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:
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
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
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.
23 //! fn divide(numerator: f64, denominator: f64) -> Option<f64> {
24 //! if denominator == 0.0 {
27 //! Some(numerator / denominator)
31 //! // The return value of the function is an option
32 //! let result = divide(2.0, 3.0);
34 //! // Pattern match to retrieve the value
36 //! // The division was valid
37 //! Some(x) => println!("Result: {x}"),
38 //! // The division was invalid
39 //! None => println!("Cannot divide by 0"),
44 // FIXME: Show how `Option` is used in practice, with lots of methods
46 //! # Options and pointers ("nullable" pointers)
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>.
52 //! [Box\<T>]: ../../std/boxed/struct.Box.html
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
61 //! let optional = None;
62 //! check_optional(optional);
64 //! let optional = Some(Box::new(9000));
65 //! check_optional(optional);
67 //! fn check_optional(optional: Option<Box<i32>>) {
69 //! Some(p) => println!("has value {p}"),
70 //! None => println!("has no value"),
75 //! # The question mark operator, `?`
77 //! Similar to the [`Result`] type, when writing code that calls many functions that return the
78 //! [`Option`] type, handling `Some`/`None` can be tedious. The question mark
79 //! operator, [`?`], hides some of the boilerplate of propagating values
80 //! up the call stack.
85 //! # #![allow(dead_code)]
86 //! fn add_last_numbers(stack: &mut Vec<i32>) -> Option<i32> {
87 //! let a = stack.pop();
88 //! let b = stack.pop();
91 //! (Some(x), Some(y)) => Some(x + y),
101 //! # #![allow(dead_code)]
102 //! fn add_last_numbers(stack: &mut Vec<i32>) -> Option<i32> {
103 //! Some(stack.pop()? + stack.pop()?)
107 //! *It's much nicer!*
109 //! Ending the expression with [`?`] will result in the [`Some`]'s unwrapped value, unless the
110 //! result is [`None`], in which case [`None`] is returned early from the enclosing function.
112 //! [`?`] can be used in functions that return [`Option`] because of the
113 //! early return of [`None`] that it provides.
115 //! [`?`]: crate::ops::Try
121 //! Rust guarantees to optimize the following types `T` such that
122 //! [`Option<T>`] has the same size as `T`:
127 //! * `fn`, `extern "C" fn`[^extern_fn]
128 //! * [`num::NonZero*`]
129 //! * [`ptr::NonNull<U>`]
130 //! * `#[repr(transparent)]` struct around one of the types in this list.
132 //! [^extern_fn]: this remains true for any other ABI: `extern "abi" fn` (_e.g._, `extern "system" fn`)
134 //! [`Box<U>`]: ../../std/boxed/struct.Box.html
135 //! [`num::NonZero*`]: crate::num
136 //! [`ptr::NonNull<U>`]: crate::ptr::NonNull
138 //! This is called the "null pointer optimization" or NPO.
140 //! It is further guaranteed that, for the cases above, one can
141 //! [`mem::transmute`] from all valid values of `T` to `Option<T>` and
142 //! from `Some::<T>(_)` to `T` (but transmuting `None::<T>` to `T`
143 //! is undefined behaviour).
145 //! # Method overview
147 //! In addition to working with pattern matching, [`Option`] provides a wide
148 //! variety of different methods.
150 //! ## Querying the variant
152 //! The [`is_some`] and [`is_none`] methods return [`true`] if the [`Option`]
153 //! is [`Some`] or [`None`], respectively.
155 //! [`is_none`]: Option::is_none
156 //! [`is_some`]: Option::is_some
158 //! ## Adapters for working with references
160 //! * [`as_ref`] converts from <code>[&][][Option]\<T></code> to <code>[Option]<[&]T></code>
161 //! * [`as_mut`] converts from <code>[&mut] [Option]\<T></code> to <code>[Option]<[&mut] T></code>
162 //! * [`as_deref`] converts from <code>[&][][Option]\<T></code> to
163 //! <code>[Option]<[&]T::[Target]></code>
164 //! * [`as_deref_mut`] converts from <code>[&mut] [Option]\<T></code> to
165 //! <code>[Option]<[&mut] T::[Target]></code>
166 //! * [`as_pin_ref`] converts from <code>[Pin]<[&][][Option]\<T>></code> to
167 //! <code>[Option]<[Pin]<[&]T>></code>
168 //! * [`as_pin_mut`] converts from <code>[Pin]<[&mut] [Option]\<T>></code> to
169 //! <code>[Option]<[Pin]<[&mut] T>></code>
171 //! [&]: reference "shared reference"
172 //! [&mut]: reference "mutable reference"
173 //! [Target]: Deref::Target "ops::Deref::Target"
174 //! [`as_deref`]: Option::as_deref
175 //! [`as_deref_mut`]: Option::as_deref_mut
176 //! [`as_mut`]: Option::as_mut
177 //! [`as_pin_mut`]: Option::as_pin_mut
178 //! [`as_pin_ref`]: Option::as_pin_ref
179 //! [`as_ref`]: Option::as_ref
181 //! ## Extracting the contained value
183 //! These methods extract the contained value in an [`Option<T>`] when it
184 //! is the [`Some`] variant. If the [`Option`] is [`None`]:
186 //! * [`expect`] panics with a provided custom message
187 //! * [`unwrap`] panics with a generic message
188 //! * [`unwrap_or`] returns the provided default value
189 //! * [`unwrap_or_default`] returns the default value of the type `T`
190 //! (which must implement the [`Default`] trait)
191 //! * [`unwrap_or_else`] returns the result of evaluating the provided
194 //! [`expect`]: Option::expect
195 //! [`unwrap`]: Option::unwrap
196 //! [`unwrap_or`]: Option::unwrap_or
197 //! [`unwrap_or_default`]: Option::unwrap_or_default
198 //! [`unwrap_or_else`]: Option::unwrap_or_else
200 //! ## Transforming contained values
202 //! These methods transform [`Option`] to [`Result`]:
204 //! * [`ok_or`] transforms [`Some(v)`] to [`Ok(v)`], and [`None`] to
205 //! [`Err(err)`] using the provided default `err` value
206 //! * [`ok_or_else`] transforms [`Some(v)`] to [`Ok(v)`], and [`None`] to
207 //! a value of [`Err`] using the provided function
208 //! * [`transpose`] transposes an [`Option`] of a [`Result`] into a
209 //! [`Result`] of an [`Option`]
211 //! [`Err(err)`]: Err
213 //! [`Some(v)`]: Some
214 //! [`ok_or`]: Option::ok_or
215 //! [`ok_or_else`]: Option::ok_or_else
216 //! [`transpose`]: Option::transpose
218 //! These methods transform the [`Some`] variant:
220 //! * [`filter`] calls the provided predicate function on the contained
221 //! value `t` if the [`Option`] is [`Some(t)`], and returns [`Some(t)`]
222 //! if the function returns `true`; otherwise, returns [`None`]
223 //! * [`flatten`] removes one level of nesting from an
224 //! [`Option<Option<T>>`]
225 //! * [`map`] transforms [`Option<T>`] to [`Option<U>`] by applying the
226 //! provided function to the contained value of [`Some`] and leaving
227 //! [`None`] values unchanged
229 //! [`Some(t)`]: Some
230 //! [`filter`]: Option::filter
231 //! [`flatten`]: Option::flatten
232 //! [`map`]: Option::map
234 //! These methods transform [`Option<T>`] to a value of a possibly
235 //! different type `U`:
237 //! * [`map_or`] applies the provided function to the contained value of
238 //! [`Some`], or returns the provided default value if the [`Option`] is
240 //! * [`map_or_else`] applies the provided function to the contained value
241 //! of [`Some`], or returns the result of evaluating the provided
242 //! fallback function if the [`Option`] is [`None`]
244 //! [`map_or`]: Option::map_or
245 //! [`map_or_else`]: Option::map_or_else
247 //! These methods combine the [`Some`] variants of two [`Option`] values:
249 //! * [`zip`] returns [`Some((s, o))`] if `self` is [`Some(s)`] and the
250 //! provided [`Option`] value is [`Some(o)`]; otherwise, returns [`None`]
251 //! * [`zip_with`] calls the provided function `f` and returns
252 //! [`Some(f(s, o))`] if `self` is [`Some(s)`] and the provided
253 //! [`Option`] value is [`Some(o)`]; otherwise, returns [`None`]
255 //! [`Some(f(s, o))`]: Some
256 //! [`Some(o)`]: Some
257 //! [`Some(s)`]: Some
258 //! [`Some((s, o))`]: Some
259 //! [`zip`]: Option::zip
260 //! [`zip_with`]: Option::zip_with
262 //! ## Boolean operators
264 //! These methods treat the [`Option`] as a boolean value, where [`Some`]
265 //! acts like [`true`] and [`None`] acts like [`false`]. There are two
266 //! categories of these methods: ones that take an [`Option`] as input, and
267 //! ones that take a function as input (to be lazily evaluated).
269 //! The [`and`], [`or`], and [`xor`] methods take another [`Option`] as
270 //! input, and produce an [`Option`] as output. Only the [`and`] method can
271 //! produce an [`Option<U>`] value having a different inner type `U` than
274 //! | method | self | input | output |
275 //! |---------|-----------|-----------|-----------|
276 //! | [`and`] | `None` | (ignored) | `None` |
277 //! | [`and`] | `Some(x)` | `None` | `None` |
278 //! | [`and`] | `Some(x)` | `Some(y)` | `Some(y)` |
279 //! | [`or`] | `None` | `None` | `None` |
280 //! | [`or`] | `None` | `Some(y)` | `Some(y)` |
281 //! | [`or`] | `Some(x)` | (ignored) | `Some(x)` |
282 //! | [`xor`] | `None` | `None` | `None` |
283 //! | [`xor`] | `None` | `Some(y)` | `Some(y)` |
284 //! | [`xor`] | `Some(x)` | `None` | `Some(x)` |
285 //! | [`xor`] | `Some(x)` | `Some(y)` | `None` |
287 //! [`and`]: Option::and
288 //! [`or`]: Option::or
289 //! [`xor`]: Option::xor
291 //! The [`and_then`] and [`or_else`] methods take a function as input, and
292 //! only evaluate the function when they need to produce a new value. Only
293 //! the [`and_then`] method can produce an [`Option<U>`] value having a
294 //! different inner type `U` than [`Option<T>`].
296 //! | method | self | function input | function result | output |
297 //! |--------------|-----------|----------------|-----------------|-----------|
298 //! | [`and_then`] | `None` | (not provided) | (not evaluated) | `None` |
299 //! | [`and_then`] | `Some(x)` | `x` | `None` | `None` |
300 //! | [`and_then`] | `Some(x)` | `x` | `Some(y)` | `Some(y)` |
301 //! | [`or_else`] | `None` | (not provided) | `None` | `None` |
302 //! | [`or_else`] | `None` | (not provided) | `Some(y)` | `Some(y)` |
303 //! | [`or_else`] | `Some(x)` | (not provided) | (not evaluated) | `Some(x)` |
305 //! [`and_then`]: Option::and_then
306 //! [`or_else`]: Option::or_else
308 //! This is an example of using methods like [`and_then`] and [`or`] in a
309 //! pipeline of method calls. Early stages of the pipeline pass failure
310 //! values ([`None`]) through unchanged, and continue processing on
311 //! success values ([`Some`]). Toward the end, [`or`] substitutes an error
312 //! message if it receives [`None`].
315 //! # use std::collections::BTreeMap;
316 //! let mut bt = BTreeMap::new();
317 //! bt.insert(20u8, "foo");
318 //! bt.insert(42u8, "bar");
319 //! let res = [0u8, 1, 11, 200, 22]
322 //! // `checked_sub()` returns `None` on error
324 //! // same with `checked_mul()`
325 //! .and_then(|x| x.checked_mul(2))
326 //! // `BTreeMap::get` returns `None` on error
327 //! .and_then(|x| bt.get(&x))
328 //! // Substitute an error message if we have `None` so far
329 //! .or(Some(&"error!"))
331 //! // Won't panic because we unconditionally used `Some` above
334 //! .collect::<Vec<_>>();
335 //! assert_eq!(res, ["error!", "error!", "foo", "error!", "bar"]);
338 //! ## Comparison operators
340 //! If `T` implements [`PartialOrd`] then [`Option<T>`] will derive its
341 //! [`PartialOrd`] implementation. With this order, [`None`] compares as
342 //! less than any [`Some`], and two [`Some`] compare the same way as their
343 //! contained values would in `T`. If `T` also implements
344 //! [`Ord`], then so does [`Option<T>`].
347 //! assert!(None < Some(0));
348 //! assert!(Some(0) < Some(1));
351 //! ## Iterating over `Option`
353 //! An [`Option`] can be iterated over. This can be helpful if you need an
354 //! iterator that is conditionally empty. The iterator will either produce
355 //! a single value (when the [`Option`] is [`Some`]), or produce no values
356 //! (when the [`Option`] is [`None`]). For example, [`into_iter`] acts like
357 //! [`once(v)`] if the [`Option`] is [`Some(v)`], and like [`empty()`] if
358 //! the [`Option`] is [`None`].
360 //! [`Some(v)`]: Some
361 //! [`empty()`]: crate::iter::empty
362 //! [`once(v)`]: crate::iter::once
364 //! Iterators over [`Option<T>`] come in three types:
366 //! * [`into_iter`] consumes the [`Option`] and produces the contained
368 //! * [`iter`] produces an immutable reference of type `&T` to the
370 //! * [`iter_mut`] produces a mutable reference of type `&mut T` to the
373 //! [`into_iter`]: Option::into_iter
374 //! [`iter`]: Option::iter
375 //! [`iter_mut`]: Option::iter_mut
377 //! An iterator over [`Option`] can be useful when chaining iterators, for
378 //! example, to conditionally insert items. (It's not always necessary to
379 //! explicitly call an iterator constructor: many [`Iterator`] methods that
380 //! accept other iterators will also accept iterable types that implement
381 //! [`IntoIterator`], which includes [`Option`].)
384 //! let yep = Some(42);
386 //! // chain() already calls into_iter(), so we don't have to do so
387 //! let nums: Vec<i32> = (0..4).chain(yep).chain(4..8).collect();
388 //! assert_eq!(nums, [0, 1, 2, 3, 42, 4, 5, 6, 7]);
389 //! let nums: Vec<i32> = (0..4).chain(nope).chain(4..8).collect();
390 //! assert_eq!(nums, [0, 1, 2, 3, 4, 5, 6, 7]);
393 //! One reason to chain iterators in this way is that a function returning
394 //! `impl Iterator` must have all possible return values be of the same
395 //! concrete type. Chaining an iterated [`Option`] can help with that.
398 //! fn make_iter(do_insert: bool) -> impl Iterator<Item = i32> {
399 //! // Explicit returns to illustrate return types matching
400 //! match do_insert {
401 //! true => return (0..4).chain(Some(42)).chain(4..8),
402 //! false => return (0..4).chain(None).chain(4..8),
405 //! println!("{:?}", make_iter(true).collect::<Vec<_>>());
406 //! println!("{:?}", make_iter(false).collect::<Vec<_>>());
409 //! If we try to do the same thing, but using [`once()`] and [`empty()`],
410 //! we can't return `impl Iterator` anymore because the concrete types of
411 //! the return values differ.
413 //! [`empty()`]: crate::iter::empty
414 //! [`once()`]: crate::iter::once
416 //! ```compile_fail,E0308
417 //! # use std::iter::{empty, once};
418 //! // This won't compile because all possible returns from the function
419 //! // must have the same concrete type.
420 //! fn make_iter(do_insert: bool) -> impl Iterator<Item = i32> {
421 //! // Explicit returns to illustrate return types not matching
422 //! match do_insert {
423 //! true => return (0..4).chain(once(42)).chain(4..8),
424 //! false => return (0..4).chain(empty()).chain(4..8),
429 //! ## Collecting into `Option`
431 //! [`Option`] implements the [`FromIterator`][impl-FromIterator] trait,
432 //! which allows an iterator over [`Option`] values to be collected into an
433 //! [`Option`] of a collection of each contained value of the original
434 //! [`Option`] values, or [`None`] if any of the elements was [`None`].
436 //! [impl-FromIterator]: Option#impl-FromIterator%3COption%3CA%3E%3E-for-Option%3CV%3E
439 //! let v = [Some(2), Some(4), None, Some(8)];
440 //! let res: Option<Vec<_>> = v.into_iter().collect();
441 //! assert_eq!(res, None);
442 //! let v = [Some(2), Some(4), Some(8)];
443 //! let res: Option<Vec<_>> = v.into_iter().collect();
444 //! assert_eq!(res, Some(vec![2, 4, 8]));
447 //! [`Option`] also implements the [`Product`][impl-Product] and
448 //! [`Sum`][impl-Sum] traits, allowing an iterator over [`Option`] values
449 //! to provide the [`product`][Iterator::product] and
450 //! [`sum`][Iterator::sum] methods.
452 //! [impl-Product]: Option#impl-Product%3COption%3CU%3E%3E-for-Option%3CT%3E
453 //! [impl-Sum]: Option#impl-Sum%3COption%3CU%3E%3E-for-Option%3CT%3E
456 //! let v = [None, Some(1), Some(2), Some(3)];
457 //! let res: Option<i32> = v.into_iter().sum();
458 //! assert_eq!(res, None);
459 //! let v = [Some(1), Some(2), Some(21)];
460 //! let res: Option<i32> = v.into_iter().product();
461 //! assert_eq!(res, Some(42));
464 //! ## Modifying an [`Option`] in-place
466 //! These methods return a mutable reference to the contained value of an
469 //! * [`insert`] inserts a value, dropping any old contents
470 //! * [`get_or_insert`] gets the current value, inserting a provided
471 //! default value if it is [`None`]
472 //! * [`get_or_insert_default`] gets the current value, inserting the
473 //! default value of type `T` (which must implement [`Default`]) if it is
475 //! * [`get_or_insert_with`] gets the current value, inserting a default
476 //! computed by the provided function if it is [`None`]
478 //! [`get_or_insert`]: Option::get_or_insert
479 //! [`get_or_insert_default`]: Option::get_or_insert_default
480 //! [`get_or_insert_with`]: Option::get_or_insert_with
481 //! [`insert`]: Option::insert
483 //! These methods transfer ownership of the contained value of an
486 //! * [`take`] takes ownership of the contained value of an [`Option`], if
487 //! any, replacing the [`Option`] with [`None`]
488 //! * [`replace`] takes ownership of the contained value of an [`Option`],
489 //! if any, replacing the [`Option`] with a [`Some`] containing the
492 //! [`replace`]: Option::replace
493 //! [`take`]: Option::take
497 //! Basic pattern matching on [`Option`]:
500 //! let msg = Some("howdy");
502 //! // Take a reference to the contained string
503 //! if let Some(m) = &msg {
504 //! println!("{}", *m);
507 //! // Remove the contained string, destroying the Option
508 //! let unwrapped_msg = msg.unwrap_or("default message");
511 //! Initialize a result to [`None`] before a loop:
514 //! enum Kingdom { Plant(u32, &'static str), Animal(u32, &'static str) }
516 //! // A list of data to search through.
517 //! let all_the_big_things = [
518 //! Kingdom::Plant(250, "redwood"),
519 //! Kingdom::Plant(230, "noble fir"),
520 //! Kingdom::Plant(229, "sugar pine"),
521 //! Kingdom::Animal(25, "blue whale"),
522 //! Kingdom::Animal(19, "fin whale"),
523 //! Kingdom::Animal(15, "north pacific right whale"),
526 //! // We're going to search for the name of the biggest animal,
527 //! // but to start with we've just got `None`.
528 //! let mut name_of_biggest_animal = None;
529 //! let mut size_of_biggest_animal = 0;
530 //! for big_thing in &all_the_big_things {
531 //! match *big_thing {
532 //! Kingdom::Animal(size, name) if size > size_of_biggest_animal => {
533 //! // Now we've found the name of some big animal
534 //! size_of_biggest_animal = size;
535 //! name_of_biggest_animal = Some(name);
537 //! Kingdom::Animal(..) | Kingdom::Plant(..) => ()
541 //! match name_of_biggest_animal {
542 //! Some(name) => println!("the biggest animal is {name}"),
543 //! None => println!("there are no animals :("),
547 #![stable(feature = "rust1", since = "1.0.0")]
549 use crate::iter::{self, FromIterator, FusedIterator, TrustedLen};
550 use crate::marker::Destruct;
551 use crate::panicking::{panic, panic_str};
555 ops::{self, ControlFlow, Deref, DerefMut},
558 /// The `Option` type. See [the module level documentation](self) for more.
559 #[derive(Copy, PartialOrd, Eq, Ord, Debug, Hash)]
560 #[rustc_diagnostic_item = "Option"]
561 #[stable(feature = "rust1", since = "1.0.0")]
565 #[stable(feature = "rust1", since = "1.0.0")]
567 /// Some value of type `T`.
569 #[stable(feature = "rust1", since = "1.0.0")]
570 Some(#[stable(feature = "rust1", since = "1.0.0")] T),
573 /////////////////////////////////////////////////////////////////////////////
574 // Type implementation
575 /////////////////////////////////////////////////////////////////////////////
578 /////////////////////////////////////////////////////////////////////////
579 // Querying the contained values
580 /////////////////////////////////////////////////////////////////////////
582 /// Returns `true` if the option is a [`Some`] value.
587 /// let x: Option<u32> = Some(2);
588 /// assert_eq!(x.is_some(), true);
590 /// let x: Option<u32> = None;
591 /// assert_eq!(x.is_some(), false);
593 #[must_use = "if you intended to assert that this has a value, consider `.unwrap()` instead"]
595 #[stable(feature = "rust1", since = "1.0.0")]
596 #[rustc_const_stable(feature = "const_option_basics", since = "1.48.0")]
597 pub const fn is_some(&self) -> bool {
598 matches!(*self, Some(_))
601 /// Returns `true` if the option is a [`Some`] and the value inside of it matches a predicate.
606 /// #![feature(is_some_and)]
608 /// let x: Option<u32> = Some(2);
609 /// assert_eq!(x.is_some_and(|x| x > 1), true);
611 /// let x: Option<u32> = Some(0);
612 /// assert_eq!(x.is_some_and(|x| x > 1), false);
614 /// let x: Option<u32> = None;
615 /// assert_eq!(x.is_some_and(|x| x > 1), false);
619 #[unstable(feature = "is_some_and", issue = "93050")]
620 pub fn is_some_and(self, f: impl FnOnce(T) -> bool) -> bool {
627 /// Returns `true` if the option is a [`None`] value.
632 /// let x: Option<u32> = Some(2);
633 /// assert_eq!(x.is_none(), false);
635 /// let x: Option<u32> = None;
636 /// assert_eq!(x.is_none(), true);
638 #[must_use = "if you intended to assert that this doesn't have a value, consider \
639 `.and_then(|_| panic!(\"`Option` had a value when expected `None`\"))` instead"]
641 #[stable(feature = "rust1", since = "1.0.0")]
642 #[rustc_const_stable(feature = "const_option_basics", since = "1.48.0")]
643 pub const fn is_none(&self) -> bool {
647 /////////////////////////////////////////////////////////////////////////
648 // Adapter for working with references
649 /////////////////////////////////////////////////////////////////////////
651 /// Converts from `&Option<T>` to `Option<&T>`.
655 /// Calculates the length of an <code>Option<[String]></code> as an <code>Option<[usize]></code>
656 /// without moving the [`String`]. The [`map`] method takes the `self` argument by value,
657 /// consuming the original, so this technique uses `as_ref` to first take an `Option` to a
658 /// reference to the value inside the original.
660 /// [`map`]: Option::map
661 /// [String]: ../../std/string/struct.String.html "String"
662 /// [`String`]: ../../std/string/struct.String.html "String"
665 /// let text: Option<String> = Some("Hello, world!".to_string());
666 /// // First, cast `Option<String>` to `Option<&String>` with `as_ref`,
667 /// // then consume *that* with `map`, leaving `text` on the stack.
668 /// let text_length: Option<usize> = text.as_ref().map(|s| s.len());
669 /// println!("still can print text: {text:?}");
672 #[rustc_const_stable(feature = "const_option_basics", since = "1.48.0")]
673 #[stable(feature = "rust1", since = "1.0.0")]
674 pub const fn as_ref(&self) -> Option<&T> {
676 Some(ref x) => Some(x),
681 /// Converts from `&mut Option<T>` to `Option<&mut T>`.
686 /// let mut x = Some(2);
687 /// match x.as_mut() {
688 /// Some(v) => *v = 42,
691 /// assert_eq!(x, Some(42));
694 #[stable(feature = "rust1", since = "1.0.0")]
695 #[rustc_const_unstable(feature = "const_option", issue = "67441")]
696 pub const fn as_mut(&mut self) -> Option<&mut T> {
698 Some(ref mut x) => Some(x),
703 /// Converts from <code>[Pin]<[&]Option\<T>></code> to <code>Option<[Pin]<[&]T>></code>.
705 /// [&]: reference "shared reference"
708 #[stable(feature = "pin", since = "1.33.0")]
709 #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
710 pub const fn as_pin_ref(self: Pin<&Self>) -> Option<Pin<&T>> {
711 match Pin::get_ref(self).as_ref() {
712 // SAFETY: `x` is guaranteed to be pinned because it comes from `self`
714 Some(x) => unsafe { Some(Pin::new_unchecked(x)) },
719 /// Converts from <code>[Pin]<[&mut] Option\<T>></code> to <code>Option<[Pin]<[&mut] T>></code>.
721 /// [&mut]: reference "mutable reference"
724 #[stable(feature = "pin", since = "1.33.0")]
725 #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
726 pub const fn as_pin_mut(self: Pin<&mut Self>) -> Option<Pin<&mut T>> {
727 // SAFETY: `get_unchecked_mut` is never used to move the `Option` inside `self`.
728 // `x` is guaranteed to be pinned because it comes from `self` which is pinned.
730 match Pin::get_unchecked_mut(self).as_mut() {
731 Some(x) => Some(Pin::new_unchecked(x)),
737 /////////////////////////////////////////////////////////////////////////
738 // Getting to contained values
739 /////////////////////////////////////////////////////////////////////////
741 /// Returns the contained [`Some`] value, consuming the `self` value.
745 /// Panics if the value is a [`None`] with a custom panic message provided by
751 /// let x = Some("value");
752 /// assert_eq!(x.expect("fruits are healthy"), "value");
756 /// let x: Option<&str> = None;
757 /// x.expect("fruits are healthy"); // panics with `fruits are healthy`
760 /// # Recommended Message Style
762 /// We recommend that `expect` messages are used to describe the reason you
763 /// _expect_ the `Option` should be `Some`.
766 /// # let slice: &[u8] = &[];
767 /// let item = slice.get(0)
768 /// .expect("slice should not be empty");
771 /// **Hint**: If you're having trouble remembering how to phrase expect
772 /// error messages remember to focus on the word "should" as in "env
773 /// variable should be set by blah" or "the given binary should be available
774 /// and executable by the current user".
776 /// For more detail on expect message styles and the reasoning behind our
777 /// recommendation please refer to the section on ["Common Message
778 /// Styles"](../../std/error/index.html#common-message-styles) in the [`std::error`](../../std/error/index.html) module docs.
781 #[stable(feature = "rust1", since = "1.0.0")]
782 #[rustc_const_unstable(feature = "const_option", issue = "67441")]
783 pub const fn expect(self, msg: &str) -> T {
786 None => expect_failed(msg),
790 /// Returns the contained [`Some`] value, consuming the `self` value.
792 /// Because this function may panic, its use is generally discouraged.
793 /// Instead, prefer to use pattern matching and handle the [`None`]
794 /// case explicitly, or call [`unwrap_or`], [`unwrap_or_else`], or
795 /// [`unwrap_or_default`].
797 /// [`unwrap_or`]: Option::unwrap_or
798 /// [`unwrap_or_else`]: Option::unwrap_or_else
799 /// [`unwrap_or_default`]: Option::unwrap_or_default
803 /// Panics if the self value equals [`None`].
808 /// let x = Some("air");
809 /// assert_eq!(x.unwrap(), "air");
813 /// let x: Option<&str> = None;
814 /// assert_eq!(x.unwrap(), "air"); // fails
818 #[stable(feature = "rust1", since = "1.0.0")]
819 #[rustc_const_unstable(feature = "const_option", issue = "67441")]
820 pub const fn unwrap(self) -> T {
823 None => panic("called `Option::unwrap()` on a `None` value"),
827 /// Returns the contained [`Some`] value or a provided default.
829 /// Arguments passed to `unwrap_or` are eagerly evaluated; if you are passing
830 /// the result of a function call, it is recommended to use [`unwrap_or_else`],
831 /// which is lazily evaluated.
833 /// [`unwrap_or_else`]: Option::unwrap_or_else
838 /// assert_eq!(Some("car").unwrap_or("bike"), "car");
839 /// assert_eq!(None.unwrap_or("bike"), "bike");
842 #[stable(feature = "rust1", since = "1.0.0")]
843 #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
844 pub const fn unwrap_or(self, default: T) -> T
854 /// Returns the contained [`Some`] value or computes it from a closure.
860 /// assert_eq!(Some(4).unwrap_or_else(|| 2 * k), 4);
861 /// assert_eq!(None.unwrap_or_else(|| 2 * k), 20);
864 #[stable(feature = "rust1", since = "1.0.0")]
865 #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
866 pub const fn unwrap_or_else<F>(self, f: F) -> T
868 F: ~const FnOnce() -> T,
877 /// Returns the contained [`Some`] value or a default.
879 /// Consumes the `self` argument then, if [`Some`], returns the contained
880 /// value, otherwise if [`None`], returns the [default value] for that
886 /// let x: Option<u32> = None;
887 /// let y: Option<u32> = Some(12);
889 /// assert_eq!(x.unwrap_or_default(), 0);
890 /// assert_eq!(y.unwrap_or_default(), 12);
893 /// [default value]: Default::default
894 /// [`parse`]: str::parse
895 /// [`FromStr`]: crate::str::FromStr
897 #[stable(feature = "rust1", since = "1.0.0")]
898 #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
899 pub const fn unwrap_or_default(self) -> T
905 None => Default::default(),
909 /// Returns the contained [`Some`] value, consuming the `self` value,
910 /// without checking that the value is not [`None`].
914 /// Calling this method on [`None`] is *[undefined behavior]*.
916 /// [undefined behavior]: https://doc.rust-lang.org/reference/behavior-considered-undefined.html
921 /// let x = Some("air");
922 /// assert_eq!(unsafe { x.unwrap_unchecked() }, "air");
926 /// let x: Option<&str> = None;
927 /// assert_eq!(unsafe { x.unwrap_unchecked() }, "air"); // Undefined behavior!
931 #[stable(feature = "option_result_unwrap_unchecked", since = "1.58.0")]
932 #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
933 pub const unsafe fn unwrap_unchecked(self) -> T {
934 debug_assert!(self.is_some());
937 // SAFETY: the safety contract must be upheld by the caller.
938 None => unsafe { hint::unreachable_unchecked() },
942 /////////////////////////////////////////////////////////////////////////
943 // Transforming contained values
944 /////////////////////////////////////////////////////////////////////////
946 /// Maps an `Option<T>` to `Option<U>` by applying a function to a contained value.
950 /// Calculates the length of an <code>Option<[String]></code> as an
951 /// <code>Option<[usize]></code>, consuming the original:
953 /// [String]: ../../std/string/struct.String.html "String"
955 /// let maybe_some_string = Some(String::from("Hello, World!"));
956 /// // `Option::map` takes self *by value*, consuming `maybe_some_string`
957 /// let maybe_some_len = maybe_some_string.map(|s| s.len());
959 /// assert_eq!(maybe_some_len, Some(13));
962 #[stable(feature = "rust1", since = "1.0.0")]
963 #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
964 pub const fn map<U, F>(self, f: F) -> Option<U>
966 F: ~const FnOnce(T) -> U,
970 Some(x) => Some(f(x)),
975 /// Calls the provided closure with a reference to the contained value (if [`Some`]).
980 /// #![feature(result_option_inspect)]
982 /// let v = vec![1, 2, 3, 4, 5];
984 /// // prints "got: 4"
985 /// let x: Option<&usize> = v.get(3).inspect(|x| println!("got: {x}"));
987 /// // prints nothing
988 /// let x: Option<&usize> = v.get(5).inspect(|x| println!("got: {x}"));
991 #[unstable(feature = "result_option_inspect", issue = "91345")]
992 #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
993 pub const fn inspect<F>(self, f: F) -> Self
995 F: ~const FnOnce(&T),
998 if let Some(ref x) = self {
1005 /// Returns the provided default result (if none),
1006 /// or applies a function to the contained value (if any).
1008 /// Arguments passed to `map_or` are eagerly evaluated; if you are passing
1009 /// the result of a function call, it is recommended to use [`map_or_else`],
1010 /// which is lazily evaluated.
1012 /// [`map_or_else`]: Option::map_or_else
1017 /// let x = Some("foo");
1018 /// assert_eq!(x.map_or(42, |v| v.len()), 3);
1020 /// let x: Option<&str> = None;
1021 /// assert_eq!(x.map_or(42, |v| v.len()), 42);
1024 #[stable(feature = "rust1", since = "1.0.0")]
1025 #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
1026 pub const fn map_or<U, F>(self, default: U, f: F) -> U
1028 F: ~const FnOnce(T) -> U,
1038 /// Computes a default function result (if none), or
1039 /// applies a different function to the contained value (if any).
1046 /// let x = Some("foo");
1047 /// assert_eq!(x.map_or_else(|| 2 * k, |v| v.len()), 3);
1049 /// let x: Option<&str> = None;
1050 /// assert_eq!(x.map_or_else(|| 2 * k, |v| v.len()), 42);
1053 #[stable(feature = "rust1", since = "1.0.0")]
1054 #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
1055 pub const fn map_or_else<U, D, F>(self, default: D, f: F) -> U
1057 D: ~const FnOnce() -> U,
1059 F: ~const FnOnce(T) -> U,
1068 /// Transforms the `Option<T>` into a [`Result<T, E>`], mapping [`Some(v)`] to
1069 /// [`Ok(v)`] and [`None`] to [`Err(err)`].
1071 /// Arguments passed to `ok_or` are eagerly evaluated; if you are passing the
1072 /// result of a function call, it is recommended to use [`ok_or_else`], which is
1073 /// lazily evaluated.
1076 /// [`Err(err)`]: Err
1077 /// [`Some(v)`]: Some
1078 /// [`ok_or_else`]: Option::ok_or_else
1083 /// let x = Some("foo");
1084 /// assert_eq!(x.ok_or(0), Ok("foo"));
1086 /// let x: Option<&str> = None;
1087 /// assert_eq!(x.ok_or(0), Err(0));
1090 #[stable(feature = "rust1", since = "1.0.0")]
1091 #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
1092 pub const fn ok_or<E>(self, err: E) -> Result<T, E>
1102 /// Transforms the `Option<T>` into a [`Result<T, E>`], mapping [`Some(v)`] to
1103 /// [`Ok(v)`] and [`None`] to [`Err(err())`].
1106 /// [`Err(err())`]: Err
1107 /// [`Some(v)`]: Some
1112 /// let x = Some("foo");
1113 /// assert_eq!(x.ok_or_else(|| 0), Ok("foo"));
1115 /// let x: Option<&str> = None;
1116 /// assert_eq!(x.ok_or_else(|| 0), Err(0));
1119 #[stable(feature = "rust1", since = "1.0.0")]
1120 #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
1121 pub const fn ok_or_else<E, F>(self, err: F) -> Result<T, E>
1123 F: ~const FnOnce() -> E,
1132 /// Converts from `Option<T>` (or `&Option<T>`) to `Option<&T::Target>`.
1134 /// Leaves the original Option in-place, creating a new one with a reference
1135 /// to the original one, additionally coercing the contents via [`Deref`].
1140 /// let x: Option<String> = Some("hey".to_owned());
1141 /// assert_eq!(x.as_deref(), Some("hey"));
1143 /// let x: Option<String> = None;
1144 /// assert_eq!(x.as_deref(), None);
1146 #[stable(feature = "option_deref", since = "1.40.0")]
1147 #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
1148 pub const fn as_deref(&self) -> Option<&T::Target>
1152 match self.as_ref() {
1153 Some(t) => Some(t.deref()),
1158 /// Converts from `Option<T>` (or `&mut Option<T>`) to `Option<&mut T::Target>`.
1160 /// Leaves the original `Option` in-place, creating a new one containing a mutable reference to
1161 /// the inner type's [`Deref::Target`] type.
1166 /// let mut x: Option<String> = Some("hey".to_owned());
1167 /// assert_eq!(x.as_deref_mut().map(|x| {
1168 /// x.make_ascii_uppercase();
1170 /// }), Some("HEY".to_owned().as_mut_str()));
1172 #[stable(feature = "option_deref", since = "1.40.0")]
1173 #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
1174 pub const fn as_deref_mut(&mut self) -> Option<&mut T::Target>
1178 match self.as_mut() {
1179 Some(t) => Some(t.deref_mut()),
1184 /////////////////////////////////////////////////////////////////////////
1185 // Iterator constructors
1186 /////////////////////////////////////////////////////////////////////////
1188 /// Returns an iterator over the possibly contained value.
1193 /// let x = Some(4);
1194 /// assert_eq!(x.iter().next(), Some(&4));
1196 /// let x: Option<u32> = None;
1197 /// assert_eq!(x.iter().next(), None);
1200 #[rustc_const_unstable(feature = "const_option", issue = "67441")]
1201 #[stable(feature = "rust1", since = "1.0.0")]
1202 pub const fn iter(&self) -> Iter<'_, T> {
1203 Iter { inner: Item { opt: self.as_ref() } }
1206 /// Returns a mutable iterator over the possibly contained value.
1211 /// let mut x = Some(4);
1212 /// match x.iter_mut().next() {
1213 /// Some(v) => *v = 42,
1216 /// assert_eq!(x, Some(42));
1218 /// let mut x: Option<u32> = None;
1219 /// assert_eq!(x.iter_mut().next(), None);
1222 #[stable(feature = "rust1", since = "1.0.0")]
1223 pub fn iter_mut(&mut self) -> IterMut<'_, T> {
1224 IterMut { inner: Item { opt: self.as_mut() } }
1227 /////////////////////////////////////////////////////////////////////////
1228 // Boolean operations on the values, eager and lazy
1229 /////////////////////////////////////////////////////////////////////////
1231 /// Returns [`None`] if the option is [`None`], otherwise returns `optb`.
1233 /// Arguments passed to `and` are eagerly evaluated; if you are passing the
1234 /// result of a function call, it is recommended to use [`and_then`], which is
1235 /// lazily evaluated.
1237 /// [`and_then`]: Option::and_then
1242 /// let x = Some(2);
1243 /// let y: Option<&str> = None;
1244 /// assert_eq!(x.and(y), None);
1246 /// let x: Option<u32> = None;
1247 /// let y = Some("foo");
1248 /// assert_eq!(x.and(y), None);
1250 /// let x = Some(2);
1251 /// let y = Some("foo");
1252 /// assert_eq!(x.and(y), Some("foo"));
1254 /// let x: Option<u32> = None;
1255 /// let y: Option<&str> = None;
1256 /// assert_eq!(x.and(y), None);
1259 #[stable(feature = "rust1", since = "1.0.0")]
1260 #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
1261 pub const fn and<U>(self, optb: Option<U>) -> Option<U>
1272 /// Returns [`None`] if the option is [`None`], otherwise calls `f` with the
1273 /// wrapped value and returns the result.
1275 /// Some languages call this operation flatmap.
1280 /// fn sq_then_to_string(x: u32) -> Option<String> {
1281 /// x.checked_mul(x).map(|sq| sq.to_string())
1284 /// assert_eq!(Some(2).and_then(sq_then_to_string), Some(4.to_string()));
1285 /// assert_eq!(Some(1_000_000).and_then(sq_then_to_string), None); // overflowed!
1286 /// assert_eq!(None.and_then(sq_then_to_string), None);
1289 /// Often used to chain fallible operations that may return [`None`].
1292 /// let arr_2d = [["A0", "A1"], ["B0", "B1"]];
1294 /// let item_0_1 = arr_2d.get(0).and_then(|row| row.get(1));
1295 /// assert_eq!(item_0_1, Some(&"A1"));
1297 /// let item_2_0 = arr_2d.get(2).and_then(|row| row.get(0));
1298 /// assert_eq!(item_2_0, None);
1301 #[stable(feature = "rust1", since = "1.0.0")]
1302 #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
1303 pub const fn and_then<U, F>(self, f: F) -> Option<U>
1305 F: ~const FnOnce(T) -> Option<U>,
1314 /// Returns [`None`] if the option is [`None`], otherwise calls `predicate`
1315 /// with the wrapped value and returns:
1317 /// - [`Some(t)`] if `predicate` returns `true` (where `t` is the wrapped
1319 /// - [`None`] if `predicate` returns `false`.
1321 /// This function works similar to [`Iterator::filter()`]. You can imagine
1322 /// the `Option<T>` being an iterator over one or zero elements. `filter()`
1323 /// lets you decide which elements to keep.
1328 /// fn is_even(n: &i32) -> bool {
1332 /// assert_eq!(None.filter(is_even), None);
1333 /// assert_eq!(Some(3).filter(is_even), None);
1334 /// assert_eq!(Some(4).filter(is_even), Some(4));
1337 /// [`Some(t)`]: Some
1339 #[stable(feature = "option_filter", since = "1.27.0")]
1340 #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
1341 pub const fn filter<P>(self, predicate: P) -> Self
1344 P: ~const FnOnce(&T) -> bool,
1347 if let Some(x) = self {
1355 /// Returns the option if it contains a value, otherwise returns `optb`.
1357 /// Arguments passed to `or` are eagerly evaluated; if you are passing the
1358 /// result of a function call, it is recommended to use [`or_else`], which is
1359 /// lazily evaluated.
1361 /// [`or_else`]: Option::or_else
1366 /// let x = Some(2);
1368 /// assert_eq!(x.or(y), Some(2));
1371 /// let y = Some(100);
1372 /// assert_eq!(x.or(y), Some(100));
1374 /// let x = Some(2);
1375 /// let y = Some(100);
1376 /// assert_eq!(x.or(y), Some(2));
1378 /// let x: Option<u32> = None;
1380 /// assert_eq!(x.or(y), None);
1383 #[stable(feature = "rust1", since = "1.0.0")]
1384 #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
1385 pub const fn or(self, optb: Option<T>) -> Option<T>
1395 /// Returns the option if it contains a value, otherwise calls `f` and
1396 /// returns the result.
1401 /// fn nobody() -> Option<&'static str> { None }
1402 /// fn vikings() -> Option<&'static str> { Some("vikings") }
1404 /// assert_eq!(Some("barbarians").or_else(vikings), Some("barbarians"));
1405 /// assert_eq!(None.or_else(vikings), Some("vikings"));
1406 /// assert_eq!(None.or_else(nobody), None);
1409 #[stable(feature = "rust1", since = "1.0.0")]
1410 #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
1411 pub const fn or_else<F>(self, f: F) -> Option<T>
1413 F: ~const FnOnce() -> Option<T>,
1422 /// Returns [`Some`] if exactly one of `self`, `optb` is [`Some`], otherwise returns [`None`].
1427 /// let x = Some(2);
1428 /// let y: Option<u32> = None;
1429 /// assert_eq!(x.xor(y), Some(2));
1431 /// let x: Option<u32> = None;
1432 /// let y = Some(2);
1433 /// assert_eq!(x.xor(y), Some(2));
1435 /// let x = Some(2);
1436 /// let y = Some(2);
1437 /// assert_eq!(x.xor(y), None);
1439 /// let x: Option<u32> = None;
1440 /// let y: Option<u32> = None;
1441 /// assert_eq!(x.xor(y), None);
1444 #[stable(feature = "option_xor", since = "1.37.0")]
1445 #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
1446 pub const fn xor(self, optb: Option<T>) -> Option<T>
1450 match (self, optb) {
1451 (Some(a), None) => Some(a),
1452 (None, Some(b)) => Some(b),
1457 /////////////////////////////////////////////////////////////////////////
1458 // Entry-like operations to insert a value and return a reference
1459 /////////////////////////////////////////////////////////////////////////
1461 /// Inserts `value` into the option, then returns a mutable reference to it.
1463 /// If the option already contains a value, the old value is dropped.
1465 /// See also [`Option::get_or_insert`], which doesn't update the value if
1466 /// the option already contains [`Some`].
1471 /// let mut opt = None;
1472 /// let val = opt.insert(1);
1473 /// assert_eq!(*val, 1);
1474 /// assert_eq!(opt.unwrap(), 1);
1475 /// let val = opt.insert(2);
1476 /// assert_eq!(*val, 2);
1478 /// assert_eq!(opt.unwrap(), 3);
1480 #[must_use = "if you intended to set a value, consider assignment instead"]
1482 #[stable(feature = "option_insert", since = "1.53.0")]
1483 #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
1484 pub const fn insert(&mut self, value: T) -> &mut T
1488 *self = Some(value);
1490 // SAFETY: the code above just filled the option
1491 unsafe { self.as_mut().unwrap_unchecked() }
1494 /// Inserts `value` into the option if it is [`None`], then
1495 /// returns a mutable reference to the contained value.
1497 /// See also [`Option::insert`], which updates the value even if
1498 /// the option already contains [`Some`].
1503 /// let mut x = None;
1506 /// let y: &mut u32 = x.get_or_insert(5);
1507 /// assert_eq!(y, &5);
1512 /// assert_eq!(x, Some(7));
1515 #[stable(feature = "option_entry", since = "1.20.0")]
1516 #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
1517 pub const fn get_or_insert(&mut self, value: T) -> &mut T
1521 if let None = *self {
1522 *self = Some(value);
1525 // SAFETY: a `None` variant for `self` would have been replaced by a `Some`
1526 // variant in the code above.
1527 unsafe { self.as_mut().unwrap_unchecked() }
1530 /// Inserts the default value into the option if it is [`None`], then
1531 /// returns a mutable reference to the contained value.
1536 /// #![feature(option_get_or_insert_default)]
1538 /// let mut x = None;
1541 /// let y: &mut u32 = x.get_or_insert_default();
1542 /// assert_eq!(y, &0);
1547 /// assert_eq!(x, Some(7));
1550 #[unstable(feature = "option_get_or_insert_default", issue = "82901")]
1551 #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
1552 pub const fn get_or_insert_default(&mut self) -> &mut T
1556 const fn default<T: ~const Default>() -> T {
1560 self.get_or_insert_with(default)
1563 /// Inserts a value computed from `f` into the option if it is [`None`],
1564 /// then returns a mutable reference to the contained value.
1569 /// let mut x = None;
1572 /// let y: &mut u32 = x.get_or_insert_with(|| 5);
1573 /// assert_eq!(y, &5);
1578 /// assert_eq!(x, Some(7));
1581 #[stable(feature = "option_entry", since = "1.20.0")]
1582 #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
1583 pub const fn get_or_insert_with<F>(&mut self, f: F) -> &mut T
1585 F: ~const FnOnce() -> T,
1588 if let None = *self {
1589 // the compiler isn't smart enough to know that we are not dropping a `T`
1590 // here and wants us to ensure `T` can be dropped at compile time.
1591 mem::forget(mem::replace(self, Some(f())))
1594 // SAFETY: a `None` variant for `self` would have been replaced by a `Some`
1595 // variant in the code above.
1596 unsafe { self.as_mut().unwrap_unchecked() }
1599 /////////////////////////////////////////////////////////////////////////
1601 /////////////////////////////////////////////////////////////////////////
1603 /// Takes the value out of the option, leaving a [`None`] in its place.
1608 /// let mut x = Some(2);
1609 /// let y = x.take();
1610 /// assert_eq!(x, None);
1611 /// assert_eq!(y, Some(2));
1613 /// let mut x: Option<u32> = None;
1614 /// let y = x.take();
1615 /// assert_eq!(x, None);
1616 /// assert_eq!(y, None);
1619 #[stable(feature = "rust1", since = "1.0.0")]
1620 #[rustc_const_unstable(feature = "const_option", issue = "67441")]
1621 pub const fn take(&mut self) -> Option<T> {
1622 // FIXME replace `mem::replace` by `mem::take` when the latter is const ready
1623 mem::replace(self, None)
1626 /// Replaces the actual value in the option by the value given in parameter,
1627 /// returning the old value if present,
1628 /// leaving a [`Some`] in its place without deinitializing either one.
1633 /// let mut x = Some(2);
1634 /// let old = x.replace(5);
1635 /// assert_eq!(x, Some(5));
1636 /// assert_eq!(old, Some(2));
1638 /// let mut x = None;
1639 /// let old = x.replace(3);
1640 /// assert_eq!(x, Some(3));
1641 /// assert_eq!(old, None);
1644 #[rustc_const_unstable(feature = "const_option", issue = "67441")]
1645 #[stable(feature = "option_replace", since = "1.31.0")]
1646 pub const fn replace(&mut self, value: T) -> Option<T> {
1647 mem::replace(self, Some(value))
1650 /// Returns `true` if the option is a [`Some`] value containing the given value.
1655 /// #![feature(option_result_contains)]
1657 /// let x: Option<u32> = Some(2);
1658 /// assert_eq!(x.contains(&2), true);
1660 /// let x: Option<u32> = Some(3);
1661 /// assert_eq!(x.contains(&2), false);
1663 /// let x: Option<u32> = None;
1664 /// assert_eq!(x.contains(&2), false);
1668 #[unstable(feature = "option_result_contains", issue = "62358")]
1669 #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
1670 pub const fn contains<U>(&self, x: &U) -> bool
1672 U: ~const PartialEq<T>,
1680 /// Zips `self` with another `Option`.
1682 /// If `self` is `Some(s)` and `other` is `Some(o)`, this method returns `Some((s, o))`.
1683 /// Otherwise, `None` is returned.
1688 /// let x = Some(1);
1689 /// let y = Some("hi");
1690 /// let z = None::<u8>;
1692 /// assert_eq!(x.zip(y), Some((1, "hi")));
1693 /// assert_eq!(x.zip(z), None);
1695 #[stable(feature = "option_zip_option", since = "1.46.0")]
1696 #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
1697 pub const fn zip<U>(self, other: Option<U>) -> Option<(T, U)>
1702 match (self, other) {
1703 (Some(a), Some(b)) => Some((a, b)),
1708 /// Zips `self` and another `Option` with function `f`.
1710 /// If `self` is `Some(s)` and `other` is `Some(o)`, this method returns `Some(f(s, o))`.
1711 /// Otherwise, `None` is returned.
1716 /// #![feature(option_zip)]
1718 /// #[derive(Debug, PartialEq)]
1725 /// fn new(x: f64, y: f64) -> Self {
1730 /// let x = Some(17.5);
1731 /// let y = Some(42.7);
1733 /// assert_eq!(x.zip_with(y, Point::new), Some(Point { x: 17.5, y: 42.7 }));
1734 /// assert_eq!(x.zip_with(None, Point::new), None);
1736 #[unstable(feature = "option_zip", issue = "70086")]
1737 #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
1738 pub const fn zip_with<U, F, R>(self, other: Option<U>, f: F) -> Option<R>
1740 F: ~const FnOnce(T, U) -> R,
1745 match (self, other) {
1746 (Some(a), Some(b)) => Some(f(a, b)),
1752 impl<T, U> Option<(T, U)> {
1753 /// Unzips an option containing a tuple of two options.
1755 /// If `self` is `Some((a, b))` this method returns `(Some(a), Some(b))`.
1756 /// Otherwise, `(None, None)` is returned.
1761 /// let x = Some((1, "hi"));
1762 /// let y = None::<(u8, u32)>;
1764 /// assert_eq!(x.unzip(), (Some(1), Some("hi")));
1765 /// assert_eq!(y.unzip(), (None, None));
1768 #[stable(feature = "unzip_option", since = "1.66.0")]
1769 #[rustc_const_unstable(feature = "const_option", issue = "67441")]
1770 pub const fn unzip(self) -> (Option<T>, Option<U>)
1776 Some((a, b)) => (Some(a), Some(b)),
1777 None => (None, None),
1782 impl<T> Option<&T> {
1783 /// Maps an `Option<&T>` to an `Option<T>` by copying the contents of the
1790 /// let opt_x = Some(&x);
1791 /// assert_eq!(opt_x, Some(&12));
1792 /// let copied = opt_x.copied();
1793 /// assert_eq!(copied, Some(12));
1795 #[must_use = "`self` will be dropped if the result is not used"]
1796 #[stable(feature = "copied", since = "1.35.0")]
1797 #[rustc_const_unstable(feature = "const_option", issue = "67441")]
1798 pub const fn copied(self) -> Option<T>
1802 // FIXME: this implementation, which sidesteps using `Option::map` since it's not const
1803 // ready yet, should be reverted when possible to avoid code repetition
1805 Some(&v) => Some(v),
1810 /// Maps an `Option<&T>` to an `Option<T>` by cloning the contents of the
1817 /// let opt_x = Some(&x);
1818 /// assert_eq!(opt_x, Some(&12));
1819 /// let cloned = opt_x.cloned();
1820 /// assert_eq!(cloned, Some(12));
1822 #[must_use = "`self` will be dropped if the result is not used"]
1823 #[stable(feature = "rust1", since = "1.0.0")]
1824 #[rustc_const_unstable(feature = "const_option_cloned", issue = "91582")]
1825 pub const fn cloned(self) -> Option<T>
1830 Some(t) => Some(t.clone()),
1836 impl<T> Option<&mut T> {
1837 /// Maps an `Option<&mut T>` to an `Option<T>` by copying the contents of the
1844 /// let opt_x = Some(&mut x);
1845 /// assert_eq!(opt_x, Some(&mut 12));
1846 /// let copied = opt_x.copied();
1847 /// assert_eq!(copied, Some(12));
1849 #[must_use = "`self` will be dropped if the result is not used"]
1850 #[stable(feature = "copied", since = "1.35.0")]
1851 #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
1852 pub const fn copied(self) -> Option<T>
1857 Some(&mut t) => Some(t),
1862 /// Maps an `Option<&mut T>` to an `Option<T>` by cloning the contents of the
1869 /// let opt_x = Some(&mut x);
1870 /// assert_eq!(opt_x, Some(&mut 12));
1871 /// let cloned = opt_x.cloned();
1872 /// assert_eq!(cloned, Some(12));
1874 #[must_use = "`self` will be dropped if the result is not used"]
1875 #[stable(since = "1.26.0", feature = "option_ref_mut_cloned")]
1876 #[rustc_const_unstable(feature = "const_option_cloned", issue = "91582")]
1877 pub const fn cloned(self) -> Option<T>
1882 Some(t) => Some(t.clone()),
1888 impl<T, E> Option<Result<T, E>> {
1889 /// Transposes an `Option` of a [`Result`] into a [`Result`] of an `Option`.
1891 /// [`None`] will be mapped to <code>[Ok]\([None])</code>.
1892 /// <code>[Some]\([Ok]\(\_))</code> and <code>[Some]\([Err]\(\_))</code> will be mapped to
1893 /// <code>[Ok]\([Some]\(\_))</code> and <code>[Err]\(\_)</code>.
1898 /// #[derive(Debug, Eq, PartialEq)]
1901 /// let x: Result<Option<i32>, SomeErr> = Ok(Some(5));
1902 /// let y: Option<Result<i32, SomeErr>> = Some(Ok(5));
1903 /// assert_eq!(x, y.transpose());
1906 #[stable(feature = "transpose_result", since = "1.33.0")]
1907 #[rustc_const_unstable(feature = "const_option", issue = "67441")]
1908 pub const fn transpose(self) -> Result<Option<T>, E> {
1910 Some(Ok(x)) => Ok(Some(x)),
1911 Some(Err(e)) => Err(e),
1917 // This is a separate function to reduce the code size of .expect() itself.
1918 #[cfg_attr(not(feature = "panic_immediate_abort"), inline(never))]
1919 #[cfg_attr(feature = "panic_immediate_abort", inline)]
1922 #[rustc_const_unstable(feature = "const_option", issue = "67441")]
1923 const fn expect_failed(msg: &str) -> ! {
1927 /////////////////////////////////////////////////////////////////////////////
1928 // Trait implementations
1929 /////////////////////////////////////////////////////////////////////////////
1931 #[stable(feature = "rust1", since = "1.0.0")]
1932 #[rustc_const_unstable(feature = "const_clone", issue = "91805")]
1933 impl<T> const Clone for Option<T>
1935 T: ~const Clone + ~const Destruct,
1938 fn clone(&self) -> Self {
1940 Some(x) => Some(x.clone()),
1946 fn clone_from(&mut self, source: &Self) {
1947 match (self, source) {
1948 (Some(to), Some(from)) => to.clone_from(from),
1949 (to, from) => *to = from.clone(),
1954 #[stable(feature = "rust1", since = "1.0.0")]
1955 #[rustc_const_unstable(feature = "const_default_impls", issue = "87864")]
1956 impl<T> const Default for Option<T> {
1957 /// Returns [`None`][Option::None].
1962 /// let opt: Option<u32> = Option::default();
1963 /// assert!(opt.is_none());
1966 fn default() -> Option<T> {
1971 #[stable(feature = "rust1", since = "1.0.0")]
1972 impl<T> IntoIterator for Option<T> {
1974 type IntoIter = IntoIter<T>;
1976 /// Returns a consuming iterator over the possibly contained value.
1981 /// let x = Some("string");
1982 /// let v: Vec<&str> = x.into_iter().collect();
1983 /// assert_eq!(v, ["string"]);
1986 /// let v: Vec<&str> = x.into_iter().collect();
1987 /// assert!(v.is_empty());
1990 fn into_iter(self) -> IntoIter<T> {
1991 IntoIter { inner: Item { opt: self } }
1995 #[stable(since = "1.4.0", feature = "option_iter")]
1996 impl<'a, T> IntoIterator for &'a Option<T> {
1998 type IntoIter = Iter<'a, T>;
2000 fn into_iter(self) -> Iter<'a, T> {
2005 #[stable(since = "1.4.0", feature = "option_iter")]
2006 impl<'a, T> IntoIterator for &'a mut Option<T> {
2007 type Item = &'a mut T;
2008 type IntoIter = IterMut<'a, T>;
2010 fn into_iter(self) -> IterMut<'a, T> {
2015 #[stable(since = "1.12.0", feature = "option_from")]
2016 #[rustc_const_unstable(feature = "const_convert", issue = "88674")]
2017 impl<T> const From<T> for Option<T> {
2018 /// Moves `val` into a new [`Some`].
2023 /// let o: Option<u8> = Option::from(67);
2025 /// assert_eq!(Some(67), o);
2027 fn from(val: T) -> Option<T> {
2032 #[stable(feature = "option_ref_from_ref_option", since = "1.30.0")]
2033 #[rustc_const_unstable(feature = "const_convert", issue = "88674")]
2034 impl<'a, T> const From<&'a Option<T>> for Option<&'a T> {
2035 /// Converts from `&Option<T>` to `Option<&T>`.
2039 /// Converts an <code>[Option]<[String]></code> into an <code>[Option]<[usize]></code>, preserving
2040 /// the original. The [`map`] method takes the `self` argument by value, consuming the original,
2041 /// so this technique uses `from` to first take an [`Option`] to a reference
2042 /// to the value inside the original.
2044 /// [`map`]: Option::map
2045 /// [String]: ../../std/string/struct.String.html "String"
2048 /// let s: Option<String> = Some(String::from("Hello, Rustaceans!"));
2049 /// let o: Option<usize> = Option::from(&s).map(|ss: &String| ss.len());
2051 /// println!("Can still print s: {s:?}");
2053 /// assert_eq!(o, Some(18));
2055 fn from(o: &'a Option<T>) -> Option<&'a T> {
2060 #[stable(feature = "option_ref_from_ref_option", since = "1.30.0")]
2061 #[rustc_const_unstable(feature = "const_convert", issue = "88674")]
2062 impl<'a, T> const From<&'a mut Option<T>> for Option<&'a mut T> {
2063 /// Converts from `&mut Option<T>` to `Option<&mut T>`
2068 /// let mut s = Some(String::from("Hello"));
2069 /// let o: Option<&mut String> = Option::from(&mut s);
2072 /// Some(t) => *t = String::from("Hello, Rustaceans!"),
2076 /// assert_eq!(s, Some(String::from("Hello, Rustaceans!")));
2078 fn from(o: &'a mut Option<T>) -> Option<&'a mut T> {
2083 #[stable(feature = "rust1", since = "1.0.0")]
2084 impl<T> crate::marker::StructuralPartialEq for Option<T> {}
2085 #[stable(feature = "rust1", since = "1.0.0")]
2086 impl<T: PartialEq> PartialEq for Option<T> {
2088 fn eq(&self, other: &Self) -> bool {
2089 SpecOptionPartialEq::eq(self, other)
2093 #[unstable(feature = "spec_option_partial_eq", issue = "none", reason = "exposed only for rustc")]
2095 pub trait SpecOptionPartialEq: Sized {
2096 fn eq(l: &Option<Self>, other: &Option<Self>) -> bool;
2099 #[unstable(feature = "spec_option_partial_eq", issue = "none", reason = "exposed only for rustc")]
2100 impl<T: PartialEq> SpecOptionPartialEq for T {
2102 default fn eq(l: &Option<T>, r: &Option<T>) -> bool {
2104 (Some(l), Some(r)) => *l == *r,
2105 (None, None) => true,
2111 macro_rules! non_zero_option {
2112 ( $( #[$stability: meta] $NZ:ty; )+ ) => {
2115 impl SpecOptionPartialEq for $NZ {
2117 fn eq(l: &Option<Self>, r: &Option<Self>) -> bool {
2118 l.map(Self::get).unwrap_or(0) == r.map(Self::get).unwrap_or(0)
2126 #[stable(feature = "nonzero", since = "1.28.0")] crate::num::NonZeroU8;
2127 #[stable(feature = "nonzero", since = "1.28.0")] crate::num::NonZeroU16;
2128 #[stable(feature = "nonzero", since = "1.28.0")] crate::num::NonZeroU32;
2129 #[stable(feature = "nonzero", since = "1.28.0")] crate::num::NonZeroU64;
2130 #[stable(feature = "nonzero", since = "1.28.0")] crate::num::NonZeroU128;
2131 #[stable(feature = "nonzero", since = "1.28.0")] crate::num::NonZeroUsize;
2132 #[stable(feature = "signed_nonzero", since = "1.34.0")] crate::num::NonZeroI8;
2133 #[stable(feature = "signed_nonzero", since = "1.34.0")] crate::num::NonZeroI16;
2134 #[stable(feature = "signed_nonzero", since = "1.34.0")] crate::num::NonZeroI32;
2135 #[stable(feature = "signed_nonzero", since = "1.34.0")] crate::num::NonZeroI64;
2136 #[stable(feature = "signed_nonzero", since = "1.34.0")] crate::num::NonZeroI128;
2137 #[stable(feature = "signed_nonzero", since = "1.34.0")] crate::num::NonZeroIsize;
2140 #[stable(feature = "nonnull", since = "1.25.0")]
2141 impl<T> SpecOptionPartialEq for crate::ptr::NonNull<T> {
2143 fn eq(l: &Option<Self>, r: &Option<Self>) -> bool {
2144 l.map(Self::as_ptr).unwrap_or_else(|| crate::ptr::null_mut())
2145 == r.map(Self::as_ptr).unwrap_or_else(|| crate::ptr::null_mut())
2149 /////////////////////////////////////////////////////////////////////////////
2150 // The Option Iterators
2151 /////////////////////////////////////////////////////////////////////////////
2153 #[derive(Clone, Debug)]
2158 impl<A> Iterator for Item<A> {
2162 fn next(&mut self) -> Option<A> {
2167 fn size_hint(&self) -> (usize, Option<usize>) {
2169 Some(_) => (1, Some(1)),
2170 None => (0, Some(0)),
2175 impl<A> DoubleEndedIterator for Item<A> {
2177 fn next_back(&mut self) -> Option<A> {
2182 impl<A> ExactSizeIterator for Item<A> {}
2183 impl<A> FusedIterator for Item<A> {}
2184 unsafe impl<A> TrustedLen for Item<A> {}
2186 /// An iterator over a reference to the [`Some`] variant of an [`Option`].
2188 /// The iterator yields one value if the [`Option`] is a [`Some`], otherwise none.
2190 /// This `struct` is created by the [`Option::iter`] function.
2191 #[stable(feature = "rust1", since = "1.0.0")]
2193 pub struct Iter<'a, A: 'a> {
2197 #[stable(feature = "rust1", since = "1.0.0")]
2198 impl<'a, A> Iterator for Iter<'a, A> {
2202 fn next(&mut self) -> Option<&'a A> {
2206 fn size_hint(&self) -> (usize, Option<usize>) {
2207 self.inner.size_hint()
2211 #[stable(feature = "rust1", since = "1.0.0")]
2212 impl<'a, A> DoubleEndedIterator for Iter<'a, A> {
2214 fn next_back(&mut self) -> Option<&'a A> {
2215 self.inner.next_back()
2219 #[stable(feature = "rust1", since = "1.0.0")]
2220 impl<A> ExactSizeIterator for Iter<'_, A> {}
2222 #[stable(feature = "fused", since = "1.26.0")]
2223 impl<A> FusedIterator for Iter<'_, A> {}
2225 #[unstable(feature = "trusted_len", issue = "37572")]
2226 unsafe impl<A> TrustedLen for Iter<'_, A> {}
2228 #[stable(feature = "rust1", since = "1.0.0")]
2229 impl<A> Clone for Iter<'_, A> {
2231 fn clone(&self) -> Self {
2232 Iter { inner: self.inner.clone() }
2236 /// An iterator over a mutable reference to the [`Some`] variant of an [`Option`].
2238 /// The iterator yields one value if the [`Option`] is a [`Some`], otherwise none.
2240 /// This `struct` is created by the [`Option::iter_mut`] function.
2241 #[stable(feature = "rust1", since = "1.0.0")]
2243 pub struct IterMut<'a, A: 'a> {
2244 inner: Item<&'a mut A>,
2247 #[stable(feature = "rust1", since = "1.0.0")]
2248 impl<'a, A> Iterator for IterMut<'a, A> {
2249 type Item = &'a mut A;
2252 fn next(&mut self) -> Option<&'a mut A> {
2256 fn size_hint(&self) -> (usize, Option<usize>) {
2257 self.inner.size_hint()
2261 #[stable(feature = "rust1", since = "1.0.0")]
2262 impl<'a, A> DoubleEndedIterator for IterMut<'a, A> {
2264 fn next_back(&mut self) -> Option<&'a mut A> {
2265 self.inner.next_back()
2269 #[stable(feature = "rust1", since = "1.0.0")]
2270 impl<A> ExactSizeIterator for IterMut<'_, A> {}
2272 #[stable(feature = "fused", since = "1.26.0")]
2273 impl<A> FusedIterator for IterMut<'_, A> {}
2274 #[unstable(feature = "trusted_len", issue = "37572")]
2275 unsafe impl<A> TrustedLen for IterMut<'_, A> {}
2277 /// An iterator over the value in [`Some`] variant of an [`Option`].
2279 /// The iterator yields one value if the [`Option`] is a [`Some`], otherwise none.
2281 /// This `struct` is created by the [`Option::into_iter`] function.
2282 #[derive(Clone, Debug)]
2283 #[stable(feature = "rust1", since = "1.0.0")]
2284 pub struct IntoIter<A> {
2288 #[stable(feature = "rust1", since = "1.0.0")]
2289 impl<A> Iterator for IntoIter<A> {
2293 fn next(&mut self) -> Option<A> {
2297 fn size_hint(&self) -> (usize, Option<usize>) {
2298 self.inner.size_hint()
2302 #[stable(feature = "rust1", since = "1.0.0")]
2303 impl<A> DoubleEndedIterator for IntoIter<A> {
2305 fn next_back(&mut self) -> Option<A> {
2306 self.inner.next_back()
2310 #[stable(feature = "rust1", since = "1.0.0")]
2311 impl<A> ExactSizeIterator for IntoIter<A> {}
2313 #[stable(feature = "fused", since = "1.26.0")]
2314 impl<A> FusedIterator for IntoIter<A> {}
2316 #[unstable(feature = "trusted_len", issue = "37572")]
2317 unsafe impl<A> TrustedLen for IntoIter<A> {}
2319 /////////////////////////////////////////////////////////////////////////////
2321 /////////////////////////////////////////////////////////////////////////////
2323 #[stable(feature = "rust1", since = "1.0.0")]
2324 impl<A, V: FromIterator<A>> FromIterator<Option<A>> for Option<V> {
2325 /// Takes each element in the [`Iterator`]: if it is [`None`][Option::None],
2326 /// no further elements are taken, and the [`None`][Option::None] is
2327 /// returned. Should no [`None`][Option::None] occur, a container of type
2328 /// `V` containing the values of each [`Option`] is returned.
2332 /// Here is an example which increments every integer in a vector.
2333 /// We use the checked variant of `add` that returns `None` when the
2334 /// calculation would result in an overflow.
2337 /// let items = vec![0_u16, 1, 2];
2339 /// let res: Option<Vec<u16>> = items
2341 /// .map(|x| x.checked_add(1))
2344 /// assert_eq!(res, Some(vec![1, 2, 3]));
2347 /// As you can see, this will return the expected, valid items.
2349 /// Here is another example that tries to subtract one from another list
2350 /// of integers, this time checking for underflow:
2353 /// let items = vec![2_u16, 1, 0];
2355 /// let res: Option<Vec<u16>> = items
2357 /// .map(|x| x.checked_sub(1))
2360 /// assert_eq!(res, None);
2363 /// Since the last element is zero, it would underflow. Thus, the resulting
2364 /// value is `None`.
2366 /// Here is a variation on the previous example, showing that no
2367 /// further elements are taken from `iter` after the first `None`.
2370 /// let items = vec![3_u16, 2, 1, 10];
2372 /// let mut shared = 0;
2374 /// let res: Option<Vec<u16>> = items
2376 /// .map(|x| { shared += x; x.checked_sub(2) })
2379 /// assert_eq!(res, None);
2380 /// assert_eq!(shared, 6);
2383 /// Since the third element caused an underflow, no further elements were taken,
2384 /// so the final value of `shared` is 6 (= `3 + 2 + 1`), not 16.
2386 fn from_iter<I: IntoIterator<Item = Option<A>>>(iter: I) -> Option<V> {
2387 // FIXME(#11084): This could be replaced with Iterator::scan when this
2388 // performance bug is closed.
2390 iter::try_process(iter.into_iter(), |i| i.collect())
2394 #[unstable(feature = "try_trait_v2", issue = "84277")]
2395 #[rustc_const_unstable(feature = "const_convert", issue = "88674")]
2396 impl<T> const ops::Try for Option<T> {
2398 type Residual = Option<convert::Infallible>;
2401 fn from_output(output: Self::Output) -> Self {
2406 fn branch(self) -> ControlFlow<Self::Residual, Self::Output> {
2408 Some(v) => ControlFlow::Continue(v),
2409 None => ControlFlow::Break(None),
2414 #[unstable(feature = "try_trait_v2", issue = "84277")]
2415 #[rustc_const_unstable(feature = "const_convert", issue = "88674")]
2416 impl<T> const ops::FromResidual for Option<T> {
2418 fn from_residual(residual: Option<convert::Infallible>) -> Self {
2425 #[unstable(feature = "try_trait_v2_yeet", issue = "96374")]
2426 impl<T> ops::FromResidual<ops::Yeet<()>> for Option<T> {
2428 fn from_residual(ops::Yeet(()): ops::Yeet<()>) -> Self {
2433 #[unstable(feature = "try_trait_v2_residual", issue = "91285")]
2434 #[rustc_const_unstable(feature = "const_try", issue = "74935")]
2435 impl<T> const ops::Residual<T> for Option<convert::Infallible> {
2436 type TryType = Option<T>;
2439 impl<T> Option<Option<T>> {
2440 /// Converts from `Option<Option<T>>` to `Option<T>`.
2447 /// let x: Option<Option<u32>> = Some(Some(6));
2448 /// assert_eq!(Some(6), x.flatten());
2450 /// let x: Option<Option<u32>> = Some(None);
2451 /// assert_eq!(None, x.flatten());
2453 /// let x: Option<Option<u32>> = None;
2454 /// assert_eq!(None, x.flatten());
2457 /// Flattening only removes one level of nesting at a time:
2460 /// let x: Option<Option<Option<u32>>> = Some(Some(Some(6)));
2461 /// assert_eq!(Some(Some(6)), x.flatten());
2462 /// assert_eq!(Some(6), x.flatten().flatten());
2465 #[stable(feature = "option_flattening", since = "1.40.0")]
2466 #[rustc_const_unstable(feature = "const_option", issue = "67441")]
2467 pub const fn flatten(self) -> Option<T> {
2469 Some(inner) => inner,