221 ///
 222 /// ```rust
 223 /// fn collect_as_strings<T>(collection: T) -> Vec<String>
 224 /// where
 225 ///     T: IntoIterator,
 226 ///     T::Item: std::fmt::Debug,
 227 /// {
 228 ///     collection
 229 ///         .into_iter()
 230 ///         .map(|item| format!("{item:?}"))
 231 ///         .collect()
 232 /// }
 233 /// ```
 234 #[rustc_diagnostic_item = "IntoIterator"]
 235 #[rustc_skip_array_during_method_dispatch]
 236 #[stable(feature = "rust1", since = "1.0.0")]
 237 pub trait IntoIterator {
 238     /// The type of the elements being iterated over.
 239     #[stable(feature = "rust1", since = "1.0.0")]
 240     type Item;
 241
 242     /// Which kind of iterator are we turning this into?
 243     #[stable(feature = "rust1", since = "1.0.0")]
 244     type IntoIter: Iterator<Item = Self::Item>;
 245
 246     /// Creates an iterator from a value.
 247     ///
 248     /// See the [module-level documentation] for more.
 249     ///
 250     /// [module-level documentation]: crate::iter
 251     ///
 252     /// # Examples
 253     ///
 254     /// Basic usage:
 255     ///
 256     /// ```
 257     /// let v = [1, 2, 3];
 258     /// let mut iter = v.into_iter();
 259     ///
 260     /// assert_eq!(Some(1), iter.next());
 261     /// assert_eq!(Some(2), iter.next());
 262     /// assert_eq!(Some(3), iter.next());
 263     /// assert_eq!(None, iter.next());
 264     /// ```
 265     #[lang = "into_iter"]
 266     #[stable(feature = "rust1", since = "1.0.0")]
 267     fn into_iter(self) -> Self::IntoIter;
 268 }
 269
 270 #[stable(feature = "rust1", since = "1.0.0")]
 271 impl<I: Iterator> IntoIterator for I {
 272     type Item = I::Item;
 273     type IntoIter = I;
 274
 275     #[inline]
 276     fn into_iter(self) -> I {
 277         self
 278     }
 279 }
 280
 281 /// Extend a collection with the contents of an iterator.
 282 ///
 283 /// Iterators produce a series of values, and collections can also be thought
 284 /// of as a series of values. The `Extend` trait bridges this gap, allowing you
 285 /// to extend a collection by including the contents of that iterator. When
 286 /// extending a collection with an already existing key, that entry is updated
 287 /// or, in the case of collections that permit multiple entries with equal
 288 /// keys, that entry is inserted.
 289 ///
 290 /// # Examples
 291 ///
 292 /// Basic usage:
 293 ///
 294 /// ```
 295 /// // You can extend a String with some chars:
 296 /// let mut message = String::from("The first three letters are: ");
 297 ///
 298 /// message.extend(&['a', 'b', 'c']);
 299 ///
 300 /// assert_eq!("abc", &message[29..32]);
 301 /// ```
 302 ///
 303 /// Implementing `Extend`:
 304 ///
 305 /// ```
 306 /// // A sample collection, that's just a wrapper over Vec<T>
 307 /// #[derive(Debug)]
 308 /// struct MyCollection(Vec<i32>);
 309 ///
 310 /// // Let's give it some methods so we can create one and add things
 311 /// // to it.
 312 /// impl MyCollection {
 313 ///     fn new() -> MyCollection {
 314 ///         MyCollection(Vec::new())
 315 ///     }
 316 ///
 317 ///     fn add(&mut self, elem: i32) {
 318 ///         self.0.push(elem);
 319 ///     }
 320 /// }
 321 ///
 322 /// // since MyCollection has a list of i32s, we implement Extend for i32
 323 /// impl Extend<i32> for MyCollection {
 324 ///
 325 ///     // This is a bit simpler with the concrete type signature: we can call
 326 ///     // extend on anything which can be turned into an Iterator which gives
 327 ///     // us i32s. Because we need i32s to put into MyCollection.
 328 ///     fn extend<T: IntoIterator<Item=i32>>(&mut self, iter: T) {
 329 ///
 330 ///         // The implementation is very straightforward: loop through the
 331 ///         // iterator, and add() each element to ourselves.
 332 ///         for elem in iter {
 333 ///             self.add(elem);
 334 ///         }
 335 ///     }
 336 /// }
 337 ///
 338 /// let mut c = MyCollection::new();
 339 ///
 340 /// c.add(5);
 341 /// c.add(6);
 342 /// c.add(7);
 343 ///
 344 /// // let's extend our collection with three more numbers
 345 /// c.extend(vec![1, 2, 3]);
 346 ///
 347 /// // we've added these elements onto the end
 348 /// assert_eq!("MyCollection([5, 6, 7, 1, 2, 3])", format!("{c:?}"));
 349 /// ```
 350 #[stable(feature = "rust1", since = "1.0.0")]
 351 pub trait Extend<A> {
 352     /// Extends a collection with the contents of an iterator.
 353     ///
 354     /// As this is the only required method for this trait, the [trait-level] docs
 355     /// contain more details.
 356     ///
 357     /// [trait-level]: Extend
 358     ///
 359     /// # Examples
 360     ///
 361     /// Basic usage:
 362     ///
 363     /// ```
 364     /// // You can extend a String with some chars:
 365     /// let mut message = String::from("abc");
 366     ///
 367     /// message.extend(['d', 'e', 'f'].iter());
 368     ///
 369     /// assert_eq!("abcdef", &message);
 370     /// ```
 371     #[stable(feature = "rust1", since = "1.0.0")]
 372     fn extend<T: IntoIterator<Item = A>>(&mut self, iter: T);
 373
 374     /// Extends a collection with exactly one element.
 375     #[unstable(feature = "extend_one", issue = "72631")]
 376     fn extend_one(&mut self, item: A) {
 377         self.extend(Some(item));
 378     }
 379
 380     /// Reserves capacity in a collection for the given number of additional elements.
 381     ///
 382     /// The default implementation does nothing.
 383     #[unstable(feature = "extend_one", issue = "72631")]
 384     fn extend_reserve(&mut self, additional: usize) {
 385         let _ = additional;
 386     }
 387 }
 388
 389 #[stable(feature = "extend_for_unit", since = "1.28.0")]
 390 impl Extend<()> for () {
 391     fn extend<T: IntoIterator<Item = ()>>(&mut self, iter: T) {
 392         iter.into_iter().for_each(drop)
 393     }
 394     fn extend_one(&mut self, _item: ()) {}
 395 }
 396
 397 #[stable(feature = "extend_for_tuple", since = "1.56.0")]
 398 impl<A, B, ExtendA, ExtendB> Extend<(A, B)> for (ExtendA, ExtendB)
 399 where
 400     ExtendA: Extend<A>,
 401     ExtendB: Extend<B>,
 402 {
 403     /// Allows to `extend` a tuple of collections that also implement `Extend`.
 404     ///
 405     /// See also: [`Iterator::unzip`]
 406     ///
 407     /// # Examples
 408     /// ```
 409     /// let mut tuple = (vec![0], vec![1]);
 410     /// tuple.extend([(2, 3), (4, 5), (6, 7)]);
 411     /// assert_eq!(tuple.0, [0, 2, 4, 6]);
 412     /// assert_eq!(tuple.1, [1, 3, 5, 7]);
 413     ///
 414     /// // also allows for arbitrarily nested tuples as elements
 415     /// let mut nested_tuple = (vec![1], (vec![2], vec![3]));
 416     /// nested_tuple.extend([(4, (5, 6)), (7, (8, 9))]);
 417     ///
 418     /// let (a, (b, c)) = nested_tuple;
 419     /// assert_eq!(a, [1, 4, 7]);
 420     /// assert_eq!(b, [2, 5, 8]);
 421     /// assert_eq!(c, [3, 6, 9]);
 422     /// ```
 423     fn extend<T: IntoIterator<Item = (A, B)>>(&mut self, into_iter: T) {
 424         let (a, b) = self;
 425         let iter = into_iter.into_iter();
 426
 427         fn extend<'a, A, B>(
 428             a: &'a mut impl Extend<A>,
 429             b: &'a mut impl Extend<B>,
 430         ) -> impl FnMut((), (A, B)) + 'a {
 431             move |(), (t, u)| {
 432                 a.extend_one(t);
 433                 b.extend_one(u);
 434             }
 435         }
 436
 437         let (lower_bound, _) = iter.size_hint();
 438         if lower_bound > 0 {
 439             a.extend_reserve(lower_bound);
 440             b.extend_reserve(lower_bound);
 441         }
 442
 443         iter.fold((), extend(a, b));
 444     }
 445
 446     fn extend_one(&mut self, item: (A, B)) {
 447         self.0.extend_one(item.0);
 448         self.1.extend_one(item.1);
 449     }
 450
 451     fn extend_reserve(&mut self, additional: usize) {
 452         self.0.extend_reserve(additional);
 453         self.1.extend_reserve(additional);
 454     }
 455 }