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	//! Under some conditions the above types `T` are also null pointer optimized when wrapped in a [`Result`][result_repr].
141	//!
142	//! [`Box<U>`]: ../../std/boxed/struct.Box.html
143	//! [`num::NonZero`]: crate::num*
144	//! [`ptr::NonNull<U>`]: crate::ptr::NonNull
145	//! [function call ABI]: ../primitive.fn.html#abi-compatibility
146	//! [result_repr]: crate::result#representation
147	//!
148	//! This is called the "null pointer optimization" or NPO.
149	//!
150	//! It is further guaranteed that, for the cases above, one can
151	//! [`mem::transmute`] from all valid values of `T` to `Option<T>` and
152	//! from `Some::<T>(_)` to `T` (but transmuting `None::<T>` to `T`
153	//! is undefined behavior).
154	//!
155	//! # Method overview
156	//!
157	//! In addition to working with pattern matching, [`Option`] provides a wide
158	//! variety of different methods.
159	//!
160	//! ## Querying the variant
161	//!
162	//! The [`is_some`] and [`is_none`] methods return [`true`] if the [`Option`]
163	//! is [`Some`] or [`None`], respectively.
164	//!
165	//! [`is_none`]: Option::is_none
166	//! [`is_some`]: Option::is_some
167	//!
168	//! ## Adapters for working with references
169	//!
170	//! * [`as_ref`] converts from <code>[&][][Option]\<T></code> to <code>[Option]<[&]T></code>
171	//! * [`as_mut`] converts from <code>[&mut] [Option]\<T></code> to <code>[Option]<[&mut] T></code>
172	//! * [`as_deref`] converts from <code>[&][][Option]\<T></code> to
173	//! <code>[Option]<[&]T::[Target]></code>
174	//! * [`as_deref_mut`] converts from <code>[&mut] [Option]\<T></code> to
175	//! <code>[Option]<[&mut] T::[Target]></code>
176	//! * [`as_pin_ref`] converts from <code>[Pin]<[&][][Option]\<T>></code> to
177	//! <code>[Option]<[Pin]<[&]T>></code>
178	//! * [`as_pin_mut`] converts from <code>[Pin]<[&mut] [Option]\<T>></code> to
179	//! <code>[Option]<[Pin]<[&mut] T>></code>
180	//!
181	//! [&]: reference "shared reference"
182	//! [&mut]: reference "mutable reference"
183	//! [Target]: Deref::Target "ops::Deref::Target"
184	//! [`as_deref`]: Option::as_deref
185	//! [`as_deref_mut`]: Option::as_deref_mut
186	//! [`as_mut`]: Option::as_mut
187	//! [`as_pin_mut`]: Option::as_pin_mut
188	//! [`as_pin_ref`]: Option::as_pin_ref
189	//! [`as_ref`]: Option::as_ref
190	//!
191	//! ## Extracting the contained value
192	//!
193	//! These methods extract the contained value in an [`Option<T>`] when it
194	//! is the [`Some`] variant. If the [`Option`] is [`None`]:
195	//!
196	//! * [`expect`] panics with a provided custom message
197	//! * [`unwrap`] panics with a generic message
198	//! * [`unwrap_or`] returns the provided default value
199	//! * [`unwrap_or_default`] returns the default value of the type `T`
200	//! (which must implement the [`Default`] trait)
201	//! * [`unwrap_or_else`] returns the result of evaluating the provided
202	//! function
203	//!
204	//! [`expect`]: Option::expect
205	//! [`unwrap`]: Option::unwrap
206	//! [`unwrap_or`]: Option::unwrap_or
207	//! [`unwrap_or_default`]: Option::unwrap_or_default
208	//! [`unwrap_or_else`]: Option::unwrap_or_else
209	//!
210	//! ## Transforming contained values
211	//!
212	//! These methods transform [`Option`] to [`Result`]:
213	//!
214	//! * [`ok_or`] transforms [`Some(v)`] to [`Ok(v)`], and [`None`] to
215	//! [`Err(err)`] using the provided default `err` value
216	//! * [`ok_or_else`] transforms [`Some(v)`] to [`Ok(v)`], and [`None`] to
217	//! a value of [`Err`] using the provided function
218	//! * [`transpose`] transposes an [`Option`] of a [`Result`] into a
219	//! [`Result`] of an [`Option`]
220	//!
221	//! [`Err(err)`]: Err
222	//! [`Ok(v)`]: Ok
223	//! [`Some(v)`]: Some
224	//! [`ok_or`]: Option::ok_or
225	//! [`ok_or_else`]: Option::ok_or_else
226	//! [`transpose`]: Option::transpose
227	//!
228	//! These methods transform the [`Some`] variant:
229	//!
230	//! * [`filter`] calls the provided predicate function on the contained
231	//! value `t` if the [`Option`] is [`Some(t)`], and returns [`Some(t)`]
232	//! if the function returns `true`; otherwise, returns [`None`]
233	//! * [`flatten`] removes one level of nesting from an
234	//! [`Option<Option<T>>`]
235	//! * [`map`] transforms [`Option<T>`] to [`Option<U>`] by applying the
236	//! provided function to the contained value of [`Some`] and leaving
237	//! [`None`] values unchanged
238	//!
239	//! [`Some(t)`]: Some
240	//! [`filter`]: Option::filter
241	//! [`flatten`]: Option::flatten
242	//! [`map`]: Option::map
243	//!
244	//! These methods transform [`Option<T>`] to a value of a possibly
245	//! different type `U`:
246	//!
247	//! * [`map_or`] applies the provided function to the contained value of
248	//! [`Some`], or returns the provided default value if the [`Option`] is
249	//! [`None`]
250	//! * [`map_or_else`] applies the provided function to the contained value
251	//! of [`Some`], or returns the result of evaluating the provided
252	//! fallback function if the [`Option`] is [`None`]
253	//!
254	//! [`map_or`]: Option::map_or
255	//! [`map_or_else`]: Option::map_or_else
256	//!
257	//! These methods combine the [`Some`] variants of two [`Option`] values:
258	//!
259	//! * [`zip`] returns [`Some((s, o))`] if `self` is [`Some(s)`] and the
260	//! provided [`Option`] value is [`Some(o)`]; otherwise, returns [`None`]
261	//! * [`zip_with`] calls the provided function `f` and returns
262	//! [`Some(f(s, o))`] if `self` is [`Some(s)`] and the provided
263	//! [`Option`] value is [`Some(o)`]; otherwise, returns [`None`]
264	//!
265	//! [`Some(f(s, o))`]: Some
266	//! [`Some(o)`]: Some
267	//! [`Some(s)`]: Some
268	//! [`Some((s, o))`]: Some
269	//! [`zip`]: Option::zip
270	//! [`zip_with`]: Option::zip_with
271	//!
272	//! ## Boolean operators
273	//!
274	//! These methods treat the [`Option`] as a boolean value, where [`Some`]
275	//! acts like [`true`] and [`None`] acts like [`false`]. There are two
276	//! categories of these methods: ones that take an [`Option`] as input, and
277	//! ones that take a function as input (to be lazily evaluated).
278	//!
279	//! The [`and`], [`or`], and [`xor`] methods take another [`Option`] as
280	//! input, and produce an [`Option`] as output. Only the [`and`] method can
281	//! produce an [`Option<U>`] value having a different inner type `U` than
282	//! [`Option<T>`].
283	//!
284	//! \| method \| self \| input \| output \|
285	//! \|---------\|-----------\|-----------\|-----------\|
286	//! \| [`and`] \| `None` \| (ignored) \| `None` \|
287	//! \| [`and`] \| `Some(x)` \| `None` \| `None` \|
288	//! \| [`and`] \| `Some(x)` \| `Some(y)` \| `Some(y)` \|
289	//! \| [`or`] \| `None` \| `None` \| `None` \|
290	//! \| [`or`] \| `None` \| `Some(y)` \| `Some(y)` \|
291	//! \| [`or`] \| `Some(x)` \| (ignored) \| `Some(x)` \|
292	//! \| [`xor`] \| `None` \| `None` \| `None` \|
293	//! \| [`xor`] \| `None` \| `Some(y)` \| `Some(y)` \|
294	//! \| [`xor`] \| `Some(x)` \| `None` \| `Some(x)` \|
295	//! \| [`xor`] \| `Some(x)` \| `Some(y)` \| `None` \|
296	//!
297	//! [`and`]: Option::and
298	//! [`or`]: Option::or
299	//! [`xor`]: Option::xor
300	//!
301	//! The [`and_then`] and [`or_else`] methods take a function as input, and
302	//! only evaluate the function when they need to produce a new value. Only
303	//! the [`and_then`] method can produce an [`Option<U>`] value having a
304	//! different inner type `U` than [`Option<T>`].
305	//!
306	//! \| method \| self \| function input \| function result \| output \|
307	//! \|--------------\|-----------\|----------------\|-----------------\|-----------\|
308	//! \| [`and_then`] \| `None` \| (not provided) \| (not evaluated) \| `None` \|
309	//! \| [`and_then`] \| `Some(x)` \| `x` \| `None` \| `None` \|
310	//! \| [`and_then`] \| `Some(x)` \| `x` \| `Some(y)` \| `Some(y)` \|
311	//! \| [`or_else`] \| `None` \| (not provided) \| `None` \| `None` \|
312	//! \| [`or_else`] \| `None` \| (not provided) \| `Some(y)` \| `Some(y)` \|
313	//! \| [`or_else`] \| `Some(x)` \| (not provided) \| (not evaluated) \| `Some(x)` \|
314	//!
315	//! [`and_then`]: Option::and_then
316	//! [`or_else`]: Option::or_else
317	//!
318	//! This is an example of using methods like [`and_then`] and [`or`] in a
319	//! pipeline of method calls. Early stages of the pipeline pass failure
320	//! values ([`None`]) through unchanged, and continue processing on
321	//! success values ([`Some`]). Toward the end, [`or`] substitutes an error
322	//! message if it receives [`None`].
323	//!
324	//! ```
325	//! # use std::collections::BTreeMap;
326	//! let mut bt = BTreeMap::new();
327	//! bt.insert(`20u8`, "foo");
328	//! bt.insert(`42u8`, "bar");
329	//! let res = [`0u8`, `1`, `11`, `200`, `22`]
330	//! .into_iter()
331	//! .map(\|x\| {
332	//! // `checked_sub()` returns `None` on error
333	//! x.checked_sub(`1`)
334	//! // same with `checked_mul()`
335	//! .and_then(\|x\| x.checked_mul(`2`))
336	//! // `BTreeMap::get` returns `None` on error
337	//! .and_then(\|x\| bt.get(&x))
338	//! // Substitute an error message if we have `None` so far
339	//! .or(Some(&"error!"))
340	//! .copied()
341	//! // Won't panic because we unconditionally used `Some` above
342	//! .unwrap()
343	//! })
344	//! .collect::<Vec<_>>();
345	//! assert_eq!(res, ["error!", "error!", "foo", "error!", "bar"]);
346	//! ```
347	//!
348	//! ## Comparison operators
349	//!
350	//! If `T` implements [`PartialOrd`] then [`Option<T>`] will derive its
351	//! [`PartialOrd`] implementation. With this order, [`None`] compares as
352	//! less than any [`Some`], and two [`Some`] compare the same way as their
353	//! contained values would in `T`. If `T` also implements
354	//! [`Ord`], then so does [`Option<T>`].
355	//!
356	//! ```
357	//! assert!(None < Some(`0`));
358	//! assert!(Some(`0`) < Some(`1`));
359	//! ```
360	//!
361	//! ## Iterating over `Option`
362	//!
363	//! An [`Option`] can be iterated over. This can be helpful if you need an
364	//! iterator that is conditionally empty. The iterator will either produce
365	//! a single value (when the [`Option`] is [`Some`]), or produce no values
366	//! (when the [`Option`] is [`None`]). For example, [`into_iter`] acts like
367	//! [`once(v)`] if the [`Option`] is [`Some(v)`], and like [`empty()`] if
368	//! the [`Option`] is [`None`].
369	//!
370	//! [`Some(v)`]: Some
371	//! [`empty()`]: crate::iter::empty
372	//! [`once(v)`]: crate::iter::once
373	//!
374	//! Iterators over [`Option<T>`] come in three types:
375	//!
376	//! * [`into_iter`] consumes the [`Option`] and produces the contained
377	//! value
378	//! * [`iter`] produces an immutable reference of type `&T` to the
379	//! contained value
380	//! * [`iter_mut`] produces a mutable reference of type `&mut T` to the
381	//! contained value
382	//!
383	//! [`into_iter`]: Option::into_iter
384	//! [`iter`]: Option::iter
385	//! [`iter_mut`]: Option::iter_mut
386	//!
387	//! An iterator over [`Option`] can be useful when chaining iterators, for
388	//! example, to conditionally insert items. (It's not always necessary to
389	//! explicitly call an iterator constructor: many [`Iterator`] methods that
390	//! accept other iterators will also accept iterable types that implement
391	//! [`IntoIterator`], which includes [`Option`].)
392	//!
393	//! ```
394	//! let yep = Some(`42`);
395	//! let nope = None;
396	//! // chain() already calls into_iter(), so we don't have to do so
397	//! let nums: Vec<i32> = (`0`..`4`).chain(yep).chain(`4`..`8`).collect();
398	//! assert_eq!(nums, [`0`, `1`, `2`, `3`, `42`, `4`, `5`, `6`, `7`]);
399	//! let nums: Vec<i32> = (`0`..`4`).chain(nope).chain(`4`..`8`).collect();
400	//! assert_eq!(nums, [`0`, `1`, `2`, `3`, `4`, `5`, `6`, `7`]);
401	//! ```
402	//!
403	//! One reason to chain iterators in this way is that a function returning
404	//! `impl Iterator` must have all possible return values be of the same
405	//! concrete type. Chaining an iterated [`Option`] can help with that.
406	//!
407	//! ```
408	//! fn make_iter(do_insert: bool) -> impl Iterator<Item = i32> {
409	//! // Explicit returns to illustrate return types matching
410	//! match do_insert {
411	//! `true` => return (`0`..`4`).chain(Some(`42`)).chain(`4`..`8`),
412	//! `false` => return (`0`..`4`).chain(None).chain(`4`..`8`),
413	//! }
414	//! }
415	//! println!("{:?}", make_iter(`true`).collect::<Vec<_>>());
416	//! println!("{:?}", make_iter(`false`).collect::<Vec<_>>());
417	//! ```
418	//!
419	//! If we try to do the same thing, but using [`once()`] and [`empty()`],
420	//! we can't return `impl Iterator` anymore because the concrete types of
421	//! the return values differ.
422	//!
423	//! [`empty()`]: crate::iter::empty
424	//! [`once()`]: crate::iter::once
425	//!
426	//! ```compile_fail,E0308
427	//! # use std::iter::{empty, once};
428	//! // This won't compile because all possible returns from the function
429	//! // must have the same concrete type.
430	//! fn make_iter(do_insert: bool) -> impl Iterator<Item = i32> {
431	//! // Explicit returns to illustrate return types not matching
432	//! match do_insert {
433	//! `true` => return (`0`..`4`).chain(once(`42`)).chain(`4`..`8`),
434	//! `false` => return (`0`..`4`).chain(empty()).chain(`4`..`8`),
435	//! }
436	//! }
437	//! ```
438	//!
439	//! ## Collecting into `Option`
440	//!
441	//! [`Option`] implements the [`FromIterator`][impl-FromIterator] trait,
442	//! which allows an iterator over [`Option`] values to be collected into an
443	//! [`Option`] of a collection of each contained value of the original
444	//! [`Option`] values, or [`None`] if any of the elements was [`None`].
445	//!
446	//! [impl-FromIterator]: Option#impl-FromIterator%3COption%3CA%3E%3E-for-Option%3CV%3E
447	//!
448	//! ```
449	//! let v = [Some(`2`), Some(`4`), None, Some(`8`)];
450	//! let res: Option<Vec<_>> = v.into_iter().collect();
451	//! assert_eq!(res, None);
452	//! let v = [Some(`2`), Some(`4`), Some(`8`)];
453	//! let res: Option<Vec<_>> = v.into_iter().collect();
454	//! assert_eq!(res, Some(vec![`2`, `4`, `8`]));
455	//! ```
456	//!
457	//! [`Option`] also implements the [`Product`][impl-Product] and
458	//! [`Sum`][impl-Sum] traits, allowing an iterator over [`Option`] values
459	//! to provide the [`product`][Iterator::product] and
460	//! [`sum`][Iterator::sum] methods.
461	//!
462	//! [impl-Product]: Option#impl-Product%3COption%3CU%3E%3E-for-Option%3CT%3E
463	//! [impl-Sum]: Option#impl-Sum%3COption%3CU%3E%3E-for-Option%3CT%3E
464	//!
465	//! ```
466	//! let v = [None, Some(`1`), Some(`2`), Some(`3`)];
467	//! let res: Option<i32> = v.into_iter().sum();
468	//! assert_eq!(res, None);
469	//! let v = [Some(`1`), Some(`2`), Some(`21`)];
470	//! let res: Option<i32> = v.into_iter().product();
471	//! assert_eq!(res, Some(`42`));
472	//! ```
473	//!
474	//! ## Modifying an [`Option`] in-place
475	//!
476	//! These methods return a mutable reference to the contained value of an
477	//! [`Option<T>`]:
478	//!
479	//! * [`insert`] inserts a value, dropping any old contents
480	//! * [`get_or_insert`] gets the current value, inserting a provided
481	//! default value if it is [`None`]
482	//! * [`get_or_insert_default`] gets the current value, inserting the
483	//! default value of type `T` (which must implement [`Default`]) if it is
484	//! [`None`]
485	//! * [`get_or_insert_with`] gets the current value, inserting a default
486	//! computed by the provided function if it is [`None`]
487	//!
488	//! [`get_or_insert`]: Option::get_or_insert
489	//! [`get_or_insert_default`]: Option::get_or_insert_default
490	//! [`get_or_insert_with`]: Option::get_or_insert_with
491	//! [`insert`]: Option::insert
492	//!
493	//! These methods transfer ownership of the contained value of an
494	//! [`Option`]:
495	//!
496	//! * [`take`] takes ownership of the contained value of an [`Option`], if
497	//! any, replacing the [`Option`] with [`None`]
498	//! * [`replace`] takes ownership of the contained value of an [`Option`],
499	//! if any, replacing the [`Option`] with a [`Some`] containing the
500	//! provided value
501	//!
502	//! [`replace`]: Option::replace
503	//! [`take`]: Option::take
504	//!
505	//! # Examples
506	//!
507	//! Basic pattern matching on [`Option`]:
508	//!
509	//! ```
510	//! let msg = Some("howdy");
511	//!
512	//! // Take a reference to the contained string
513	//! if let Some(m) = &msg {
514	//! println!("{}", *m);
515	//! }
516	//!
517	//! // Remove the contained string, destroying the Option
518	//! let unwrapped_msg = msg.unwrap_or("default message");
519	//! ```
520	//!
521	//! Initialize a result to [`None`] before a loop:
522	//!
523	//! ```
524	//! enum Kingdom { Plant(u32, &'static str), Animal(u32, &'static str) }
525	//!
526	//! // A list of data to search through.
527	//! let all_the_big_things = [
528	//! Kingdom::Plant(`250`, "redwood"),
529	//! Kingdom::Plant(`230`, "noble fir"),
530	//! Kingdom::Plant(`229`, "sugar pine"),
531	//! Kingdom::Animal(`25`, "blue whale"),
532	//! Kingdom::Animal(`19`, "fin whale"),
533	//! Kingdom::Animal(`15`, "north pacific right whale"),
534	//! ];
535	//!
536	//! // We're going to search for the name of the biggest animal,
537	//! // but to start with we've just got `None`.
538	//! let mut name_of_biggest_animal = None;
539	//! let mut size_of_biggest_animal = `0`;
540	//! for big_thing in &all_the_big_things {
541	//! match *big_thing {
542	//! Kingdom::Animal(size, name) if size > size_of_biggest_animal => {
543	//! // Now we've found the name of some big animal
544	//! size_of_biggest_animal = size;
545	//! name_of_biggest_animal = Some(name);
546	//! }
547	//! Kingdom::Animal(..) \| Kingdom::Plant(..) => ()
548	//! }
549	//! }
550	//!
551	//! match name_of_biggest_animal {
552	//! Some(name) => println!("the biggest animal is {name}"),
553	//! None => println!("there are no animals :("),
554	//! }
555	//! ```
556
557	#![stable(feature = "rust1", since = "1.0.0")]
558
559	use crate::iter::{self, FusedIterator, TrustedLen};
560	use crate::ops::{self, ControlFlow, Deref, DerefMut};
561	use crate::panicking::{panic, panic_display};
562	use crate::pin::Pin;
563	use crate::{cmp, convert, hint, mem, slice};
564
565	/// The `Option` type. See [the module level documentation](self) for more.
566	#[doc(search_unbox)]
567	#[derive(Copy, Eq, Debug, Hash)]
568	#[rustc_diagnostic_item = "Option"]
569	#[lang = "Option"]
570	#[stable(feature = "rust1", since = "1.0.0")]
571	#[allow(clippy::derived_hash_with_manual_eq)] // PartialEq is manually implemented equivalently
572	pub enum Option<T> {
573	/// No value.
574	#[lang = "None"]
575	#[stable(feature = "rust1", since = "1.0.0")]
576	None,
577	/// Some value of type `T`.
578	#[lang = "Some"]
579	#[stable(feature = "rust1", since = "1.0.0")]
580	Some(#[stable(feature = "rust1", since = "1.0.0")] T),
581	}
582
583	/////////////////////////////////////////////////////////////////////////////
584	// Type implementation
585	/////////////////////////////////////////////////////////////////////////////
586
587	impl<T> Option<T> {
588	/////////////////////////////////////////////////////////////////////////
589	// Querying the contained values
590	/////////////////////////////////////////////////////////////////////////
591
592	/// Returns `true` if the option is a [`Some`] value.
593	///
594	/// # Examples
595	///
596	/// ```
597	/// let x: Option<u32> = Some(`2`);
598	/// assert_eq!(x.is_some(), `true`);
599	///
600	/// let x: Option<u32> = None;
601	/// assert_eq!(x.is_some(), `false`);
602	/// ```
603	#[must_use = "if you intended to assert that this has a value, consider `.unwrap()` instead"]
604	#[inline]
605	#[stable(feature = "rust1", since = "1.0.0")]
606	#[rustc_const_stable(feature = "const_option_basics", since = "1.48.0")]
607	pub const fn is_some(&self) -> bool {
608	matches!(*self, Some(_))
609	}
610
611	/// Returns `true` if the option is a [`Some`] and the value inside of it matches a predicate.
612	///
613	/// # Examples
614	///
615	/// ```
616	/// let x: Option<u32> = Some(`2`);
617	/// assert_eq!(x.is_some_and(\|x\| x > `1`), `true`);
618	///
619	/// let x: Option<u32> = Some(`0`);
620	/// assert_eq!(x.is_some_and(\|x\| x > `1`), `false`);
621	///
622	/// let x: Option<u32> = None;
623	/// assert_eq!(x.is_some_and(\|x\| x > `1`), `false`);
624	/// ```
625	#[must_use]
626	#[inline]
627	#[stable(feature = "is_some_and", since = "1.70.0")]
628	pub fn is_some_and(self, f: impl FnOnce(T) -> bool) -> bool {
629	match self {
630	None => `false`,
631	Some(x) => f(x),
632	}
633	}
634
635	/// Returns `true` if the option is a [`None`] value.
636	///
637	/// # Examples
638	///
639	/// ```
640	/// let x: Option<u32> = Some(`2`);
641	/// assert_eq!(x.is_none(), `false`);
642	///
643	/// let x: Option<u32> = None;
644	/// assert_eq!(x.is_none(), `true`);
645	/// ```
646	#[must_use = "if you intended to assert that this doesn't have a value, consider \
647	wrapping this in an `assert!()` instead"]
648	#[inline]
649	#[stable(feature = "rust1", since = "1.0.0")]
650	#[rustc_const_stable(feature = "const_option_basics", since = "1.48.0")]
651	pub const fn is_none(&self) -> bool {
652	!self.is_some()
653	}
654
655	/// Returns `true` if the option is a [`None`] or the value inside of it matches a predicate.
656	///
657	/// # Examples
658	///
659	/// ```
660	/// let x: Option<u32> = Some(`2`);
661	/// assert_eq!(x.is_none_or(\|x\| x > `1`), `true`);
662	///
663	/// let x: Option<u32> = Some(`0`);
664	/// assert_eq!(x.is_none_or(\|x\| x > `1`), `false`);
665	///
666	/// let x: Option<u32> = None;
667	/// assert_eq!(x.is_none_or(\|x\| x > `1`), `true`);
668	/// ```
669	#[must_use]
670	#[inline]
671	#[stable(feature = "is_none_or", since = "1.82.0")]
672	pub fn is_none_or(self, f: impl FnOnce(T) -> bool) -> bool {
673	match self {
674	None => `true`,
675	Some(x) => f(x),
676	}
677	}
678
679	/////////////////////////////////////////////////////////////////////////
680	// Adapter for working with references
681	/////////////////////////////////////////////////////////////////////////
682
683	/// Converts from `&Option<T>` to `Option<&T>`.
684	///
685	/// # Examples
686	///
687	/// Calculates the length of an <code>Option<[String]></code> as an <code>Option<[usize]></code>
688	/// without moving the [`String`]. The [`map`] method takes the `self` argument by value,
689	/// consuming the original, so this technique uses `as_ref` to first take an `Option` to a
690	/// reference to the value inside the original.
691	///
692	/// [`map`]: Option::map
693	/// [String]: ../../std/string/struct.String.html "String"
694	/// [`String`]: ../../std/string/struct.String.html "String"
695	///
696	/// ```
697	/// let text: Option<String> = Some("Hello, world!".to_string());
698	/// // First, cast `Option<String>` to `Option<&String>` with `as_ref`,
699	/// // then consume that* with `map`, leaving `text` on the stack.*
700	/// let text_length: Option<usize> = text.as_ref().map(\|s\| s.len());
701	/// println!("still can print text: {text:?}");
702	/// ```
703	#[inline]
704	#[rustc_const_stable(feature = "const_option_basics", since = "1.48.0")]
705	#[stable(feature = "rust1", since = "1.0.0")]
706	pub const fn as_ref(&self) -> Option<&T> {
707	match *self {
708	Some(ref x) => Some(x),
709	None => None,
710	}
711	}
712
713	/// Converts from `&mut Option<T>` to `Option<&mut T>`.
714	///
715	/// # Examples
716	///
717	/// ```
718	/// let mut x = Some(`2`);
719	/// match x.as_mut() {
720	/// Some(v) => *v = `42`,
721	/// None => {},
722	/// }
723	/// assert_eq!(x, Some(`42`));
724	/// ```
725	#[inline]
726	#[stable(feature = "rust1", since = "1.0.0")]
727	#[rustc_const_stable(feature = "const_option", since = "1.83.0")]
728	pub const fn as_mut(&mut self) -> Option<&mut T> {
729	match *self {
730	Some(ref mut x) => Some(x),
731	None => None,
732	}
733	}
734
735	/// Converts from <code>[Pin]<[&]Option\<T>></code> to <code>Option<[Pin]<[&]T>></code>.
736	///
737	/// [&]: reference "shared reference"
738	#[inline]
739	#[must_use]
740	#[stable(feature = "pin", since = "1.33.0")]
741	#[rustc_const_stable(feature = "const_option_ext", since = "1.84.0")]
742	pub const fn as_pin_ref(self: Pin<&Self>) -> Option<Pin<&T>> {
743	// FIXME(const-hack): use `map` once that is possible
744	match Pin::get_ref(self).as_ref() {
745	// SAFETY: `x` is guaranteed to be pinned because it comes from `self`
746	// which is pinned.
747	Some(x) => unsafe { Some(Pin::new_unchecked(x)) },
748	None => None,
749	}
750	}
751
752	/// Converts from <code>[Pin]<[&mut] Option\<T>></code> to <code>Option<[Pin]<[&mut] T>></code>.
753	///
754	/// [&mut]: reference "mutable reference"
755	#[inline]
756	#[must_use]
757	#[stable(feature = "pin", since = "1.33.0")]
758	#[rustc_const_stable(feature = "const_option_ext", since = "1.84.0")]
759	pub const fn as_pin_mut(self: Pin<&mut Self>) -> Option<Pin<&mut T>> {
760	// SAFETY: `get_unchecked_mut` is never used to move the `Option` inside `self`.
761	// `x` is guaranteed to be pinned because it comes from `self` which is pinned.
762	unsafe {
763	// FIXME(const-hack): use `map` once that is possible
764	match Pin::get_unchecked_mut(self).as_mut() {
765	Some(x) => Some(Pin::new_unchecked(x)),
766	None => None,
767	}
768	}
769	}
770
771	#[inline]
772	const fn len(&self) -> usize {
773	// Using the intrinsic avoids emitting a branch to get the 0 or 1.
774	let discriminant: isize = crate::intrinsics::discriminant_value(self);
775	discriminant as usize
776	}
777
778	/// Returns a slice of the contained value, if any. If this is `None`, an
779	/// empty slice is returned. This can be useful to have a single type of
780	/// iterator over an `Option` or slice.
781	///
782	/// Note: Should you have an `Option<&T>` and wish to get a slice of `T`,
783	/// you can unpack it via `opt.map_or(&[], std::slice::from_ref)`.
784	///
785	/// # Examples
786	///
787	/// ```rust
788	/// assert_eq!(
789	/// [Some(`1234`).as_slice(), None.as_slice()],
790	/// [&[`1234`][..], &[][..]],
791	/// );
792	/// ```
793	///
794	/// The inverse of this function is (discounting
795	/// borrowing) [`[_]::first`](slice::first):
796	///
797	/// ```rust
798	/// for i in [Some(`1234_u16`), None] {
799	/// assert_eq!(i.as_ref(), i.as_slice().first());
800	/// }
801	/// ```
802	#[inline]
803	#[must_use]
804	#[stable(feature = "option_as_slice", since = "1.75.0")]
805	#[rustc_const_stable(feature = "const_option_ext", since = "1.84.0")]
806	pub const fn as_slice(&self) -> &[T] {
807	// SAFETY: When the `Option` is `Some`, we're using the actual pointer
808	// to the payload, with a length of 1, so this is equivalent to
809	// `slice::from_ref`, and thus is safe.
810	// When the `Option` is `None`, the length used is 0, so to be safe it
811	// just needs to be aligned, which it is because `&self` is aligned and
812	// the offset used is a multiple of alignment.
813	//
814	// In the new version, the intrinsic always returns a pointer to an
815	// in-bounds and correctly aligned position for a `T` (even if in the
816	// `None` case it's just padding).
817	unsafe {
818	slice::from_raw_parts(
819	(self as *const Self).byte_add(core::mem::offset_of!(Self, Some.0)).cast(),
820	self.len(),
821	)
822	}
823	}
824
825	/// Returns a mutable slice of the contained value, if any. If this is
826	/// `None`, an empty slice is returned. This can be useful to have a
827	/// single type of iterator over an `Option` or slice.
828	///
829	/// Note: Should you have an `Option<&mut T>` instead of a
830	/// `&mut Option<T>`, which this method takes, you can obtain a mutable
831	/// slice via `opt.map_or(&mut [], std::slice::from_mut)`.
832	///
833	/// # Examples
834	///
835	/// ```rust
836	/// assert_eq!(
837	/// [Some(`1234`).as_mut_slice(), None.as_mut_slice()],
838	/// [&mut [`1234`][..], &mut [][..]],
839	/// );
840	/// ```
841	///
842	/// The result is a mutable slice of zero or one items that points into
843	/// our original `Option`:
844	///
845	/// ```rust
846	/// let mut x = Some(`1234`);
847	/// x.as_mut_slice()[`0`] += `1`;
848	/// assert_eq!(x, Some(`1235`));
849	/// ```
850	///
851	/// The inverse of this method (discounting borrowing)
852	/// is [`[_]::first_mut`](slice::first_mut):
853	///
854	/// ```rust
855	/// assert_eq!(Some(`123`).as_mut_slice().first_mut(), Some(&mut `123`))
856	/// ```
857	#[inline]
858	#[must_use]
859	#[stable(feature = "option_as_slice", since = "1.75.0")]
860	#[rustc_const_stable(feature = "const_option_ext", since = "1.84.0")]
861	pub const fn as_mut_slice(&mut self) -> &mut [T] {
862	// SAFETY: When the `Option` is `Some`, we're using the actual pointer
863	// to the payload, with a length of 1, so this is equivalent to
864	// `slice::from_mut`, and thus is safe.
865	// When the `Option` is `None`, the length used is 0, so to be safe it
866	// just needs to be aligned, which it is because `&self` is aligned and
867	// the offset used is a multiple of alignment.
868	//
869	// In the new version, the intrinsic creates a `const T` from a*
870	// mutable reference so it is safe to cast back to a mutable pointer
871	// here. As with `as_slice`, the intrinsic always returns a pointer to
872	// an in-bounds and correctly aligned position for a `T` (even if in
873	// the `None` case it's just padding).
874	unsafe {
875	slice::from_raw_parts_mut(
876	(self as *mut Self).byte_add(core::mem::offset_of!(Self, Some.0)).cast(),
877	self.len(),
878	)
879	}
880	}
881
882	/////////////////////////////////////////////////////////////////////////
883	// Getting to contained values
884	/////////////////////////////////////////////////////////////////////////
885
886	/// Returns the contained [`Some`] value, consuming the `self` value.
887	///
888	/// # Panics
889	///
890	/// Panics if the value is a [`None`] with a custom panic message provided by
891	/// `msg`.
892	///
893	/// # Examples
894	///
895	/// ```
896	/// let x = Some("value");
897	/// assert_eq!(x.expect("fruits are healthy"), "value");
898	/// ```
899	///
900	/// ```should_panic
901	/// let x: Option<&str> = None;
902	/// x.expect("fruits are healthy"); // panics with `fruits are healthy`
903	/// ```
904	///
905	/// # Recommended Message Style
906	///
907	/// We recommend that `expect` messages are used to describe the reason you
908	/// _expect_ the `Option` should be `Some`.
909	///
910	/// ```should_panic
911	/// # let slice: &[u8] = &[];
912	/// let item = slice.get(`0`)
913	/// .expect("slice should not be empty");
914	/// ```
915	///
916	/// Hint: If you're having trouble remembering how to phrase expect
917	/// error messages remember to focus on the word "should" as in "env
918	/// variable should be set by blah" or "the given binary should be available
919	/// and executable by the current user".
920	///
921	/// For more detail on expect message styles and the reasoning behind our
922	/// recommendation please refer to the section on ["Common Message
923	/// Styles"](../../std/error/index.html#common-message-styles) in the [`std::error`](../../std/error/index.html) module docs.
924	#[inline]
925	#[track_caller]
926	#[stable(feature = "rust1", since = "1.0.0")]
927	#[rustc_diagnostic_item = "option_expect"]
928	#[rustc_allow_const_fn_unstable(const_precise_live_drops)]
929	#[rustc_const_stable(feature = "const_option", since = "1.83.0")]
930	pub const fn expect(self, msg: &str) -> T {
931	match self {
932	Some(val) => val,
933	None => expect_failed(msg),
934	}
935	}
936
937	/// Returns the contained [`Some`] value, consuming the `self` value.
938	///
939	/// Because this function may panic, its use is generally discouraged.
940	/// Panics are meant for unrecoverable errors, and
941	/// [may abort the entire program][panic-abort].
942	///
943	/// Instead, prefer to use pattern matching and handle the [`None`]
944	/// case explicitly, or call [`unwrap_or`], [`unwrap_or_else`], or
945	/// [`unwrap_or_default`]. In functions returning `Option`, you can use
946	/// [the `?` (try) operator][try-option].
947	///
948	/// [panic-abort]: https://doc.rust-lang.org/book/ch09-01-unrecoverable-errors-with-panic.html
949	/// [try-option]: https://doc.rust-lang.org/book/ch09-02-recoverable-errors-with-result.html#where-the--operator-can-be-used
950	/// [`unwrap_or`]: Option::unwrap_or
951	/// [`unwrap_or_else`]: Option::unwrap_or_else
952	/// [`unwrap_or_default`]: Option::unwrap_or_default
953	///
954	/// # Panics
955	///
956	/// Panics if the self value equals [`None`].
957	///
958	/// # Examples
959	///
960	/// ```
961	/// let x = Some("air");
962	/// assert_eq!(x.unwrap(), "air");
963	/// ```
964	///
965	/// ```should_panic
966	/// let x: Option<&str> = None;
967	/// assert_eq!(x.unwrap(), "air"); // fails
968	/// ```
969	#[inline(always)]
970	#[track_caller]
971	#[stable(feature = "rust1", since = "1.0.0")]
972	#[rustc_diagnostic_item = "option_unwrap"]
973	#[rustc_allow_const_fn_unstable(const_precise_live_drops)]
974	#[rustc_const_stable(feature = "const_option", since = "1.83.0")]
975	pub const fn unwrap(self) -> T {
976	match self {
977	Some(val) => val,
978	None => unwrap_failed(),
979	}
980	}
981
982	/// Returns the contained [`Some`] value or a provided default.
983	///
984	/// Arguments passed to `unwrap_or` are eagerly evaluated; if you are passing
985	/// the result of a function call, it is recommended to use [`unwrap_or_else`],
986	/// which is lazily evaluated.
987	///
988	/// [`unwrap_or_else`]: Option::unwrap_or_else
989	///
990	/// # Examples
991	///
992	/// ```
993	/// assert_eq!(Some("car").unwrap_or("bike"), "car");
994	/// assert_eq!(None.unwrap_or("bike"), "bike");
995	/// ```
996	#[inline]
997	#[stable(feature = "rust1", since = "1.0.0")]
998	pub fn unwrap_or(self, default: T) -> T {
999	match self {
1000	Some(x) => x,
1001	None => default,
1002	}
1003	}
1004
1005	/// Returns the contained [`Some`] value or computes it from a closure.
1006	///
1007	/// # Examples
1008	///
1009	/// ```
1010	/// let k = `10`;
1011	/// assert_eq!(Some(`4`).unwrap_or_else(\|\| `2` * k), `4`);
1012	/// assert_eq!(None.unwrap_or_else(\|\| `2` * k), `20`);
1013	/// ```
1014	#[inline]
1015	#[track_caller]
1016	#[stable(feature = "rust1", since = "1.0.0")]
1017	pub fn unwrap_or_else<F>(self, f: F) -> T
1018	where
1019	F: FnOnce() -> T,
1020	{
1021	match self {
1022	Some(x) => x,
1023	None => f(),
1024	}
1025	}
1026
1027	/// Returns the contained [`Some`] value or a default.
1028	///
1029	/// Consumes the `self` argument then, if [`Some`], returns the contained
1030	/// value, otherwise if [`None`], returns the [default value] for that
1031	/// type.
1032	///
1033	/// # Examples
1034	///
1035	/// ```
1036	/// let x: Option<u32> = None;
1037	/// let y: Option<u32> = Some(`12`);
1038	///
1039	/// assert_eq!(x.unwrap_or_default(), `0`);
1040	/// assert_eq!(y.unwrap_or_default(), `12`);
1041	/// ```
1042	///
1043	/// [default value]: Default::default
1044	/// [`parse`]: str::parse
1045	/// [`FromStr`]: crate::str::FromStr
1046	#[inline]
1047	#[stable(feature = "rust1", since = "1.0.0")]
1048	pub fn unwrap_or_default(self) -> T
1049	where
1050	T: Default,
1051	{
1052	match self {
1053	Some(x) => x,
1054	None => T::default(),
1055	}
1056	}
1057
1058	/// Returns the contained [`Some`] value, consuming the `self` value,
1059	/// without checking that the value is not [`None`].
1060	///
1061	/// # Safety
1062	///
1063	/// Calling this method on [`None`] is [undefined behavior].
1064	///
1065	/// [undefined behavior]: https://doc.rust-lang.org/reference/behavior-considered-undefined.html
1066	///
1067	/// # Examples
1068	///
1069	/// ```
1070	/// let x = Some("air");
1071	/// assert_eq!(unsafe { x.unwrap_unchecked() }, "air");
1072	/// ```
1073	///
1074	/// ```no_run
1075	/// let x: Option<&str> = None;
1076	/// assert_eq!(unsafe { x.unwrap_unchecked() }, "air"); // Undefined behavior!
1077	/// ```
1078	#[inline]
1079	#[track_caller]
1080	#[stable(feature = "option_result_unwrap_unchecked", since = "1.58.0")]
1081	#[rustc_allow_const_fn_unstable(const_precise_live_drops)]
1082	#[rustc_const_stable(feature = "const_option", since = "1.83.0")]
1083	pub const unsafe fn unwrap_unchecked(self) -> T {
1084	match self {
1085	Some(val) => val,
1086	// SAFETY: the safety contract must be upheld by the caller.
1087	None => unsafe { hint::unreachable_unchecked() },
1088	}
1089	}
1090
1091	/////////////////////////////////////////////////////////////////////////
1092	// Transforming contained values
1093	/////////////////////////////////////////////////////////////////////////
1094
1095	/// Maps an `Option<T>` to `Option<U>` by applying a function to a contained value (if `Some`) or returns `None` (if `None`).
1096	///
1097	/// # Examples
1098	///
1099	/// Calculates the length of an <code>Option<[String]></code> as an
1100	/// <code>Option<[usize]></code>, consuming the original:
1101	///
1102	/// [String]: ../../std/string/struct.String.html "String"
1103	/// ```
1104	/// let maybe_some_string = Some(String::from("Hello, World!"));
1105	/// // `Option::map` takes self by value, consuming `maybe_some_string`
1106	/// let maybe_some_len = maybe_some_string.map(\|s\| s.len());
1107	/// assert_eq!(maybe_some_len, Some(`13`));
1108	///
1109	/// let x: Option<&str> = None;
1110	/// assert_eq!(x.map(\|s\| s.len()), None);
1111	/// ```
1112	#[inline]
1113	#[stable(feature = "rust1", since = "1.0.0")]
1114	pub fn map<U, F>(self, f: F) -> Option<U>
1115	where
1116	F: FnOnce(T) -> U,
1117	{
1118	match self {
1119	Some(x) => Some(f(x)),
1120	None => None,
1121	}
1122	}
1123
1124	/// Calls a function with a reference to the contained value if [`Some`].
1125	///
1126	/// Returns the original option.
1127	///
1128	/// # Examples
1129	///
1130	/// ```
1131	/// let list = vec![`1`, `2`, `3`];
1132	///
1133	/// // prints "got: 2"
1134	/// let x = list
1135	/// .get(`1`)
1136	/// .inspect(\|x\| println!("got: {x}"))
1137	/// .expect("list should be long enough");
1138	///
1139	/// // prints nothing
1140	/// list.get(`5`).inspect(\|x\| println!("got: {x}"));
1141	/// ```
1142	#[inline]
1143	#[stable(feature = "result_option_inspect", since = "1.76.0")]
1144	pub fn inspect<F: FnOnce(&T)>(self, f: F) -> Self {
1145	if let Some(ref x) = self {
1146	f(x);
1147	}
1148
1149	self
1150	}
1151
1152	/// Returns the provided default result (if none),
1153	/// or applies a function to the contained value (if any).
1154	///
1155	/// Arguments passed to `map_or` are eagerly evaluated; if you are passing
1156	/// the result of a function call, it is recommended to use [`map_or_else`],
1157	/// which is lazily evaluated.
1158	///
1159	/// [`map_or_else`]: Option::map_or_else
1160	///
1161	/// # Examples
1162	///
1163	/// ```
1164	/// let x = Some("foo");
1165	/// assert_eq!(x.map_or(`42`, \|v\| v.len()), `3`);
1166	///
1167	/// let x: Option<&str> = None;
1168	/// assert_eq!(x.map_or(`42`, \|v\| v.len()), `42`);
1169	/// ```
1170	#[inline]
1171	#[stable(feature = "rust1", since = "1.0.0")]
1172	#[must_use = "if you don't need the returned value, use `if let` instead"]
1173	pub fn map_or<U, F>(self, default: U, f: F) -> U
1174	where
1175	F: FnOnce(T) -> U,
1176	{
1177	match self {
1178	Some(t) => f(t),
1179	None => default,
1180	}
1181	}
1182
1183	/// Computes a default function result (if none), or
1184	/// applies a different function to the contained value (if any).
1185	///
1186	/// # Basic examples
1187	///
1188	/// ```
1189	/// let k = `21`;
1190	///
1191	/// let x = Some("foo");
1192	/// assert_eq!(x.map_or_else(\|\| `2` * k, \|v\| v.len()), `3`);
1193	///
1194	/// let x: Option<&str> = None;
1195	/// assert_eq!(x.map_or_else(\|\| `2` * k, \|v\| v.len()), `42`);
1196	/// ```
1197	///
1198	/// # Handling a Result-based fallback
1199	///
1200	/// A somewhat common occurrence when dealing with optional values
1201	/// in combination with [`Result<T, E>`] is the case where one wants to invoke
1202	/// a fallible fallback if the option is not present. This example
1203	/// parses a command line argument (if present), or the contents of a file to
1204	/// an integer. However, unlike accessing the command line argument, reading
1205	/// the file is fallible, so it must be wrapped with `Ok`.
1206	///
1207	/// ```no_run
1208	/// # fn main() -> Result<(), Box<dyn std::error::Error>> {
1209	/// let v: u64 = std::env::args()
1210	/// .nth(`1`)
1211	/// .map_or_else(\|\| std::fs::read_to_string("/etc/someconfig.conf"), Ok)?
1212	/// .parse()?;
1213	/// # Ok(())
1214	/// # }
1215	/// ```
1216	#[inline]
1217	#[stable(feature = "rust1", since = "1.0.0")]
1218	pub fn map_or_else<U, D, F>(self, default: D, f: F) -> U
1219	where
1220	D: FnOnce() -> U,
1221	F: FnOnce(T) -> U,
1222	{
1223	match self {
1224	Some(t) => f(t),
1225	None => default(),
1226	}
1227	}
1228
1229	/// Transforms the `Option<T>` into a [`Result<T, E>`], mapping [`Some(v)`] to
1230	/// [`Ok(v)`] and [`None`] to [`Err(err)`].
1231	///
1232	/// Arguments passed to `ok_or` are eagerly evaluated; if you are passing the
1233	/// result of a function call, it is recommended to use [`ok_or_else`], which is
1234	/// lazily evaluated.
1235	///
1236	/// [`Ok(v)`]: Ok
1237	/// [`Err(err)`]: Err
1238	/// [`Some(v)`]: Some
1239	/// [`ok_or_else`]: Option::ok_or_else
1240	///
1241	/// # Examples
1242	///
1243	/// ```
1244	/// let x = Some("foo");
1245	/// assert_eq!(x.ok_or(`0`), Ok("foo"));
1246	///
1247	/// let x: Option<&str> = None;
1248	/// assert_eq!(x.ok_or(`0`), Err(`0`));
1249	/// ```
1250	#[inline]
1251	#[stable(feature = "rust1", since = "1.0.0")]
1252	pub fn ok_or<E>(self, err: E) -> Result<T, E> {
1253	match self {
1254	Some(v) => Ok(v),
1255	None => Err(err),
1256	}
1257	}
1258
1259	/// Transforms the `Option<T>` into a [`Result<T, E>`], mapping [`Some(v)`] to
1260	/// [`Ok(v)`] and [`None`] to [`Err(err())`].
1261	///
1262	/// [`Ok(v)`]: Ok
1263	/// [`Err(err())`]: Err
1264	/// [`Some(v)`]: Some
1265	///
1266	/// # Examples
1267	///
1268	/// ```
1269	/// let x = Some("foo");
1270	/// assert_eq!(x.ok_or_else(\|\| `0`), Ok("foo"));
1271	///
1272	/// let x: Option<&str> = None;
1273	/// assert_eq!(x.ok_or_else(\|\| `0`), Err(`0`));
1274	/// ```
1275	#[inline]
1276	#[stable(feature = "rust1", since = "1.0.0")]
1277	pub fn ok_or_else<E, F>(self, err: F) -> Result<T, E>
1278	where
1279	F: FnOnce() -> E,
1280	{
1281	match self {
1282	Some(v) => Ok(v),
1283	None => Err(err()),
1284	}
1285	}
1286
1287	/// Converts from `Option<T>` (or `&Option<T>`) to `Option<&T::Target>`.
1288	///
1289	/// Leaves the original Option in-place, creating a new one with a reference
1290	/// to the original one, additionally coercing the contents via [`Deref`].
1291	///
1292	/// # Examples
1293	///
1294	/// ```
1295	/// let x: Option<String> = Some("hey".to_owned());
1296	/// assert_eq!(x.as_deref(), Some("hey"));
1297	///
1298	/// let x: Option<String> = None;
1299	/// assert_eq!(x.as_deref(), None);
1300	/// ```
1301	#[inline]
1302	#[stable(feature = "option_deref", since = "1.40.0")]
1303	pub fn as_deref(&self) -> Option<&T::Target>
1304	where
1305	T: Deref,
1306	{
1307	self.as_ref().map(\|t\| t.deref())
1308	}
1309
1310	/// Converts from `Option<T>` (or `&mut Option<T>`) to `Option<&mut T::Target>`.
1311	///
1312	/// Leaves the original `Option` in-place, creating a new one containing a mutable reference to
1313	/// the inner type's [`Deref::Target`] type.
1314	///
1315	/// # Examples
1316	///
1317	/// ```
1318	/// let mut x: Option<String> = Some("hey".to_owned());
1319	/// assert_eq!(x.as_deref_mut().map(\|x\| {
1320	/// x.make_ascii_uppercase();
1321	/// x
1322	/// }), Some("HEY".to_owned().as_mut_str()));
1323	/// ```
1324	#[inline]
1325	#[stable(feature = "option_deref", since = "1.40.0")]
1326	pub fn as_deref_mut(&mut self) -> Option<&mut T::Target>
1327	where
1328	T: DerefMut,
1329	{
1330	self.as_mut().map(\|t\| t.deref_mut())
1331	}
1332
1333	/////////////////////////////////////////////////////////////////////////
1334	// Iterator constructors
1335	/////////////////////////////////////////////////////////////////////////
1336
1337	/// Returns an iterator over the possibly contained value.
1338	///
1339	/// # Examples
1340	///
1341	/// ```
1342	/// let x = Some(`4`);
1343	/// assert_eq!(x.iter().next(), Some(&`4`));
1344	///
1345	/// let x: Option<u32> = None;
1346	/// assert_eq!(x.iter().next(), None);
1347	/// ```
1348	#[inline]
1349	#[stable(feature = "rust1", since = "1.0.0")]
1350	pub fn iter(&self) -> Iter<'_, T> {
1351	Iter { inner: Item { opt: self.as_ref() } }
1352	}
1353
1354	/// Returns a mutable iterator over the possibly contained value.
1355	///
1356	/// # Examples
1357	///
1358	/// ```
1359	/// let mut x = Some(`4`);
1360	/// match x.iter_mut().next() {
1361	/// Some(v) => *v = `42`,
1362	/// None => {},
1363	/// }
1364	/// assert_eq!(x, Some(`42`));
1365	///
1366	/// let mut x: Option<u32> = None;
1367	/// assert_eq!(x.iter_mut().next(), None);
1368	/// ```
1369	#[inline]
1370	#[stable(feature = "rust1", since = "1.0.0")]
1371	pub fn iter_mut(&mut self) -> IterMut<'_, T> {
1372	IterMut { inner: Item { opt: self.as_mut() } }
1373	}
1374
1375	/////////////////////////////////////////////////////////////////////////
1376	// Boolean operations on the values, eager and lazy
1377	/////////////////////////////////////////////////////////////////////////
1378
1379	/// Returns [`None`] if the option is [`None`], otherwise returns `optb`.
1380	///
1381	/// Arguments passed to `and` are eagerly evaluated; if you are passing the
1382	/// result of a function call, it is recommended to use [`and_then`], which is
1383	/// lazily evaluated.
1384	///
1385	/// [`and_then`]: Option::and_then
1386	///
1387	/// # Examples
1388	///
1389	/// ```
1390	/// let x = Some(`2`);
1391	/// let y: Option<&str> = None;
1392	/// assert_eq!(x.and(y), None);
1393	///
1394	/// let x: Option<u32> = None;
1395	/// let y = Some("foo");
1396	/// assert_eq!(x.and(y), None);
1397	///
1398	/// let x = Some(`2`);
1399	/// let y = Some("foo");
1400	/// assert_eq!(x.and(y), Some("foo"));
1401	///
1402	/// let x: Option<u32> = None;
1403	/// let y: Option<&str> = None;
1404	/// assert_eq!(x.and(y), None);
1405	/// ```
1406	#[inline]
1407	#[stable(feature = "rust1", since = "1.0.0")]
1408	pub fn and<U>(self, optb: Option<U>) -> Option<U> {
1409	match self {
1410	Some(_) => optb,
1411	None => None,
1412	}
1413	}
1414
1415	/// Returns [`None`] if the option is [`None`], otherwise calls `f` with the
1416	/// wrapped value and returns the result.
1417	///
1418	/// Some languages call this operation flatmap.
1419	///
1420	/// # Examples
1421	///
1422	/// ```
1423	/// fn sq_then_to_string(x: u32) -> Option<String> {
1424	/// x.checked_mul(x).map(\|sq\| sq.to_string())
1425	/// }
1426	///
1427	/// assert_eq!(Some(`2`).and_then(sq_then_to_string), Some(`4`.to_string()));
1428	/// assert_eq!(Some(`1_000_000`).and_then(sq_then_to_string), None); // overflowed!
1429	/// assert_eq!(None.and_then(sq_then_to_string), None);
1430	/// ```
1431	///
1432	/// Often used to chain fallible operations that may return [`None`].
1433	///
1434	/// ```
1435	/// let arr_2d = [["A0", "A1"], ["B0", "B1"]];
1436	///
1437	/// let item_0_1 = arr_2d.get(`0`).and_then(\|row\| row.get(`1`));
1438	/// assert_eq!(item_0_1, Some(&"A1"));
1439	///
1440	/// let item_2_0 = arr_2d.get(`2`).and_then(\|row\| row.get(`0`));
1441	/// assert_eq!(item_2_0, None);
1442	/// ```
1443	#[doc(alias = "flatmap")]
1444	#[inline]
1445	#[stable(feature = "rust1", since = "1.0.0")]
1446	#[rustc_confusables("flat_map", "flatmap")]
1447	pub fn and_then<U, F>(self, f: F) -> Option<U>
1448	where
1449	F: FnOnce(T) -> Option<U>,
1450	{
1451	match self {
1452	Some(x) => f(x),
1453	None => None,
1454	}
1455	}
1456
1457	/// Returns [`None`] if the option is [`None`], otherwise calls `predicate`
1458	/// with the wrapped value and returns:
1459	///
1460	/// - [`Some(t)`] if `predicate` returns `true` (where `t` is the wrapped
1461	/// value), and
1462	/// - [`None`] if `predicate` returns `false`.
1463	///
1464	/// This function works similar to [`Iterator::filter()`]. You can imagine
1465	/// the `Option<T>` being an iterator over one or zero elements. `filter()`
1466	/// lets you decide which elements to keep.
1467	///
1468	/// # Examples
1469	///
1470	/// ```rust
1471	/// fn is_even(n: &i32) -> bool {
1472	/// n % `2` == `0`
1473	/// }
1474	///
1475	/// assert_eq!(None.filter(is_even), None);
1476	/// assert_eq!(Some(`3`).filter(is_even), None);
1477	/// assert_eq!(Some(`4`).filter(is_even), Some(`4`));
1478	/// ```
1479	///
1480	/// [`Some(t)`]: Some
1481	#[inline]
1482	#[stable(feature = "option_filter", since = "1.27.0")]
1483	pub fn filter<P>(self, predicate: P) -> Self
1484	where
1485	P: FnOnce(&T) -> bool,
1486	{
1487	if let Some(x) = self {
1488	if predicate(&x) {
1489	return Some(x);
1490	}
1491	}
1492	None
1493	}
1494
1495	/// Returns the option if it contains a value, otherwise returns `optb`.
1496	///
1497	/// Arguments passed to `or` are eagerly evaluated; if you are passing the
1498	/// result of a function call, it is recommended to use [`or_else`], which is
1499	/// lazily evaluated.
1500	///
1501	/// [`or_else`]: Option::or_else
1502	///
1503	/// # Examples
1504	///
1505	/// ```
1506	/// let x = Some(`2`);
1507	/// let y = None;
1508	/// assert_eq!(x.or(y), Some(`2`));
1509	///
1510	/// let x = None;
1511	/// let y = Some(`100`);
1512	/// assert_eq!(x.or(y), Some(`100`));
1513	///
1514	/// let x = Some(`2`);
1515	/// let y = Some(`100`);
1516	/// assert_eq!(x.or(y), Some(`2`));
1517	///
1518	/// let x: Option<u32> = None;
1519	/// let y = None;
1520	/// assert_eq!(x.or(y), None);
1521	/// ```
1522	#[inline]
1523	#[stable(feature = "rust1", since = "1.0.0")]
1524	pub fn or(self, optb: Option<T>) -> Option<T> {
1525	match self {
1526	x @ Some(_) => x,
1527	None => optb,
1528	}
1529	}
1530
1531	/// Returns the option if it contains a value, otherwise calls `f` and
1532	/// returns the result.
1533	///
1534	/// # Examples
1535	///
1536	/// ```
1537	/// fn nobody() -> Option<&'static str> { None }
1538	/// fn vikings() -> Option<&'static str> { Some("vikings") }
1539	///
1540	/// assert_eq!(Some("barbarians").or_else(vikings), Some("barbarians"));
1541	/// assert_eq!(None.or_else(vikings), Some("vikings"));
1542	/// assert_eq!(None.or_else(nobody), None);
1543	/// ```
1544	#[inline]
1545	#[stable(feature = "rust1", since = "1.0.0")]
1546	pub fn or_else<F>(self, f: F) -> Option<T>
1547	where
1548	F: FnOnce() -> Option<T>,
1549	{
1550	match self {
1551	x @ Some(_) => x,
1552	None => f(),
1553	}
1554	}
1555
1556	/// Returns [`Some`] if exactly one of `self`, `optb` is [`Some`], otherwise returns [`None`].
1557	///
1558	/// # Examples
1559	///
1560	/// ```
1561	/// let x = Some(`2`);
1562	/// let y: Option<u32> = None;
1563	/// assert_eq!(x.xor(y), Some(`2`));
1564	///
1565	/// let x: Option<u32> = None;
1566	/// let y = Some(`2`);
1567	/// assert_eq!(x.xor(y), Some(`2`));
1568	///
1569	/// let x = Some(`2`);
1570	/// let y = Some(`2`);
1571	/// assert_eq!(x.xor(y), None);
1572	///
1573	/// let x: Option<u32> = None;
1574	/// let y: Option<u32> = None;
1575	/// assert_eq!(x.xor(y), None);
1576	/// ```
1577	#[inline]
1578	#[stable(feature = "option_xor", since = "1.37.0")]
1579	pub fn xor(self, optb: Option<T>) -> Option<T> {
1580	match (self, optb) {
1581	(a @ Some(_), None) => a,
1582	(None, b @ Some(_)) => b,
1583	_ => None,
1584	}
1585	}
1586
1587	/////////////////////////////////////////////////////////////////////////
1588	// Entry-like operations to insert a value and return a reference
1589	/////////////////////////////////////////////////////////////////////////
1590
1591	/// Inserts `value` into the option, then returns a mutable reference to it.
1592	///
1593	/// If the option already contains a value, the old value is dropped.
1594	///
1595	/// See also [`Option::get_or_insert`], which doesn't update the value if
1596	/// the option already contains [`Some`].
1597	///
1598	/// # Example
1599	///
1600	/// ```
1601	/// let mut opt = None;
1602	/// let val = opt.insert(`1`);
1603	/// assert_eq!(*val, `1`);
1604	/// assert_eq!(opt.unwrap(), `1`);
1605	/// let val = opt.insert(`2`);
1606	/// assert_eq!(*val, `2`);
1607	/// *val = `3`;
1608	/// assert_eq!(opt.unwrap(), `3`);
1609	/// ```
1610	#[must_use = "if you intended to set a value, consider assignment instead"]
1611	#[inline]
1612	#[stable(feature = "option_insert", since = "1.53.0")]
1613	pub fn insert(&mut self, value: T) -> &mut T {
1614	*self = Some(value);
1615
1616	// SAFETY: the code above just filled the option
1617	unsafe { self.as_mut().unwrap_unchecked() }
1618	}
1619
1620	/// Inserts `value` into the option if it is [`None`], then
1621	/// returns a mutable reference to the contained value.
1622	///
1623	/// See also [`Option::insert`], which updates the value even if
1624	/// the option already contains [`Some`].
1625	///
1626	/// # Examples
1627	///
1628	/// ```
1629	/// let mut x = None;
1630	///
1631	/// {
1632	/// let y: &mut u32 = x.get_or_insert(`5`);
1633	/// assert_eq!(y, &`5`);
1634	///
1635	/// *y = `7`;
1636	/// }
1637	///
1638	/// assert_eq!(x, Some(`7`));
1639	/// ```
1640	#[inline]
1641	#[stable(feature = "option_entry", since = "1.20.0")]
1642	pub fn get_or_insert(&mut self, value: T) -> &mut T {
1643	self.get_or_insert_with(\|\| value)
1644	}
1645
1646	/// Inserts the default value into the option if it is [`None`], then
1647	/// returns a mutable reference to the contained value.
1648	///
1649	/// # Examples
1650	///
1651	/// ```
1652	/// let mut x = None;
1653	///
1654	/// {
1655	/// let y: &mut u32 = x.get_or_insert_default();
1656	/// assert_eq!(y, &`0`);
1657	///
1658	/// *y = `7`;
1659	/// }
1660	///
1661	/// assert_eq!(x, Some(`7`));
1662	/// ```
1663	#[inline]
1664	#[stable(feature = "option_get_or_insert_default", since = "1.83.0")]
1665	pub fn get_or_insert_default(&mut self) -> &mut T
1666	where
1667	T: Default,
1668	{
1669	self.get_or_insert_with(T::default)
1670	}
1671
1672	/// Inserts a value computed from `f` into the option if it is [`None`],
1673	/// then returns a mutable reference to the contained value.
1674	///
1675	/// # Examples
1676	///
1677	/// ```
1678	/// let mut x = None;
1679	///
1680	/// {
1681	/// let y: &mut u32 = x.get_or_insert_with(\|\| `5`);
1682	/// assert_eq!(y, &`5`);
1683	///
1684	/// *y = `7`;
1685	/// }
1686	///
1687	/// assert_eq!(x, Some(`7`));
1688	/// ```
1689	#[inline]
1690	#[stable(feature = "option_entry", since = "1.20.0")]
1691	pub fn get_or_insert_with<F>(&mut self, f: F) -> &mut T
1692	where
1693	F: FnOnce() -> T,
1694	{
1695	if let None = self {
1696	*self = Some(f());
1697	}
1698
1699	// SAFETY: a `None` variant for `self` would have been replaced by a `Some`
1700	// variant in the code above.
1701	unsafe { self.as_mut().unwrap_unchecked() }
1702	}
1703
1704	/////////////////////////////////////////////////////////////////////////
1705	// Misc
1706	/////////////////////////////////////////////////////////////////////////
1707
1708	/// Takes the value out of the option, leaving a [`None`] in its place.
1709	///
1710	/// # Examples
1711	///
1712	/// ```
1713	/// let mut x = Some(`2`);
1714	/// let y = x.take();
1715	/// assert_eq!(x, None);
1716	/// assert_eq!(y, Some(`2`));
1717	///
1718	/// let mut x: Option<u32> = None;
1719	/// let y = x.take();
1720	/// assert_eq!(x, None);
1721	/// assert_eq!(y, None);
1722	/// ```
1723	#[inline]
1724	#[stable(feature = "rust1", since = "1.0.0")]
1725	#[rustc_const_stable(feature = "const_option", since = "1.83.0")]
1726	pub const fn take(&mut self) -> Option<T> {
1727	// FIXME(const-hack) replace `mem::replace` by `mem::take` when the latter is const ready
1728	mem::replace(self, None)
1729	}
1730
1731	/// Takes the value out of the option, but only if the predicate evaluates to
1732	/// `true` on a mutable reference to the value.
1733	///
1734	/// In other words, replaces `self` with `None` if the predicate returns `true`.
1735	/// This method operates similar to [`Option::take`] but conditional.
1736	///
1737	/// # Examples
1738	///
1739	/// ```
1740	/// let mut x = Some(`42`);
1741	///
1742	/// let prev = x.take_if(\|v\| if *v == `42` {
1743	/// *v += `1`;
1744	/// `false`
1745	/// } else {
1746	/// `false`
1747	/// });
1748	/// assert_eq!(x, Some(`43`));
1749	/// assert_eq!(prev, None);
1750	///
1751	/// let prev = x.take_if(\|v\| *v == `43`);
1752	/// assert_eq!(x, None);
1753	/// assert_eq!(prev, Some(`43`));
1754	/// ```
1755	#[inline]
1756	#[stable(feature = "option_take_if", since = "1.80.0")]
1757	pub fn take_if<P>(&mut self, predicate: P) -> Option<T>
1758	where
1759	P: FnOnce(&mut T) -> bool,
1760	{
1761	if self.as_mut().map_or(`false`, predicate) { self.take() } else { None }
1762	}
1763
1764	/// Replaces the actual value in the option by the value given in parameter,
1765	/// returning the old value if present,
1766	/// leaving a [`Some`] in its place without deinitializing either one.
1767	///
1768	/// # Examples
1769	///
1770	/// ```
1771	/// let mut x = Some(`2`);
1772	/// let old = x.replace(`5`);
1773	/// assert_eq!(x, Some(`5`));
1774	/// assert_eq!(old, Some(`2`));
1775	///
1776	/// let mut x = None;
1777	/// let old = x.replace(`3`);
1778	/// assert_eq!(x, Some(`3`));
1779	/// assert_eq!(old, None);
1780	/// ```
1781	#[inline]
1782	#[stable(feature = "option_replace", since = "1.31.0")]
1783	#[rustc_const_stable(feature = "const_option", since = "1.83.0")]
1784	pub const fn replace(&mut self, value: T) -> Option<T> {
1785	mem::replace(self, Some(value))
1786	}
1787
1788	/// Zips `self` with another `Option`.
1789	///
1790	/// If `self` is `Some(s)` and `other` is `Some(o)`, this method returns `Some((s, o))`.
1791	/// Otherwise, `None` is returned.
1792	///
1793	/// # Examples
1794	///
1795	/// ```
1796	/// let x = Some(`1`);
1797	/// let y = Some("hi");
1798	/// let z = None::<u8>;
1799	///
1800	/// assert_eq!(x.zip(y), Some((`1`, "hi")));
1801	/// assert_eq!(x.zip(z), None);
1802	/// ```
1803	#[stable(feature = "option_zip_option", since = "1.46.0")]
1804	pub fn zip<U>(self, other: Option<U>) -> Option<(T, U)> {
1805	match (self, other) {
1806	(Some(a), Some(b)) => Some((a, b)),
1807	_ => None,
1808	}
1809	}
1810
1811	/// Zips `self` and another `Option` with function `f`.
1812	///
1813	/// If `self` is `Some(s)` and `other` is `Some(o)`, this method returns `Some(f(s, o))`.
1814	/// Otherwise, `None` is returned.
1815	///
1816	/// # Examples
1817	///
1818	/// ```
1819	/// #![feature(option_zip)]
1820	///
1821	/// #[derive(Debug, PartialEq)]
1822	/// struct Point {
1823	/// x: f64,
1824	/// y: f64,
1825	/// }
1826	///
1827	/// impl Point {
1828	/// fn new(x: f64, y: f64) -> Self {
1829	/// Self { x, y }
1830	/// }
1831	/// }
1832	///
1833	/// let x = Some(`17.5`);
1834	/// let y = Some(`42.7`);
1835	///
1836	/// assert_eq!(x.zip_with(y, Point::new), Some(Point { x: `17.5`, y: `42.7` }));
1837	/// assert_eq!(x.zip_with(None, Point::new), None);
1838	/// ```
1839	#[unstable(feature = "option_zip", issue = "70086")]
1840	pub fn zip_with<U, F, R>(self, other: Option<U>, f: F) -> Option<R>
1841	where
1842	F: FnOnce(T, U) -> R,
1843	{
1844	match (self, other) {
1845	(Some(a), Some(b)) => Some(f(a, b)),
1846	_ => None,
1847	}
1848	}
1849	}
1850
1851	impl<T, U> Option<(T, U)> {
1852	/// Unzips an option containing a tuple of two options.
1853	///
1854	/// If `self` is `Some((a, b))` this method returns `(Some(a), Some(b))`.
1855	/// Otherwise, `(None, None)` is returned.
1856	///
1857	/// # Examples
1858	///
1859	/// ```
1860	/// let x = Some((`1`, "hi"));
1861	/// let y = None::<(u8, u32)>;
1862	///
1863	/// assert_eq!(x.unzip(), (Some(`1`), Some("hi")));
1864	/// assert_eq!(y.unzip(), (None, None));
1865	/// ```
1866	#[inline]
1867	#[stable(feature = "unzip_option", since = "1.66.0")]
1868	pub fn unzip(self) -> (Option<T>, Option<U>) {
1869	match self {
1870	Some((a: T, b: U)) => (Some(a), Some(b)),
1871	None => (None, None),
1872	}
1873	}
1874	}
1875
1876	impl<T> Option<&T> {
1877	/// Maps an `Option<&T>` to an `Option<T>` by copying the contents of the
1878	/// option.
1879	///
1880	/// # Examples
1881	///
1882	/// ```
1883	/// let x = `12`;
1884	/// let opt_x = Some(&x);
1885	/// assert_eq!(opt_x, Some(&`12`));
1886	/// let copied = opt_x.copied();
1887	/// assert_eq!(copied, Some(`12`));
1888	/// ```
1889	#[must_use = "`self` will be dropped if the result is not used"]
1890	#[stable(feature = "copied", since = "1.35.0")]
1891	#[rustc_const_stable(feature = "const_option", since = "1.83.0")]
1892	pub const fn copied(self) -> Option<T>
1893	where
1894	T: Copy,
1895	{
1896	// FIXME(const-hack): this implementation, which sidesteps using `Option::map` since it's not const
1897	// ready yet, should be reverted when possible to avoid code repetition
1898	match self {
1899	Some(&v) => Some(v),
1900	None => None,
1901	}
1902	}
1903
1904	/// Maps an `Option<&T>` to an `Option<T>` by cloning the contents of the
1905	/// option.
1906	///
1907	/// # Examples
1908	///
1909	/// ```
1910	/// let x = `12`;
1911	/// let opt_x = Some(&x);
1912	/// assert_eq!(opt_x, Some(&`12`));
1913	/// let cloned = opt_x.cloned();
1914	/// assert_eq!(cloned, Some(`12`));
1915	/// ```
1916	#[must_use = "`self` will be dropped if the result is not used"]
1917	#[stable(feature = "rust1", since = "1.0.0")]
1918	pub fn cloned(self) -> Option<T>
1919	where
1920	T: Clone,
1921	{
1922	match self {
1923	Some(t) => Some(t.clone()),
1924	None => None,
1925	}
1926	}
1927	}
1928
1929	impl<T> Option<&mut T> {
1930	/// Maps an `Option<&mut T>` to an `Option<T>` by copying the contents of the
1931	/// option.
1932	///
1933	/// # Examples
1934	///
1935	/// ```
1936	/// let mut x = `12`;
1937	/// let opt_x = Some(&mut x);
1938	/// assert_eq!(opt_x, Some(&mut `12`));
1939	/// let copied = opt_x.copied();
1940	/// assert_eq!(copied, Some(`12`));
1941	/// ```
1942	#[must_use = "`self` will be dropped if the result is not used"]
1943	#[stable(feature = "copied", since = "1.35.0")]
1944	#[rustc_const_stable(feature = "const_option", since = "1.83.0")]
1945	pub const fn copied(self) -> Option<T>
1946	where
1947	T: Copy,
1948	{
1949	match self {
1950	Some(&mut t) => Some(t),
1951	None => None,
1952	}
1953	}
1954
1955	/// Maps an `Option<&mut T>` to an `Option<T>` by cloning the contents of the
1956	/// option.
1957	///
1958	/// # Examples
1959	///
1960	/// ```
1961	/// let mut x = `12`;
1962	/// let opt_x = Some(&mut x);
1963	/// assert_eq!(opt_x, Some(&mut `12`));
1964	/// let cloned = opt_x.cloned();
1965	/// assert_eq!(cloned, Some(`12`));
1966	/// ```
1967	#[must_use = "`self` will be dropped if the result is not used"]
1968	#[stable(since = "1.26.0", feature = "option_ref_mut_cloned")]
1969	pub fn cloned(self) -> Option<T>
1970	where
1971	T: Clone,
1972	{
1973	match self {
1974	Some(t) => Some(t.clone()),
1975	None => None,
1976	}
1977	}
1978	}
1979
1980	impl<T, E> Option<Result<T, E>> {
1981	/// Transposes an `Option` of a [`Result`] into a [`Result`] of an `Option`.
1982	///
1983	/// [`None`] will be mapped to <code>[Ok]\([None])</code>.
1984	/// <code>[Some]\([Ok]\(\_))</code> and <code>[Some]\([Err]\(\_))</code> will be mapped to
1985	/// <code>[Ok]\([Some]\(\_))</code> and <code>[Err]\(\_)</code>.
1986	///
1987	/// # Examples
1988	///
1989	/// ```
1990	/// #[derive(Debug, Eq, PartialEq)]
1991	/// struct SomeErr;
1992	///
1993	/// let x: Result<Option<i32>, SomeErr> = Ok(Some(`5`));
1994	/// let y: Option<Result<i32, SomeErr>> = Some(Ok(`5`));
1995	/// assert_eq!(x, y.transpose());
1996	/// ```
1997	#[inline]
1998	#[stable(feature = "transpose_result", since = "1.33.0")]
1999	#[rustc_allow_const_fn_unstable(const_precise_live_drops)]
2000	#[rustc_const_stable(feature = "const_option", since = "1.83.0")]
2001	pub const fn transpose(self) -> Result<Option<T>, E> {
2002	match self {
2003	Some(Ok(x)) => Ok(Some(x)),
2004	Some(Err(e)) => Err(e),
2005	None => Ok(None),
2006	}
2007	}
2008	}
2009
2010	#[cfg_attr(not(feature = "panic_immediate_abort"), inline(never))]
2011	#[cfg_attr(feature = "panic_immediate_abort", inline)]
2012	#[cold]
2013	#[track_caller]
2014	const fn unwrap_failed() -> ! {
2015	panic(expr:"called `Option::unwrap()` on a `None` value")
2016	}
2017
2018	// This is a separate function to reduce the code size of .expect() itself.
2019	#[cfg_attr(not(feature = "panic_immediate_abort"), inline(never))]
2020	#[cfg_attr(feature = "panic_immediate_abort", inline)]
2021	#[cold]
2022	#[track_caller]
2023	const fn expect_failed(msg: &str) -> ! {
2024	panic_display(&msg)
2025	}
2026
2027	/////////////////////////////////////////////////////////////////////////////
2028	// Trait implementations
2029	/////////////////////////////////////////////////////////////////////////////
2030
2031	#[stable(feature = "rust1", since = "1.0.0")]
2032	impl<T> Clone for Option<T>
2033	where
2034	T: Clone,
2035	{
2036	#[inline]
2037	fn clone(&self) -> Self {
2038	match self {
2039	Some(x: &T) => Some(x.clone()),
2040	None => None,
2041	}
2042	}
2043
2044	#[inline]
2045	fn clone_from(&mut self, source: &Self) {
2046	match (self, source) {
2047	(Some(to: &mut T), Some(from: &T)) => to.clone_from(source:from),
2048	(to: &mut Option, from: &Option) => *to = from.clone(),
2049	}
2050	}
2051	}
2052
2053	#[unstable(feature = "ergonomic_clones", issue = "132290")]
2054	impl<T> crate::clone::UseCloned for Option<T> where T: crate::clone::UseCloned {}
2055
2056	#[stable(feature = "rust1", since = "1.0.0")]
2057	impl<T> Default for Option<T> {
2058	/// Returns [`None`][Option::None].
2059	///
2060	/// # Examples
2061	///
2062	/// ```
2063	/// let opt: Option<u32> = Option::default();
2064	/// assert!(opt.is_none());
2065	/// ```
2066	#[inline]
2067	fn default() -> Option<T> {
2068	None
2069	}
2070	}
2071
2072	#[stable(feature = "rust1", since = "1.0.0")]
2073	impl<T> IntoIterator for Option<T> {
2074	type Item = T;
2075	type IntoIter = IntoIter<T>;
2076
2077	/// Returns a consuming iterator over the possibly contained value.
2078	///
2079	/// # Examples
2080	///
2081	/// ```
2082	/// let x = Some("string");
2083	/// let v: Vec<&str> = x.into_iter().collect();
2084	/// assert_eq!(v, ["string"]);
2085	///
2086	/// let x = None;
2087	/// let v: Vec<&str> = x.into_iter().collect();
2088	/// assert!(v.is_empty());
2089	/// ```
2090	#[inline]
2091	fn into_iter(self) -> IntoIter<T> {
2092	IntoIter { inner: Item { opt: self } }
2093	}
2094	}
2095
2096	#[stable(since = "1.4.0", feature = "option_iter")]
2097	impl<'a, T> IntoIterator for &'a Option<T> {
2098	type Item = &'a T;
2099	type IntoIter = Iter<'a, T>;
2100
2101	fn into_iter(self) -> Iter<'a, T> {
2102	self.iter()
2103	}
2104	}
2105
2106	#[stable(since = "1.4.0", feature = "option_iter")]
2107	impl<'a, T> IntoIterator for &'a mut Option<T> {
2108	type Item = &'a mut T;
2109	type IntoIter = IterMut<'a, T>;
2110
2111	fn into_iter(self) -> IterMut<'a, T> {
2112	self.iter_mut()
2113	}
2114	}
2115
2116	#[stable(since = "1.12.0", feature = "option_from")]
2117	impl<T> From<T> for Option<T> {
2118	/// Moves `val` into a new [`Some`].
2119	///
2120	/// # Examples
2121	///
2122	/// ```
2123	/// let o: Option<u8> = Option::from(`67`);
2124	///
2125	/// assert_eq!(Some(`67`), o);
2126	/// ```
2127	fn from(val: T) -> Option<T> {
2128	Some(val)
2129	}
2130	}
2131
2132	#[stable(feature = "option_ref_from_ref_option", since = "1.30.0")]
2133	impl<'a, T> From<&'a Option<T>> for Option<&'a T> {
2134	/// Converts from `&Option<T>` to `Option<&T>`.
2135	///
2136	/// # Examples
2137	///
2138	/// Converts an <code>[Option]<[String]></code> into an <code>[Option]<[usize]></code>, preserving
2139	/// the original. The [`map`] method takes the `self` argument by value, consuming the original,
2140	/// so this technique uses `from` to first take an [`Option`] to a reference
2141	/// to the value inside the original.
2142	///
2143	/// [`map`]: Option::map
2144	/// [String]: ../../std/string/struct.String.html "String"
2145	///
2146	/// ```
2147	/// let s: Option<String> = Some(String::from("Hello, Rustaceans!"));
2148	/// let o: Option<usize> = Option::from(&s).map(\|ss: &String\| ss.len());
2149	///
2150	/// println!("Can still print s: {s:?}");
2151	///
2152	/// assert_eq!(o, Some(`18`));
2153	/// ```
2154	fn from(o: &'a Option<T>) -> Option<&'a T> {
2155	o.as_ref()
2156	}
2157	}
2158
2159	#[stable(feature = "option_ref_from_ref_option", since = "1.30.0")]
2160	impl<'a, T> From<&'a mut Option<T>> for Option<&'a mut T> {
2161	/// Converts from `&mut Option<T>` to `Option<&mut T>`
2162	///
2163	/// # Examples
2164	///
2165	/// ```
2166	/// let mut s = Some(String::from("Hello"));
2167	/// let o: Option<&mut String> = Option::from(&mut s);
2168	///
2169	/// match o {
2170	/// Some(t) => *t = String::from("Hello, Rustaceans!"),
2171	/// None => (),
2172	/// }
2173	///
2174	/// assert_eq!(s, Some(String::from("Hello, Rustaceans!")));
2175	/// ```
2176	fn from(o: &'a mut Option<T>) -> Option<&'a mut T> {
2177	o.as_mut()
2178	}
2179	}
2180
2181	// Ideally, LLVM should be able to optimize our derive code to this.
2182	// Once https://github.com/llvm/llvm-project/issues/52622 is fixed, we can
2183	// go back to deriving `PartialEq`.
2184	#[stable(feature = "rust1", since = "1.0.0")]
2185	impl<T> crate::marker::StructuralPartialEq for Option<T> {}
2186	#[stable(feature = "rust1", since = "1.0.0")]
2187	impl<T: PartialEq> PartialEq for Option<T> {
2188	#[inline]
2189	fn eq(&self, other: &Self) -> bool {
2190	// Spelling out the cases explicitly optimizes better than
2191	// `_ => false`
2192	match (self, other) {
2193	(Some(l: &T), Some(r: &T)) => l == r,
2194	(Some(_), None) => `false`,
2195	(None, Some(_)) => `false`,
2196	(None, None) => `true`,
2197	}
2198	}
2199	}
2200
2201	// Manually implementing here somewhat improves codegen for
2202	// https://github.com/rust-lang/rust/issues/49892, although still
2203	// not optimal.
2204	#[stable(feature = "rust1", since = "1.0.0")]
2205	impl<T: PartialOrd> PartialOrd for Option<T> {
2206	#[inline]
2207	fn partial_cmp(&self, other: &Self) -> Option<cmp::Ordering> {
2208	match (self, other) {
2209	(Some(l: &T), Some(r: &T)) => l.partial_cmp(r),
2210	(Some(_), None) => Some(cmp::Ordering::Greater),
2211	(None, Some(_)) => Some(cmp::Ordering::Less),
2212	(None, None) => Some(cmp::Ordering::Equal),
2213	}
2214	}
2215	}
2216
2217	#[stable(feature = "rust1", since = "1.0.0")]
2218	impl<T: Ord> Ord for Option<T> {
2219	#[inline]
2220	fn cmp(&self, other: &Self) -> cmp::Ordering {
2221	match (self, other) {
2222	(Some(l: &T), Some(r: &T)) => l.cmp(r),
2223	(Some(_), None) => cmp::Ordering::Greater,
2224	(None, Some(_)) => cmp::Ordering::Less,
2225	(None, None) => cmp::Ordering::Equal,
2226	}
2227	}
2228	}
2229
2230	/////////////////////////////////////////////////////////////////////////////
2231	// The Option Iterators
2232	/////////////////////////////////////////////////////////////////////////////
2233
2234	#[derive(Clone, Debug)]
2235	struct Item<A> {
2236	opt: Option<A>,
2237	}
2238
2239	impl<A> Iterator for Item<A> {
2240	type Item = A;
2241
2242	#[inline]
2243	fn next(&mut self) -> Option<A> {
2244	self.opt.take()
2245	}
2246
2247	#[inline]
2248	fn size_hint(&self) -> (usize, Option<usize>) {
2249	let len: usize = self.len();
2250	(len, Some(len))
2251	}
2252	}
2253
2254	impl<A> DoubleEndedIterator for Item<A> {
2255	#[inline]
2256	fn next_back(&mut self) -> Option<A> {
2257	self.opt.take()
2258	}
2259	}
2260
2261	impl<A> ExactSizeIterator for Item<A> {
2262	#[inline]
2263	fn len(&self) -> usize {
2264	self.opt.len()
2265	}
2266	}
2267	impl<A> FusedIterator for Item<A> {}
2268	unsafe impl<A> TrustedLen for Item<A> {}
2269
2270	/// An iterator over a reference to the [`Some`] variant of an [`Option`].
2271	///
2272	/// The iterator yields one value if the [`Option`] is a [`Some`], otherwise none.
2273	///
2274	/// This `struct` is created by the [`Option::iter`] function.
2275	#[stable(feature = "rust1", since = "1.0.0")]
2276	#[derive(Debug)]
2277	pub struct Iter<'a, A: 'a> {
2278	inner: Item<&'a A>,
2279	}
2280
2281	#[stable(feature = "rust1", since = "1.0.0")]
2282	impl<'a, A> Iterator for Iter<'a, A> {
2283	type Item = &'a A;
2284
2285	#[inline]
2286	fn next(&mut self) -> Option<&'a A> {
2287	self.inner.next()
2288	}
2289	#[inline]
2290	fn size_hint(&self) -> (usize, Option<usize>) {
2291	self.inner.size_hint()
2292	}
2293	}
2294
2295	#[stable(feature = "rust1", since = "1.0.0")]
2296	impl<'a, A> DoubleEndedIterator for Iter<'a, A> {
2297	#[inline]
2298	fn next_back(&mut self) -> Option<&'a A> {
2299	self.inner.next_back()
2300	}
2301	}
2302
2303	#[stable(feature = "rust1", since = "1.0.0")]
2304	impl<A> ExactSizeIterator for Iter<'_, A> {}
2305
2306	#[stable(feature = "fused", since = "1.26.0")]
2307	impl<A> FusedIterator for Iter<'_, A> {}
2308
2309	#[unstable(feature = "trusted_len", issue = "37572")]
2310	unsafe impl<A> TrustedLen for Iter<'_, A> {}
2311
2312	#[stable(feature = "rust1", since = "1.0.0")]
2313	impl<A> Clone for Iter<'_, A> {
2314	#[inline]
2315	fn clone(&self) -> Self {
2316	Iter { inner: self.inner.clone() }
2317	}
2318	}
2319
2320	/// An iterator over a mutable reference to the [`Some`] variant of an [`Option`].
2321	///
2322	/// The iterator yields one value if the [`Option`] is a [`Some`], otherwise none.
2323	///
2324	/// This `struct` is created by the [`Option::iter_mut`] function.
2325	#[stable(feature = "rust1", since = "1.0.0")]
2326	#[derive(Debug)]
2327	pub struct IterMut<'a, A: 'a> {
2328	inner: Item<&'a mut A>,
2329	}
2330
2331	#[stable(feature = "rust1", since = "1.0.0")]
2332	impl<'a, A> Iterator for IterMut<'a, A> {
2333	type Item = &'a mut A;
2334
2335	#[inline]
2336	fn next(&mut self) -> Option<&'a mut A> {
2337	self.inner.next()
2338	}
2339	#[inline]
2340	fn size_hint(&self) -> (usize, Option<usize>) {
2341	self.inner.size_hint()
2342	}
2343	}
2344
2345	#[stable(feature = "rust1", since = "1.0.0")]
2346	impl<'a, A> DoubleEndedIterator for IterMut<'a, A> {
2347	#[inline]
2348	fn next_back(&mut self) -> Option<&'a mut A> {
2349	self.inner.next_back()
2350	}
2351	}
2352
2353	#[stable(feature = "rust1", since = "1.0.0")]
2354	impl<A> ExactSizeIterator for IterMut<'_, A> {}
2355
2356	#[stable(feature = "fused", since = "1.26.0")]
2357	impl<A> FusedIterator for IterMut<'_, A> {}
2358	#[unstable(feature = "trusted_len", issue = "37572")]
2359	unsafe impl<A> TrustedLen for IterMut<'_, A> {}
2360
2361	/// An iterator over the value in [`Some`] variant of an [`Option`].
2362	///
2363	/// The iterator yields one value if the [`Option`] is a [`Some`], otherwise none.
2364	///
2365	/// This `struct` is created by the [`Option::into_iter`] function.
2366	#[derive(Clone, Debug)]
2367	#[stable(feature = "rust1", since = "1.0.0")]
2368	pub struct IntoIter<A> {
2369	inner: Item<A>,
2370	}
2371
2372	#[stable(feature = "rust1", since = "1.0.0")]
2373	impl<A> Iterator for IntoIter<A> {
2374	type Item = A;
2375
2376	#[inline]
2377	fn next(&mut self) -> Option<A> {
2378	self.inner.next()
2379	}
2380	#[inline]
2381	fn size_hint(&self) -> (usize, Option<usize>) {
2382	self.inner.size_hint()
2383	}
2384	}
2385
2386	#[stable(feature = "rust1", since = "1.0.0")]
2387	impl<A> DoubleEndedIterator for IntoIter<A> {
2388	#[inline]
2389	fn next_back(&mut self) -> Option<A> {
2390	self.inner.next_back()
2391	}
2392	}
2393
2394	#[stable(feature = "rust1", since = "1.0.0")]
2395	impl<A> ExactSizeIterator for IntoIter<A> {}
2396
2397	#[stable(feature = "fused", since = "1.26.0")]
2398	impl<A> FusedIterator for IntoIter<A> {}
2399
2400	#[unstable(feature = "trusted_len", issue = "37572")]
2401	unsafe impl<A> TrustedLen for IntoIter<A> {}
2402
2403	/////////////////////////////////////////////////////////////////////////////
2404	// FromIterator
2405	/////////////////////////////////////////////////////////////////////////////
2406
2407	#[stable(feature = "rust1", since = "1.0.0")]
2408	impl<A, V: FromIterator<A>> FromIterator<Option<A>> for Option<V> {
2409	/// Takes each element in the [`Iterator`]: if it is [`None`][Option::None],
2410	/// no further elements are taken, and the [`None`][Option::None] is
2411	/// returned. Should no [`None`][Option::None] occur, a container of type
2412	/// `V` containing the values of each [`Option`] is returned.
2413	///
2414	/// # Examples
2415	///
2416	/// Here is an example which increments every integer in a vector.
2417	/// We use the checked variant of `add` that returns `None` when the
2418	/// calculation would result in an overflow.
2419	///
2420	/// ```
2421	/// let items = vec![`0_u16`, `1`, `2`];
2422	///
2423	/// let res: Option<Vec<u16>> = items
2424	/// .iter()
2425	/// .map(\|x\| x.checked_add(`1`))
2426	/// .collect();
2427	///
2428	/// assert_eq!(res, Some(vec![`1`, `2`, `3`]));
2429	/// ```
2430	///
2431	/// As you can see, this will return the expected, valid items.
2432	///
2433	/// Here is another example that tries to subtract one from another list
2434	/// of integers, this time checking for underflow:
2435	///
2436	/// ```
2437	/// let items = vec![`2_u16`, `1`, `0`];
2438	///
2439	/// let res: Option<Vec<u16>> = items
2440	/// .iter()
2441	/// .map(\|x\| x.checked_sub(`1`))
2442	/// .collect();
2443	///
2444	/// assert_eq!(res, None);
2445	/// ```
2446	///
2447	/// Since the last element is zero, it would underflow. Thus, the resulting
2448	/// value is `None`.
2449	///
2450	/// Here is a variation on the previous example, showing that no
2451	/// further elements are taken from `iter` after the first `None`.
2452	///
2453	/// ```
2454	/// let items = vec![`3_u16`, `2`, `1`, `10`];
2455	///
2456	/// let mut shared = `0`;
2457	///
2458	/// let res: Option<Vec<u16>> = items
2459	/// .iter()
2460	/// .map(\|x\| { shared += x; x.checked_sub(`2`) })
2461	/// .collect();
2462	///
2463	/// assert_eq!(res, None);
2464	/// assert_eq!(shared, `6`);
2465	/// ```
2466	///
2467	/// Since the third element caused an underflow, no further elements were taken,
2468	/// so the final value of `shared` is 6 (= `3 + 2 + 1`), not 16.
2469	#[inline]
2470	fn from_iter<I: IntoIterator<Item = Option<A>>>(iter: I) -> Option<V> {
2471	// FIXME(#11084): This could be replaced with Iterator::scan when this
2472	// performance bug is closed.
2473
2474	iter::try_process(iter.into_iter(), \|i\| i.collect())
2475	}
2476	}
2477
2478	#[unstable(feature = "try_trait_v2", issue = "84277")]
2479	impl<T> ops::Try for Option<T> {
2480	type Output = T;
2481	type Residual = Option<convert::Infallible>;
2482
2483	#[inline]
2484	fn from_output(output: Self::Output) -> Self {
2485	Some(output)
2486	}
2487
2488	#[inline]
2489	fn branch(self) -> ControlFlow<Self::Residual, Self::Output> {
2490	match self {
2491	Some(v: T) => ControlFlow::Continue(v),
2492	None => ControlFlow::Break(None),
2493	}
2494	}
2495	}
2496
2497	#[unstable(feature = "try_trait_v2", issue = "84277")]
2498	// Note: manually specifying the residual type instead of using the default to work around
2499	// https://github.com/rust-lang/rust/issues/99940
2500	impl<T> ops::FromResidual<Option<convert::Infallible>> for Option<T> {
2501	#[inline]
2502	fn from_residual(residual: Option<convert::Infallible>) -> Self {
2503	match residual {
2504	None => None,
2505	}
2506	}
2507	}
2508
2509	#[diagnostic::do_not_recommend]
2510	#[unstable(feature = "try_trait_v2_yeet", issue = "96374")]
2511	impl<T> ops::FromResidual<ops::Yeet<()>> for Option<T> {
2512	#[inline]
2513	fn from_residual(ops::Yeet(()): ops::Yeet<()>) -> Self {
2514	None
2515	}
2516	}
2517
2518	#[unstable(feature = "try_trait_v2_residual", issue = "91285")]
2519	impl<T> ops::Residual<T> for Option<convert::Infallible> {
2520	type TryType = Option<T>;
2521	}
2522
2523	impl<T> Option<Option<T>> {
2524	/// Converts from `Option<Option<T>>` to `Option<T>`.
2525	///
2526	/// # Examples
2527	///
2528	/// Basic usage:
2529	///
2530	/// ```
2531	/// let x: Option<Option<u32>> = Some(Some(`6`));
2532	/// assert_eq!(Some(`6`), x.flatten());
2533	///
2534	/// let x: Option<Option<u32>> = Some(None);
2535	/// assert_eq!(None, x.flatten());
2536	///
2537	/// let x: Option<Option<u32>> = None;
2538	/// assert_eq!(None, x.flatten());
2539	/// ```
2540	///
2541	/// Flattening only removes one level of nesting at a time:
2542	///
2543	/// ```
2544	/// let x: Option<Option<Option<u32>>> = Some(Some(Some(`6`)));
2545	/// assert_eq!(Some(Some(`6`)), x.flatten());
2546	/// assert_eq!(Some(`6`), x.flatten().flatten());
2547	/// ```
2548	#[inline]
2549	#[stable(feature = "option_flattening", since = "1.40.0")]
2550	#[rustc_allow_const_fn_unstable(const_precise_live_drops)]
2551	#[rustc_const_stable(feature = "const_option", since = "1.83.0")]
2552	pub const fn flatten(self) -> Option<T> {
2553	// FIXME(const-hack): could be written with `and_then`
2554	match self {
2555	Some(inner) => inner,
2556	None => None,
2557	}
2558	}
2559	}
2560
2561	impl<T, const N: usize> [Option<T>; N] {
2562	/// Transposes a `[Option<T>; N]` into a `Option<[T; N]>`.
2563	///
2564	/// # Examples
2565	///
2566	/// ```
2567	/// #![feature(option_array_transpose)]
2568	/// # use std::option::Option;
2569	///
2570	/// let data = [Some(`0`); `1000`];
2571	/// let data: Option<[u8; `1000`]> = data.transpose();
2572	/// assert_eq!(data, Some([`0`; `1000`]));
2573	///
2574	/// let data = [Some(`0`), None];
2575	/// let data: Option<[u8; `2`]> = data.transpose();
2576	/// assert_eq!(data, None);
2577	/// ```
2578	#[inline]
2579	#[unstable(feature = "option_array_transpose", issue = "130828")]
2580	pub fn transpose(self) -> Option<[T; N]> {
2581	self.try_map(core::convert::identity)
2582	}
2583	}
2584