option.rs source code [crates/core/src/option.rs]

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	//! # The question mark operator, `?`
76	//!
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.
81	//!
82	//! It replaces this:
83	//!
84	//! ```
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();
89	//!
90	//! match (a, b) {
91	//! (Some(x), Some(y)) => Some(x + y),
92	//! _ => None,
93	//! }
94	//! }
95	//!
96	//! ```
97	//!
98	//! With this:
99	//!
100	//! ```
101	//! # #![allow(dead_code)]
102	//! fn add_last_numbers(stack: &mut Vec<i32>) -> Option<i32> {
103	//! Some(stack.pop()? + stack.pop()?)
104	//! }
105	//! ```
106	//!
107	//! It's much nicer!
108	//!
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.
111	//!
112	//! [`?`] can be used in functions that return [`Option`] because of the
113	//! early return of [`None`] that it provides.
114	//!
115	//! [`?`]: crate::ops::Try
116	//! [`Some`]: Some
117	//! [`None`]: None
118	//!
119	//! # Representation
120	//!
121	//! Rust guarantees to optimize the following types `T` such that
122	//! [`Option<T>`] has the same size, alignment, and [function call ABI] as `T`. In some
123	//! of these cases, Rust further guarantees that
124	//! `transmute::<_, Option<T>>([0u8; size_of::<T>()])` is sound and
125	//! produces `Option::<T>::None`. These cases are identified by the
126	//! second column:
127	//!
128	//! \| `T` \| `transmute::<_, Option<T>>([0u8; size_of::<T>()])` sound? \|
129	//! \|---------------------------------------------------------------------\|----------------------------------------------------------------------\|
130	//! \| [`Box<U>`] (specifically, only `Box<U, Global>`) \| when `U: Sized` \|
131	//! \| `&U` \| when `U: Sized` \|
132	//! \| `&mut U` \| when `U: Sized` \|
133	//! \| `fn`, `extern "C" fn`[^extern_fn] \| always \|
134	//! \| [`num::NonZero`] \| always \|*
135	//! \| [`ptr::NonNull<U>`] \| when `U: Sized` \|
136	//! \| `#[repr(transparent)]` struct around one of the types in this list. \| when it holds for the inner type \|
137	//!
138	//! [^extern_fn]: this remains true for any argument/return types and any other ABI: `extern "abi" fn` (_e.g._, `extern "system" fn`)
139	//!
140	//! [`Box<U>`]: ../../std/boxed/struct.Box.html
141	//! [`num::NonZero`]: crate::num*
142	//! [`ptr::NonNull<U>`]: crate::ptr::NonNull
143	//! [function call ABI]: ../primitive.fn.html#abi-compatibility
144	//!
145	//! This is called the "null pointer optimization" or NPO.
146	//!
147	//! It is further guaranteed that, for the cases above, one can
148	//! [`mem::transmute`] from all valid values of `T` to `Option<T>` and
149	//! from `Some::<T>(_)` to `T` (but transmuting `None::<T>` to `T`
150	//! is undefined behaviour).
151	//!
152	//! # Method overview
153	//!
154	//! In addition to working with pattern matching, [`Option`] provides a wide
155	//! variety of different methods.
156	//!
157	//! ## Querying the variant
158	//!
159	//! The [`is_some`] and [`is_none`] methods return [`true`] if the [`Option`]
160	//! is [`Some`] or [`None`], respectively.
161	//!
162	//! [`is_none`]: Option::is_none
163	//! [`is_some`]: Option::is_some
164	//!
165	//! ## Adapters for working with references
166	//!
167	//! * [`as_ref`] converts from <code>[&][][Option]\<T></code> to <code>[Option]<[&]T></code>
168	//! * [`as_mut`] converts from <code>[&mut] [Option]\<T></code> to <code>[Option]<[&mut] T></code>
169	//! * [`as_deref`] converts from <code>[&][][Option]\<T></code> to
170	//! <code>[Option]<[&]T::[Target]></code>
171	//! * [`as_deref_mut`] converts from <code>[&mut] [Option]\<T></code> to
172	//! <code>[Option]<[&mut] T::[Target]></code>
173	//! * [`as_pin_ref`] converts from <code>[Pin]<[&][][Option]\<T>></code> to
174	//! <code>[Option]<[Pin]<[&]T>></code>
175	//! * [`as_pin_mut`] converts from <code>[Pin]<[&mut] [Option]\<T>></code> to
176	//! <code>[Option]<[Pin]<[&mut] T>></code>
177	//!
178	//! [&]: reference "shared reference"
179	//! [&mut]: reference "mutable reference"
180	//! [Target]: Deref::Target "ops::Deref::Target"
181	//! [`as_deref`]: Option::as_deref
182	//! [`as_deref_mut`]: Option::as_deref_mut
183	//! [`as_mut`]: Option::as_mut
184	//! [`as_pin_mut`]: Option::as_pin_mut
185	//! [`as_pin_ref`]: Option::as_pin_ref
186	//! [`as_ref`]: Option::as_ref
187	//!
188	//! ## Extracting the contained value
189	//!
190	//! These methods extract the contained value in an [`Option<T>`] when it
191	//! is the [`Some`] variant. If the [`Option`] is [`None`]:
192	//!
193	//! * [`expect`] panics with a provided custom message
194	//! * [`unwrap`] panics with a generic message
195	//! * [`unwrap_or`] returns the provided default value
196	//! * [`unwrap_or_default`] returns the default value of the type `T`
197	//! (which must implement the [`Default`] trait)
198	//! * [`unwrap_or_else`] returns the result of evaluating the provided
199	//! function
200	//!
201	//! [`expect`]: Option::expect
202	//! [`unwrap`]: Option::unwrap
203	//! [`unwrap_or`]: Option::unwrap_or
204	//! [`unwrap_or_default`]: Option::unwrap_or_default
205	//! [`unwrap_or_else`]: Option::unwrap_or_else
206	//!
207	//! ## Transforming contained values
208	//!
209	//! These methods transform [`Option`] to [`Result`]:
210	//!
211	//! * [`ok_or`] transforms [`Some(v)`] to [`Ok(v)`], and [`None`] to
212	//! [`Err(err)`] using the provided default `err` value
213	//! * [`ok_or_else`] transforms [`Some(v)`] to [`Ok(v)`], and [`None`] to
214	//! a value of [`Err`] using the provided function
215	//! * [`transpose`] transposes an [`Option`] of a [`Result`] into a
216	//! [`Result`] of an [`Option`]
217	//!
218	//! [`Err(err)`]: Err
219	//! [`Ok(v)`]: Ok
220	//! [`Some(v)`]: Some
221	//! [`ok_or`]: Option::ok_or
222	//! [`ok_or_else`]: Option::ok_or_else
223	//! [`transpose`]: Option::transpose
224	//!
225	//! These methods transform the [`Some`] variant:
226	//!
227	//! * [`filter`] calls the provided predicate function on the contained
228	//! value `t` if the [`Option`] is [`Some(t)`], and returns [`Some(t)`]
229	//! if the function returns `true`; otherwise, returns [`None`]
230	//! * [`flatten`] removes one level of nesting from an
231	//! [`Option<Option<T>>`]
232	//! * [`map`] transforms [`Option<T>`] to [`Option<U>`] by applying the
233	//! provided function to the contained value of [`Some`] and leaving
234	//! [`None`] values unchanged
235	//!
236	//! [`Some(t)`]: Some
237	//! [`filter`]: Option::filter
238	//! [`flatten`]: Option::flatten
239	//! [`map`]: Option::map
240	//!
241	//! These methods transform [`Option<T>`] to a value of a possibly
242	//! different type `U`:
243	//!
244	//! * [`map_or`] applies the provided function to the contained value of
245	//! [`Some`], or returns the provided default value if the [`Option`] is
246	//! [`None`]
247	//! * [`map_or_else`] applies the provided function to the contained value
248	//! of [`Some`], or returns the result of evaluating the provided
249	//! fallback function if the [`Option`] is [`None`]
250	//!
251	//! [`map_or`]: Option::map_or
252	//! [`map_or_else`]: Option::map_or_else
253	//!
254	//! These methods combine the [`Some`] variants of two [`Option`] values:
255	//!
256	//! * [`zip`] returns [`Some((s, o))`] if `self` is [`Some(s)`] and the
257	//! provided [`Option`] value is [`Some(o)`]; otherwise, returns [`None`]
258	//! * [`zip_with`] calls the provided function `f` and returns
259	//! [`Some(f(s, o))`] if `self` is [`Some(s)`] and the provided
260	//! [`Option`] value is [`Some(o)`]; otherwise, returns [`None`]
261	//!
262	//! [`Some(f(s, o))`]: Some
263	//! [`Some(o)`]: Some
264	//! [`Some(s)`]: Some
265	//! [`Some((s, o))`]: Some
266	//! [`zip`]: Option::zip
267	//! [`zip_with`]: Option::zip_with
268	//!
269	//! ## Boolean operators
270	//!
271	//! These methods treat the [`Option`] as a boolean value, where [`Some`]
272	//! acts like [`true`] and [`None`] acts like [`false`]. There are two
273	//! categories of these methods: ones that take an [`Option`] as input, and
274	//! ones that take a function as input (to be lazily evaluated).
275	//!
276	//! The [`and`], [`or`], and [`xor`] methods take another [`Option`] as
277	//! input, and produce an [`Option`] as output. Only the [`and`] method can
278	//! produce an [`Option<U>`] value having a different inner type `U` than
279	//! [`Option<T>`].
280	//!
281	//! \| method \| self \| input \| output \|
282	//! \|---------\|-----------\|-----------\|-----------\|
283	//! \| [`and`] \| `None` \| (ignored) \| `None` \|
284	//! \| [`and`] \| `Some(x)` \| `None` \| `None` \|
285	//! \| [`and`] \| `Some(x)` \| `Some(y)` \| `Some(y)` \|
286	//! \| [`or`] \| `None` \| `None` \| `None` \|
287	//! \| [`or`] \| `None` \| `Some(y)` \| `Some(y)` \|
288	//! \| [`or`] \| `Some(x)` \| (ignored) \| `Some(x)` \|
289	//! \| [`xor`] \| `None` \| `None` \| `None` \|
290	//! \| [`xor`] \| `None` \| `Some(y)` \| `Some(y)` \|
291	//! \| [`xor`] \| `Some(x)` \| `None` \| `Some(x)` \|
292	//! \| [`xor`] \| `Some(x)` \| `Some(y)` \| `None` \|
293	//!
294	//! [`and`]: Option::and
295	//! [`or`]: Option::or
296	//! [`xor`]: Option::xor
297	//!
298	//! The [`and_then`] and [`or_else`] methods take a function as input, and
299	//! only evaluate the function when they need to produce a new value. Only
300	//! the [`and_then`] method can produce an [`Option<U>`] value having a
301	//! different inner type `U` than [`Option<T>`].
302	//!
303	//! \| method \| self \| function input \| function result \| output \|
304	//! \|--------------\|-----------\|----------------\|-----------------\|-----------\|
305	//! \| [`and_then`] \| `None` \| (not provided) \| (not evaluated) \| `None` \|
306	//! \| [`and_then`] \| `Some(x)` \| `x` \| `None` \| `None` \|
307	//! \| [`and_then`] \| `Some(x)` \| `x` \| `Some(y)` \| `Some(y)` \|
308	//! \| [`or_else`] \| `None` \| (not provided) \| `None` \| `None` \|
309	//! \| [`or_else`] \| `None` \| (not provided) \| `Some(y)` \| `Some(y)` \|
310	//! \| [`or_else`] \| `Some(x)` \| (not provided) \| (not evaluated) \| `Some(x)` \|
311	//!
312	//! [`and_then`]: Option::and_then
313	//! [`or_else`]: Option::or_else
314	//!
315	//! This is an example of using methods like [`and_then`] and [`or`] in a
316	//! pipeline of method calls. Early stages of the pipeline pass failure
317	//! values ([`None`]) through unchanged, and continue processing on
318	//! success values ([`Some`]). Toward the end, [`or`] substitutes an error
319	//! message if it receives [`None`].
320	//!
321	//! ```
322	//! # use std::collections::BTreeMap;
323	//! let mut bt = BTreeMap::new();
324	//! bt.insert(`20u8`, "foo");
325	//! bt.insert(`42u8`, "bar");
326	//! let res = [`0u8`, `1`, `11`, `200`, `22`]
327	//! .into_iter()
328	//! .map(\|x\| {
329	//! // `checked_sub()` returns `None` on error
330	//! x.checked_sub(`1`)
331	//! // same with `checked_mul()`
332	//! .and_then(\|x\| x.checked_mul(`2`))
333	//! // `BTreeMap::get` returns `None` on error
334	//! .and_then(\|x\| bt.get(&x))
335	//! // Substitute an error message if we have `None` so far
336	//! .or(Some(&"error!"))
337	//! .copied()
338	//! // Won't panic because we unconditionally used `Some` above
339	//! .unwrap()
340	//! })
341	//! .collect::<Vec<_>>();
342	//! assert_eq!(res, ["error!", "error!", "foo", "error!", "bar"]);
343	//! ```
344	//!
345	//! ## Comparison operators
346	//!
347	//! If `T` implements [`PartialOrd`] then [`Option<T>`] will derive its
348	//! [`PartialOrd`] implementation. With this order, [`None`] compares as
349	//! less than any [`Some`], and two [`Some`] compare the same way as their
350	//! contained values would in `T`. If `T` also implements
351	//! [`Ord`], then so does [`Option<T>`].
352	//!
353	//! ```
354	//! assert!(None < Some(`0`));
355	//! assert!(Some(`0`) < Some(`1`));
356	//! ```
357	//!
358	//! ## Iterating over `Option`
359	//!
360	//! An [`Option`] can be iterated over. This can be helpful if you need an
361	//! iterator that is conditionally empty. The iterator will either produce
362	//! a single value (when the [`Option`] is [`Some`]), or produce no values
363	//! (when the [`Option`] is [`None`]). For example, [`into_iter`] acts like
364	//! [`once(v)`] if the [`Option`] is [`Some(v)`], and like [`empty()`] if
365	//! the [`Option`] is [`None`].
366	//!
367	//! [`Some(v)`]: Some
368	//! [`empty()`]: crate::iter::empty
369	//! [`once(v)`]: crate::iter::once
370	//!
371	//! Iterators over [`Option<T>`] come in three types:
372	//!
373	//! * [`into_iter`] consumes the [`Option`] and produces the contained
374	//! value
375	//! * [`iter`] produces an immutable reference of type `&T` to the
376	//! contained value
377	//! * [`iter_mut`] produces a mutable reference of type `&mut T` to the
378	//! contained value
379	//!
380	//! [`into_iter`]: Option::into_iter
381	//! [`iter`]: Option::iter
382	//! [`iter_mut`]: Option::iter_mut
383	//!
384	//! An iterator over [`Option`] can be useful when chaining iterators, for
385	//! example, to conditionally insert items. (It's not always necessary to
386	//! explicitly call an iterator constructor: many [`Iterator`] methods that
387	//! accept other iterators will also accept iterable types that implement
388	//! [`IntoIterator`], which includes [`Option`].)
389	//!
390	//! ```
391	//! let yep = Some(`42`);
392	//! let nope = None;
393	//! // chain() already calls into_iter(), so we don't have to do so
394	//! let nums: Vec<i32> = (`0`..`4`).chain(yep).chain(`4`..`8`).collect();
395	//! assert_eq!(nums, [`0`, `1`, `2`, `3`, `42`, `4`, `5`, `6`, `7`]);
396	//! let nums: Vec<i32> = (`0`..`4`).chain(nope).chain(`4`..`8`).collect();
397	//! assert_eq!(nums, [`0`, `1`, `2`, `3`, `4`, `5`, `6`, `7`]);
398	//! ```
399	//!
400	//! One reason to chain iterators in this way is that a function returning
401	//! `impl Iterator` must have all possible return values be of the same
402	//! concrete type. Chaining an iterated [`Option`] can help with that.
403	//!
404	//! ```
405	//! fn make_iter(do_insert: bool) -> impl Iterator<Item = i32> {
406	//! // Explicit returns to illustrate return types matching
407	//! match do_insert {
408	//! `true` => return (`0`..`4`).chain(Some(`42`)).chain(`4`..`8`),
409	//! `false` => return (`0`..`4`).chain(None).chain(`4`..`8`),
410	//! }
411	//! }
412	//! println!("{:?}", make_iter(`true`).collect::<Vec<_>>());
413	//! println!("{:?}", make_iter(`false`).collect::<Vec<_>>());
414	//! ```
415	//!
416	//! If we try to do the same thing, but using [`once()`] and [`empty()`],
417	//! we can't return `impl Iterator` anymore because the concrete types of
418	//! the return values differ.
419	//!
420	//! [`empty()`]: crate::iter::empty
421	//! [`once()`]: crate::iter::once
422	//!
423	//! ```compile_fail,E0308
424	//! # use std::iter::{empty, once};
425	//! // This won't compile because all possible returns from the function
426	//! // must have the same concrete type.
427	//! fn make_iter(do_insert: bool) -> impl Iterator<Item = i32> {
428	//! // Explicit returns to illustrate return types not matching
429	//! match do_insert {
430	//! `true` => return (`0`..`4`).chain(once(`42`)).chain(`4`..`8`),
431	//! `false` => return (`0`..`4`).chain(empty()).chain(`4`..`8`),
432	//! }
433	//! }
434	//! ```
435	//!
436	//! ## Collecting into `Option`
437	//!
438	//! [`Option`] implements the [`FromIterator`][impl-FromIterator] trait,
439	//! which allows an iterator over [`Option`] values to be collected into an
440	//! [`Option`] of a collection of each contained value of the original
441	//! [`Option`] values, or [`None`] if any of the elements was [`None`].
442	//!
443	//! [impl-FromIterator]: Option#impl-FromIterator%3COption%3CA%3E%3E-for-Option%3CV%3E
444	//!
445	//! ```
446	//! let v = [Some(`2`), Some(`4`), None, Some(`8`)];
447	//! let res: Option<Vec<_>> = v.into_iter().collect();
448	//! assert_eq!(res, None);
449	//! let v = [Some(`2`), Some(`4`), Some(`8`)];
450	//! let res: Option<Vec<_>> = v.into_iter().collect();
451	//! assert_eq!(res, Some(vec![`2`, `4`, `8`]));
452	//! ```
453	//!
454	//! [`Option`] also implements the [`Product`][impl-Product] and
455	//! [`Sum`][impl-Sum] traits, allowing an iterator over [`Option`] values
456	//! to provide the [`product`][Iterator::product] and
457	//! [`sum`][Iterator::sum] methods.
458	//!
459	//! [impl-Product]: Option#impl-Product%3COption%3CU%3E%3E-for-Option%3CT%3E
460	//! [impl-Sum]: Option#impl-Sum%3COption%3CU%3E%3E-for-Option%3CT%3E
461	//!
462	//! ```
463	//! let v = [None, Some(`1`), Some(`2`), Some(`3`)];
464	//! let res: Option<i32> = v.into_iter().sum();
465	//! assert_eq!(res, None);
466	//! let v = [Some(`1`), Some(`2`), Some(`21`)];
467	//! let res: Option<i32> = v.into_iter().product();
468	//! assert_eq!(res, Some(`42`));
469	//! ```
470	//!
471	//! ## Modifying an [`Option`] in-place
472	//!
473	//! These methods return a mutable reference to the contained value of an
474	//! [`Option<T>`]:
475	//!
476	//! * [`insert`] inserts a value, dropping any old contents
477	//! * [`get_or_insert`] gets the current value, inserting a provided
478	//! default value if it is [`None`]
479	//! * [`get_or_insert_default`] gets the current value, inserting the
480	//! default value of type `T` (which must implement [`Default`]) if it is
481	//! [`None`]
482	//! * [`get_or_insert_with`] gets the current value, inserting a default
483	//! computed by the provided function if it is [`None`]
484	//!
485	//! [`get_or_insert`]: Option::get_or_insert
486	//! [`get_or_insert_default`]: Option::get_or_insert_default
487	//! [`get_or_insert_with`]: Option::get_or_insert_with
488	//! [`insert`]: Option::insert
489	//!
490	//! These methods transfer ownership of the contained value of an
491	//! [`Option`]:
492	//!
493	//! * [`take`] takes ownership of the contained value of an [`Option`], if
494	//! any, replacing the [`Option`] with [`None`]
495	//! * [`replace`] takes ownership of the contained value of an [`Option`],
496	//! if any, replacing the [`Option`] with a [`Some`] containing the
497	//! provided value
498	//!
499	//! [`replace`]: Option::replace
500	//! [`take`]: Option::take
501	//!
502	//! # Examples
503	//!
504	//! Basic pattern matching on [`Option`]:
505	//!
506	//! ```
507	//! let msg = Some("howdy");
508	//!
509	//! // Take a reference to the contained string
510	//! if let Some(m) = &msg {
511	//! println!("{}", *m);
512	//! }
513	//!
514	//! // Remove the contained string, destroying the Option
515	//! let unwrapped_msg = msg.unwrap_or("default message");
516	//! ```
517	//!
518	//! Initialize a result to [`None`] before a loop:
519	//!
520	//! ```
521	//! enum Kingdom { Plant(u32, &'static str), Animal(u32, &'static str) }
522	//!
523	//! // A list of data to search through.
524	//! let all_the_big_things = [
525	//! Kingdom::Plant(`250`, "redwood"),
526	//! Kingdom::Plant(`230`, "noble fir"),
527	//! Kingdom::Plant(`229`, "sugar pine"),
528	//! Kingdom::Animal(`25`, "blue whale"),
529	//! Kingdom::Animal(`19`, "fin whale"),
530	//! Kingdom::Animal(`15`, "north pacific right whale"),
531	//! ];
532	//!
533	//! // We're going to search for the name of the biggest animal,
534	//! // but to start with we've just got `None`.
535	//! let mut name_of_biggest_animal = None;
536	//! let mut size_of_biggest_animal = `0`;
537	//! for big_thing in &all_the_big_things {
538	//! match *big_thing {
539	//! Kingdom::Animal(size, name) if size > size_of_biggest_animal => {
540	//! // Now we've found the name of some big animal
541	//! size_of_biggest_animal = size;
542	//! name_of_biggest_animal = Some(name);
543	//! }
544	//! Kingdom::Animal(..) \| Kingdom::Plant(..) => ()
545	//! }
546	//! }
547	//!
548	//! match name_of_biggest_animal {
549	//! Some(name) => println!("the biggest animal is {name}"),
550	//! None => println!("there are no animals :("),
551	//! }
552	//! ```
553
554	#![stable(feature = "rust1", since = "1.0.0")]
555
556	use crate::iter::{self, FromIterator, FusedIterator, TrustedLen};
557	use crate::panicking::{panic, panic_str};
558	use crate::pin::Pin;
559	use crate::{
560	cmp, convert, hint, mem,
561	ops::{self, ControlFlow, Deref, DerefMut},
562	slice,
563	};
564
565	/// The `Option` type. See [the module level documentation](self) for more.
566	#[derive(Copy, PartialOrd, Eq, Ord, Debug, Hash)]
567	#[rustc_diagnostic_item = "Option"]
568	#[lang = "Option"]
569	#[stable(feature = "rust1", since = "1.0.0")]
570	pub enum Option<T> {
571	/// No value.
572	#[lang = "None"]
573	#[stable(feature = "rust1", since = "1.0.0")]
574	None,
575	/// Some value of type `T`.
576	#[lang = "Some"]
577	#[stable(feature = "rust1", since = "1.0.0")]
578	Some(#[stable(feature = "rust1", since = "1.0.0")] T),
579	}
580
581	/////////////////////////////////////////////////////////////////////////////
582	// Type implementation
583	/////////////////////////////////////////////////////////////////////////////
584
585	impl<T> Option<T> {
586	/////////////////////////////////////////////////////////////////////////
587	// Querying the contained values
588	/////////////////////////////////////////////////////////////////////////
589
590	/// Returns `true` if the option is a [`Some`] value.
591	///
592	/// # Examples
593	///
594	/// ```
595	/// let x: Option<u32> = Some(`2`);
596	/// assert_eq!(x.is_some(), `true`);
597	///
598	/// let x: Option<u32> = None;
599	/// assert_eq!(x.is_some(), `false`);
600	/// ```
601	#[must_use = "if you intended to assert that this has a value, consider `.unwrap()` instead"]
602	#[inline]
603	#[stable(feature = "rust1", since = "1.0.0")]
604	#[rustc_const_stable(feature = "const_option_basics", since = "1.48.0")]
605	pub const fn is_some(&self) -> bool {
606	matches!(*self, Some(_))
607	}
608
609	/// Returns `true` if the option is a [`Some`] and the value inside of it matches a predicate.
610	///
611	/// # Examples
612	///
613	/// ```
614	/// let x: Option<u32> = Some(`2`);
615	/// assert_eq!(x.is_some_and(\|x\| x > `1`), `true`);
616	///
617	/// let x: Option<u32> = Some(`0`);
618	/// assert_eq!(x.is_some_and(\|x\| x > `1`), `false`);
619	///
620	/// let x: Option<u32> = None;
621	/// assert_eq!(x.is_some_and(\|x\| x > `1`), `false`);
622	/// ```
623	#[must_use]
624	#[inline]
625	#[stable(feature = "is_some_and", since = "1.70.0")]
626	pub fn is_some_and(self, f: impl FnOnce(T) -> bool) -> bool {
627	match self {
628	None => `false`,
629	Some(x) => f(x),
630	}
631	}
632
633	/// Returns `true` if the option is a [`None`] value.
634	///
635	/// # Examples
636	///
637	/// ```
638	/// let x: Option<u32> = Some(`2`);
639	/// assert_eq!(x.is_none(), `false`);
640	///
641	/// let x: Option<u32> = None;
642	/// assert_eq!(x.is_none(), `true`);
643	/// ```
644	#[must_use = "if you intended to assert that this doesn't have a value, consider \
645	wrapping this in an `assert!()` instead"]
646	#[inline]
647	#[stable(feature = "rust1", since = "1.0.0")]
648	#[rustc_const_stable(feature = "const_option_basics", since = "1.48.0")]
649	pub const fn is_none(&self) -> bool {
650	!self.is_some()
651	}
652
653	/////////////////////////////////////////////////////////////////////////
654	// Adapter for working with references
655	/////////////////////////////////////////////////////////////////////////
656
657	/// Converts from `&Option<T>` to `Option<&T>`.
658	///
659	/// # Examples
660	///
661	/// Calculates the length of an <code>Option<[String]></code> as an <code>Option<[usize]></code>
662	/// without moving the [`String`]. The [`map`] method takes the `self` argument by value,
663	/// consuming the original, so this technique uses `as_ref` to first take an `Option` to a
664	/// reference to the value inside the original.
665	///
666	/// [`map`]: Option::map
667	/// [String]: ../../std/string/struct.String.html "String"
668	/// [`String`]: ../../std/string/struct.String.html "String"
669	///
670	/// ```
671	/// let text: Option<String> = Some("Hello, world!".to_string());
672	/// // First, cast `Option<String>` to `Option<&String>` with `as_ref`,
673	/// // then consume that* with `map`, leaving `text` on the stack.*
674	/// let text_length: Option<usize> = text.as_ref().map(\|s\| s.len());
675	/// println!("still can print text: {text:?}");
676	/// ```
677	#[inline]
678	#[rustc_const_stable(feature = "const_option_basics", since = "1.48.0")]
679	#[stable(feature = "rust1", since = "1.0.0")]
680	pub const fn as_ref(&self) -> Option<&T> {
681	match *self {
682	Some(ref x) => Some(x),
683	None => None,
684	}
685	}
686
687	/// Converts from `&mut Option<T>` to `Option<&mut T>`.
688	///
689	/// # Examples
690	///
691	/// ```
692	/// let mut x = Some(`2`);
693	/// match x.as_mut() {
694	/// Some(v) => *v = `42`,
695	/// None => {},
696	/// }
697	/// assert_eq!(x, Some(`42`));
698	/// ```
699	#[inline]
700	#[stable(feature = "rust1", since = "1.0.0")]
701	#[rustc_const_unstable(feature = "const_option", issue = "67441")]
702	pub const fn as_mut(&mut self) -> Option<&mut T> {
703	match *self {
704	Some(ref mut x) => Some(x),
705	None => None,
706	}
707	}
708
709	/// Converts from <code>[Pin]<[&]Option\<T>></code> to <code>Option<[Pin]<[&]T>></code>.
710	///
711	/// [&]: reference "shared reference"
712	#[inline]
713	#[must_use]
714	#[stable(feature = "pin", since = "1.33.0")]
715	#[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
716	pub const fn as_pin_ref(self: Pin<&Self>) -> Option<Pin<&T>> {
717	match Pin::get_ref(self).as_ref() {
718	// SAFETY: `x` is guaranteed to be pinned because it comes from `self`
719	// which is pinned.
720	Some(x) => unsafe { Some(Pin::new_unchecked(x)) },
721	None => None,
722	}
723	}
724
725	/// Converts from <code>[Pin]<[&mut] Option\<T>></code> to <code>Option<[Pin]<[&mut] T>></code>.
726	///
727	/// [&mut]: reference "mutable reference"
728	#[inline]
729	#[must_use]
730	#[stable(feature = "pin", since = "1.33.0")]
731	#[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
732	pub const fn as_pin_mut(self: Pin<&mut Self>) -> Option<Pin<&mut T>> {
733	// SAFETY: `get_unchecked_mut` is never used to move the `Option` inside `self`.
734	// `x` is guaranteed to be pinned because it comes from `self` which is pinned.
735	unsafe {
736	match Pin::get_unchecked_mut(self).as_mut() {
737	Some(x) => Some(Pin::new_unchecked(x)),
738	None => None,
739	}
740	}
741	}
742
743	/// Returns a slice of the contained value, if any. If this is `None`, an
744	/// empty slice is returned. This can be useful to have a single type of
745	/// iterator over an `Option` or slice.
746	///
747	/// Note: Should you have an `Option<&T>` and wish to get a slice of `T`,
748	/// you can unpack it via `opt.map_or(&[], std::slice::from_ref)`.
749	///
750	/// # Examples
751	///
752	/// ```rust
753	/// assert_eq!(
754	/// [Some(`1234`).as_slice(), None.as_slice()],
755	/// [&[`1234`][..], &[][..]],
756	/// );
757	/// ```
758	///
759	/// The inverse of this function is (discounting
760	/// borrowing) [`[_]::first`](slice::first):
761	///
762	/// ```rust
763	/// for i in [Some(`1234_u16`), None] {
764	/// assert_eq!(i.as_ref(), i.as_slice().first());
765	/// }
766	/// ```
767	#[inline]
768	#[must_use]
769	#[stable(feature = "option_as_slice", since = "1.75.0")]
770	pub fn as_slice(&self) -> &[T] {
771	// SAFETY: When the `Option` is `Some`, we're using the actual pointer
772	// to the payload, with a length of 1, so this is equivalent to
773	// `slice::from_ref`, and thus is safe.
774	// When the `Option` is `None`, the length used is 0, so to be safe it
775	// just needs to be aligned, which it is because `&self` is aligned and
776	// the offset used is a multiple of alignment.
777	//
778	// In the new version, the intrinsic always returns a pointer to an
779	// in-bounds and correctly aligned position for a `T` (even if in the
780	// `None` case it's just padding).
781	unsafe {
782	slice::from_raw_parts(
783	(self as *const Self).byte_add(core::mem::offset_of!(Self, Some.0)).cast(),
784	usize::from(self.is_some()),
785	)
786	}
787	}
788
789	/// Returns a mutable slice of the contained value, if any. If this is
790	/// `None`, an empty slice is returned. This can be useful to have a
791	/// single type of iterator over an `Option` or slice.
792	///
793	/// Note: Should you have an `Option<&mut T>` instead of a
794	/// `&mut Option<T>`, which this method takes, you can obtain a mutable
795	/// slice via `opt.map_or(&mut [], std::slice::from_mut)`.
796	///
797	/// # Examples
798	///
799	/// ```rust
800	/// assert_eq!(
801	/// [Some(`1234`).as_mut_slice(), None.as_mut_slice()],
802	/// [&mut [`1234`][..], &mut [][..]],
803	/// );
804	/// ```
805	///
806	/// The result is a mutable slice of zero or one items that points into
807	/// our original `Option`:
808	///
809	/// ```rust
810	/// let mut x = Some(`1234`);
811	/// x.as_mut_slice()[`0`] += `1`;
812	/// assert_eq!(x, Some(`1235`));
813	/// ```
814	///
815	/// The inverse of this method (discounting borrowing)
816	/// is [`[_]::first_mut`](slice::first_mut):
817	///
818	/// ```rust
819	/// assert_eq!(Some(`123`).as_mut_slice().first_mut(), Some(&mut `123`))
820	/// ```
821	#[inline]
822	#[must_use]
823	#[stable(feature = "option_as_slice", since = "1.75.0")]
824	pub fn as_mut_slice(&mut self) -> &mut [T] {
825	// SAFETY: When the `Option` is `Some`, we're using the actual pointer
826	// to the payload, with a length of 1, so this is equivalent to
827	// `slice::from_mut`, and thus is safe.
828	// When the `Option` is `None`, the length used is 0, so to be safe it
829	// just needs to be aligned, which it is because `&self` is aligned and
830	// the offset used is a multiple of alignment.
831	//
832	// In the new version, the intrinsic creates a `const T` from a*
833	// mutable reference so it is safe to cast back to a mutable pointer
834	// here. As with `as_slice`, the intrinsic always returns a pointer to
835	// an in-bounds and correctly aligned position for a `T` (even if in
836	// the `None` case it's just padding).
837	unsafe {
838	slice::from_raw_parts_mut(
839	(self as *mut Self).byte_add(core::mem::offset_of!(Self, Some.0)).cast(),
840	usize::from(self.is_some()),
841	)
842	}
843	}
844
845	/////////////////////////////////////////////////////////////////////////
846	// Getting to contained values
847	/////////////////////////////////////////////////////////////////////////
848
849	/// Returns the contained [`Some`] value, consuming the `self` value.
850	///
851	/// # Panics
852	///
853	/// Panics if the value is a [`None`] with a custom panic message provided by
854	/// `msg`.
855	///
856	/// # Examples
857	///
858	/// ```
859	/// let x = Some("value");
860	/// assert_eq!(x.expect("fruits are healthy"), "value");
861	/// ```
862	///
863	/// ```should_panic
864	/// let x: Option<&str> = None;
865	/// x.expect("fruits are healthy"); // panics with `fruits are healthy`
866	/// ```
867	///
868	/// # Recommended Message Style
869	///
870	/// We recommend that `expect` messages are used to describe the reason you
871	/// _expect_ the `Option` should be `Some`.
872	///
873	/// ```should_panic
874	/// # let slice: &[u8] = &[];
875	/// let item = slice.get(`0`)
876	/// .expect("slice should not be empty");
877	/// ```
878	///
879	/// Hint: If you're having trouble remembering how to phrase expect
880	/// error messages remember to focus on the word "should" as in "env
881	/// variable should be set by blah" or "the given binary should be available
882	/// and executable by the current user".
883	///
884	/// For more detail on expect message styles and the reasoning behind our
885	/// recommendation please refer to the section on ["Common Message
886	/// Styles"](../../std/error/index.html#common-message-styles) in the [`std::error`](../../std/error/index.html) module docs.
887	#[inline]
888	#[track_caller]
889	#[stable(feature = "rust1", since = "1.0.0")]
890	#[rustc_const_unstable(feature = "const_option", issue = "67441")]
891	pub const fn expect(self, msg: &str) -> T {
892	match self {
893	Some(val) => val,
894	None => expect_failed(msg),
895	}
896	}
897
898	/// Returns the contained [`Some`] value, consuming the `self` value.
899	///
900	/// Because this function may panic, its use is generally discouraged.
901	/// Instead, prefer to use pattern matching and handle the [`None`]
902	/// case explicitly, or call [`unwrap_or`], [`unwrap_or_else`], or
903	/// [`unwrap_or_default`].
904	///
905	/// [`unwrap_or`]: Option::unwrap_or
906	/// [`unwrap_or_else`]: Option::unwrap_or_else
907	/// [`unwrap_or_default`]: Option::unwrap_or_default
908	///
909	/// # Panics
910	///
911	/// Panics if the self value equals [`None`].
912	///
913	/// # Examples
914	///
915	/// ```
916	/// let x = Some("air");
917	/// assert_eq!(x.unwrap(), "air");
918	/// ```
919	///
920	/// ```should_panic
921	/// let x: Option<&str> = None;
922	/// assert_eq!(x.unwrap(), "air"); // fails
923	/// ```
924	#[inline(always)]
925	#[track_caller]
926	#[stable(feature = "rust1", since = "1.0.0")]
927	#[rustc_const_unstable(feature = "const_option", issue = "67441")]
928	pub const fn unwrap(self) -> T {
929	match self {
930	Some(val) => val,
931	None => unwrap_failed(),
932	}
933	}
934
935	/// Returns the contained [`Some`] value or a provided default.
936	///
937	/// Arguments passed to `unwrap_or` are eagerly evaluated; if you are passing
938	/// the result of a function call, it is recommended to use [`unwrap_or_else`],
939	/// which is lazily evaluated.
940	///
941	/// [`unwrap_or_else`]: Option::unwrap_or_else
942	///
943	/// # Examples
944	///
945	/// ```
946	/// assert_eq!(Some("car").unwrap_or("bike"), "car");
947	/// assert_eq!(None.unwrap_or("bike"), "bike");
948	/// ```
949	#[inline]
950	#[stable(feature = "rust1", since = "1.0.0")]
951	pub fn unwrap_or(self, default: T) -> T {
952	match self {
953	Some(x) => x,
954	None => default,
955	}
956	}
957
958	/// Returns the contained [`Some`] value or computes it from a closure.
959	///
960	/// # Examples
961	///
962	/// ```
963	/// let k = `10`;
964	/// assert_eq!(Some(`4`).unwrap_or_else(\|\| `2` * k), `4`);
965	/// assert_eq!(None.unwrap_or_else(\|\| `2` * k), `20`);
966	/// ```
967	#[inline]
968	#[track_caller]
969	#[stable(feature = "rust1", since = "1.0.0")]
970	pub fn unwrap_or_else<F>(self, f: F) -> T
971	where
972	F: FnOnce() -> T,
973	{
974	match self {
975	Some(x) => x,
976	None => f(),
977	}
978	}
979
980	/// Returns the contained [`Some`] value or a default.
981	///
982	/// Consumes the `self` argument then, if [`Some`], returns the contained
983	/// value, otherwise if [`None`], returns the [default value] for that
984	/// type.
985	///
986	/// # Examples
987	///
988	/// ```
989	/// let x: Option<u32> = None;
990	/// let y: Option<u32> = Some(`12`);
991	///
992	/// assert_eq!(x.unwrap_or_default(), `0`);
993	/// assert_eq!(y.unwrap_or_default(), `12`);
994	/// ```
995	///
996	/// [default value]: Default::default
997	/// [`parse`]: str::parse
998	/// [`FromStr`]: crate::str::FromStr
999	#[inline]
1000	#[stable(feature = "rust1", since = "1.0.0")]
1001	pub fn unwrap_or_default(self) -> T
1002	where
1003	T: Default,
1004	{
1005	match self {
1006	Some(x) => x,
1007	None => T::default(),
1008	}
1009	}
1010
1011	/// Returns the contained [`Some`] value, consuming the `self` value,
1012	/// without checking that the value is not [`None`].
1013	///
1014	/// # Safety
1015	///
1016	/// Calling this method on [`None`] is [undefined behavior].
1017	///
1018	/// [undefined behavior]: https://doc.rust-lang.org/reference/behavior-considered-undefined.html
1019	///
1020	/// # Examples
1021	///
1022	/// ```
1023	/// let x = Some("air");
1024	/// assert_eq!(unsafe { x.unwrap_unchecked() }, "air");
1025	/// ```
1026	///
1027	/// ```no_run
1028	/// let x: Option<&str> = None;
1029	/// assert_eq!(unsafe { x.unwrap_unchecked() }, "air"); // Undefined behavior!
1030	/// ```
1031	#[inline]
1032	#[track_caller]
1033	#[stable(feature = "option_result_unwrap_unchecked", since = "1.58.0")]
1034	#[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
1035	pub const unsafe fn unwrap_unchecked(self) -> T {
1036	debug_assert!(self.is_some());
1037	match self {
1038	Some(val) => val,
1039	// SAFETY: the safety contract must be upheld by the caller.
1040	None => unsafe { hint::unreachable_unchecked() },
1041	}
1042	}
1043
1044	/////////////////////////////////////////////////////////////////////////
1045	// Transforming contained values
1046	/////////////////////////////////////////////////////////////////////////
1047
1048	/// Maps an `Option<T>` to `Option<U>` by applying a function to a contained value (if `Some`) or returns `None` (if `None`).
1049	///
1050	/// # Examples
1051	///
1052	/// Calculates the length of an <code>Option<[String]></code> as an
1053	/// <code>Option<[usize]></code>, consuming the original:
1054	///
1055	/// [String]: ../../std/string/struct.String.html "String"
1056	/// ```
1057	/// let maybe_some_string = Some(String::from("Hello, World!"));
1058	/// // `Option::map` takes self by value, consuming `maybe_some_string`
1059	/// let maybe_some_len = maybe_some_string.map(\|s\| s.len());
1060	/// assert_eq!(maybe_some_len, Some(`13`));
1061	///
1062	/// let x: Option<&str> = None;
1063	/// assert_eq!(x.map(\|s\| s.len()), None);
1064	/// ```
1065	#[inline]
1066	#[stable(feature = "rust1", since = "1.0.0")]
1067	pub fn map<U, F>(self, f: F) -> Option<U>
1068	where
1069	F: FnOnce(T) -> U,
1070	{
1071	match self {
1072	Some(x) => Some(f(x)),
1073	None => None,
1074	}
1075	}
1076
1077	/// Calls the provided closure with a reference to the contained value (if [`Some`]).
1078	///
1079	/// # Examples
1080	///
1081	/// ```
1082	/// let v = vec![`1`, `2`, `3`, `4`, `5`];
1083	///
1084	/// // prints "got: 4"
1085	/// let x: Option<&usize> = v.get(`3`).inspect(\|x\| println!("got: {x}"));
1086	///
1087	/// // prints nothing
1088	/// let x: Option<&usize> = v.get(`5`).inspect(\|x\| println!("got: {x}"));
1089	/// ```
1090	#[inline]
1091	#[stable(feature = "result_option_inspect", since = "1.76.0")]
1092	pub fn inspect<F: FnOnce(&T)>(self, f: F) -> Self {
1093	if let Some(ref x) = self {
1094	f(x);
1095	}
1096
1097	self
1098	}
1099
1100	/// Returns the provided default result (if none),
1101	/// or applies a function to the contained value (if any).
1102	///
1103	/// Arguments passed to `map_or` are eagerly evaluated; if you are passing
1104	/// the result of a function call, it is recommended to use [`map_or_else`],
1105	/// which is lazily evaluated.
1106	///
1107	/// [`map_or_else`]: Option::map_or_else
1108	///
1109	/// # Examples
1110	///
1111	/// ```
1112	/// let x = Some("foo");
1113	/// assert_eq!(x.map_or(`42`, \|v\| v.len()), `3`);
1114	///
1115	/// let x: Option<&str> = None;
1116	/// assert_eq!(x.map_or(`42`, \|v\| v.len()), `42`);
1117	/// ```
1118	#[inline]
1119	#[stable(feature = "rust1", since = "1.0.0")]
1120	#[must_use = "if you don't need the returned value, use `if let` instead"]
1121	pub fn map_or<U, F>(self, default: U, f: F) -> U
1122	where
1123	F: FnOnce(T) -> U,
1124	{
1125	match self {
1126	Some(t) => f(t),
1127	None => default,
1128	}
1129	}
1130
1131	/// Computes a default function result (if none), or
1132	/// applies a different function to the contained value (if any).
1133	///
1134	/// # Basic examples
1135	///
1136	/// ```
1137	/// let k = `21`;
1138	///
1139	/// let x = Some("foo");
1140	/// assert_eq!(x.map_or_else(\|\| `2` * k, \|v\| v.len()), `3`);
1141	///
1142	/// let x: Option<&str> = None;
1143	/// assert_eq!(x.map_or_else(\|\| `2` * k, \|v\| v.len()), `42`);
1144	/// ```
1145	///
1146	/// # Handling a Result-based fallback
1147	///
1148	/// A somewhat common occurrence when dealing with optional values
1149	/// in combination with [`Result<T, E>`] is the case where one wants to invoke
1150	/// a fallible fallback if the option is not present. This example
1151	/// parses a command line argument (if present), or the contents of a file to
1152	/// an integer. However, unlike accessing the command line argument, reading
1153	/// the file is fallible, so it must be wrapped with `Ok`.
1154	///
1155	/// ```no_run
1156	/// # fn main() -> Result<(), Box<dyn std::error::Error>> {
1157	/// let v: u64 = std::env::args()
1158	/// .nth(`1`)
1159	/// .map_or_else(\|\| std::fs::read_to_string("/etc/someconfig.conf"), Ok)?
1160	/// .parse()?;
1161	/// # Ok(())
1162	/// # }
1163	/// ```
1164	#[inline]
1165	#[stable(feature = "rust1", since = "1.0.0")]
1166	pub fn map_or_else<U, D, F>(self, default: D, f: F) -> U
1167	where
1168	D: FnOnce() -> U,
1169	F: FnOnce(T) -> U,
1170	{
1171	match self {
1172	Some(t) => f(t),
1173	None => default(),
1174	}
1175	}
1176
1177	/// Transforms the `Option<T>` into a [`Result<T, E>`], mapping [`Some(v)`] to
1178	/// [`Ok(v)`] and [`None`] to [`Err(err)`].
1179	///
1180	/// Arguments passed to `ok_or` are eagerly evaluated; if you are passing the
1181	/// result of a function call, it is recommended to use [`ok_or_else`], which is
1182	/// lazily evaluated.
1183	///
1184	/// [`Ok(v)`]: Ok
1185	/// [`Err(err)`]: Err
1186	/// [`Some(v)`]: Some
1187	/// [`ok_or_else`]: Option::ok_or_else
1188	///
1189	/// # Examples
1190	///
1191	/// ```
1192	/// let x = Some("foo");
1193	/// assert_eq!(x.ok_or(`0`), Ok("foo"));
1194	///
1195	/// let x: Option<&str> = None;
1196	/// assert_eq!(x.ok_or(`0`), Err(`0`));
1197	/// ```
1198	#[inline]
1199	#[stable(feature = "rust1", since = "1.0.0")]
1200	pub fn ok_or<E>(self, err: E) -> Result<T, E> {
1201	match self {
1202	Some(v) => Ok(v),
1203	None => Err(err),
1204	}
1205	}
1206
1207	/// Transforms the `Option<T>` into a [`Result<T, E>`], mapping [`Some(v)`] to
1208	/// [`Ok(v)`] and [`None`] to [`Err(err())`].
1209	///
1210	/// [`Ok(v)`]: Ok
1211	/// [`Err(err())`]: Err
1212	/// [`Some(v)`]: Some
1213	///
1214	/// # Examples
1215	///
1216	/// ```
1217	/// let x = Some("foo");
1218	/// assert_eq!(x.ok_or_else(\|\| `0`), Ok("foo"));
1219	///
1220	/// let x: Option<&str> = None;
1221	/// assert_eq!(x.ok_or_else(\|\| `0`), Err(`0`));
1222	/// ```
1223	#[inline]
1224	#[stable(feature = "rust1", since = "1.0.0")]
1225	pub fn ok_or_else<E, F>(self, err: F) -> Result<T, E>
1226	where
1227	F: FnOnce() -> E,
1228	{
1229	match self {
1230	Some(v) => Ok(v),
1231	None => Err(err()),
1232	}
1233	}
1234
1235	/// Converts from `Option<T>` (or `&Option<T>`) to `Option<&T::Target>`.
1236	///
1237	/// Leaves the original Option in-place, creating a new one with a reference
1238	/// to the original one, additionally coercing the contents via [`Deref`].
1239	///
1240	/// # Examples
1241	///
1242	/// ```
1243	/// let x: Option<String> = Some("hey".to_owned());
1244	/// assert_eq!(x.as_deref(), Some("hey"));
1245	///
1246	/// let x: Option<String> = None;
1247	/// assert_eq!(x.as_deref(), None);
1248	/// ```
1249	#[inline]
1250	#[stable(feature = "option_deref", since = "1.40.0")]
1251	pub fn as_deref(&self) -> Option<&T::Target>
1252	where
1253	T: Deref,
1254	{
1255	match self.as_ref() {
1256	Some(t) => Some(t.deref()),
1257	None => None,
1258	}
1259	}
1260
1261	/// Converts from `Option<T>` (or `&mut Option<T>`) to `Option<&mut T::Target>`.
1262	///
1263	/// Leaves the original `Option` in-place, creating a new one containing a mutable reference to
1264	/// the inner type's [`Deref::Target`] type.
1265	///
1266	/// # Examples
1267	///
1268	/// ```
1269	/// let mut x: Option<String> = Some("hey".to_owned());
1270	/// assert_eq!(x.as_deref_mut().map(\|x\| {
1271	/// x.make_ascii_uppercase();
1272	/// x
1273	/// }), Some("HEY".to_owned().as_mut_str()));
1274	/// ```
1275	#[inline]
1276	#[stable(feature = "option_deref", since = "1.40.0")]
1277	pub fn as_deref_mut(&mut self) -> Option<&mut T::Target>
1278	where
1279	T: DerefMut,
1280	{
1281	match self.as_mut() {
1282	Some(t) => Some(t.deref_mut()),
1283	None => None,
1284	}
1285	}
1286
1287	/////////////////////////////////////////////////////////////////////////
1288	// Iterator constructors
1289	/////////////////////////////////////////////////////////////////////////
1290
1291	/// Returns an iterator over the possibly contained value.
1292	///
1293	/// # Examples
1294	///
1295	/// ```
1296	/// let x = Some(`4`);
1297	/// assert_eq!(x.iter().next(), Some(&`4`));
1298	///
1299	/// let x: Option<u32> = None;
1300	/// assert_eq!(x.iter().next(), None);
1301	/// ```
1302	#[inline]
1303	#[rustc_const_unstable(feature = "const_option", issue = "67441")]
1304	#[stable(feature = "rust1", since = "1.0.0")]
1305	pub const fn iter(&self) -> Iter<'_, T> {
1306	Iter { inner: Item { opt: self.as_ref() } }
1307	}
1308
1309	/// Returns a mutable iterator over the possibly contained value.
1310	///
1311	/// # Examples
1312	///
1313	/// ```
1314	/// let mut x = Some(`4`);
1315	/// match x.iter_mut().next() {
1316	/// Some(v) => *v = `42`,
1317	/// None => {},
1318	/// }
1319	/// assert_eq!(x, Some(`42`));
1320	///
1321	/// let mut x: Option<u32> = None;
1322	/// assert_eq!(x.iter_mut().next(), None);
1323	/// ```
1324	#[inline]
1325	#[stable(feature = "rust1", since = "1.0.0")]
1326	pub fn iter_mut(&mut self) -> IterMut<'_, T> {
1327	IterMut { inner: Item { opt: self.as_mut() } }
1328	}
1329
1330	/////////////////////////////////////////////////////////////////////////
1331	// Boolean operations on the values, eager and lazy
1332	/////////////////////////////////////////////////////////////////////////
1333
1334	/// Returns [`None`] if the option is [`None`], otherwise returns `optb`.
1335	///
1336	/// Arguments passed to `and` are eagerly evaluated; if you are passing the
1337	/// result of a function call, it is recommended to use [`and_then`], which is
1338	/// lazily evaluated.
1339	///
1340	/// [`and_then`]: Option::and_then
1341	///
1342	/// # Examples
1343	///
1344	/// ```
1345	/// let x = Some(`2`);
1346	/// let y: Option<&str> = None;
1347	/// assert_eq!(x.and(y), None);
1348	///
1349	/// let x: Option<u32> = None;
1350	/// let y = Some("foo");
1351	/// assert_eq!(x.and(y), None);
1352	///
1353	/// let x = Some(`2`);
1354	/// let y = Some("foo");
1355	/// assert_eq!(x.and(y), Some("foo"));
1356	///
1357	/// let x: Option<u32> = None;
1358	/// let y: Option<&str> = None;
1359	/// assert_eq!(x.and(y), None);
1360	/// ```
1361	#[inline]
1362	#[stable(feature = "rust1", since = "1.0.0")]
1363	pub fn and<U>(self, optb: Option<U>) -> Option<U> {
1364	match self {
1365	Some(_) => optb,
1366	None => None,
1367	}
1368	}
1369
1370	/// Returns [`None`] if the option is [`None`], otherwise calls `f` with the
1371	/// wrapped value and returns the result.
1372	///
1373	/// Some languages call this operation flatmap.
1374	///
1375	/// # Examples
1376	///
1377	/// ```
1378	/// fn sq_then_to_string(x: u32) -> Option<String> {
1379	/// x.checked_mul(x).map(\|sq\| sq.to_string())
1380	/// }
1381	///
1382	/// assert_eq!(Some(`2`).and_then(sq_then_to_string), Some(`4`.to_string()));
1383	/// assert_eq!(Some(`1_000_000`).and_then(sq_then_to_string), None); // overflowed!
1384	/// assert_eq!(None.and_then(sq_then_to_string), None);
1385	/// ```
1386	///
1387	/// Often used to chain fallible operations that may return [`None`].
1388	///
1389	/// ```
1390	/// let arr_2d = [["A0", "A1"], ["B0", "B1"]];
1391	///
1392	/// let item_0_1 = arr_2d.get(`0`).and_then(\|row\| row.get(`1`));
1393	/// assert_eq!(item_0_1, Some(&"A1"));
1394	///
1395	/// let item_2_0 = arr_2d.get(`2`).and_then(\|row\| row.get(`0`));
1396	/// assert_eq!(item_2_0, None);
1397	/// ```
1398	#[doc(alias = "flatmap")]
1399	#[inline]
1400	#[stable(feature = "rust1", since = "1.0.0")]
1401	pub fn and_then<U, F>(self, f: F) -> Option<U>
1402	where
1403	F: FnOnce(T) -> Option<U>,
1404	{
1405	match self {
1406	Some(x) => f(x),
1407	None => None,
1408	}
1409	}
1410
1411	/// Returns [`None`] if the option is [`None`], otherwise calls `predicate`
1412	/// with the wrapped value and returns:
1413	///
1414	/// - [`Some(t)`] if `predicate` returns `true` (where `t` is the wrapped
1415	/// value), and
1416	/// - [`None`] if `predicate` returns `false`.
1417	///
1418	/// This function works similar to [`Iterator::filter()`]. You can imagine
1419	/// the `Option<T>` being an iterator over one or zero elements. `filter()`
1420	/// lets you decide which elements to keep.
1421	///
1422	/// # Examples
1423	///
1424	/// ```rust
1425	/// fn is_even(n: &i32) -> bool {
1426	/// n % `2` == `0`
1427	/// }
1428	///
1429	/// assert_eq!(None.filter(is_even), None);
1430	/// assert_eq!(Some(`3`).filter(is_even), None);
1431	/// assert_eq!(Some(`4`).filter(is_even), Some(`4`));
1432	/// ```
1433	///
1434	/// [`Some(t)`]: Some
1435	#[inline]
1436	#[stable(feature = "option_filter", since = "1.27.0")]
1437	pub fn filter<P>(self, predicate: P) -> Self
1438	where
1439	P: FnOnce(&T) -> bool,
1440	{
1441	if let Some(x) = self {
1442	if predicate(&x) {
1443	return Some(x);
1444	}
1445	}
1446	None
1447	}
1448
1449	/// Returns the option if it contains a value, otherwise returns `optb`.
1450	///
1451	/// Arguments passed to `or` are eagerly evaluated; if you are passing the
1452	/// result of a function call, it is recommended to use [`or_else`], which is
1453	/// lazily evaluated.
1454	///
1455	/// [`or_else`]: Option::or_else
1456	///
1457	/// # Examples
1458	///
1459	/// ```
1460	/// let x = Some(`2`);
1461	/// let y = None;
1462	/// assert_eq!(x.or(y), Some(`2`));
1463	///
1464	/// let x = None;
1465	/// let y = Some(`100`);
1466	/// assert_eq!(x.or(y), Some(`100`));
1467	///
1468	/// let x = Some(`2`);
1469	/// let y = Some(`100`);
1470	/// assert_eq!(x.or(y), Some(`2`));
1471	///
1472	/// let x: Option<u32> = None;
1473	/// let y = None;
1474	/// assert_eq!(x.or(y), None);
1475	/// ```
1476	#[inline]
1477	#[stable(feature = "rust1", since = "1.0.0")]
1478	pub fn or(self, optb: Option<T>) -> Option<T> {
1479	match self {
1480	x @ Some(_) => x,
1481	None => optb,
1482	}
1483	}
1484
1485	/// Returns the option if it contains a value, otherwise calls `f` and
1486	/// returns the result.
1487	///
1488	/// # Examples
1489	///
1490	/// ```
1491	/// fn nobody() -> Option<&'static str> { None }
1492	/// fn vikings() -> Option<&'static str> { Some("vikings") }
1493	///
1494	/// assert_eq!(Some("barbarians").or_else(vikings), Some("barbarians"));
1495	/// assert_eq!(None.or_else(vikings), Some("vikings"));
1496	/// assert_eq!(None.or_else(nobody), None);
1497	/// ```
1498	#[inline]
1499	#[stable(feature = "rust1", since = "1.0.0")]
1500	pub fn or_else<F>(self, f: F) -> Option<T>
1501	where
1502	F: FnOnce() -> Option<T>,
1503	{
1504	match self {
1505	x @ Some(_) => x,
1506	None => f(),
1507	}
1508	}
1509
1510	/// Returns [`Some`] if exactly one of `self`, `optb` is [`Some`], otherwise returns [`None`].
1511	///
1512	/// # Examples
1513	///
1514	/// ```
1515	/// let x = Some(`2`);
1516	/// let y: Option<u32> = None;
1517	/// assert_eq!(x.xor(y), Some(`2`));
1518	///
1519	/// let x: Option<u32> = None;
1520	/// let y = Some(`2`);
1521	/// assert_eq!(x.xor(y), Some(`2`));
1522	///
1523	/// let x = Some(`2`);
1524	/// let y = Some(`2`);
1525	/// assert_eq!(x.xor(y), None);
1526	///
1527	/// let x: Option<u32> = None;
1528	/// let y: Option<u32> = None;
1529	/// assert_eq!(x.xor(y), None);
1530	/// ```
1531	#[inline]
1532	#[stable(feature = "option_xor", since = "1.37.0")]
1533	pub fn xor(self, optb: Option<T>) -> Option<T> {
1534	match (self, optb) {
1535	(a @ Some(_), None) => a,
1536	(None, b @ Some(_)) => b,
1537	_ => None,
1538	}
1539	}
1540
1541	/////////////////////////////////////////////////////////////////////////
1542	// Entry-like operations to insert a value and return a reference
1543	/////////////////////////////////////////////////////////////////////////
1544
1545	/// Inserts `value` into the option, then returns a mutable reference to it.
1546	///
1547	/// If the option already contains a value, the old value is dropped.
1548	///
1549	/// See also [`Option::get_or_insert`], which doesn't update the value if
1550	/// the option already contains [`Some`].
1551	///
1552	/// # Example
1553	///
1554	/// ```
1555	/// let mut opt = None;
1556	/// let val = opt.insert(`1`);
1557	/// assert_eq!(*val, `1`);
1558	/// assert_eq!(opt.unwrap(), `1`);
1559	/// let val = opt.insert(`2`);
1560	/// assert_eq!(*val, `2`);
1561	/// *val = `3`;
1562	/// assert_eq!(opt.unwrap(), `3`);
1563	/// ```
1564	#[must_use = "if you intended to set a value, consider assignment instead"]
1565	#[inline]
1566	#[stable(feature = "option_insert", since = "1.53.0")]
1567	pub fn insert(&mut self, value: T) -> &mut T {
1568	*self = Some(value);
1569
1570	// SAFETY: the code above just filled the option
1571	unsafe { self.as_mut().unwrap_unchecked() }
1572	}
1573
1574	/// Inserts `value` into the option if it is [`None`], then
1575	/// returns a mutable reference to the contained value.
1576	///
1577	/// See also [`Option::insert`], which updates the value even if
1578	/// the option already contains [`Some`].
1579	///
1580	/// # Examples
1581	///
1582	/// ```
1583	/// let mut x = None;
1584	///
1585	/// {
1586	/// let y: &mut u32 = x.get_or_insert(`5`);
1587	/// assert_eq!(y, &`5`);
1588	///
1589	/// *y = `7`;
1590	/// }
1591	///
1592	/// assert_eq!(x, Some(`7`));
1593	/// ```
1594	#[inline]
1595	#[stable(feature = "option_entry", since = "1.20.0")]
1596	pub fn get_or_insert(&mut self, value: T) -> &mut T {
1597	if let None = *self {
1598	*self = Some(value);
1599	}
1600
1601	// SAFETY: a `None` variant for `self` would have been replaced by a `Some`
1602	// variant in the code above.
1603	unsafe { self.as_mut().unwrap_unchecked() }
1604	}
1605
1606	/// Inserts the default value into the option if it is [`None`], then
1607	/// returns a mutable reference to the contained value.
1608	///
1609	/// # Examples
1610	///
1611	/// ```
1612	/// #![feature(option_get_or_insert_default)]
1613	///
1614	/// let mut x = None;
1615	///
1616	/// {
1617	/// let y: &mut u32 = x.get_or_insert_default();
1618	/// assert_eq!(y, &`0`);
1619	///
1620	/// *y = `7`;
1621	/// }
1622	///
1623	/// assert_eq!(x, Some(`7`));
1624	/// ```
1625	#[inline]
1626	#[unstable(feature = "option_get_or_insert_default", issue = "82901")]
1627	pub fn get_or_insert_default(&mut self) -> &mut T
1628	where
1629	T: Default,
1630	{
1631	self.get_or_insert_with(T::default)
1632	}
1633
1634	/// Inserts a value computed from `f` into the option if it is [`None`],
1635	/// then returns a mutable reference to the contained value.
1636	///
1637	/// # Examples
1638	///
1639	/// ```
1640	/// let mut x = None;
1641	///
1642	/// {
1643	/// let y: &mut u32 = x.get_or_insert_with(\|\| `5`);
1644	/// assert_eq!(y, &`5`);
1645	///
1646	/// *y = `7`;
1647	/// }
1648	///
1649	/// assert_eq!(x, Some(`7`));
1650	/// ```
1651	#[inline]
1652	#[stable(feature = "option_entry", since = "1.20.0")]
1653	pub fn get_or_insert_with<F>(&mut self, f: F) -> &mut T
1654	where
1655	F: FnOnce() -> T,
1656	{
1657	if let None = self {
1658	*self = Some(f());
1659	}
1660
1661	// SAFETY: a `None` variant for `self` would have been replaced by a `Some`
1662	// variant in the code above.
1663	unsafe { self.as_mut().unwrap_unchecked() }
1664	}
1665
1666	/////////////////////////////////////////////////////////////////////////
1667	// Misc
1668	/////////////////////////////////////////////////////////////////////////
1669
1670	/// Takes the value out of the option, leaving a [`None`] in its place.
1671	///
1672	/// # Examples
1673	///
1674	/// ```
1675	/// let mut x = Some(`2`);
1676	/// let y = x.take();
1677	/// assert_eq!(x, None);
1678	/// assert_eq!(y, Some(`2`));
1679	///
1680	/// let mut x: Option<u32> = None;
1681	/// let y = x.take();
1682	/// assert_eq!(x, None);
1683	/// assert_eq!(y, None);
1684	/// ```
1685	#[inline]
1686	#[stable(feature = "rust1", since = "1.0.0")]
1687	#[rustc_const_unstable(feature = "const_option", issue = "67441")]
1688	pub const fn take(&mut self) -> Option<T> {
1689	// FIXME replace `mem::replace` by `mem::take` when the latter is const ready
1690	mem::replace(self, None)
1691	}
1692
1693	/// Takes the value out of the option, but only if the predicate evaluates to
1694	/// `true` on a mutable reference to the value.
1695	///
1696	/// In other words, replaces `self` with `None` if the predicate returns `true`.
1697	/// This method operates similar to [`Option::take`] but conditional.
1698	///
1699	/// # Examples
1700	///
1701	/// ```
1702	/// #![feature(option_take_if)]
1703	///
1704	/// let mut x = Some(`42`);
1705	///
1706	/// let prev = x.take_if(\|v\| if *v == `42` {
1707	/// *v += `1`;
1708	/// `false`
1709	/// } else {
1710	/// `false`
1711	/// });
1712	/// assert_eq!(x, Some(`43`));
1713	/// assert_eq!(prev, None);
1714	///
1715	/// let prev = x.take_if(\|v\| *v == `43`);
1716	/// assert_eq!(x, None);
1717	/// assert_eq!(prev, Some(`43`));
1718	/// ```
1719	#[inline]
1720	#[unstable(feature = "option_take_if", issue = "98934")]
1721	pub fn take_if<P>(&mut self, predicate: P) -> Option<T>
1722	where
1723	P: FnOnce(&mut T) -> bool,
1724	{
1725	if self.as_mut().map_or(`false`, predicate) { self.take() } else { None }
1726	}
1727
1728	/// Replaces the actual value in the option by the value given in parameter,
1729	/// returning the old value if present,
1730	/// leaving a [`Some`] in its place without deinitializing either one.
1731	///
1732	/// # Examples
1733	///
1734	/// ```
1735	/// let mut x = Some(`2`);
1736	/// let old = x.replace(`5`);
1737	/// assert_eq!(x, Some(`5`));
1738	/// assert_eq!(old, Some(`2`));
1739	///
1740	/// let mut x = None;
1741	/// let old = x.replace(`3`);
1742	/// assert_eq!(x, Some(`3`));
1743	/// assert_eq!(old, None);
1744	/// ```
1745	#[inline]
1746	#[rustc_const_unstable(feature = "const_option", issue = "67441")]
1747	#[stable(feature = "option_replace", since = "1.31.0")]
1748	pub const fn replace(&mut self, value: T) -> Option<T> {
1749	mem::replace(self, Some(value))
1750	}
1751
1752	/// Zips `self` with another `Option`.
1753	///
1754	/// If `self` is `Some(s)` and `other` is `Some(o)`, this method returns `Some((s, o))`.
1755	/// Otherwise, `None` is returned.
1756	///
1757	/// # Examples
1758	///
1759	/// ```
1760	/// let x = Some(`1`);
1761	/// let y = Some("hi");
1762	/// let z = None::<u8>;
1763	///
1764	/// assert_eq!(x.zip(y), Some((`1`, "hi")));
1765	/// assert_eq!(x.zip(z), None);
1766	/// ```
1767	#[stable(feature = "option_zip_option", since = "1.46.0")]
1768	pub fn zip<U>(self, other: Option<U>) -> Option<(T, U)> {
1769	match (self, other) {
1770	(Some(a), Some(b)) => Some((a, b)),
1771	_ => None,
1772	}
1773	}
1774
1775	/// Zips `self` and another `Option` with function `f`.
1776	///
1777	/// If `self` is `Some(s)` and `other` is `Some(o)`, this method returns `Some(f(s, o))`.
1778	/// Otherwise, `None` is returned.
1779	///
1780	/// # Examples
1781	///
1782	/// ```
1783	/// #![feature(option_zip)]
1784	///
1785	/// #[derive(Debug, PartialEq)]
1786	/// struct Point {
1787	/// x: f64,
1788	/// y: f64,
1789	/// }
1790	///
1791	/// impl Point {
1792	/// fn new(x: f64, y: f64) -> Self {
1793	/// Self { x, y }
1794	/// }
1795	/// }
1796	///
1797	/// let x = Some(`17.5`);
1798	/// let y = Some(`42.7`);
1799	///
1800	/// assert_eq!(x.zip_with(y, Point::new), Some(Point { x: `17.5`, y: `42.7` }));
1801	/// assert_eq!(x.zip_with(None, Point::new), None);
1802	/// ```
1803	#[unstable(feature = "option_zip", issue = "70086")]
1804	pub fn zip_with<U, F, R>(self, other: Option<U>, f: F) -> Option<R>
1805	where
1806	F: FnOnce(T, U) -> R,
1807	{
1808	match (self, other) {
1809	(Some(a), Some(b)) => Some(f(a, b)),
1810	_ => None,
1811	}
1812	}
1813	}
1814
1815	impl<T, U> Option<(T, U)> {
1816	/// Unzips an option containing a tuple of two options.
1817	///
1818	/// If `self` is `Some((a, b))` this method returns `(Some(a), Some(b))`.
1819	/// Otherwise, `(None, None)` is returned.
1820	///
1821	/// # Examples
1822	///
1823	/// ```
1824	/// let x = Some((`1`, "hi"));
1825	/// let y = None::<(u8, u32)>;
1826	///
1827	/// assert_eq!(x.unzip(), (Some(`1`), Some("hi")));
1828	/// assert_eq!(y.unzip(), (None, None));
1829	/// ```
1830	#[inline]
1831	#[stable(feature = "unzip_option", since = "1.66.0")]
1832	pub fn unzip(self) -> (Option<T>, Option<U>) {
1833	match self {
1834	Some((a: T, b: U)) => (Some(a), Some(b)),
1835	None => (None, None),
1836	}
1837	}
1838	}
1839
1840	impl<T> Option<&T> {
1841	/// Maps an `Option<&T>` to an `Option<T>` by copying the contents of the
1842	/// option.
1843	///
1844	/// # Examples
1845	///
1846	/// ```
1847	/// let x = `12`;
1848	/// let opt_x = Some(&x);
1849	/// assert_eq!(opt_x, Some(&`12`));
1850	/// let copied = opt_x.copied();
1851	/// assert_eq!(copied, Some(`12`));
1852	/// ```
1853	#[must_use = "`self` will be dropped if the result is not used"]
1854	#[stable(feature = "copied", since = "1.35.0")]
1855	#[rustc_const_unstable(feature = "const_option", issue = "67441")]
1856	pub const fn copied(self) -> Option<T>
1857	where
1858	T: Copy,
1859	{
1860	// FIXME: this implementation, which sidesteps using `Option::map` since it's not const
1861	// ready yet, should be reverted when possible to avoid code repetition
1862	match self {
1863	Some(&v) => Some(v),
1864	None => None,
1865	}
1866	}
1867
1868	/// Maps an `Option<&T>` to an `Option<T>` by cloning the contents of the
1869	/// option.
1870	///
1871	/// # Examples
1872	///
1873	/// ```
1874	/// let x = `12`;
1875	/// let opt_x = Some(&x);
1876	/// assert_eq!(opt_x, Some(&`12`));
1877	/// let cloned = opt_x.cloned();
1878	/// assert_eq!(cloned, Some(`12`));
1879	/// ```
1880	#[must_use = "`self` will be dropped if the result is not used"]
1881	#[stable(feature = "rust1", since = "1.0.0")]
1882	pub fn cloned(self) -> Option<T>
1883	where
1884	T: Clone,
1885	{
1886	match self {
1887	Some(t) => Some(t.clone()),
1888	None => None,
1889	}
1890	}
1891	}
1892
1893	impl<T> Option<&mut T> {
1894	/// Maps an `Option<&mut T>` to an `Option<T>` by copying the contents of the
1895	/// option.
1896	///
1897	/// # Examples
1898	///
1899	/// ```
1900	/// let mut x = `12`;
1901	/// let opt_x = Some(&mut x);
1902	/// assert_eq!(opt_x, Some(&mut `12`));
1903	/// let copied = opt_x.copied();
1904	/// assert_eq!(copied, Some(`12`));
1905	/// ```
1906	#[must_use = "`self` will be dropped if the result is not used"]
1907	#[stable(feature = "copied", since = "1.35.0")]
1908	#[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
1909	pub const fn copied(self) -> Option<T>
1910	where
1911	T: Copy,
1912	{
1913	match self {
1914	Some(&mut t) => Some(t),
1915	None => None,
1916	}
1917	}
1918
1919	/// Maps an `Option<&mut T>` to an `Option<T>` by cloning the contents of the
1920	/// option.
1921	///
1922	/// # Examples
1923	///
1924	/// ```
1925	/// let mut x = `12`;
1926	/// let opt_x = Some(&mut x);
1927	/// assert_eq!(opt_x, Some(&mut `12`));
1928	/// let cloned = opt_x.cloned();
1929	/// assert_eq!(cloned, Some(`12`));
1930	/// ```
1931	#[must_use = "`self` will be dropped if the result is not used"]
1932	#[stable(since = "1.26.0", feature = "option_ref_mut_cloned")]
1933	pub fn cloned(self) -> Option<T>
1934	where
1935	T: Clone,
1936	{
1937	match self {
1938	Some(t) => Some(t.clone()),
1939	None => None,
1940	}
1941	}
1942	}
1943
1944	impl<T, E> Option<Result<T, E>> {
1945	/// Transposes an `Option` of a [`Result`] into a [`Result`] of an `Option`.
1946	///
1947	/// [`None`] will be mapped to <code>[Ok]\([None])</code>.
1948	/// <code>[Some]\([Ok]\(\_))</code> and <code>[Some]\([Err]\(\_))</code> will be mapped to
1949	/// <code>[Ok]\([Some]\(\_))</code> and <code>[Err]\(\_)</code>.
1950	///
1951	/// # Examples
1952	///
1953	/// ```
1954	/// #[derive(Debug, Eq, PartialEq)]
1955	/// struct SomeErr;
1956	///
1957	/// let x: Result<Option<i32>, SomeErr> = Ok(Some(`5`));
1958	/// let y: Option<Result<i32, SomeErr>> = Some(Ok(`5`));
1959	/// assert_eq!(x, y.transpose());
1960	/// ```
1961	#[inline]
1962	#[stable(feature = "transpose_result", since = "1.33.0")]
1963	#[rustc_const_unstable(feature = "const_option", issue = "67441")]
1964	pub const fn transpose(self) -> Result<Option<T>, E> {
1965	match self {
1966	Some(Ok(x)) => Ok(Some(x)),
1967	Some(Err(e)) => Err(e),
1968	None => Ok(None),
1969	}
1970	}
1971	}
1972
1973	#[cfg_attr(not(feature = "panic_immediate_abort"), inline(never))]
1974	#[cfg_attr(feature = "panic_immediate_abort", inline)]
1975	#[cold]
1976	#[track_caller]
1977	const fn unwrap_failed() -> ! {
1978	panic(expr:"called `Option::unwrap()` on a `None` value")
1979	}
1980
1981	// This is a separate function to reduce the code size of .expect() itself.
1982	#[cfg_attr(not(feature = "panic_immediate_abort"), inline(never))]
1983	#[cfg_attr(feature = "panic_immediate_abort", inline)]
1984	#[cold]
1985	#[track_caller]
1986	#[rustc_const_unstable(feature = "const_option", issue = "67441")]
1987	const fn expect_failed(msg: &str) -> ! {
1988	panic_str(expr:msg)
1989	}
1990
1991	/////////////////////////////////////////////////////////////////////////////
1992	// Trait implementations
1993	/////////////////////////////////////////////////////////////////////////////
1994
1995	#[stable(feature = "rust1", since = "1.0.0")]
1996	impl<T> Clone for Option<T>
1997	where
1998	T: Clone,
1999	{
2000	#[inline]
2001	fn clone(&self) -> Self {
2002	match self {
2003	Some(x: &T) => Some(x.clone()),
2004	None => None,
2005	}
2006	}
2007
2008	#[inline]
2009	fn clone_from(&mut self, source: &Self) {
2010	match (self, source) {
2011	(Some(to: &mut T), Some(from: &T)) => to.clone_from(source:from),
2012	(to: &mut Option, from: &Option) => *to = from.clone(),
2013	}
2014	}
2015	}
2016
2017	#[stable(feature = "rust1", since = "1.0.0")]
2018	impl<T> Default for Option<T> {
2019	/// Returns [`None`][Option::None].
2020	///
2021	/// # Examples
2022	///
2023	/// ```
2024	/// let opt: Option<u32> = Option::default();
2025	/// assert!(opt.is_none());
2026	/// ```
2027	#[inline]
2028	fn default() -> Option<T> {
2029	None
2030	}
2031	}
2032
2033	#[stable(feature = "rust1", since = "1.0.0")]
2034	impl<T> IntoIterator for Option<T> {
2035	type Item = T;
2036	type IntoIter = IntoIter<T>;
2037
2038	/// Returns a consuming iterator over the possibly contained value.
2039	///
2040	/// # Examples
2041	///
2042	/// ```
2043	/// let x = Some("string");
2044	/// let v: Vec<&str> = x.into_iter().collect();
2045	/// assert_eq!(v, ["string"]);
2046	///
2047	/// let x = None;
2048	/// let v: Vec<&str> = x.into_iter().collect();
2049	/// assert!(v.is_empty());
2050	/// ```
2051	#[inline]
2052	fn into_iter(self) -> IntoIter<T> {
2053	IntoIter { inner: Item { opt: self } }
2054	}
2055	}
2056
2057	#[stable(since = "1.4.0", feature = "option_iter")]
2058	impl<'a, T> IntoIterator for &'a Option<T> {
2059	type Item = &'a T;
2060	type IntoIter = Iter<'a, T>;
2061
2062	fn into_iter(self) -> Iter<'a, T> {
2063	self.iter()
2064	}
2065	}
2066
2067	#[stable(since = "1.4.0", feature = "option_iter")]
2068	impl<'a, T> IntoIterator for &'a mut Option<T> {
2069	type Item = &'a mut T;
2070	type IntoIter = IterMut<'a, T>;
2071
2072	fn into_iter(self) -> IterMut<'a, T> {
2073	self.iter_mut()
2074	}
2075	}
2076
2077	#[stable(since = "1.12.0", feature = "option_from")]
2078	impl<T> From<T> for Option<T> {
2079	/// Moves `val` into a new [`Some`].
2080	///
2081	/// # Examples
2082	///
2083	/// ```
2084	/// let o: Option<u8> = Option::from(`67`);
2085	///
2086	/// assert_eq!(Some(`67`), o);
2087	/// ```
2088	fn from(val: T) -> Option<T> {
2089	Some(val)
2090	}
2091	}
2092
2093	#[stable(feature = "option_ref_from_ref_option", since = "1.30.0")]
2094	impl<'a, T> From<&'a Option<T>> for Option<&'a T> {
2095	/// Converts from `&Option<T>` to `Option<&T>`.
2096	///
2097	/// # Examples
2098	///
2099	/// Converts an <code>[Option]<[String]></code> into an <code>[Option]<[usize]></code>, preserving
2100	/// the original. The [`map`] method takes the `self` argument by value, consuming the original,
2101	/// so this technique uses `from` to first take an [`Option`] to a reference
2102	/// to the value inside the original.
2103	///
2104	/// [`map`]: Option::map
2105	/// [String]: ../../std/string/struct.String.html "String"
2106	///
2107	/// ```
2108	/// let s: Option<String> = Some(String::from("Hello, Rustaceans!"));
2109	/// let o: Option<usize> = Option::from(&s).map(\|ss: &String\| ss.len());
2110	///
2111	/// println!("Can still print s: {s:?}");
2112	///
2113	/// assert_eq!(o, Some(`18`));
2114	/// ```
2115	fn from(o: &'a Option<T>) -> Option<&'a T> {
2116	o.as_ref()
2117	}
2118	}
2119
2120	#[stable(feature = "option_ref_from_ref_option", since = "1.30.0")]
2121	impl<'a, T> From<&'a mut Option<T>> for Option<&'a mut T> {
2122	/// Converts from `&mut Option<T>` to `Option<&mut T>`
2123	///
2124	/// # Examples
2125	///
2126	/// ```
2127	/// let mut s = Some(String::from("Hello"));
2128	/// let o: Option<&mut String> = Option::from(&mut s);
2129	///
2130	/// match o {
2131	/// Some(t) => *t = String::from("Hello, Rustaceans!"),
2132	/// None => (),
2133	/// }
2134	///
2135	/// assert_eq!(s, Some(String::from("Hello, Rustaceans!")));
2136	/// ```
2137	fn from(o: &'a mut Option<T>) -> Option<&'a mut T> {
2138	o.as_mut()
2139	}
2140	}
2141
2142	#[stable(feature = "rust1", since = "1.0.0")]
2143	impl<T> crate::marker::StructuralPartialEq for Option<T> {}
2144	#[stable(feature = "rust1", since = "1.0.0")]
2145	impl<T: PartialEq> PartialEq for Option<T> {
2146	#[inline]
2147	fn eq(&self, other: &Self) -> bool {
2148	SpecOptionPartialEq::eq(self, other)
2149	}
2150	}
2151
2152	/// This specialization trait is a workaround for LLVM not currently (2023-01)
2153	/// being able to optimize this itself, even though Alive confirms that it would
2154	/// be legal to do so: <https://github.com/llvm/llvm-project/issues/52622>
2155	///
2156	/// Once that's fixed, `Option` should go back to deriving `PartialEq`, as
2157	/// it used to do before <https://github.com/rust-lang/rust/pull/103556>.
2158	/// The comment regarding this trait on the `newtype_index` macro should be removed if this is done.
2159	#[unstable(feature = "spec_option_partial_eq", issue = "none", reason = "exposed only for rustc")]
2160	#[doc(hidden)]
2161	pub trait SpecOptionPartialEq: Sized {
2162	fn eq(l: &Option<Self>, other: &Option<Self>) -> bool;
2163	}
2164
2165	#[unstable(feature = "spec_option_partial_eq", issue = "none", reason = "exposed only for rustc")]
2166	impl<T: PartialEq> SpecOptionPartialEq for T {
2167	#[inline]
2168	default fn eq(l: &Option<T>, r: &Option<T>) -> bool {
2169	match (l, r) {
2170	(Some(l: &T), Some(r: &T)) => l == r,
2171	(None, None) => `true`,
2172	_ => `false`,
2173	}
2174	}
2175	}
2176
2177	macro_rules! non_zero_option {
2178	( $( #[$stability: meta] $NZ:ty; )+ ) => {
2179	$(
2180	#[$stability]
2181	impl SpecOptionPartialEq for $NZ {
2182	#[inline]
2183	fn eq(l: &Option<Self>, r: &Option<Self>) -> bool {
2184	l.map(Self::get).unwrap_or(`0`) == r.map(Self::get).unwrap_or(`0`)
2185	}
2186	}
2187	)+
2188	};
2189	}
2190
2191	non_zero_option! {
2192	#[stable(feature = "nonzero", since = "1.28.0")] crate::num::NonZeroU8;
2193	#[stable(feature = "nonzero", since = "1.28.0")] crate::num::NonZeroU16;
2194	#[stable(feature = "nonzero", since = "1.28.0")] crate::num::NonZeroU32;
2195	#[stable(feature = "nonzero", since = "1.28.0")] crate::num::NonZeroU64;
2196	#[stable(feature = "nonzero", since = "1.28.0")] crate::num::NonZeroU128;
2197	#[stable(feature = "nonzero", since = "1.28.0")] crate::num::NonZeroUsize;
2198	#[stable(feature = "signed_nonzero", since = "1.34.0")] crate::num::NonZeroI8;
2199	#[stable(feature = "signed_nonzero", since = "1.34.0")] crate::num::NonZeroI16;
2200	#[stable(feature = "signed_nonzero", since = "1.34.0")] crate::num::NonZeroI32;
2201	#[stable(feature = "signed_nonzero", since = "1.34.0")] crate::num::NonZeroI64;
2202	#[stable(feature = "signed_nonzero", since = "1.34.0")] crate::num::NonZeroI128;
2203	#[stable(feature = "signed_nonzero", since = "1.34.0")] crate::num::NonZeroIsize;
2204	}
2205
2206	#[stable(feature = "nonnull", since = "1.25.0")]
2207	impl<T> SpecOptionPartialEq for crate::ptr::NonNull<T> {
2208	#[inline]
2209	fn eq(l: &Option<Self>, r: &Option<Self>) -> bool {
2210	l.map(Self::as_ptr).unwrap_or_else(\|\| crate::ptr::null_mut())
2211	== r.map(Self::as_ptr).unwrap_or_else(\|\| crate::ptr::null_mut())
2212	}
2213	}
2214
2215	#[stable(feature = "rust1", since = "1.0.0")]
2216	impl SpecOptionPartialEq for cmp::Ordering {
2217	#[inline]
2218	fn eq(l: &Option<Self>, r: &Option<Self>) -> bool {
2219	l.map_or(default:`2`, \|x: Ordering\| x as i8) == r.map_or(default:`2`, \|x: Ordering\| x as i8)
2220	}
2221	}
2222
2223	/////////////////////////////////////////////////////////////////////////////
2224	// The Option Iterators
2225	/////////////////////////////////////////////////////////////////////////////
2226
2227	#[derive(Clone, Debug)]
2228	struct Item<A> {
2229	opt: Option<A>,
2230	}
2231
2232	impl<A> Iterator for Item<A> {
2233	type Item = A;
2234
2235	#[inline]
2236	fn next(&mut self) -> Option<A> {
2237	self.opt.take()
2238	}
2239
2240	#[inline]
2241	fn size_hint(&self) -> (usize, Option<usize>) {
2242	match self.opt {
2243	Some(_) => (`1`, Some(`1`)),
2244	None => (`0`, Some(`0`)),
2245	}
2246	}
2247	}
2248
2249	impl<A> DoubleEndedIterator for Item<A> {
2250	#[inline]
2251	fn next_back(&mut self) -> Option<A> {
2252	self.opt.take()
2253	}
2254	}
2255
2256	impl<A> ExactSizeIterator for Item<A> {}
2257	impl<A> FusedIterator for Item<A> {}
2258	unsafe impl<A> TrustedLen for Item<A> {}
2259
2260	/// An iterator over a reference to the [`Some`] variant of an [`Option`].
2261	///
2262	/// The iterator yields one value if the [`Option`] is a [`Some`], otherwise none.
2263	///
2264	/// This `struct` is created by the [`Option::iter`] function.
2265	#[stable(feature = "rust1", since = "1.0.0")]
2266	#[derive(Debug)]
2267	pub struct Iter<'a, A: 'a> {
2268	inner: Item<&'a A>,
2269	}
2270
2271	#[stable(feature = "rust1", since = "1.0.0")]
2272	impl<'a, A> Iterator for Iter<'a, A> {
2273	type Item = &'a A;
2274
2275	#[inline]
2276	fn next(&mut self) -> Option<&'a A> {
2277	self.inner.next()
2278	}
2279	#[inline]
2280	fn size_hint(&self) -> (usize, Option<usize>) {
2281	self.inner.size_hint()
2282	}
2283	}
2284
2285	#[stable(feature = "rust1", since = "1.0.0")]
2286	impl<'a, A> DoubleEndedIterator for Iter<'a, A> {
2287	#[inline]
2288	fn next_back(&mut self) -> Option<&'a A> {
2289	self.inner.next_back()
2290	}
2291	}
2292
2293	#[stable(feature = "rust1", since = "1.0.0")]
2294	impl<A> ExactSizeIterator for Iter<'_, A> {}
2295
2296	#[stable(feature = "fused", since = "1.26.0")]
2297	impl<A> FusedIterator for Iter<'_, A> {}
2298
2299	#[unstable(feature = "trusted_len", issue = "37572")]
2300	unsafe impl<A> TrustedLen for Iter<'_, A> {}
2301
2302	#[stable(feature = "rust1", since = "1.0.0")]
2303	impl<A> Clone for Iter<'_, A> {
2304	#[inline]
2305	fn clone(&self) -> Self {
2306	Iter { inner: self.inner.clone() }
2307	}
2308	}
2309
2310	/// An iterator over a mutable reference to the [`Some`] variant of an [`Option`].
2311	///
2312	/// The iterator yields one value if the [`Option`] is a [`Some`], otherwise none.
2313	///
2314	/// This `struct` is created by the [`Option::iter_mut`] function.
2315	#[stable(feature = "rust1", since = "1.0.0")]
2316	#[derive(Debug)]
2317	pub struct IterMut<'a, A: 'a> {
2318	inner: Item<&'a mut A>,
2319	}
2320
2321	#[stable(feature = "rust1", since = "1.0.0")]
2322	impl<'a, A> Iterator for IterMut<'a, A> {
2323	type Item = &'a mut A;
2324
2325	#[inline]
2326	fn next(&mut self) -> Option<&'a mut A> {
2327	self.inner.next()
2328	}
2329	#[inline]
2330	fn size_hint(&self) -> (usize, Option<usize>) {
2331	self.inner.size_hint()
2332	}
2333	}
2334
2335	#[stable(feature = "rust1", since = "1.0.0")]
2336	impl<'a, A> DoubleEndedIterator for IterMut<'a, A> {
2337	#[inline]
2338	fn next_back(&mut self) -> Option<&'a mut A> {
2339	self.inner.next_back()
2340	}
2341	}
2342
2343	#[stable(feature = "rust1", since = "1.0.0")]
2344	impl<A> ExactSizeIterator for IterMut<'_, A> {}
2345
2346	#[stable(feature = "fused", since = "1.26.0")]
2347	impl<A> FusedIterator for IterMut<'_, A> {}
2348	#[unstable(feature = "trusted_len", issue = "37572")]
2349	unsafe impl<A> TrustedLen for IterMut<'_, A> {}
2350
2351	/// An iterator over the value in [`Some`] variant of an [`Option`].
2352	///
2353	/// The iterator yields one value if the [`Option`] is a [`Some`], otherwise none.
2354	///
2355	/// This `struct` is created by the [`Option::into_iter`] function.
2356	#[derive(Clone, Debug)]
2357	#[stable(feature = "rust1", since = "1.0.0")]
2358	pub struct IntoIter<A> {
2359	inner: Item<A>,
2360	}
2361
2362	#[stable(feature = "rust1", since = "1.0.0")]
2363	impl<A> Iterator for IntoIter<A> {
2364	type Item = A;
2365
2366	#[inline]
2367	fn next(&mut self) -> Option<A> {
2368	self.inner.next()
2369	}
2370	#[inline]
2371	fn size_hint(&self) -> (usize, Option<usize>) {
2372	self.inner.size_hint()
2373	}
2374	}
2375
2376	#[stable(feature = "rust1", since = "1.0.0")]
2377	impl<A> DoubleEndedIterator for IntoIter<A> {
2378	#[inline]
2379	fn next_back(&mut self) -> Option<A> {
2380	self.inner.next_back()
2381	}
2382	}
2383
2384	#[stable(feature = "rust1", since = "1.0.0")]
2385	impl<A> ExactSizeIterator for IntoIter<A> {}
2386
2387	#[stable(feature = "fused", since = "1.26.0")]
2388	impl<A> FusedIterator for IntoIter<A> {}
2389
2390	#[unstable(feature = "trusted_len", issue = "37572")]
2391	unsafe impl<A> TrustedLen for IntoIter<A> {}
2392
2393	/////////////////////////////////////////////////////////////////////////////
2394	// FromIterator
2395	/////////////////////////////////////////////////////////////////////////////
2396
2397	#[stable(feature = "rust1", since = "1.0.0")]
2398	impl<A, V: FromIterator<A>> FromIterator<Option<A>> for Option<V> {
2399	/// Takes each element in the [`Iterator`]: if it is [`None`][Option::None],
2400	/// no further elements are taken, and the [`None`][Option::None] is
2401	/// returned. Should no [`None`][Option::None] occur, a container of type
2402	/// `V` containing the values of each [`Option`] is returned.
2403	///
2404	/// # Examples
2405	///
2406	/// Here is an example which increments every integer in a vector.
2407	/// We use the checked variant of `add` that returns `None` when the
2408	/// calculation would result in an overflow.
2409	///
2410	/// ```
2411	/// let items = vec![`0_u16`, `1`, `2`];
2412	///
2413	/// let res: Option<Vec<u16>> = items
2414	/// .iter()
2415	/// .map(\|x\| x.checked_add(`1`))
2416	/// .collect();
2417	///
2418	/// assert_eq!(res, Some(vec![`1`, `2`, `3`]));
2419	/// ```
2420	///
2421	/// As you can see, this will return the expected, valid items.
2422	///
2423	/// Here is another example that tries to subtract one from another list
2424	/// of integers, this time checking for underflow:
2425	///
2426	/// ```
2427	/// let items = vec![`2_u16`, `1`, `0`];
2428	///
2429	/// let res: Option<Vec<u16>> = items
2430	/// .iter()
2431	/// .map(\|x\| x.checked_sub(`1`))
2432	/// .collect();
2433	///
2434	/// assert_eq!(res, None);
2435	/// ```
2436	///
2437	/// Since the last element is zero, it would underflow. Thus, the resulting
2438	/// value is `None`.
2439	///
2440	/// Here is a variation on the previous example, showing that no
2441	/// further elements are taken from `iter` after the first `None`.
2442	///
2443	/// ```
2444	/// let items = vec![`3_u16`, `2`, `1`, `10`];
2445	///
2446	/// let mut shared = `0`;
2447	///
2448	/// let res: Option<Vec<u16>> = items
2449	/// .iter()
2450	/// .map(\|x\| { shared += x; x.checked_sub(`2`) })
2451	/// .collect();
2452	///
2453	/// assert_eq!(res, None);
2454	/// assert_eq!(shared, `6`);
2455	/// ```
2456	///
2457	/// Since the third element caused an underflow, no further elements were taken,
2458	/// so the final value of `shared` is 6 (= `3 + 2 + 1`), not 16.
2459	#[inline]
2460	fn from_iter<I: IntoIterator<Item = Option<A>>>(iter: I) -> Option<V> {
2461	// FIXME(#11084): This could be replaced with Iterator::scan when this
2462	// performance bug is closed.
2463
2464	iter::try_process(iter.into_iter(), \|i\| i.collect())
2465	}
2466	}
2467
2468	#[unstable(feature = "try_trait_v2", issue = "84277")]
2469	impl<T> ops::Try for Option<T> {
2470	type Output = T;
2471	type Residual = Option<convert::Infallible>;
2472
2473	#[inline]
2474	fn from_output(output: Self::Output) -> Self {
2475	Some(output)
2476	}
2477
2478	#[inline]
2479	fn branch(self) -> ControlFlow<Self::Residual, Self::Output> {
2480	match self {
2481	Some(v: T) => ControlFlow::Continue(v),
2482	None => ControlFlow::Break(None),
2483	}
2484	}
2485	}
2486
2487	#[unstable(feature = "try_trait_v2", issue = "84277")]
2488	impl<T> ops::FromResidual for Option<T> {
2489	#[inline]
2490	fn from_residual(residual: Option<convert::Infallible>) -> Self {
2491	match residual {
2492	None => None,
2493	}
2494	}
2495	}
2496
2497	#[unstable(feature = "try_trait_v2_yeet", issue = "96374")]
2498	impl<T> ops::FromResidual<ops::Yeet<()>> for Option<T> {
2499	#[inline]
2500	fn from_residual(ops::Yeet(()): ops::Yeet<()>) -> Self {
2501	None
2502	}
2503	}
2504
2505	#[unstable(feature = "try_trait_v2_residual", issue = "91285")]
2506	impl<T> ops::Residual<T> for Option<convert::Infallible> {
2507	type TryType = Option<T>;
2508	}
2509
2510	impl<T> Option<Option<T>> {
2511	/// Converts from `Option<Option<T>>` to `Option<T>`.
2512	///
2513	/// # Examples
2514	///
2515	/// Basic usage:
2516	///
2517	/// ```
2518	/// let x: Option<Option<u32>> = Some(Some(`6`));
2519	/// assert_eq!(Some(`6`), x.flatten());
2520	///
2521	/// let x: Option<Option<u32>> = Some(None);
2522	/// assert_eq!(None, x.flatten());
2523	///
2524	/// let x: Option<Option<u32>> = None;
2525	/// assert_eq!(None, x.flatten());
2526	/// ```
2527	///
2528	/// Flattening only removes one level of nesting at a time:
2529	///
2530	/// ```
2531	/// let x: Option<Option<Option<u32>>> = Some(Some(Some(`6`)));
2532	/// assert_eq!(Some(Some(`6`)), x.flatten());
2533	/// assert_eq!(Some(`6`), x.flatten().flatten());
2534	/// ```
2535	#[inline]
2536	#[stable(feature = "option_flattening", since = "1.40.0")]
2537	#[rustc_const_unstable(feature = "const_option", issue = "67441")]
2538	pub const fn flatten(self) -> Option<T> {
2539	match self {
2540	Some(inner) => inner,
2541	None => None,
2542	}
2543	}
2544	}
2545