1 //! Optional values.
2 //!
3 //! Type [`Option`] represents an optional value: every [`Option`]
4 //! is either [`Some`] and contains a value, or [`None`], and
5 //! does not. [`Option`] types are very common in Rust code, as
6 //! they have a number of uses:
7 //!
8 //! * Initial values
9 //! * Return values for functions that are not defined
10 //! over their entire input range (partial functions)
11 //! * Return value for otherwise reporting simple errors, where [`None`] is
12 //! returned on error
13 //! * Optional struct fields
14 //! * Struct fields that can be loaned or "taken"
15 //! * Optional function arguments
16 //! * Nullable pointers
17 //! * Swapping things out of difficult situations
18 //!
19 //! [`Option`]s are commonly paired with pattern matching to query the presence
20 //! of a value and take action, always accounting for the [`None`] case.
21 //!
22 //! ```
23 //! fn divide(numerator: f64, denominator: f64) -> Option<f64> {
24 //! if denominator == 0.0 {
25 //! None
26 //! } else {
27 //! Some(numerator / denominator)
28 //! }
29 //! }
30 //!
31 //! // The return value of the function is an option
32 //! let result = divide(2.0, 3.0);
33 //!
34 //! // Pattern match to retrieve the value
35 //! match result {
36 //! // The division was valid
37 //! Some(x) => println!("Result: {}", x),
38 //! // The division was invalid
39 //! None => println!("Cannot divide by 0"),
40 //! }
41 //! ```
42 //!
43 //
44 // FIXME: Show how `Option` is used in practice, with lots of methods
45 //
46 //! # Options and pointers ("nullable" pointers)
47 //!
48 //! Rust's pointer types must always point to a valid location; there are
49 //! no "null" references. Instead, Rust has *optional* pointers, like
50 //! the optional owned box, <code>[Option]<[Box\<T>]></code>.
51 //!
52 //! [Box\<T>]: ../../std/boxed/struct.Box.html
53 //!
54 //! The following example uses [`Option`] to create an optional box of
55 //! [`i32`]. Notice that in order to use the inner [`i32`] value, the
56 //! `check_optional` function first needs to use pattern matching to
57 //! determine whether the box has a value (i.e., it is [`Some(...)`][`Some`]) or
58 //! not ([`None`]).
59 //!
60 //! ```
61 //! let optional = None;
62 //! check_optional(optional);
63 //!
64 //! let optional = Some(Box::new(9000));
65 //! check_optional(optional);
66 //!
67 //! fn check_optional(optional: Option<Box<i32>>) {
68 //! match optional {
69 //! Some(p) => println!("has value {}", p),
70 //! None => println!("has no value"),
71 //! }
72 //! }
73 //! ```
74 //!
75 //! # Representation
76 //!
77 //! Rust guarantees to optimize the following types `T` such that
78 //! [`Option<T>`] has the same size as `T`:
79 //!
80 //! * [`Box<U>`]
81 //! * `&U`
82 //! * `&mut U`
83 //! * `fn`, `extern "C" fn`
84 //! * [`num::NonZero*`]
85 //! * [`ptr::NonNull<U>`]
86 //! * `#[repr(transparent)]` struct around one of the types in this list.
87 //!
88 //! [`Box<U>`]: ../../std/boxed/struct.Box.html
89 //! [`num::NonZero*`]: crate::num
90 //! [`ptr::NonNull<U>`]: crate::ptr::NonNull
91 //!
92 //! This is called the "null pointer optimization" or NPO.
93 //!
94 //! It is further guaranteed that, for the cases above, one can
95 //! [`mem::transmute`] from all valid values of `T` to `Option<T>` and
96 //! from `Some::<T>(_)` to `T` (but transmuting `None::<T>` to `T`
97 //! is undefined behaviour).
98 //!
99 //! # Method overview
100 //!
101 //! In addition to working with pattern matching, [`Option`] provides a wide
102 //! variety of different methods.
103 //!
104 //! ## Querying the variant
105 //!
106 //! The [`is_some`] and [`is_none`] methods return [`true`] if the [`Option`]
107 //! is [`Some`] or [`None`], respectively.
108 //!
109 //! [`is_none`]: Option::is_none
110 //! [`is_some`]: Option::is_some
111 //!
112 //! ## Adapters for working with references
113 //!
114 //! * [`as_ref`] converts from <code>[&][][Option]\<T></code> to <code>[Option]<[&]T></code>
115 //! * [`as_mut`] converts from <code>[&mut] [Option]\<T></code> to <code>[Option]<[&mut] T></code>
116 //! * [`as_deref`] converts from <code>[&][][Option]\<T></code> to
117 //! <code>[Option]<[&]T::[Target]></code>
118 //! * [`as_deref_mut`] converts from <code>[&mut] [Option]\<T></code> to
119 //! <code>[Option]<[&mut] T::[Target]></code>
120 //! * [`as_pin_ref`] converts from <code>[Pin]<[&][][Option]\<T>></code> to
121 //! <code>[Option]<[Pin]<[&]T>></code>
122 //! * [`as_pin_mut`] converts from <code>[Pin]<[&mut] [Option]\<T>></code> to
123 //! <code>[Option]<[Pin]<[&mut] T>></code>
124 //!
125 //! [&]: reference "shared reference"
126 //! [&mut]: reference "mutable reference"
127 //! [Target]: Deref::Target "ops::Deref::Target"
128 //! [`as_deref`]: Option::as_deref
129 //! [`as_deref_mut`]: Option::as_deref_mut
130 //! [`as_mut`]: Option::as_mut
131 //! [`as_pin_mut`]: Option::as_pin_mut
132 //! [`as_pin_ref`]: Option::as_pin_ref
133 //! [`as_ref`]: Option::as_ref
134 //!
135 //! ## Extracting the contained value
136 //!
137 //! These methods extract the contained value in an [`Option<T>`] when it
138 //! is the [`Some`] variant. If the [`Option`] is [`None`]:
139 //!
140 //! * [`expect`] panics with a provided custom message
141 //! * [`unwrap`] panics with a generic message
142 //! * [`unwrap_or`] returns the provided default value
143 //! * [`unwrap_or_default`] returns the default value of the type `T`
144 //! (which must implement the [`Default`] trait)
145 //! * [`unwrap_or_else`] returns the result of evaluating the provided
146 //! function
147 //!
148 //! [`expect`]: Option::expect
149 //! [`unwrap`]: Option::unwrap
150 //! [`unwrap_or`]: Option::unwrap_or
151 //! [`unwrap_or_default`]: Option::unwrap_or_default
152 //! [`unwrap_or_else`]: Option::unwrap_or_else
153 //!
154 //! ## Transforming contained values
155 //!
156 //! These methods transform [`Option`] to [`Result`]:
157 //!
158 //! * [`ok_or`] transforms [`Some(v)`] to [`Ok(v)`], and [`None`] to
159 //! [`Err(err)`] using the provided default `err` value
160 //! * [`ok_or_else`] transforms [`Some(v)`] to [`Ok(v)`], and [`None`] to
161 //! a value of [`Err`] using the provided function
162 //! * [`transpose`] transposes an [`Option`] of a [`Result`] into a
163 //! [`Result`] of an [`Option`]
164 //!
165 //! [`Err(err)`]: Err
166 //! [`Ok(v)`]: Ok
167 //! [`Some(v)`]: Some
168 //! [`ok_or`]: Option::ok_or
169 //! [`ok_or_else`]: Option::ok_or_else
170 //! [`transpose`]: Option::transpose
171 //!
172 //! These methods transform the [`Some`] variant:
173 //!
174 //! * [`filter`] calls the provided predicate function on the contained
175 //! value `t` if the [`Option`] is [`Some(t)`], and returns [`Some(t)`]
176 //! if the function returns `true`; otherwise, returns [`None`]
177 //! * [`flatten`] removes one level of nesting from an
178 //! [`Option<Option<T>>`]
179 //! * [`map`] transforms [`Option<T>`] to [`Option<U>`] by applying the
180 //! provided function to the contained value of [`Some`] and leaving
181 //! [`None`] values unchanged
182 //!
183 //! [`Some(t)`]: Some
184 //! [`filter`]: Option::filter
185 //! [`flatten`]: Option::flatten
186 //! [`map`]: Option::map
187 //!
188 //! These methods transform [`Option<T>`] to a value of a possibly
189 //! different type `U`:
190 //!
191 //! * [`map_or`] applies the provided function to the contained value of
192 //! [`Some`], or returns the provided default value if the [`Option`] is
193 //! [`None`]
194 //! * [`map_or_else`] applies the provided function to the contained value
195 //! of [`Some`], or returns the result of evaluating the provided
196 //! fallback function if the [`Option`] is [`None`]
197 //!
198 //! [`map_or`]: Option::map_or
199 //! [`map_or_else`]: Option::map_or_else
200 //!
201 //! These methods combine the [`Some`] variants of two [`Option`] values:
202 //!
203 //! * [`zip`] returns [`Some((s, o))`] if `self` is [`Some(s)`] and the
204 //! provided [`Option`] value is [`Some(o)`]; otherwise, returns [`None`]
205 //! * [`zip_with`] calls the provided function `f` and returns
206 //! [`Some(f(s, o))`] if `self` is [`Some(s)`] and the provided
207 //! [`Option`] value is [`Some(o)`]; otherwise, returns [`None`]
208 //!
209 //! [`Some(f(s, o))`]: Some
210 //! [`Some(o)`]: Some
211 //! [`Some(s)`]: Some
212 //! [`Some((s, o))`]: Some
213 //! [`zip`]: Option::zip
214 //! [`zip_with`]: Option::zip_with
215 //!
216 //! ## Boolean operators
217 //!
218 //! These methods treat the [`Option`] as a boolean value, where [`Some`]
219 //! acts like [`true`] and [`None`] acts like [`false`]. There are two
220 //! categories of these methods: ones that take an [`Option`] as input, and
221 //! ones that take a function as input (to be lazily evaluated).
222 //!
223 //! The [`and`], [`or`], and [`xor`] methods take another [`Option`] as
224 //! input, and produce an [`Option`] as output. Only the [`and`] method can
225 //! produce an [`Option<U>`] value having a different inner type `U` than
226 //! [`Option<T>`].
227 //!
228 //! | method | self | input | output |
229 //! |---------|-----------|-----------|-----------|
230 //! | [`and`] | `None` | (ignored) | `None` |
231 //! | [`and`] | `Some(x)` | `None` | `None` |
232 //! | [`and`] | `Some(x)` | `Some(y)` | `Some(y)` |
233 //! | [`or`] | `None` | `None` | `None` |
234 //! | [`or`] | `None` | `Some(y)` | `Some(y)` |
235 //! | [`or`] | `Some(x)` | (ignored) | `Some(x)` |
236 //! | [`xor`] | `None` | `None` | `None` |
237 //! | [`xor`] | `None` | `Some(y)` | `Some(y)` |
238 //! | [`xor`] | `Some(x)` | `None` | `Some(x)` |
239 //! | [`xor`] | `Some(x)` | `Some(y)` | `None` |
240 //!
241 //! [`and`]: Option::and
242 //! [`or`]: Option::or
243 //! [`xor`]: Option::xor
244 //!
245 //! The [`and_then`] and [`or_else`] methods take a function as input, and
246 //! only evaluate the function when they need to produce a new value. Only
247 //! the [`and_then`] method can produce an [`Option<U>`] value having a
248 //! different inner type `U` than [`Option<T>`].
249 //!
250 //! | method | self | function input | function result | output |
251 //! |--------------|-----------|----------------|-----------------|-----------|
252 //! | [`and_then`] | `None` | (not provided) | (not evaluated) | `None` |
253 //! | [`and_then`] | `Some(x)` | `x` | `None` | `None` |
254 //! | [`and_then`] | `Some(x)` | `x` | `Some(y)` | `Some(y)` |
255 //! | [`or_else`] | `None` | (not provided) | `None` | `None` |
256 //! | [`or_else`] | `None` | (not provided) | `Some(y)` | `Some(y)` |
257 //! | [`or_else`] | `Some(x)` | (not provided) | (not evaluated) | `Some(x)` |
258 //!
259 //! [`and_then`]: Option::and_then
260 //! [`or_else`]: Option::or_else
261 //!
262 //! This is an example of using methods like [`and_then`] and [`or`] in a
263 //! pipeline of method calls. Early stages of the pipeline pass failure
264 //! values ([`None`]) through unchanged, and continue processing on
265 //! success values ([`Some`]). Toward the end, [`or`] substitutes an error
266 //! message if it receives [`None`].
267 //!
268 //! ```
269 //! # use std::collections::BTreeMap;
270 //! let mut bt = BTreeMap::new();
271 //! bt.insert(20u8, "foo");
272 //! bt.insert(42u8, "bar");
273 //! let res = vec![0u8, 1, 11, 200, 22]
274 //! .into_iter()
275 //! .map(|x| {
276 //! // `checked_sub()` returns `None` on error
277 //! x.checked_sub(1)
278 //! // same with `checked_mul()`
279 //! .and_then(|x| x.checked_mul(2))
280 //! // `BTreeMap::get` returns `None` on error
281 //! .and_then(|x| bt.get(&x))
282 //! // Substitute an error message if we have `None` so far
283 //! .or(Some(&"error!"))
284 //! .copied()
285 //! // Won't panic because we unconditionally used `Some` above
286 //! .unwrap()
287 //! })
288 //! .collect::<Vec<_>>();
289 //! assert_eq!(res, ["error!", "error!", "foo", "error!", "bar"]);
290 //! ```
291 //!
292 //! ## Comparison operators
293 //!
294 //! If `T` implements [`PartialOrd`] then [`Option<T>`] will derive its
295 //! [`PartialOrd`] implementation. With this order, [`None`] compares as
296 //! less than any [`Some`], and two [`Some`] compare the same way as their
297 //! contained values would in `T`. If `T` also implements
298 //! [`Ord`], then so does [`Option<T>`].
299 //!
300 //! ```
301 //! assert!(None < Some(0));
302 //! assert!(Some(0) < Some(1));
303 //! ```
304 //!
305 //! ## Iterating over `Option`
306 //!
307 //! An [`Option`] can be iterated over. This can be helpful if you need an
308 //! iterator that is conditionally empty. The iterator will either produce
309 //! a single value (when the [`Option`] is [`Some`]), or produce no values
310 //! (when the [`Option`] is [`None`]). For example, [`into_iter`] acts like
311 //! [`once(v)`] if the [`Option`] is [`Some(v)`], and like [`empty()`] if
312 //! the [`Option`] is [`None`].
313 //!
314 //! [`Some(v)`]: Some
315 //! [`empty()`]: crate::iter::empty
316 //! [`once(v)`]: crate::iter::once
317 //!
318 //! Iterators over [`Option<T>`] come in three types:
319 //!
320 //! * [`into_iter`] consumes the [`Option`] and produces the contained
321 //! value
322 //! * [`iter`] produces an immutable reference of type `&T` to the
323 //! contained value
324 //! * [`iter_mut`] produces a mutable reference of type `&mut T` to the
325 //! contained value
326 //!
327 //! [`into_iter`]: Option::into_iter
328 //! [`iter`]: Option::iter
329 //! [`iter_mut`]: Option::iter_mut
330 //!
331 //! An iterator over [`Option`] can be useful when chaining iterators, for
332 //! example, to conditionally insert items. (It's not always necessary to
333 //! explicitly call an iterator constructor: many [`Iterator`] methods that
334 //! accept other iterators will also accept iterable types that implement
335 //! [`IntoIterator`], which includes [`Option`].)
336 //!
337 //! ```
338 //! let yep = Some(42);
339 //! let nope = None;
340 //! // chain() already calls into_iter(), so we don't have to do so
341 //! let nums: Vec<i32> = (0..4).chain(yep).chain(4..8).collect();
342 //! assert_eq!(nums, [0, 1, 2, 3, 42, 4, 5, 6, 7]);
343 //! let nums: Vec<i32> = (0..4).chain(nope).chain(4..8).collect();
344 //! assert_eq!(nums, [0, 1, 2, 3, 4, 5, 6, 7]);
345 //! ```
346 //!
347 //! One reason to chain iterators in this way is that a function returning
348 //! `impl Iterator` must have all possible return values be of the same
349 //! concrete type. Chaining an iterated [`Option`] can help with that.
350 //!
351 //! ```
352 //! fn make_iter(do_insert: bool) -> impl Iterator<Item = i32> {
353 //! // Explicit returns to illustrate return types matching
354 //! match do_insert {
355 //! true => return (0..4).chain(Some(42)).chain(4..8),
356 //! false => return (0..4).chain(None).chain(4..8),
357 //! }
358 //! }
359 //! println!("{:?}", make_iter(true).collect::<Vec<_>>());
360 //! println!("{:?}", make_iter(false).collect::<Vec<_>>());
361 //! ```
362 //!
363 //! If we try to do the same thing, but using [`once()`] and [`empty()`],
364 //! we can't return `impl Iterator` anymore because the concrete types of
365 //! the return values differ.
366 //!
367 //! [`empty()`]: crate::iter::empty
368 //! [`once()`]: crate::iter::once
369 //!
370 //! ```compile_fail,E0308
371 //! # use std::iter::{empty, once};
372 //! // This won't compile because all possible returns from the function
373 //! // must have the same concrete type.
374 //! fn make_iter(do_insert: bool) -> impl Iterator<Item = i32> {
375 //! // Explicit returns to illustrate return types not matching
376 //! match do_insert {
377 //! true => return (0..4).chain(once(42)).chain(4..8),
378 //! false => return (0..4).chain(empty()).chain(4..8),
379 //! }
380 //! }
381 //! ```
382 //!
383 //! ## Collecting into `Option`
384 //!
385 //! [`Option`] implements the [`FromIterator`][impl-FromIterator] trait,
386 //! which allows an iterator over [`Option`] values to be collected into an
387 //! [`Option`] of a collection of each contained value of the original
388 //! [`Option`] values, or [`None`] if any of the elements was [`None`].
389 //!
390 //! [impl-FromIterator]: Option#impl-FromIterator%3COption%3CA%3E%3E
391 //!
392 //! ```
393 //! let v = vec![Some(2), Some(4), None, Some(8)];
394 //! let res: Option<Vec<_>> = v.into_iter().collect();
395 //! assert_eq!(res, None);
396 //! let v = vec![Some(2), Some(4), Some(8)];
397 //! let res: Option<Vec<_>> = v.into_iter().collect();
398 //! assert_eq!(res, Some(vec![2, 4, 8]));
399 //! ```
400 //!
401 //! [`Option`] also implements the [`Product`][impl-Product] and
402 //! [`Sum`][impl-Sum] traits, allowing an iterator over [`Option`] values
403 //! to provide the [`product`][Iterator::product] and
404 //! [`sum`][Iterator::sum] methods.
405 //!
406 //! [impl-Product]: Option#impl-Product%3COption%3CU%3E%3E
407 //! [impl-Sum]: Option#impl-Sum%3COption%3CU%3E%3E
408 //!
409 //! ```
410 //! let v = vec![None, Some(1), Some(2), Some(3)];
411 //! let res: Option<i32> = v.into_iter().sum();
412 //! assert_eq!(res, None);
413 //! let v = vec![Some(1), Some(2), Some(21)];
414 //! let res: Option<i32> = v.into_iter().product();
415 //! assert_eq!(res, Some(42));
416 //! ```
417 //!
418 //! ## Modifying an [`Option`] in-place
419 //!
420 //! These methods return a mutable reference to the contained value of an
421 //! [`Option<T>`]:
422 //!
423 //! * [`insert`] inserts a value, dropping any old contents
424 //! * [`get_or_insert`] gets the current value, inserting a provided
425 //! default value if it is [`None`]
426 //! * [`get_or_insert_default`] gets the current value, inserting the
427 //! default value of type `T` (which must implement [`Default`]) if it is
428 //! [`None`]
429 //! * [`get_or_insert_with`] gets the current value, inserting a default
430 //! computed by the provided function if it is [`None`]
431 //!
432 //! [`get_or_insert`]: Option::get_or_insert
433 //! [`get_or_insert_default`]: Option::get_or_insert_default
434 //! [`get_or_insert_with`]: Option::get_or_insert_with
435 //! [`insert`]: Option::insert
436 //!
437 //! These methods transfer ownership of the contained value of an
438 //! [`Option`]:
439 //!
440 //! * [`take`] takes ownership of the contained value of an [`Option`], if
441 //! any, replacing the [`Option`] with [`None`]
442 //! * [`replace`] takes ownership of the contained value of an [`Option`],
443 //! if any, replacing the [`Option`] with a [`Some`] containing the
444 //! provided value
445 //!
446 //! [`replace`]: Option::replace
447 //! [`take`]: Option::take
448 //!
449 //! # Examples
450 //!
451 //! Basic pattern matching on [`Option`]:
452 //!
453 //! ```
454 //! let msg = Some("howdy");
455 //!
456 //! // Take a reference to the contained string
457 //! if let Some(m) = &msg {
458 //! println!("{}", *m);
459 //! }
460 //!
461 //! // Remove the contained string, destroying the Option
462 //! let unwrapped_msg = msg.unwrap_or("default message");
463 //! ```
464 //!
465 //! Initialize a result to [`None`] before a loop:
466 //!
467 //! ```
468 //! enum Kingdom { Plant(u32, &'static str), Animal(u32, &'static str) }
469 //!
470 //! // A list of data to search through.
471 //! let all_the_big_things = [
472 //! Kingdom::Plant(250, "redwood"),
473 //! Kingdom::Plant(230, "noble fir"),
474 //! Kingdom::Plant(229, "sugar pine"),
475 //! Kingdom::Animal(25, "blue whale"),
476 //! Kingdom::Animal(19, "fin whale"),
477 //! Kingdom::Animal(15, "north pacific right whale"),
478 //! ];
479 //!
480 //! // We're going to search for the name of the biggest animal,
481 //! // but to start with we've just got `None`.
482 //! let mut name_of_biggest_animal = None;
483 //! let mut size_of_biggest_animal = 0;
484 //! for big_thing in &all_the_big_things {
485 //! match *big_thing {
486 //! Kingdom::Animal(size, name) if size > size_of_biggest_animal => {
487 //! // Now we've found the name of some big animal
488 //! size_of_biggest_animal = size;
489 //! name_of_biggest_animal = Some(name);
490 //! }
491 //! Kingdom::Animal(..) | Kingdom::Plant(..) => ()
492 //! }
493 //! }
494 //!
495 //! match name_of_biggest_animal {
496 //! Some(name) => println!("the biggest animal is {}", name),
497 //! None => println!("there are no animals :("),
498 //! }
499 //! ```
500
501 #![stable(feature = "rust1", since = "1.0.0")]
502
503 use crate::iter::{FromIterator, FusedIterator, TrustedLen};
504 use crate::pin::Pin;
505 use crate::{
506 convert, hint, mem,
507 ops::{self, ControlFlow, Deref, DerefMut},
508 };
509
510 /// The `Option` type. See [the module level documentation](self) for more.
511 #[derive(Copy, PartialEq, PartialOrd, Eq, Ord, Debug, Hash)]
512 #[rustc_diagnostic_item = "Option"]
513 #[stable(feature = "rust1", since = "1.0.0")]
514 pub enum Option<T> {
515 /// No value
516 #[lang = "None"]
517 #[stable(feature = "rust1", since = "1.0.0")]
518 None,
519 /// Some value `T`
520 #[lang = "Some"]
521 #[stable(feature = "rust1", since = "1.0.0")]
522 Some(#[stable(feature = "rust1", since = "1.0.0")] T),
523 }
524
525 /////////////////////////////////////////////////////////////////////////////
526 // Type implementation
527 /////////////////////////////////////////////////////////////////////////////
528
529 impl<T> Option<T> {
530 /////////////////////////////////////////////////////////////////////////
531 // Querying the contained values
532 /////////////////////////////////////////////////////////////////////////
533
534 /// Returns `true` if the option is a [`Some`] value.
535 ///
536 /// # Examples
537 ///
538 /// ```
539 /// let x: Option<u32> = Some(2);
540 /// assert_eq!(x.is_some(), true);
541 ///
542 /// let x: Option<u32> = None;
543 /// assert_eq!(x.is_some(), false);
544 /// ```
545 #[must_use = "if you intended to assert that this has a value, consider `.unwrap()` instead"]
546 #[inline]
547 #[stable(feature = "rust1", since = "1.0.0")]
548 #[rustc_const_stable(feature = "const_option", since = "1.48.0")]
is_some(&self) -> bool549 pub const fn is_some(&self) -> bool {
550 matches!(*self, Some(_))
551 }
552
553 /// Returns `true` if the option is a [`None`] value.
554 ///
555 /// # Examples
556 ///
557 /// ```
558 /// let x: Option<u32> = Some(2);
559 /// assert_eq!(x.is_none(), false);
560 ///
561 /// let x: Option<u32> = None;
562 /// assert_eq!(x.is_none(), true);
563 /// ```
564 #[must_use = "if you intended to assert that this doesn't have a value, consider \
565 `.and_then(|_| panic!(\"`Option` had a value when expected `None`\"))` instead"]
566 #[inline]
567 #[stable(feature = "rust1", since = "1.0.0")]
568 #[rustc_const_stable(feature = "const_option", since = "1.48.0")]
is_none(&self) -> bool569 pub const fn is_none(&self) -> bool {
570 !self.is_some()
571 }
572
573 /// Returns `true` if the option is a [`Some`] value containing the given value.
574 ///
575 /// # Examples
576 ///
577 /// ```
578 /// #![feature(option_result_contains)]
579 ///
580 /// let x: Option<u32> = Some(2);
581 /// assert_eq!(x.contains(&2), true);
582 ///
583 /// let x: Option<u32> = Some(3);
584 /// assert_eq!(x.contains(&2), false);
585 ///
586 /// let x: Option<u32> = None;
587 /// assert_eq!(x.contains(&2), false);
588 /// ```
589 #[must_use]
590 #[inline]
591 #[unstable(feature = "option_result_contains", issue = "62358")]
contains<U>(&self, x: &U) -> bool where U: PartialEq<T>,592 pub fn contains<U>(&self, x: &U) -> bool
593 where
594 U: PartialEq<T>,
595 {
596 match self {
597 Some(y) => x == y,
598 None => false,
599 }
600 }
601
602 /////////////////////////////////////////////////////////////////////////
603 // Adapter for working with references
604 /////////////////////////////////////////////////////////////////////////
605
606 /// Converts from `&Option<T>` to `Option<&T>`.
607 ///
608 /// # Examples
609 ///
610 /// Converts an <code>Option<[String]></code> into an <code>Option<[usize]></code>, preserving
611 /// the original. The [`map`] method takes the `self` argument by value, consuming the original,
612 /// so this technique uses `as_ref` to first take an `Option` to a reference
613 /// to the value inside the original.
614 ///
615 /// [`map`]: Option::map
616 /// [String]: ../../std/string/struct.String.html "String"
617 ///
618 /// ```
619 /// let text: Option<String> = Some("Hello, world!".to_string());
620 /// // First, cast `Option<String>` to `Option<&String>` with `as_ref`,
621 /// // then consume *that* with `map`, leaving `text` on the stack.
622 /// let text_length: Option<usize> = text.as_ref().map(|s| s.len());
623 /// println!("still can print text: {:?}", text);
624 /// ```
625 #[inline]
626 #[rustc_const_stable(feature = "const_option", since = "1.48.0")]
627 #[stable(feature = "rust1", since = "1.0.0")]
as_ref(&self) -> Option<&T>628 pub const fn as_ref(&self) -> Option<&T> {
629 match *self {
630 Some(ref x) => Some(x),
631 None => None,
632 }
633 }
634
635 /// Converts from `&mut Option<T>` to `Option<&mut T>`.
636 ///
637 /// # Examples
638 ///
639 /// ```
640 /// let mut x = Some(2);
641 /// match x.as_mut() {
642 /// Some(v) => *v = 42,
643 /// None => {},
644 /// }
645 /// assert_eq!(x, Some(42));
646 /// ```
647 #[inline]
648 #[stable(feature = "rust1", since = "1.0.0")]
649 #[rustc_const_unstable(feature = "const_option", issue = "67441")]
as_mut(&mut self) -> Option<&mut T>650 pub const fn as_mut(&mut self) -> Option<&mut T> {
651 match *self {
652 Some(ref mut x) => Some(x),
653 None => None,
654 }
655 }
656
657 /// Converts from <code>[Pin]<[&]Option\<T>></code> to <code>Option<[Pin]<[&]T>></code>.
658 ///
659 /// [&]: reference "shared reference"
660 #[inline]
661 #[must_use]
662 #[stable(feature = "pin", since = "1.33.0")]
as_pin_ref(self: Pin<&Self>) -> Option<Pin<&T>>663 pub fn as_pin_ref(self: Pin<&Self>) -> Option<Pin<&T>> {
664 // SAFETY: `x` is guaranteed to be pinned because it comes from `self`
665 // which is pinned.
666 unsafe { Pin::get_ref(self).as_ref().map(|x| Pin::new_unchecked(x)) }
667 }
668
669 /// Converts from <code>[Pin]<[&mut] Option\<T>></code> to <code>Option<[Pin]<[&mut] T>></code>.
670 ///
671 /// [&mut]: reference "mutable reference"
672 #[inline]
673 #[must_use]
674 #[stable(feature = "pin", since = "1.33.0")]
as_pin_mut(self: Pin<&mut Self>) -> Option<Pin<&mut T>>675 pub fn as_pin_mut(self: Pin<&mut Self>) -> Option<Pin<&mut T>> {
676 // SAFETY: `get_unchecked_mut` is never used to move the `Option` inside `self`.
677 // `x` is guaranteed to be pinned because it comes from `self` which is pinned.
678 unsafe { Pin::get_unchecked_mut(self).as_mut().map(|x| Pin::new_unchecked(x)) }
679 }
680
681 /////////////////////////////////////////////////////////////////////////
682 // Getting to contained values
683 /////////////////////////////////////////////////////////////////////////
684
685 /// Returns the contained [`Some`] value, consuming the `self` value.
686 ///
687 /// # Panics
688 ///
689 /// Panics if the value is a [`None`] with a custom panic message provided by
690 /// `msg`.
691 ///
692 /// # Examples
693 ///
694 /// ```
695 /// let x = Some("value");
696 /// assert_eq!(x.expect("fruits are healthy"), "value");
697 /// ```
698 ///
699 /// ```should_panic
700 /// let x: Option<&str> = None;
701 /// x.expect("fruits are healthy"); // panics with `fruits are healthy`
702 /// ```
703 #[inline]
704 #[track_caller]
705 #[stable(feature = "rust1", since = "1.0.0")]
expect(self, msg: &str) -> T706 pub fn expect(self, msg: &str) -> T {
707 match self {
708 Some(val) => val,
709 None => expect_failed(msg),
710 }
711 }
712
713 /// Returns the contained [`Some`] value, consuming the `self` value.
714 ///
715 /// Because this function may panic, its use is generally discouraged.
716 /// Instead, prefer to use pattern matching and handle the [`None`]
717 /// case explicitly, or call [`unwrap_or`], [`unwrap_or_else`], or
718 /// [`unwrap_or_default`].
719 ///
720 /// [`unwrap_or`]: Option::unwrap_or
721 /// [`unwrap_or_else`]: Option::unwrap_or_else
722 /// [`unwrap_or_default`]: Option::unwrap_or_default
723 ///
724 /// # Panics
725 ///
726 /// Panics if the self value equals [`None`].
727 ///
728 /// # Examples
729 ///
730 /// ```
731 /// let x = Some("air");
732 /// assert_eq!(x.unwrap(), "air");
733 /// ```
734 ///
735 /// ```should_panic
736 /// let x: Option<&str> = None;
737 /// assert_eq!(x.unwrap(), "air"); // fails
738 /// ```
739 #[inline]
740 #[track_caller]
741 #[stable(feature = "rust1", since = "1.0.0")]
742 #[rustc_const_unstable(feature = "const_option", issue = "67441")]
unwrap(self) -> T743 pub const fn unwrap(self) -> T {
744 match self {
745 Some(val) => val,
746 None => panic!("called `Option::unwrap()` on a `None` value"),
747 }
748 }
749
750 /// Returns the contained [`Some`] value or a provided default.
751 ///
752 /// Arguments passed to `unwrap_or` are eagerly evaluated; if you are passing
753 /// the result of a function call, it is recommended to use [`unwrap_or_else`],
754 /// which is lazily evaluated.
755 ///
756 /// [`unwrap_or_else`]: Option::unwrap_or_else
757 ///
758 /// # Examples
759 ///
760 /// ```
761 /// assert_eq!(Some("car").unwrap_or("bike"), "car");
762 /// assert_eq!(None.unwrap_or("bike"), "bike");
763 /// ```
764 #[inline]
765 #[stable(feature = "rust1", since = "1.0.0")]
unwrap_or(self, default: T) -> T766 pub fn unwrap_or(self, default: T) -> T {
767 match self {
768 Some(x) => x,
769 None => default,
770 }
771 }
772
773 /// Returns the contained [`Some`] value or computes it from a closure.
774 ///
775 /// # Examples
776 ///
777 /// ```
778 /// let k = 10;
779 /// assert_eq!(Some(4).unwrap_or_else(|| 2 * k), 4);
780 /// assert_eq!(None.unwrap_or_else(|| 2 * k), 20);
781 /// ```
782 #[inline]
783 #[stable(feature = "rust1", since = "1.0.0")]
unwrap_or_else<F: FnOnce() -> T>(self, f: F) -> T784 pub fn unwrap_or_else<F: FnOnce() -> T>(self, f: F) -> T {
785 match self {
786 Some(x) => x,
787 None => f(),
788 }
789 }
790
791 /// Returns the contained [`Some`] value, consuming the `self` value,
792 /// without checking that the value is not [`None`].
793 ///
794 /// # Safety
795 ///
796 /// Calling this method on [`None`] is *[undefined behavior]*.
797 ///
798 /// [undefined behavior]: https://doc.rust-lang.org/reference/behavior-considered-undefined.html
799 ///
800 /// # Examples
801 ///
802 /// ```
803 /// let x = Some("air");
804 /// assert_eq!(unsafe { x.unwrap_unchecked() }, "air");
805 /// ```
806 ///
807 /// ```no_run
808 /// let x: Option<&str> = None;
809 /// assert_eq!(unsafe { x.unwrap_unchecked() }, "air"); // Undefined behavior!
810 /// ```
811 #[inline]
812 #[track_caller]
813 #[stable(feature = "option_result_unwrap_unchecked", since = "1.58.0")]
unwrap_unchecked(self) -> T814 pub unsafe fn unwrap_unchecked(self) -> T {
815 debug_assert!(self.is_some());
816 match self {
817 Some(val) => val,
818 // SAFETY: the safety contract must be upheld by the caller.
819 None => unsafe { hint::unreachable_unchecked() },
820 }
821 }
822
823 /////////////////////////////////////////////////////////////////////////
824 // Transforming contained values
825 /////////////////////////////////////////////////////////////////////////
826
827 /// Maps an `Option<T>` to `Option<U>` by applying a function to a contained value.
828 ///
829 /// # Examples
830 ///
831 /// Converts an <code>Option<[String]></code> into an <code>Option<[usize]></code>, consuming
832 /// the original:
833 ///
834 /// [String]: ../../std/string/struct.String.html "String"
835 /// ```
836 /// let maybe_some_string = Some(String::from("Hello, World!"));
837 /// // `Option::map` takes self *by value*, consuming `maybe_some_string`
838 /// let maybe_some_len = maybe_some_string.map(|s| s.len());
839 ///
840 /// assert_eq!(maybe_some_len, Some(13));
841 /// ```
842 #[inline]
843 #[stable(feature = "rust1", since = "1.0.0")]
map<U, F: FnOnce(T) -> U>(self, f: F) -> Option<U>844 pub fn map<U, F: FnOnce(T) -> U>(self, f: F) -> Option<U> {
845 match self {
846 Some(x) => Some(f(x)),
847 None => None,
848 }
849 }
850
851 /// Returns the provided default result (if none),
852 /// or applies a function to the contained value (if any).
853 ///
854 /// Arguments passed to `map_or` are eagerly evaluated; if you are passing
855 /// the result of a function call, it is recommended to use [`map_or_else`],
856 /// which is lazily evaluated.
857 ///
858 /// [`map_or_else`]: Option::map_or_else
859 ///
860 /// # Examples
861 ///
862 /// ```
863 /// let x = Some("foo");
864 /// assert_eq!(x.map_or(42, |v| v.len()), 3);
865 ///
866 /// let x: Option<&str> = None;
867 /// assert_eq!(x.map_or(42, |v| v.len()), 42);
868 /// ```
869 #[inline]
870 #[stable(feature = "rust1", since = "1.0.0")]
map_or<U, F: FnOnce(T) -> U>(self, default: U, f: F) -> U871 pub fn map_or<U, F: FnOnce(T) -> U>(self, default: U, f: F) -> U {
872 match self {
873 Some(t) => f(t),
874 None => default,
875 }
876 }
877
878 /// Computes a default function result (if none), or
879 /// applies a different function to the contained value (if any).
880 ///
881 /// # Examples
882 ///
883 /// ```
884 /// let k = 21;
885 ///
886 /// let x = Some("foo");
887 /// assert_eq!(x.map_or_else(|| 2 * k, |v| v.len()), 3);
888 ///
889 /// let x: Option<&str> = None;
890 /// assert_eq!(x.map_or_else(|| 2 * k, |v| v.len()), 42);
891 /// ```
892 #[inline]
893 #[stable(feature = "rust1", since = "1.0.0")]
map_or_else<U, D: FnOnce() -> U, F: FnOnce(T) -> U>(self, default: D, f: F) -> U894 pub fn map_or_else<U, D: FnOnce() -> U, F: FnOnce(T) -> U>(self, default: D, f: F) -> U {
895 match self {
896 Some(t) => f(t),
897 None => default(),
898 }
899 }
900
901 /// Transforms the `Option<T>` into a [`Result<T, E>`], mapping [`Some(v)`] to
902 /// [`Ok(v)`] and [`None`] to [`Err(err)`].
903 ///
904 /// Arguments passed to `ok_or` are eagerly evaluated; if you are passing the
905 /// result of a function call, it is recommended to use [`ok_or_else`], which is
906 /// lazily evaluated.
907 ///
908 /// [`Ok(v)`]: Ok
909 /// [`Err(err)`]: Err
910 /// [`Some(v)`]: Some
911 /// [`ok_or_else`]: Option::ok_or_else
912 ///
913 /// # Examples
914 ///
915 /// ```
916 /// let x = Some("foo");
917 /// assert_eq!(x.ok_or(0), Ok("foo"));
918 ///
919 /// let x: Option<&str> = None;
920 /// assert_eq!(x.ok_or(0), Err(0));
921 /// ```
922 #[inline]
923 #[stable(feature = "rust1", since = "1.0.0")]
ok_or<E>(self, err: E) -> Result<T, E>924 pub fn ok_or<E>(self, err: E) -> Result<T, E> {
925 match self {
926 Some(v) => Ok(v),
927 None => Err(err),
928 }
929 }
930
931 /// Transforms the `Option<T>` into a [`Result<T, E>`], mapping [`Some(v)`] to
932 /// [`Ok(v)`] and [`None`] to [`Err(err())`].
933 ///
934 /// [`Ok(v)`]: Ok
935 /// [`Err(err())`]: Err
936 /// [`Some(v)`]: Some
937 ///
938 /// # Examples
939 ///
940 /// ```
941 /// let x = Some("foo");
942 /// assert_eq!(x.ok_or_else(|| 0), Ok("foo"));
943 ///
944 /// let x: Option<&str> = None;
945 /// assert_eq!(x.ok_or_else(|| 0), Err(0));
946 /// ```
947 #[inline]
948 #[stable(feature = "rust1", since = "1.0.0")]
ok_or_else<E, F: FnOnce() -> E>(self, err: F) -> Result<T, E>949 pub fn ok_or_else<E, F: FnOnce() -> E>(self, err: F) -> Result<T, E> {
950 match self {
951 Some(v) => Ok(v),
952 None => Err(err()),
953 }
954 }
955
956 /////////////////////////////////////////////////////////////////////////
957 // Iterator constructors
958 /////////////////////////////////////////////////////////////////////////
959
960 /// Returns an iterator over the possibly contained value.
961 ///
962 /// # Examples
963 ///
964 /// ```
965 /// let x = Some(4);
966 /// assert_eq!(x.iter().next(), Some(&4));
967 ///
968 /// let x: Option<u32> = None;
969 /// assert_eq!(x.iter().next(), None);
970 /// ```
971 #[inline]
972 #[rustc_const_unstable(feature = "const_option", issue = "67441")]
973 #[stable(feature = "rust1", since = "1.0.0")]
iter(&self) -> Iter<'_, T>974 pub const fn iter(&self) -> Iter<'_, T> {
975 Iter { inner: Item { opt: self.as_ref() } }
976 }
977
978 /// Returns a mutable iterator over the possibly contained value.
979 ///
980 /// # Examples
981 ///
982 /// ```
983 /// let mut x = Some(4);
984 /// match x.iter_mut().next() {
985 /// Some(v) => *v = 42,
986 /// None => {},
987 /// }
988 /// assert_eq!(x, Some(42));
989 ///
990 /// let mut x: Option<u32> = None;
991 /// assert_eq!(x.iter_mut().next(), None);
992 /// ```
993 #[inline]
994 #[stable(feature = "rust1", since = "1.0.0")]
iter_mut(&mut self) -> IterMut<'_, T>995 pub fn iter_mut(&mut self) -> IterMut<'_, T> {
996 IterMut { inner: Item { opt: self.as_mut() } }
997 }
998
999 /////////////////////////////////////////////////////////////////////////
1000 // Boolean operations on the values, eager and lazy
1001 /////////////////////////////////////////////////////////////////////////
1002
1003 /// Returns [`None`] if the option is [`None`], otherwise returns `optb`.
1004 ///
1005 /// # Examples
1006 ///
1007 /// ```
1008 /// let x = Some(2);
1009 /// let y: Option<&str> = None;
1010 /// assert_eq!(x.and(y), None);
1011 ///
1012 /// let x: Option<u32> = None;
1013 /// let y = Some("foo");
1014 /// assert_eq!(x.and(y), None);
1015 ///
1016 /// let x = Some(2);
1017 /// let y = Some("foo");
1018 /// assert_eq!(x.and(y), Some("foo"));
1019 ///
1020 /// let x: Option<u32> = None;
1021 /// let y: Option<&str> = None;
1022 /// assert_eq!(x.and(y), None);
1023 /// ```
1024 #[inline]
1025 #[stable(feature = "rust1", since = "1.0.0")]
and<U>(self, optb: Option<U>) -> Option<U>1026 pub fn and<U>(self, optb: Option<U>) -> Option<U> {
1027 match self {
1028 Some(_) => optb,
1029 None => None,
1030 }
1031 }
1032
1033 /// Returns [`None`] if the option is [`None`], otherwise calls `f` with the
1034 /// wrapped value and returns the result.
1035 ///
1036 /// Some languages call this operation flatmap.
1037 ///
1038 /// # Examples
1039 ///
1040 /// ```
1041 /// fn sq(x: u32) -> Option<u32> { Some(x * x) }
1042 /// fn nope(_: u32) -> Option<u32> { None }
1043 ///
1044 /// assert_eq!(Some(2).and_then(sq).and_then(sq), Some(16));
1045 /// assert_eq!(Some(2).and_then(sq).and_then(nope), None);
1046 /// assert_eq!(Some(2).and_then(nope).and_then(sq), None);
1047 /// assert_eq!(None.and_then(sq).and_then(sq), None);
1048 /// ```
1049 #[inline]
1050 #[stable(feature = "rust1", since = "1.0.0")]
and_then<U, F: FnOnce(T) -> Option<U>>(self, f: F) -> Option<U>1051 pub fn and_then<U, F: FnOnce(T) -> Option<U>>(self, f: F) -> Option<U> {
1052 match self {
1053 Some(x) => f(x),
1054 None => None,
1055 }
1056 }
1057
1058 /// Returns [`None`] if the option is [`None`], otherwise calls `predicate`
1059 /// with the wrapped value and returns:
1060 ///
1061 /// - [`Some(t)`] if `predicate` returns `true` (where `t` is the wrapped
1062 /// value), and
1063 /// - [`None`] if `predicate` returns `false`.
1064 ///
1065 /// This function works similar to [`Iterator::filter()`]. You can imagine
1066 /// the `Option<T>` being an iterator over one or zero elements. `filter()`
1067 /// lets you decide which elements to keep.
1068 ///
1069 /// # Examples
1070 ///
1071 /// ```rust
1072 /// fn is_even(n: &i32) -> bool {
1073 /// n % 2 == 0
1074 /// }
1075 ///
1076 /// assert_eq!(None.filter(is_even), None);
1077 /// assert_eq!(Some(3).filter(is_even), None);
1078 /// assert_eq!(Some(4).filter(is_even), Some(4));
1079 /// ```
1080 ///
1081 /// [`Some(t)`]: Some
1082 #[inline]
1083 #[stable(feature = "option_filter", since = "1.27.0")]
filter<P: FnOnce(&T) -> bool>(self, predicate: P) -> Self1084 pub fn filter<P: FnOnce(&T) -> bool>(self, predicate: P) -> Self {
1085 if let Some(x) = self {
1086 if predicate(&x) {
1087 return Some(x);
1088 }
1089 }
1090 None
1091 }
1092
1093 /// Returns the option if it contains a value, otherwise returns `optb`.
1094 ///
1095 /// Arguments passed to `or` are eagerly evaluated; if you are passing the
1096 /// result of a function call, it is recommended to use [`or_else`], which is
1097 /// lazily evaluated.
1098 ///
1099 /// [`or_else`]: Option::or_else
1100 ///
1101 /// # Examples
1102 ///
1103 /// ```
1104 /// let x = Some(2);
1105 /// let y = None;
1106 /// assert_eq!(x.or(y), Some(2));
1107 ///
1108 /// let x = None;
1109 /// let y = Some(100);
1110 /// assert_eq!(x.or(y), Some(100));
1111 ///
1112 /// let x = Some(2);
1113 /// let y = Some(100);
1114 /// assert_eq!(x.or(y), Some(2));
1115 ///
1116 /// let x: Option<u32> = None;
1117 /// let y = None;
1118 /// assert_eq!(x.or(y), None);
1119 /// ```
1120 #[inline]
1121 #[stable(feature = "rust1", since = "1.0.0")]
or(self, optb: Option<T>) -> Option<T>1122 pub fn or(self, optb: Option<T>) -> Option<T> {
1123 match self {
1124 Some(_) => self,
1125 None => optb,
1126 }
1127 }
1128
1129 /// Returns the option if it contains a value, otherwise calls `f` and
1130 /// returns the result.
1131 ///
1132 /// # Examples
1133 ///
1134 /// ```
1135 /// fn nobody() -> Option<&'static str> { None }
1136 /// fn vikings() -> Option<&'static str> { Some("vikings") }
1137 ///
1138 /// assert_eq!(Some("barbarians").or_else(vikings), Some("barbarians"));
1139 /// assert_eq!(None.or_else(vikings), Some("vikings"));
1140 /// assert_eq!(None.or_else(nobody), None);
1141 /// ```
1142 #[inline]
1143 #[stable(feature = "rust1", since = "1.0.0")]
or_else<F: FnOnce() -> Option<T>>(self, f: F) -> Option<T>1144 pub fn or_else<F: FnOnce() -> Option<T>>(self, f: F) -> Option<T> {
1145 match self {
1146 Some(_) => self,
1147 None => f(),
1148 }
1149 }
1150
1151 /// Returns [`Some`] if exactly one of `self`, `optb` is [`Some`], otherwise returns [`None`].
1152 ///
1153 /// # Examples
1154 ///
1155 /// ```
1156 /// let x = Some(2);
1157 /// let y: Option<u32> = None;
1158 /// assert_eq!(x.xor(y), Some(2));
1159 ///
1160 /// let x: Option<u32> = None;
1161 /// let y = Some(2);
1162 /// assert_eq!(x.xor(y), Some(2));
1163 ///
1164 /// let x = Some(2);
1165 /// let y = Some(2);
1166 /// assert_eq!(x.xor(y), None);
1167 ///
1168 /// let x: Option<u32> = None;
1169 /// let y: Option<u32> = None;
1170 /// assert_eq!(x.xor(y), None);
1171 /// ```
1172 #[inline]
1173 #[stable(feature = "option_xor", since = "1.37.0")]
xor(self, optb: Option<T>) -> Option<T>1174 pub fn xor(self, optb: Option<T>) -> Option<T> {
1175 match (self, optb) {
1176 (Some(a), None) => Some(a),
1177 (None, Some(b)) => Some(b),
1178 _ => None,
1179 }
1180 }
1181
1182 /////////////////////////////////////////////////////////////////////////
1183 // Entry-like operations to insert a value and return a reference
1184 /////////////////////////////////////////////////////////////////////////
1185
1186 /// Inserts `value` into the option, then returns a mutable reference to it.
1187 ///
1188 /// If the option already contains a value, the old value is dropped.
1189 ///
1190 /// See also [`Option::get_or_insert`], which doesn't update the value if
1191 /// the option already contains [`Some`].
1192 ///
1193 /// # Example
1194 ///
1195 /// ```
1196 /// let mut opt = None;
1197 /// let val = opt.insert(1);
1198 /// assert_eq!(*val, 1);
1199 /// assert_eq!(opt.unwrap(), 1);
1200 /// let val = opt.insert(2);
1201 /// assert_eq!(*val, 2);
1202 /// *val = 3;
1203 /// assert_eq!(opt.unwrap(), 3);
1204 /// ```
1205 #[must_use = "if you intended to set a value, consider assignment instead"]
1206 #[inline]
1207 #[stable(feature = "option_insert", since = "1.53.0")]
insert(&mut self, value: T) -> &mut T1208 pub fn insert(&mut self, value: T) -> &mut T {
1209 *self = Some(value);
1210
1211 // SAFETY: the code above just filled the option
1212 unsafe { self.as_mut().unwrap_unchecked() }
1213 }
1214
1215 /// Inserts `value` into the option if it is [`None`], then
1216 /// returns a mutable reference to the contained value.
1217 ///
1218 /// See also [`Option::insert`], which updates the value even if
1219 /// the option already contains [`Some`].
1220 ///
1221 /// # Examples
1222 ///
1223 /// ```
1224 /// let mut x = None;
1225 ///
1226 /// {
1227 /// let y: &mut u32 = x.get_or_insert(5);
1228 /// assert_eq!(y, &5);
1229 ///
1230 /// *y = 7;
1231 /// }
1232 ///
1233 /// assert_eq!(x, Some(7));
1234 /// ```
1235 #[inline]
1236 #[stable(feature = "option_entry", since = "1.20.0")]
get_or_insert(&mut self, value: T) -> &mut T1237 pub fn get_or_insert(&mut self, value: T) -> &mut T {
1238 self.get_or_insert_with(|| value)
1239 }
1240
1241 /// Inserts the default value into the option if it is [`None`], then
1242 /// returns a mutable reference to the contained value.
1243 ///
1244 /// # Examples
1245 ///
1246 /// ```
1247 /// #![feature(option_get_or_insert_default)]
1248 ///
1249 /// let mut x = None;
1250 ///
1251 /// {
1252 /// let y: &mut u32 = x.get_or_insert_default();
1253 /// assert_eq!(y, &0);
1254 ///
1255 /// *y = 7;
1256 /// }
1257 ///
1258 /// assert_eq!(x, Some(7));
1259 /// ```
1260 #[inline]
1261 #[unstable(feature = "option_get_or_insert_default", issue = "82901")]
get_or_insert_default(&mut self) -> &mut T where T: Default,1262 pub fn get_or_insert_default(&mut self) -> &mut T
1263 where
1264 T: Default,
1265 {
1266 self.get_or_insert_with(Default::default)
1267 }
1268
1269 /// Inserts a value computed from `f` into the option if it is [`None`],
1270 /// then returns a mutable reference to the contained value.
1271 ///
1272 /// # Examples
1273 ///
1274 /// ```
1275 /// let mut x = None;
1276 ///
1277 /// {
1278 /// let y: &mut u32 = x.get_or_insert_with(|| 5);
1279 /// assert_eq!(y, &5);
1280 ///
1281 /// *y = 7;
1282 /// }
1283 ///
1284 /// assert_eq!(x, Some(7));
1285 /// ```
1286 #[inline]
1287 #[stable(feature = "option_entry", since = "1.20.0")]
get_or_insert_with<F: FnOnce() -> T>(&mut self, f: F) -> &mut T1288 pub fn get_or_insert_with<F: FnOnce() -> T>(&mut self, f: F) -> &mut T {
1289 if let None = *self {
1290 *self = Some(f());
1291 }
1292
1293 match self {
1294 Some(v) => v,
1295 // SAFETY: a `None` variant for `self` would have been replaced by a `Some`
1296 // variant in the code above.
1297 None => unsafe { hint::unreachable_unchecked() },
1298 }
1299 }
1300
1301 /////////////////////////////////////////////////////////////////////////
1302 // Misc
1303 /////////////////////////////////////////////////////////////////////////
1304
1305 /// Takes the value out of the option, leaving a [`None`] in its place.
1306 ///
1307 /// # Examples
1308 ///
1309 /// ```
1310 /// let mut x = Some(2);
1311 /// let y = x.take();
1312 /// assert_eq!(x, None);
1313 /// assert_eq!(y, Some(2));
1314 ///
1315 /// let mut x: Option<u32> = None;
1316 /// let y = x.take();
1317 /// assert_eq!(x, None);
1318 /// assert_eq!(y, None);
1319 /// ```
1320 #[inline]
1321 #[stable(feature = "rust1", since = "1.0.0")]
1322 #[rustc_const_unstable(feature = "const_option", issue = "67441")]
take(&mut self) -> Option<T>1323 pub const fn take(&mut self) -> Option<T> {
1324 // FIXME replace `mem::replace` by `mem::take` when the latter is const ready
1325 mem::replace(self, None)
1326 }
1327
1328 /// Replaces the actual value in the option by the value given in parameter,
1329 /// returning the old value if present,
1330 /// leaving a [`Some`] in its place without deinitializing either one.
1331 ///
1332 /// # Examples
1333 ///
1334 /// ```
1335 /// let mut x = Some(2);
1336 /// let old = x.replace(5);
1337 /// assert_eq!(x, Some(5));
1338 /// assert_eq!(old, Some(2));
1339 ///
1340 /// let mut x = None;
1341 /// let old = x.replace(3);
1342 /// assert_eq!(x, Some(3));
1343 /// assert_eq!(old, None);
1344 /// ```
1345 #[inline]
1346 #[rustc_const_unstable(feature = "const_option", issue = "67441")]
1347 #[stable(feature = "option_replace", since = "1.31.0")]
replace(&mut self, value: T) -> Option<T>1348 pub const fn replace(&mut self, value: T) -> Option<T> {
1349 mem::replace(self, Some(value))
1350 }
1351
1352 /// Zips `self` with another `Option`.
1353 ///
1354 /// If `self` is `Some(s)` and `other` is `Some(o)`, this method returns `Some((s, o))`.
1355 /// Otherwise, `None` is returned.
1356 ///
1357 /// # Examples
1358 ///
1359 /// ```
1360 /// let x = Some(1);
1361 /// let y = Some("hi");
1362 /// let z = None::<u8>;
1363 ///
1364 /// assert_eq!(x.zip(y), Some((1, "hi")));
1365 /// assert_eq!(x.zip(z), None);
1366 /// ```
1367 #[stable(feature = "option_zip_option", since = "1.46.0")]
zip<U>(self, other: Option<U>) -> Option<(T, U)>1368 pub fn zip<U>(self, other: Option<U>) -> Option<(T, U)> {
1369 match (self, other) {
1370 (Some(a), Some(b)) => Some((a, b)),
1371 _ => None,
1372 }
1373 }
1374
1375 /// Zips `self` and another `Option` with function `f`.
1376 ///
1377 /// If `self` is `Some(s)` and `other` is `Some(o)`, this method returns `Some(f(s, o))`.
1378 /// Otherwise, `None` is returned.
1379 ///
1380 /// # Examples
1381 ///
1382 /// ```
1383 /// #![feature(option_zip)]
1384 ///
1385 /// #[derive(Debug, PartialEq)]
1386 /// struct Point {
1387 /// x: f64,
1388 /// y: f64,
1389 /// }
1390 ///
1391 /// impl Point {
1392 /// fn new(x: f64, y: f64) -> Self {
1393 /// Self { x, y }
1394 /// }
1395 /// }
1396 ///
1397 /// let x = Some(17.5);
1398 /// let y = Some(42.7);
1399 ///
1400 /// assert_eq!(x.zip_with(y, Point::new), Some(Point { x: 17.5, y: 42.7 }));
1401 /// assert_eq!(x.zip_with(None, Point::new), None);
1402 /// ```
1403 #[unstable(feature = "option_zip", issue = "70086")]
zip_with<U, F, R>(self, other: Option<U>, f: F) -> Option<R> where F: FnOnce(T, U) -> R,1404 pub fn zip_with<U, F, R>(self, other: Option<U>, f: F) -> Option<R>
1405 where
1406 F: FnOnce(T, U) -> R,
1407 {
1408 Some(f(self?, other?))
1409 }
1410 }
1411
1412 impl<T, U> Option<(T, U)> {
1413 /// Unzips an option containing a tuple of two options.
1414 ///
1415 /// If `self` is `Some((a, b))` this method returns `(Some(a), Some(b))`.
1416 /// Otherwise, `(None, None)` is returned.
1417 ///
1418 /// # Examples
1419 ///
1420 /// ```
1421 /// #![feature(unzip_option)]
1422 ///
1423 /// let x = Some((1, "hi"));
1424 /// let y = None::<(u8, u32)>;
1425 ///
1426 /// assert_eq!(x.unzip(), (Some(1), Some("hi")));
1427 /// assert_eq!(y.unzip(), (None, None));
1428 /// ```
1429 #[inline]
1430 #[unstable(feature = "unzip_option", issue = "87800", reason = "recently added")]
unzip(self) -> (Option<T>, Option<U>)1431 pub const fn unzip(self) -> (Option<T>, Option<U>) {
1432 match self {
1433 Some((a, b)) => (Some(a), Some(b)),
1434 None => (None, None),
1435 }
1436 }
1437 }
1438
1439 impl<T: Copy> Option<&T> {
1440 /// Maps an `Option<&T>` to an `Option<T>` by copying the contents of the
1441 /// option.
1442 ///
1443 /// # Examples
1444 ///
1445 /// ```
1446 /// let x = 12;
1447 /// let opt_x = Some(&x);
1448 /// assert_eq!(opt_x, Some(&12));
1449 /// let copied = opt_x.copied();
1450 /// assert_eq!(copied, Some(12));
1451 /// ```
1452 #[must_use = "`self` will be dropped if the result is not used"]
1453 #[stable(feature = "copied", since = "1.35.0")]
1454 #[rustc_const_unstable(feature = "const_option", issue = "67441")]
copied(self) -> Option<T>1455 pub const fn copied(self) -> Option<T> {
1456 // FIXME: this implementation, which sidesteps using `Option::map` since it's not const
1457 // ready yet, should be reverted when possible to avoid code repetition
1458 match self {
1459 Some(&v) => Some(v),
1460 None => None,
1461 }
1462 }
1463 }
1464
1465 impl<T: Copy> Option<&mut T> {
1466 /// Maps an `Option<&mut T>` to an `Option<T>` by copying the contents of the
1467 /// option.
1468 ///
1469 /// # Examples
1470 ///
1471 /// ```
1472 /// let mut x = 12;
1473 /// let opt_x = Some(&mut x);
1474 /// assert_eq!(opt_x, Some(&mut 12));
1475 /// let copied = opt_x.copied();
1476 /// assert_eq!(copied, Some(12));
1477 /// ```
1478 #[must_use = "`self` will be dropped if the result is not used"]
1479 #[stable(feature = "copied", since = "1.35.0")]
copied(self) -> Option<T>1480 pub fn copied(self) -> Option<T> {
1481 self.map(|&mut t| t)
1482 }
1483 }
1484
1485 impl<T: Clone> Option<&T> {
1486 /// Maps an `Option<&T>` to an `Option<T>` by cloning the contents of the
1487 /// option.
1488 ///
1489 /// # Examples
1490 ///
1491 /// ```
1492 /// let x = 12;
1493 /// let opt_x = Some(&x);
1494 /// assert_eq!(opt_x, Some(&12));
1495 /// let cloned = opt_x.cloned();
1496 /// assert_eq!(cloned, Some(12));
1497 /// ```
1498 #[must_use = "`self` will be dropped if the result is not used"]
1499 #[stable(feature = "rust1", since = "1.0.0")]
cloned(self) -> Option<T>1500 pub fn cloned(self) -> Option<T> {
1501 self.map(|t| t.clone())
1502 }
1503 }
1504
1505 impl<T: Clone> Option<&mut T> {
1506 /// Maps an `Option<&mut T>` to an `Option<T>` by cloning the contents of the
1507 /// option.
1508 ///
1509 /// # Examples
1510 ///
1511 /// ```
1512 /// let mut x = 12;
1513 /// let opt_x = Some(&mut x);
1514 /// assert_eq!(opt_x, Some(&mut 12));
1515 /// let cloned = opt_x.cloned();
1516 /// assert_eq!(cloned, Some(12));
1517 /// ```
1518 #[stable(since = "1.26.0", feature = "option_ref_mut_cloned")]
cloned(self) -> Option<T>1519 pub fn cloned(self) -> Option<T> {
1520 self.map(|t| t.clone())
1521 }
1522 }
1523
1524 impl<T: Default> Option<T> {
1525 /// Returns the contained [`Some`] value or a default.
1526 ///
1527 /// Consumes the `self` argument then, if [`Some`], returns the contained
1528 /// value, otherwise if [`None`], returns the [default value] for that
1529 /// type.
1530 ///
1531 /// # Examples
1532 ///
1533 /// Converts a string to an integer, turning poorly-formed strings
1534 /// into 0 (the default value for integers). [`parse`] converts
1535 /// a string to any other type that implements [`FromStr`], returning
1536 /// [`None`] on error.
1537 ///
1538 /// ```
1539 /// let good_year_from_input = "1909";
1540 /// let bad_year_from_input = "190blarg";
1541 /// let good_year = good_year_from_input.parse().ok().unwrap_or_default();
1542 /// let bad_year = bad_year_from_input.parse().ok().unwrap_or_default();
1543 ///
1544 /// assert_eq!(1909, good_year);
1545 /// assert_eq!(0, bad_year);
1546 /// ```
1547 ///
1548 /// [default value]: Default::default
1549 /// [`parse`]: str::parse
1550 /// [`FromStr`]: crate::str::FromStr
1551 #[inline]
1552 #[stable(feature = "rust1", since = "1.0.0")]
unwrap_or_default(self) -> T1553 pub fn unwrap_or_default(self) -> T {
1554 match self {
1555 Some(x) => x,
1556 None => Default::default(),
1557 }
1558 }
1559 }
1560
1561 impl<T: Deref> Option<T> {
1562 /// Converts from `Option<T>` (or `&Option<T>`) to `Option<&T::Target>`.
1563 ///
1564 /// Leaves the original Option in-place, creating a new one with a reference
1565 /// to the original one, additionally coercing the contents via [`Deref`].
1566 ///
1567 /// # Examples
1568 ///
1569 /// ```
1570 /// let x: Option<String> = Some("hey".to_owned());
1571 /// assert_eq!(x.as_deref(), Some("hey"));
1572 ///
1573 /// let x: Option<String> = None;
1574 /// assert_eq!(x.as_deref(), None);
1575 /// ```
1576 #[stable(feature = "option_deref", since = "1.40.0")]
as_deref(&self) -> Option<&T::Target>1577 pub fn as_deref(&self) -> Option<&T::Target> {
1578 self.as_ref().map(|t| t.deref())
1579 }
1580 }
1581
1582 impl<T: DerefMut> Option<T> {
1583 /// Converts from `Option<T>` (or `&mut Option<T>`) to `Option<&mut T::Target>`.
1584 ///
1585 /// Leaves the original `Option` in-place, creating a new one containing a mutable reference to
1586 /// the inner type's [`Deref::Target`] type.
1587 ///
1588 /// # Examples
1589 ///
1590 /// ```
1591 /// let mut x: Option<String> = Some("hey".to_owned());
1592 /// assert_eq!(x.as_deref_mut().map(|x| {
1593 /// x.make_ascii_uppercase();
1594 /// x
1595 /// }), Some("HEY".to_owned().as_mut_str()));
1596 /// ```
1597 #[stable(feature = "option_deref", since = "1.40.0")]
as_deref_mut(&mut self) -> Option<&mut T::Target>1598 pub fn as_deref_mut(&mut self) -> Option<&mut T::Target> {
1599 self.as_mut().map(|t| t.deref_mut())
1600 }
1601 }
1602
1603 impl<T, E> Option<Result<T, E>> {
1604 /// Transposes an `Option` of a [`Result`] into a [`Result`] of an `Option`.
1605 ///
1606 /// [`None`] will be mapped to <code>[Ok]\([None])</code>.
1607 /// <code>[Some]\([Ok]\(\_))</code> and <code>[Some]\([Err]\(\_))</code> will be mapped to
1608 /// <code>[Ok]\([Some]\(\_))</code> and <code>[Err]\(\_)</code>.
1609 ///
1610 /// # Examples
1611 ///
1612 /// ```
1613 /// #[derive(Debug, Eq, PartialEq)]
1614 /// struct SomeErr;
1615 ///
1616 /// let x: Result<Option<i32>, SomeErr> = Ok(Some(5));
1617 /// let y: Option<Result<i32, SomeErr>> = Some(Ok(5));
1618 /// assert_eq!(x, y.transpose());
1619 /// ```
1620 #[inline]
1621 #[stable(feature = "transpose_result", since = "1.33.0")]
1622 #[rustc_const_unstable(feature = "const_option", issue = "67441")]
transpose(self) -> Result<Option<T>, E>1623 pub const fn transpose(self) -> Result<Option<T>, E> {
1624 match self {
1625 Some(Ok(x)) => Ok(Some(x)),
1626 Some(Err(e)) => Err(e),
1627 None => Ok(None),
1628 }
1629 }
1630 }
1631
1632 // This is a separate function to reduce the code size of .expect() itself.
1633 #[inline(never)]
1634 #[cold]
1635 #[track_caller]
expect_failed(msg: &str) -> !1636 fn expect_failed(msg: &str) -> ! {
1637 panic!("{}", msg)
1638 }
1639
1640 /////////////////////////////////////////////////////////////////////////////
1641 // Trait implementations
1642 /////////////////////////////////////////////////////////////////////////////
1643
1644 #[stable(feature = "rust1", since = "1.0.0")]
1645 impl<T: Clone> Clone for Option<T> {
1646 #[inline]
clone(&self) -> Self1647 fn clone(&self) -> Self {
1648 match self {
1649 Some(x) => Some(x.clone()),
1650 None => None,
1651 }
1652 }
1653
1654 #[inline]
clone_from(&mut self, source: &Self)1655 fn clone_from(&mut self, source: &Self) {
1656 match (self, source) {
1657 (Some(to), Some(from)) => to.clone_from(from),
1658 (to, from) => *to = from.clone(),
1659 }
1660 }
1661 }
1662
1663 #[stable(feature = "rust1", since = "1.0.0")]
1664 #[rustc_const_unstable(feature = "const_default_impls", issue = "87864")]
1665 impl<T> const Default for Option<T> {
1666 /// Returns [`None`][Option::None].
1667 ///
1668 /// # Examples
1669 ///
1670 /// ```
1671 /// let opt: Option<u32> = Option::default();
1672 /// assert!(opt.is_none());
1673 /// ```
1674 #[inline]
default() -> Option<T>1675 fn default() -> Option<T> {
1676 None
1677 }
1678 }
1679
1680 #[stable(feature = "rust1", since = "1.0.0")]
1681 impl<T> IntoIterator for Option<T> {
1682 type Item = T;
1683 type IntoIter = IntoIter<T>;
1684
1685 /// Returns a consuming iterator over the possibly contained value.
1686 ///
1687 /// # Examples
1688 ///
1689 /// ```
1690 /// let x = Some("string");
1691 /// let v: Vec<&str> = x.into_iter().collect();
1692 /// assert_eq!(v, ["string"]);
1693 ///
1694 /// let x = None;
1695 /// let v: Vec<&str> = x.into_iter().collect();
1696 /// assert!(v.is_empty());
1697 /// ```
1698 #[inline]
into_iter(self) -> IntoIter<T>1699 fn into_iter(self) -> IntoIter<T> {
1700 IntoIter { inner: Item { opt: self } }
1701 }
1702 }
1703
1704 #[stable(since = "1.4.0", feature = "option_iter")]
1705 impl<'a, T> IntoIterator for &'a Option<T> {
1706 type Item = &'a T;
1707 type IntoIter = Iter<'a, T>;
1708
into_iter(self) -> Iter<'a, T>1709 fn into_iter(self) -> Iter<'a, T> {
1710 self.iter()
1711 }
1712 }
1713
1714 #[stable(since = "1.4.0", feature = "option_iter")]
1715 impl<'a, T> IntoIterator for &'a mut Option<T> {
1716 type Item = &'a mut T;
1717 type IntoIter = IterMut<'a, T>;
1718
into_iter(self) -> IterMut<'a, T>1719 fn into_iter(self) -> IterMut<'a, T> {
1720 self.iter_mut()
1721 }
1722 }
1723
1724 #[stable(since = "1.12.0", feature = "option_from")]
1725 #[rustc_const_unstable(feature = "const_convert", issue = "88674")]
1726 impl<T> const From<T> for Option<T> {
1727 /// Moves `val` into a new [`Some`].
1728 ///
1729 /// # Examples
1730 ///
1731 /// ```
1732 /// let o: Option<u8> = Option::from(67);
1733 ///
1734 /// assert_eq!(Some(67), o);
1735 /// ```
from(val: T) -> Option<T>1736 fn from(val: T) -> Option<T> {
1737 Some(val)
1738 }
1739 }
1740
1741 #[stable(feature = "option_ref_from_ref_option", since = "1.30.0")]
1742 #[rustc_const_unstable(feature = "const_convert", issue = "88674")]
1743 impl<'a, T> const From<&'a Option<T>> for Option<&'a T> {
1744 /// Converts from `&Option<T>` to `Option<&T>`.
1745 ///
1746 /// # Examples
1747 ///
1748 /// Converts an <code>[Option]<[String]></code> into an <code>[Option]<[usize]></code>, preserving
1749 /// the original. The [`map`] method takes the `self` argument by value, consuming the original,
1750 /// so this technique uses `from` to first take an [`Option`] to a reference
1751 /// to the value inside the original.
1752 ///
1753 /// [`map`]: Option::map
1754 /// [String]: ../../std/string/struct.String.html "String"
1755 ///
1756 /// ```
1757 /// let s: Option<String> = Some(String::from("Hello, Rustaceans!"));
1758 /// let o: Option<usize> = Option::from(&s).map(|ss: &String| ss.len());
1759 ///
1760 /// println!("Can still print s: {:?}", s);
1761 ///
1762 /// assert_eq!(o, Some(18));
1763 /// ```
from(o: &'a Option<T>) -> Option<&'a T>1764 fn from(o: &'a Option<T>) -> Option<&'a T> {
1765 o.as_ref()
1766 }
1767 }
1768
1769 #[stable(feature = "option_ref_from_ref_option", since = "1.30.0")]
1770 #[rustc_const_unstable(feature = "const_convert", issue = "88674")]
1771 impl<'a, T> const From<&'a mut Option<T>> for Option<&'a mut T> {
1772 /// Converts from `&mut Option<T>` to `Option<&mut T>`
1773 ///
1774 /// # Examples
1775 ///
1776 /// ```
1777 /// let mut s = Some(String::from("Hello"));
1778 /// let o: Option<&mut String> = Option::from(&mut s);
1779 ///
1780 /// match o {
1781 /// Some(t) => *t = String::from("Hello, Rustaceans!"),
1782 /// None => (),
1783 /// }
1784 ///
1785 /// assert_eq!(s, Some(String::from("Hello, Rustaceans!")));
1786 /// ```
from(o: &'a mut Option<T>) -> Option<&'a mut T>1787 fn from(o: &'a mut Option<T>) -> Option<&'a mut T> {
1788 o.as_mut()
1789 }
1790 }
1791
1792 /////////////////////////////////////////////////////////////////////////////
1793 // The Option Iterators
1794 /////////////////////////////////////////////////////////////////////////////
1795
1796 #[derive(Clone, Debug)]
1797 struct Item<A> {
1798 opt: Option<A>,
1799 }
1800
1801 impl<A> Iterator for Item<A> {
1802 type Item = A;
1803
1804 #[inline]
next(&mut self) -> Option<A>1805 fn next(&mut self) -> Option<A> {
1806 self.opt.take()
1807 }
1808
1809 #[inline]
size_hint(&self) -> (usize, Option<usize>)1810 fn size_hint(&self) -> (usize, Option<usize>) {
1811 match self.opt {
1812 Some(_) => (1, Some(1)),
1813 None => (0, Some(0)),
1814 }
1815 }
1816 }
1817
1818 impl<A> DoubleEndedIterator for Item<A> {
1819 #[inline]
next_back(&mut self) -> Option<A>1820 fn next_back(&mut self) -> Option<A> {
1821 self.opt.take()
1822 }
1823 }
1824
1825 impl<A> ExactSizeIterator for Item<A> {}
1826 impl<A> FusedIterator for Item<A> {}
1827 unsafe impl<A> TrustedLen for Item<A> {}
1828
1829 /// An iterator over a reference to the [`Some`] variant of an [`Option`].
1830 ///
1831 /// The iterator yields one value if the [`Option`] is a [`Some`], otherwise none.
1832 ///
1833 /// This `struct` is created by the [`Option::iter`] function.
1834 #[stable(feature = "rust1", since = "1.0.0")]
1835 #[derive(Debug)]
1836 pub struct Iter<'a, A: 'a> {
1837 inner: Item<&'a A>,
1838 }
1839
1840 #[stable(feature = "rust1", since = "1.0.0")]
1841 impl<'a, A> Iterator for Iter<'a, A> {
1842 type Item = &'a A;
1843
1844 #[inline]
next(&mut self) -> Option<&'a A>1845 fn next(&mut self) -> Option<&'a A> {
1846 self.inner.next()
1847 }
1848 #[inline]
size_hint(&self) -> (usize, Option<usize>)1849 fn size_hint(&self) -> (usize, Option<usize>) {
1850 self.inner.size_hint()
1851 }
1852 }
1853
1854 #[stable(feature = "rust1", since = "1.0.0")]
1855 impl<'a, A> DoubleEndedIterator for Iter<'a, A> {
1856 #[inline]
next_back(&mut self) -> Option<&'a A>1857 fn next_back(&mut self) -> Option<&'a A> {
1858 self.inner.next_back()
1859 }
1860 }
1861
1862 #[stable(feature = "rust1", since = "1.0.0")]
1863 impl<A> ExactSizeIterator for Iter<'_, A> {}
1864
1865 #[stable(feature = "fused", since = "1.26.0")]
1866 impl<A> FusedIterator for Iter<'_, A> {}
1867
1868 #[unstable(feature = "trusted_len", issue = "37572")]
1869 unsafe impl<A> TrustedLen for Iter<'_, A> {}
1870
1871 #[stable(feature = "rust1", since = "1.0.0")]
1872 impl<A> Clone for Iter<'_, A> {
1873 #[inline]
clone(&self) -> Self1874 fn clone(&self) -> Self {
1875 Iter { inner: self.inner.clone() }
1876 }
1877 }
1878
1879 /// An iterator over a mutable reference to the [`Some`] variant of an [`Option`].
1880 ///
1881 /// The iterator yields one value if the [`Option`] is a [`Some`], otherwise none.
1882 ///
1883 /// This `struct` is created by the [`Option::iter_mut`] function.
1884 #[stable(feature = "rust1", since = "1.0.0")]
1885 #[derive(Debug)]
1886 pub struct IterMut<'a, A: 'a> {
1887 inner: Item<&'a mut A>,
1888 }
1889
1890 #[stable(feature = "rust1", since = "1.0.0")]
1891 impl<'a, A> Iterator for IterMut<'a, A> {
1892 type Item = &'a mut A;
1893
1894 #[inline]
next(&mut self) -> Option<&'a mut A>1895 fn next(&mut self) -> Option<&'a mut A> {
1896 self.inner.next()
1897 }
1898 #[inline]
size_hint(&self) -> (usize, Option<usize>)1899 fn size_hint(&self) -> (usize, Option<usize>) {
1900 self.inner.size_hint()
1901 }
1902 }
1903
1904 #[stable(feature = "rust1", since = "1.0.0")]
1905 impl<'a, A> DoubleEndedIterator for IterMut<'a, A> {
1906 #[inline]
next_back(&mut self) -> Option<&'a mut A>1907 fn next_back(&mut self) -> Option<&'a mut A> {
1908 self.inner.next_back()
1909 }
1910 }
1911
1912 #[stable(feature = "rust1", since = "1.0.0")]
1913 impl<A> ExactSizeIterator for IterMut<'_, A> {}
1914
1915 #[stable(feature = "fused", since = "1.26.0")]
1916 impl<A> FusedIterator for IterMut<'_, A> {}
1917 #[unstable(feature = "trusted_len", issue = "37572")]
1918 unsafe impl<A> TrustedLen for IterMut<'_, A> {}
1919
1920 /// An iterator over the value in [`Some`] variant of an [`Option`].
1921 ///
1922 /// The iterator yields one value if the [`Option`] is a [`Some`], otherwise none.
1923 ///
1924 /// This `struct` is created by the [`Option::into_iter`] function.
1925 #[derive(Clone, Debug)]
1926 #[stable(feature = "rust1", since = "1.0.0")]
1927 pub struct IntoIter<A> {
1928 inner: Item<A>,
1929 }
1930
1931 #[stable(feature = "rust1", since = "1.0.0")]
1932 impl<A> Iterator for IntoIter<A> {
1933 type Item = A;
1934
1935 #[inline]
next(&mut self) -> Option<A>1936 fn next(&mut self) -> Option<A> {
1937 self.inner.next()
1938 }
1939 #[inline]
size_hint(&self) -> (usize, Option<usize>)1940 fn size_hint(&self) -> (usize, Option<usize>) {
1941 self.inner.size_hint()
1942 }
1943 }
1944
1945 #[stable(feature = "rust1", since = "1.0.0")]
1946 impl<A> DoubleEndedIterator for IntoIter<A> {
1947 #[inline]
next_back(&mut self) -> Option<A>1948 fn next_back(&mut self) -> Option<A> {
1949 self.inner.next_back()
1950 }
1951 }
1952
1953 #[stable(feature = "rust1", since = "1.0.0")]
1954 impl<A> ExactSizeIterator for IntoIter<A> {}
1955
1956 #[stable(feature = "fused", since = "1.26.0")]
1957 impl<A> FusedIterator for IntoIter<A> {}
1958
1959 #[unstable(feature = "trusted_len", issue = "37572")]
1960 unsafe impl<A> TrustedLen for IntoIter<A> {}
1961
1962 /////////////////////////////////////////////////////////////////////////////
1963 // FromIterator
1964 /////////////////////////////////////////////////////////////////////////////
1965
1966 #[stable(feature = "rust1", since = "1.0.0")]
1967 impl<A, V: FromIterator<A>> FromIterator<Option<A>> for Option<V> {
1968 /// Takes each element in the [`Iterator`]: if it is [`None`][Option::None],
1969 /// no further elements are taken, and the [`None`][Option::None] is
1970 /// returned. Should no [`None`][Option::None] occur, a container of type
1971 /// `V` containing the values of each [`Option`] is returned.
1972 ///
1973 /// # Examples
1974 ///
1975 /// Here is an example which increments every integer in a vector.
1976 /// We use the checked variant of `add` that returns `None` when the
1977 /// calculation would result in an overflow.
1978 ///
1979 /// ```
1980 /// let items = vec![0_u16, 1, 2];
1981 ///
1982 /// let res: Option<Vec<u16>> = items
1983 /// .iter()
1984 /// .map(|x| x.checked_add(1))
1985 /// .collect();
1986 ///
1987 /// assert_eq!(res, Some(vec![1, 2, 3]));
1988 /// ```
1989 ///
1990 /// As you can see, this will return the expected, valid items.
1991 ///
1992 /// Here is another example that tries to subtract one from another list
1993 /// of integers, this time checking for underflow:
1994 ///
1995 /// ```
1996 /// let items = vec![2_u16, 1, 0];
1997 ///
1998 /// let res: Option<Vec<u16>> = items
1999 /// .iter()
2000 /// .map(|x| x.checked_sub(1))
2001 /// .collect();
2002 ///
2003 /// assert_eq!(res, None);
2004 /// ```
2005 ///
2006 /// Since the last element is zero, it would underflow. Thus, the resulting
2007 /// value is `None`.
2008 ///
2009 /// Here is a variation on the previous example, showing that no
2010 /// further elements are taken from `iter` after the first `None`.
2011 ///
2012 /// ```
2013 /// let items = vec![3_u16, 2, 1, 10];
2014 ///
2015 /// let mut shared = 0;
2016 ///
2017 /// let res: Option<Vec<u16>> = items
2018 /// .iter()
2019 /// .map(|x| { shared += x; x.checked_sub(2) })
2020 /// .collect();
2021 ///
2022 /// assert_eq!(res, None);
2023 /// assert_eq!(shared, 6);
2024 /// ```
2025 ///
2026 /// Since the third element caused an underflow, no further elements were taken,
2027 /// so the final value of `shared` is 6 (= `3 + 2 + 1`), not 16.
2028 #[inline]
from_iter<I: IntoIterator<Item = Option<A>>>(iter: I) -> Option<V>2029 fn from_iter<I: IntoIterator<Item = Option<A>>>(iter: I) -> Option<V> {
2030 // FIXME(#11084): This could be replaced with Iterator::scan when this
2031 // performance bug is closed.
2032
2033 iter.into_iter().map(|x| x.ok_or(())).collect::<Result<_, _>>().ok()
2034 }
2035 }
2036
2037 #[unstable(feature = "try_trait_v2", issue = "84277")]
2038 impl<T> ops::Try for Option<T> {
2039 type Output = T;
2040 type Residual = Option<convert::Infallible>;
2041
2042 #[inline]
from_output(output: Self::Output) -> Self2043 fn from_output(output: Self::Output) -> Self {
2044 Some(output)
2045 }
2046
2047 #[inline]
branch(self) -> ControlFlow<Self::Residual, Self::Output>2048 fn branch(self) -> ControlFlow<Self::Residual, Self::Output> {
2049 match self {
2050 Some(v) => ControlFlow::Continue(v),
2051 None => ControlFlow::Break(None),
2052 }
2053 }
2054 }
2055
2056 #[unstable(feature = "try_trait_v2", issue = "84277")]
2057 impl<T> const ops::FromResidual for Option<T> {
2058 #[inline]
from_residual(residual: Option<convert::Infallible>) -> Self2059 fn from_residual(residual: Option<convert::Infallible>) -> Self {
2060 match residual {
2061 None => None,
2062 }
2063 }
2064 }
2065
2066 impl<T> Option<Option<T>> {
2067 /// Converts from `Option<Option<T>>` to `Option<T>`.
2068 ///
2069 /// # Examples
2070 ///
2071 /// Basic usage:
2072 ///
2073 /// ```
2074 /// let x: Option<Option<u32>> = Some(Some(6));
2075 /// assert_eq!(Some(6), x.flatten());
2076 ///
2077 /// let x: Option<Option<u32>> = Some(None);
2078 /// assert_eq!(None, x.flatten());
2079 ///
2080 /// let x: Option<Option<u32>> = None;
2081 /// assert_eq!(None, x.flatten());
2082 /// ```
2083 ///
2084 /// Flattening only removes one level of nesting at a time:
2085 ///
2086 /// ```
2087 /// let x: Option<Option<Option<u32>>> = Some(Some(Some(6)));
2088 /// assert_eq!(Some(Some(6)), x.flatten());
2089 /// assert_eq!(Some(6), x.flatten().flatten());
2090 /// ```
2091 #[inline]
2092 #[stable(feature = "option_flattening", since = "1.40.0")]
2093 #[rustc_const_unstable(feature = "const_option", issue = "67441")]
flatten(self) -> Option<T>2094 pub const fn flatten(self) -> Option<T> {
2095 match self {
2096 Some(inner) => inner,
2097 None => None,
2098 }
2099 }
2100 }
2101