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 the following:
124	//! - `transmute::<_, Option<T>>([0u8; size_of::<T>()])` is sound and produces
125	//! `Option::<T>::None`
126	//! - `transmute::<_, [u8; size_of::<T>()]>(Option::<T>::None)` is sound and produces
127	//! `[0u8; size_of::<T>()]`
128	//! These cases are identified by the second column:
129	//!
130	//! \| `T` \| Transmuting between `[0u8; size_of::<T>()]` and `Option::<T>::None` sound? \|
131	//! \|---------------------------------------------------------------------\|----------------------------------------------------------------------------\|
132	//! \| [`Box<U>`] (specifically, only `Box<U, Global>`) \| when `U: Sized` \|
133	//! \| `&U` \| when `U: Sized` \|
134	//! \| `&mut U` \| when `U: Sized` \|
135	//! \| `fn`, `extern "C" fn`[^extern_fn] \| always \|
136	//! \| [`num::NonZero`] \| always \|*
137	//! \| [`ptr::NonNull<U>`] \| when `U: Sized` \|
138	//! \| `#[repr(transparent)]` struct around one of the types in this list. \| when it holds for the inner type \|
139	//!
140	//! [^extern_fn]: this remains true for `unsafe` variants, any argument/return types, and any other ABI: `[unsafe] extern "abi" fn` (_e.g._, `extern "system" fn`)
141	//!
142	//! Under some conditions the above types `T` are also null pointer optimized when wrapped in a [`Result`][result_repr].
143	//!
144	//! [`Box<U>`]: ../../std/boxed/struct.Box.html
145	//! [`num::NonZero`]: crate::num*
146	//! [`ptr::NonNull<U>`]: crate::ptr::NonNull
147	//! [function call ABI]: ../primitive.fn.html#abi-compatibility
148	//! [result_repr]: crate::result#representation
149	//!
150	//! This is called the "null pointer optimization" or NPO.
151	//!
152	//! It is further guaranteed that, for the cases above, one can
153	//! [`mem::transmute`] from all valid values of `T` to `Option<T>` and
154	//! from `Some::<T>(_)` to `T` (but transmuting `None::<T>` to `T`
155	//! is undefined behavior).
156	//!
157	//! # Method overview
158	//!
159	//! In addition to working with pattern matching, [`Option`] provides a wide
160	//! variety of different methods.
161	//!
162	//! ## Querying the variant
163	//!
164	//! The [`is_some`] and [`is_none`] methods return [`true`] if the [`Option`]
165	//! is [`Some`] or [`None`], respectively.
166	//!
167	//! The [`is_some_and`] and [`is_none_or`] methods apply the provided function
168	//! to the contents of the [`Option`] to produce a boolean value.
169	//! If this is [`None`] then a default result is returned instead without executing the function.
170	//!
171	//! [`is_none`]: Option::is_none
172	//! [`is_some`]: Option::is_some
173	//! [`is_some_and`]: Option::is_some_and
174	//! [`is_none_or`]: Option::is_none_or
175	//!
176	//! ## Adapters for working with references
177	//!
178	//! * [`as_ref`] converts from <code>[&][][Option]\<T></code> to <code>[Option]<[&]T></code>
179	//! * [`as_mut`] converts from <code>[&mut] [Option]\<T></code> to <code>[Option]<[&mut] T></code>
180	//! * [`as_deref`] converts from <code>[&][][Option]\<T></code> to
181	//! <code>[Option]<[&]T::[Target]></code>
182	//! * [`as_deref_mut`] converts from <code>[&mut] [Option]\<T></code> to
183	//! <code>[Option]<[&mut] T::[Target]></code>
184	//! * [`as_pin_ref`] converts from <code>[Pin]<[&][][Option]\<T>></code> to
185	//! <code>[Option]<[Pin]<[&]T>></code>
186	//! * [`as_pin_mut`] converts from <code>[Pin]<[&mut] [Option]\<T>></code> to
187	//! <code>[Option]<[Pin]<[&mut] T>></code>
188	//! * [`as_slice`] returns a one-element slice of the contained value, if any.
189	//! If this is [`None`], an empty slice is returned.
190	//! * [`as_mut_slice`] returns a mutable one-element slice of the contained value, if any.
191	//! If this is [`None`], an empty slice is returned.
192	//!
193	//! [&]: reference "shared reference"
194	//! [&mut]: reference "mutable reference"
195	//! [Target]: Deref::Target "ops::Deref::Target"
196	//! [`as_deref`]: Option::as_deref
197	//! [`as_deref_mut`]: Option::as_deref_mut
198	//! [`as_mut`]: Option::as_mut
199	//! [`as_pin_mut`]: Option::as_pin_mut
200	//! [`as_pin_ref`]: Option::as_pin_ref
201	//! [`as_ref`]: Option::as_ref
202	//! [`as_slice`]: Option::as_slice
203	//! [`as_mut_slice`]: Option::as_mut_slice
204	//!
205	//! ## Extracting the contained value
206	//!
207	//! These methods extract the contained value in an [`Option<T>`] when it
208	//! is the [`Some`] variant. If the [`Option`] is [`None`]:
209	//!
210	//! * [`expect`] panics with a provided custom message
211	//! * [`unwrap`] panics with a generic message
212	//! * [`unwrap_or`] returns the provided default value
213	//! * [`unwrap_or_default`] returns the default value of the type `T`
214	//! (which must implement the [`Default`] trait)
215	//! * [`unwrap_or_else`] returns the result of evaluating the provided
216	//! function
217	//! * [`unwrap_unchecked`] produces [undefined behavior]
218	//!
219	//! [`expect`]: Option::expect
220	//! [`unwrap`]: Option::unwrap
221	//! [`unwrap_or`]: Option::unwrap_or
222	//! [`unwrap_or_default`]: Option::unwrap_or_default
223	//! [`unwrap_or_else`]: Option::unwrap_or_else
224	//! [`unwrap_unchecked`]: Option::unwrap_unchecked
225	//! [undefined behavior]: https://doc.rust-lang.org/reference/behavior-considered-undefined.html
226	//!
227	//! ## Transforming contained values
228	//!
229	//! These methods transform [`Option`] to [`Result`]:
230	//!
231	//! * [`ok_or`] transforms [`Some(v)`] to [`Ok(v)`], and [`None`] to
232	//! [`Err(err)`] using the provided default `err` value
233	//! * [`ok_or_else`] transforms [`Some(v)`] to [`Ok(v)`], and [`None`] to
234	//! a value of [`Err`] using the provided function
235	//! * [`transpose`] transposes an [`Option`] of a [`Result`] into a
236	//! [`Result`] of an [`Option`]
237	//!
238	//! [`Err(err)`]: Err
239	//! [`Ok(v)`]: Ok
240	//! [`Some(v)`]: Some
241	//! [`ok_or`]: Option::ok_or
242	//! [`ok_or_else`]: Option::ok_or_else
243	//! [`transpose`]: Option::transpose
244	//!
245	//! These methods transform the [`Some`] variant:
246	//!
247	//! * [`filter`] calls the provided predicate function on the contained
248	//! value `t` if the [`Option`] is [`Some(t)`], and returns [`Some(t)`]
249	//! if the function returns `true`; otherwise, returns [`None`]
250	//! * [`flatten`] removes one level of nesting from an [`Option<Option<T>>`]
251	//! * [`inspect`] method takes ownership of the [`Option`] and applies
252	//! the provided function to the contained value by reference if [`Some`]
253	//! * [`map`] transforms [`Option<T>`] to [`Option<U>`] by applying the
254	//! provided function to the contained value of [`Some`] and leaving
255	//! [`None`] values unchanged
256	//!
257	//! [`Some(t)`]: Some
258	//! [`filter`]: Option::filter
259	//! [`flatten`]: Option::flatten
260	//! [`inspect`]: Option::inspect
261	//! [`map`]: Option::map
262	//!
263	//! These methods transform [`Option<T>`] to a value of a possibly
264	//! different type `U`:
265	//!
266	//! * [`map_or`] applies the provided function to the contained value of
267	//! [`Some`], or returns the provided default value if the [`Option`] is
268	//! [`None`]
269	//! * [`map_or_else`] applies the provided function to the contained value
270	//! of [`Some`], or returns the result of evaluating the provided
271	//! fallback function if the [`Option`] is [`None`]
272	//!
273	//! [`map_or`]: Option::map_or
274	//! [`map_or_else`]: Option::map_or_else
275	//!
276	//! These methods combine the [`Some`] variants of two [`Option`] values:
277	//!
278	//! * [`zip`] returns [`Some((s, o))`] if `self` is [`Some(s)`] and the
279	//! provided [`Option`] value is [`Some(o)`]; otherwise, returns [`None`]
280	//! * [`zip_with`] calls the provided function `f` and returns
281	//! [`Some(f(s, o))`] if `self` is [`Some(s)`] and the provided
282	//! [`Option`] value is [`Some(o)`]; otherwise, returns [`None`]
283	//!
284	//! [`Some(f(s, o))`]: Some
285	//! [`Some(o)`]: Some
286	//! [`Some(s)`]: Some
287	//! [`Some((s, o))`]: Some
288	//! [`zip`]: Option::zip
289	//! [`zip_with`]: Option::zip_with
290	//!
291	//! ## Boolean operators
292	//!
293	//! These methods treat the [`Option`] as a boolean value, where [`Some`]
294	//! acts like [`true`] and [`None`] acts like [`false`]. There are two
295	//! categories of these methods: ones that take an [`Option`] as input, and
296	//! ones that take a function as input (to be lazily evaluated).
297	//!
298	//! The [`and`], [`or`], and [`xor`] methods take another [`Option`] as
299	//! input, and produce an [`Option`] as output. Only the [`and`] method can
300	//! produce an [`Option<U>`] value having a different inner type `U` than
301	//! [`Option<T>`].
302	//!
303	//! \| method \| self \| input \| output \|
304	//! \|---------\|-----------\|-----------\|-----------\|
305	//! \| [`and`] \| `None` \| (ignored) \| `None` \|
306	//! \| [`and`] \| `Some(x)` \| `None` \| `None` \|
307	//! \| [`and`] \| `Some(x)` \| `Some(y)` \| `Some(y)` \|
308	//! \| [`or`] \| `None` \| `None` \| `None` \|
309	//! \| [`or`] \| `None` \| `Some(y)` \| `Some(y)` \|
310	//! \| [`or`] \| `Some(x)` \| (ignored) \| `Some(x)` \|
311	//! \| [`xor`] \| `None` \| `None` \| `None` \|
312	//! \| [`xor`] \| `None` \| `Some(y)` \| `Some(y)` \|
313	//! \| [`xor`] \| `Some(x)` \| `None` \| `Some(x)` \|
314	//! \| [`xor`] \| `Some(x)` \| `Some(y)` \| `None` \|
315	//!
316	//! [`and`]: Option::and
317	//! [`or`]: Option::or
318	//! [`xor`]: Option::xor
319	//!
320	//! The [`and_then`] and [`or_else`] methods take a function as input, and
321	//! only evaluate the function when they need to produce a new value. Only
322	//! the [`and_then`] method can produce an [`Option<U>`] value having a
323	//! different inner type `U` than [`Option<T>`].
324	//!
325	//! \| method \| self \| function input \| function result \| output \|
326	//! \|--------------\|-----------\|----------------\|-----------------\|-----------\|
327	//! \| [`and_then`] \| `None` \| (not provided) \| (not evaluated) \| `None` \|
328	//! \| [`and_then`] \| `Some(x)` \| `x` \| `None` \| `None` \|
329	//! \| [`and_then`] \| `Some(x)` \| `x` \| `Some(y)` \| `Some(y)` \|
330	//! \| [`or_else`] \| `None` \| (not provided) \| `None` \| `None` \|
331	//! \| [`or_else`] \| `None` \| (not provided) \| `Some(y)` \| `Some(y)` \|
332	//! \| [`or_else`] \| `Some(x)` \| (not provided) \| (not evaluated) \| `Some(x)` \|
333	//!
334	//! [`and_then`]: Option::and_then
335	//! [`or_else`]: Option::or_else
336	//!
337	//! This is an example of using methods like [`and_then`] and [`or`] in a
338	//! pipeline of method calls. Early stages of the pipeline pass failure
339	//! values ([`None`]) through unchanged, and continue processing on
340	//! success values ([`Some`]). Toward the end, [`or`] substitutes an error
341	//! message if it receives [`None`].
342	//!
343	//! ```
344	//! # use std::collections::BTreeMap;
345	//! let mut bt = BTreeMap::new();
346	//! bt.insert(`20u8`, "foo");
347	//! bt.insert(`42u8`, "bar");
348	//! let res = [`0u8`, `1`, `11`, `200`, `22`]
349	//! .into_iter()
350	//! .map(\|x\| {
351	//! // `checked_sub()` returns `None` on error
352	//! x.checked_sub(`1`)
353	//! // same with `checked_mul()`
354	//! .and_then(\|x\| x.checked_mul(`2`))
355	//! // `BTreeMap::get` returns `None` on error
356	//! .and_then(\|x\| bt.get(&x))
357	//! // Substitute an error message if we have `None` so far
358	//! .or(Some(&"error!"))
359	//! .copied()
360	//! // Won't panic because we unconditionally used `Some` above
361	//! .unwrap()
362	//! })
363	//! .collect::<Vec<_>>();
364	//! assert_eq!(res, ["error!", "error!", "foo", "error!", "bar"]);
365	//! ```
366	//!
367	//! ## Comparison operators
368	//!
369	//! If `T` implements [`PartialOrd`] then [`Option<T>`] will derive its
370	//! [`PartialOrd`] implementation. With this order, [`None`] compares as
371	//! less than any [`Some`], and two [`Some`] compare the same way as their
372	//! contained values would in `T`. If `T` also implements
373	//! [`Ord`], then so does [`Option<T>`].
374	//!
375	//! ```
376	//! assert!(None < Some(`0`));
377	//! assert!(Some(`0`) < Some(`1`));
378	//! ```
379	//!
380	//! ## Iterating over `Option`
381	//!
382	//! An [`Option`] can be iterated over. This can be helpful if you need an
383	//! iterator that is conditionally empty. The iterator will either produce
384	//! a single value (when the [`Option`] is [`Some`]), or produce no values
385	//! (when the [`Option`] is [`None`]). For example, [`into_iter`] acts like
386	//! [`once(v)`] if the [`Option`] is [`Some(v)`], and like [`empty()`] if
387	//! the [`Option`] is [`None`].
388	//!
389	//! [`Some(v)`]: Some
390	//! [`empty()`]: crate::iter::empty
391	//! [`once(v)`]: crate::iter::once
392	//!
393	//! Iterators over [`Option<T>`] come in three types:
394	//!
395	//! * [`into_iter`] consumes the [`Option`] and produces the contained
396	//! value
397	//! * [`iter`] produces an immutable reference of type `&T` to the
398	//! contained value
399	//! * [`iter_mut`] produces a mutable reference of type `&mut T` to the
400	//! contained value
401	//!
402	//! [`into_iter`]: Option::into_iter
403	//! [`iter`]: Option::iter
404	//! [`iter_mut`]: Option::iter_mut
405	//!
406	//! An iterator over [`Option`] can be useful when chaining iterators, for
407	//! example, to conditionally insert items. (It's not always necessary to
408	//! explicitly call an iterator constructor: many [`Iterator`] methods that
409	//! accept other iterators will also accept iterable types that implement
410	//! [`IntoIterator`], which includes [`Option`].)
411	//!
412	//! ```
413	//! let yep = Some(`42`);
414	//! let nope = None;
415	//! // chain() already calls into_iter(), so we don't have to do so
416	//! let nums: Vec<i32> = (`0`..`4`).chain(yep).chain(`4`..`8`).collect();
417	//! assert_eq!(nums, [`0`, `1`, `2`, `3`, `42`, `4`, `5`, `6`, `7`]);
418	//! let nums: Vec<i32> = (`0`..`4`).chain(nope).chain(`4`..`8`).collect();
419	//! assert_eq!(nums, [`0`, `1`, `2`, `3`, `4`, `5`, `6`, `7`]);
420	//! ```
421	//!
422	//! One reason to chain iterators in this way is that a function returning
423	//! `impl Iterator` must have all possible return values be of the same
424	//! concrete type. Chaining an iterated [`Option`] can help with that.
425	//!
426	//! ```
427	//! fn make_iter(do_insert: bool) -> impl Iterator<Item = i32> {
428	//! // Explicit returns to illustrate return types matching
429	//! match do_insert {
430	//! `true` => return (`0`..`4`).chain(Some(`42`)).chain(`4`..`8`),
431	//! `false` => return (`0`..`4`).chain(None).chain(`4`..`8`),
432	//! }
433	//! }
434	//! println!("{:?}", make_iter(`true`).collect::<Vec<_>>());
435	//! println!("{:?}", make_iter(`false`).collect::<Vec<_>>());
436	//! ```
437	//!
438	//! If we try to do the same thing, but using [`once()`] and [`empty()`],
439	//! we can't return `impl Iterator` anymore because the concrete types of
440	//! the return values differ.
441	//!
442	//! [`empty()`]: crate::iter::empty
443	//! [`once()`]: crate::iter::once
444	//!
445	//! ```compile_fail,E0308
446	//! # use std::iter::{empty, once};
447	//! // This won't compile because all possible returns from the function
448	//! // must have the same concrete type.
449	//! fn make_iter(do_insert: bool) -> impl Iterator<Item = i32> {
450	//! // Explicit returns to illustrate return types not matching
451	//! match do_insert {
452	//! `true` => return (`0`..`4`).chain(once(`42`)).chain(`4`..`8`),
453	//! `false` => return (`0`..`4`).chain(empty()).chain(`4`..`8`),
454	//! }
455	//! }
456	//! ```
457	//!
458	//! ## Collecting into `Option`
459	//!
460	//! [`Option`] implements the [`FromIterator`][impl-FromIterator] trait,
461	//! which allows an iterator over [`Option`] values to be collected into an
462	//! [`Option`] of a collection of each contained value of the original
463	//! [`Option`] values, or [`None`] if any of the elements was [`None`].
464	//!
465	//! [impl-FromIterator]: Option#impl-FromIterator%3COption%3CA%3E%3E-for-Option%3CV%3E
466	//!
467	//! ```
468	//! let v = [Some(`2`), Some(`4`), None, Some(`8`)];
469	//! let res: Option<Vec<_>> = v.into_iter().collect();
470	//! assert_eq!(res, None);
471	//! let v = [Some(`2`), Some(`4`), Some(`8`)];
472	//! let res: Option<Vec<_>> = v.into_iter().collect();
473	//! assert_eq!(res, Some(vec![`2`, `4`, `8`]));
474	//! ```
475	//!
476	//! [`Option`] also implements the [`Product`][impl-Product] and
477	//! [`Sum`][impl-Sum] traits, allowing an iterator over [`Option`] values
478	//! to provide the [`product`][Iterator::product] and
479	//! [`sum`][Iterator::sum] methods.
480	//!
481	//! [impl-Product]: Option#impl-Product%3COption%3CU%3E%3E-for-Option%3CT%3E
482	//! [impl-Sum]: Option#impl-Sum%3COption%3CU%3E%3E-for-Option%3CT%3E
483	//!
484	//! ```
485	//! let v = [None, Some(`1`), Some(`2`), Some(`3`)];
486	//! let res: Option<i32> = v.into_iter().sum();
487	//! assert_eq!(res, None);
488	//! let v = [Some(`1`), Some(`2`), Some(`21`)];
489	//! let res: Option<i32> = v.into_iter().product();
490	//! assert_eq!(res, Some(`42`));
491	//! ```
492	//!
493	//! ## Modifying an [`Option`] in-place
494	//!
495	//! These methods return a mutable reference to the contained value of an
496	//! [`Option<T>`]:
497	//!
498	//! * [`insert`] inserts a value, dropping any old contents
499	//! * [`get_or_insert`] gets the current value, inserting a provided
500	//! default value if it is [`None`]
501	//! * [`get_or_insert_default`] gets the current value, inserting the
502	//! default value of type `T` (which must implement [`Default`]) if it is
503	//! [`None`]
504	//! * [`get_or_insert_with`] gets the current value, inserting a default
505	//! computed by the provided function if it is [`None`]
506	//!
507	//! [`get_or_insert`]: Option::get_or_insert
508	//! [`get_or_insert_default`]: Option::get_or_insert_default
509	//! [`get_or_insert_with`]: Option::get_or_insert_with
510	//! [`insert`]: Option::insert
511	//!
512	//! These methods transfer ownership of the contained value of an
513	//! [`Option`]:
514	//!
515	//! * [`take`] takes ownership of the contained value of an [`Option`], if
516	//! any, replacing the [`Option`] with [`None`]
517	//! * [`replace`] takes ownership of the contained value of an [`Option`],
518	//! if any, replacing the [`Option`] with a [`Some`] containing the
519	//! provided value
520	//!
521	//! [`replace`]: Option::replace
522	//! [`take`]: Option::take
523	//!
524	//! # Examples
525	//!
526	//! Basic pattern matching on [`Option`]:
527	//!
528	//! ```
529	//! let msg = Some("howdy");
530	//!
531	//! // Take a reference to the contained string
532	//! if let Some(m) = &msg {
533	//! println!("{}", *m);
534	//! }
535	//!
536	//! // Remove the contained string, destroying the Option
537	//! let unwrapped_msg = msg.unwrap_or("default message");
538	//! ```
539	//!
540	//! Initialize a result to [`None`] before a loop:
541	//!
542	//! ```
543	//! enum Kingdom { Plant(u32, &'static str), Animal(u32, &'static str) }
544	//!
545	//! // A list of data to search through.
546	//! let all_the_big_things = [
547	//! Kingdom::Plant(`250`, "redwood"),
548	//! Kingdom::Plant(`230`, "noble fir"),
549	//! Kingdom::Plant(`229`, "sugar pine"),
550	//! Kingdom::Animal(`25`, "blue whale"),
551	//! Kingdom::Animal(`19`, "fin whale"),
552	//! Kingdom::Animal(`15`, "north pacific right whale"),
553	//! ];
554	//!
555	//! // We're going to search for the name of the biggest animal,
556	//! // but to start with we've just got `None`.
557	//! let mut name_of_biggest_animal = None;
558	//! let mut size_of_biggest_animal = `0`;
559	//! for big_thing in &all_the_big_things {
560	//! match *big_thing {
561	//! Kingdom::Animal(size, name) if size > size_of_biggest_animal => {
562	//! // Now we've found the name of some big animal
563	//! size_of_biggest_animal = size;
564	//! name_of_biggest_animal = Some(name);
565	//! }
566	//! Kingdom::Animal(..) \| Kingdom::Plant(..) => ()
567	//! }
568	//! }
569	//!
570	//! match name_of_biggest_animal {
571	//! Some(name) => println!("the biggest animal is {name}"),
572	//! None => println!("there are no animals :("),
573	//! }
574	//! ```
575
576	#![stable(feature = "rust1", since = "1.0.0")]
577
578	use crate::iter::{self, FusedIterator, TrustedLen};
579	use crate::ops::{self, ControlFlow, Deref, DerefMut};
580	use crate::panicking::{panic, panic_display};
581	use crate::pin::Pin;
582	use crate::{cmp, convert, hint, mem, slice};
583
584	/// The `Option` type. See [the module level documentation](self) for more.
585	#[doc(search_unbox)]
586	#[derive(Copy, Eq, Debug, Hash)]
587	#[rustc_diagnostic_item = "Option"]
588	#[lang = "Option"]
589	#[stable(feature = "rust1", since = "1.0.0")]
590	#[allow(clippy::derived_hash_with_manual_eq)] // PartialEq is manually implemented equivalently
591	pub enum Option<T> {
592	/// No value.
593	#[lang = "None"]
594	#[stable(feature = "rust1", since = "1.0.0")]
595	None,
596	/// Some value of type `T`.
597	#[lang = "Some"]
598	#[stable(feature = "rust1", since = "1.0.0")]
599	Some(#[stable(feature = "rust1", since = "1.0.0")] T),
600	}
601
602	/////////////////////////////////////////////////////////////////////////////
603	// Type implementation
604	/////////////////////////////////////////////////////////////////////////////
605
606	impl<T> Option<T> {
607	/////////////////////////////////////////////////////////////////////////
608	// Querying the contained values
609	/////////////////////////////////////////////////////////////////////////
610
611	/// Returns `true` if the option is a [`Some`] value.
612	///
613	/// # Examples
614	///
615	/// ```
616	/// let x: Option<u32> = Some(`2`);
617	/// assert_eq!(x.is_some(), `true`);
618	///
619	/// let x: Option<u32> = None;
620	/// assert_eq!(x.is_some(), `false`);
621	/// ```
622	#[must_use = "if you intended to assert that this has a value, consider `.unwrap()` instead"]
623	#[inline]
624	#[stable(feature = "rust1", since = "1.0.0")]
625	#[rustc_const_stable(feature = "const_option_basics", since = "1.48.0")]
626	pub const fn is_some(&self) -> bool {
627	matches!(*self, Some(_))
628	}
629
630	/// Returns `true` if the option is a [`Some`] and the value inside of it matches a predicate.
631	///
632	/// # Examples
633	///
634	/// ```
635	/// let x: Option<u32> = Some(`2`);
636	/// assert_eq!(x.is_some_and(\|x\| x > `1`), `true`);
637	///
638	/// let x: Option<u32> = Some(`0`);
639	/// assert_eq!(x.is_some_and(\|x\| x > `1`), `false`);
640	///
641	/// let x: Option<u32> = None;
642	/// assert_eq!(x.is_some_and(\|x\| x > `1`), `false`);
643	///
644	/// let x: Option<String> = Some("ownership".to_string());
645	/// assert_eq!(x.as_ref().is_some_and(\|x\| x.len() > `1`), `true`);
646	/// println!("still alive {:?}", x);
647	/// ```
648	#[must_use]
649	#[inline]
650	#[stable(feature = "is_some_and", since = "1.70.0")]
651	pub fn is_some_and(self, f: impl FnOnce(T) -> bool) -> bool {
652	match self {
653	None => `false`,
654	Some(x) => f(x),
655	}
656	}
657
658	/// Returns `true` if the option is a [`None`] value.
659	///
660	/// # Examples
661	///
662	/// ```
663	/// let x: Option<u32> = Some(`2`);
664	/// assert_eq!(x.is_none(), `false`);
665	///
666	/// let x: Option<u32> = None;
667	/// assert_eq!(x.is_none(), `true`);
668	/// ```
669	#[must_use = "if you intended to assert that this doesn't have a value, consider \
670	wrapping this in an `assert!()` instead"]
671	#[inline]
672	#[stable(feature = "rust1", since = "1.0.0")]
673	#[rustc_const_stable(feature = "const_option_basics", since = "1.48.0")]
674	pub const fn is_none(&self) -> bool {
675	!self.is_some()
676	}
677
678	/// Returns `true` if the option is a [`None`] or the value inside of it matches a predicate.
679	///
680	/// # Examples
681	///
682	/// ```
683	/// let x: Option<u32> = Some(`2`);
684	/// assert_eq!(x.is_none_or(\|x\| x > `1`), `true`);
685	///
686	/// let x: Option<u32> = Some(`0`);
687	/// assert_eq!(x.is_none_or(\|x\| x > `1`), `false`);
688	///
689	/// let x: Option<u32> = None;
690	/// assert_eq!(x.is_none_or(\|x\| x > `1`), `true`);
691	///
692	/// let x: Option<String> = Some("ownership".to_string());
693	/// assert_eq!(x.as_ref().is_none_or(\|x\| x.len() > `1`), `true`);
694	/// println!("still alive {:?}", x);
695	/// ```
696	#[must_use]
697	#[inline]
698	#[stable(feature = "is_none_or", since = "1.82.0")]
699	pub fn is_none_or(self, f: impl FnOnce(T) -> bool) -> bool {
700	match self {
701	None => `true`,
702	Some(x) => f(x),
703	}
704	}
705
706	/////////////////////////////////////////////////////////////////////////
707	// Adapter for working with references
708	/////////////////////////////////////////////////////////////////////////
709
710	/// Converts from `&Option<T>` to `Option<&T>`.
711	///
712	/// # Examples
713	///
714	/// Calculates the length of an <code>Option<[String]></code> as an <code>Option<[usize]></code>
715	/// without moving the [`String`]. The [`map`] method takes the `self` argument by value,
716	/// consuming the original, so this technique uses `as_ref` to first take an `Option` to a
717	/// reference to the value inside the original.
718	///
719	/// [`map`]: Option::map
720	/// [String]: ../../std/string/struct.String.html "String"
721	/// [`String`]: ../../std/string/struct.String.html "String"
722	///
723	/// ```
724	/// let text: Option<String> = Some("Hello, world!".to_string());
725	/// // First, cast `Option<String>` to `Option<&String>` with `as_ref`,
726	/// // then consume that* with `map`, leaving `text` on the stack.*
727	/// let text_length: Option<usize> = text.as_ref().map(\|s\| s.len());
728	/// println!("still can print text: {text:?}");
729	/// ```
730	#[inline]
731	#[rustc_const_stable(feature = "const_option_basics", since = "1.48.0")]
732	#[stable(feature = "rust1", since = "1.0.0")]
733	pub const fn as_ref(&self) -> Option<&T> {
734	match *self {
735	Some(ref x) => Some(x),
736	None => None,
737	}
738	}
739
740	/// Converts from `&mut Option<T>` to `Option<&mut T>`.
741	///
742	/// # Examples
743	///
744	/// ```
745	/// let mut x = Some(`2`);
746	/// match x.as_mut() {
747	/// Some(v) => *v = `42`,
748	/// None => {},
749	/// }
750	/// assert_eq!(x, Some(`42`));
751	/// ```
752	#[inline]
753	#[stable(feature = "rust1", since = "1.0.0")]
754	#[rustc_const_stable(feature = "const_option", since = "1.83.0")]
755	pub const fn as_mut(&mut self) -> Option<&mut T> {
756	match *self {
757	Some(ref mut x) => Some(x),
758	None => None,
759	}
760	}
761
762	/// Converts from <code>[Pin]<[&]Option\<T>></code> to <code>Option<[Pin]<[&]T>></code>.
763	///
764	/// [&]: reference "shared reference"
765	#[inline]
766	#[must_use]
767	#[stable(feature = "pin", since = "1.33.0")]
768	#[rustc_const_stable(feature = "const_option_ext", since = "1.84.0")]
769	pub const fn as_pin_ref(self: Pin<&Self>) -> Option<Pin<&T>> {
770	// FIXME(const-hack): use `map` once that is possible
771	match Pin::get_ref(self).as_ref() {
772	// SAFETY: `x` is guaranteed to be pinned because it comes from `self`
773	// which is pinned.
774	Some(x) => unsafe { Some(Pin::new_unchecked(x)) },
775	None => None,
776	}
777	}
778
779	/// Converts from <code>[Pin]<[&mut] Option\<T>></code> to <code>Option<[Pin]<[&mut] T>></code>.
780	///
781	/// [&mut]: reference "mutable reference"
782	#[inline]
783	#[must_use]
784	#[stable(feature = "pin", since = "1.33.0")]
785	#[rustc_const_stable(feature = "const_option_ext", since = "1.84.0")]
786	pub const fn as_pin_mut(self: Pin<&mut Self>) -> Option<Pin<&mut T>> {
787	// SAFETY: `get_unchecked_mut` is never used to move the `Option` inside `self`.
788	// `x` is guaranteed to be pinned because it comes from `self` which is pinned.
789	unsafe {
790	// FIXME(const-hack): use `map` once that is possible
791	match Pin::get_unchecked_mut(self).as_mut() {
792	Some(x) => Some(Pin::new_unchecked(x)),
793	None => None,
794	}
795	}
796	}
797
798	#[inline]
799	const fn len(&self) -> usize {
800	// Using the intrinsic avoids emitting a branch to get the 0 or 1.
801	let discriminant: isize = crate::intrinsics::discriminant_value(self);
802	discriminant as usize
803	}
804
805	/// Returns a slice of the contained value, if any. If this is `None`, an
806	/// empty slice is returned. This can be useful to have a single type of
807	/// iterator over an `Option` or slice.
808	///
809	/// Note: Should you have an `Option<&T>` and wish to get a slice of `T`,
810	/// you can unpack it via `opt.map_or(&[], std::slice::from_ref)`.
811	///
812	/// # Examples
813	///
814	/// ```rust
815	/// assert_eq!(
816	/// [Some(`1234`).as_slice(), None.as_slice()],
817	/// [&[`1234`][..], &[][..]],
818	/// );
819	/// ```
820	///
821	/// The inverse of this function is (discounting
822	/// borrowing) [`[_]::first`](slice::first):
823	///
824	/// ```rust
825	/// for i in [Some(`1234_u16`), None] {
826	/// assert_eq!(i.as_ref(), i.as_slice().first());
827	/// }
828	/// ```
829	#[inline]
830	#[must_use]
831	#[stable(feature = "option_as_slice", since = "1.75.0")]
832	#[rustc_const_stable(feature = "const_option_ext", since = "1.84.0")]
833	pub const fn as_slice(&self) -> &[T] {
834	// SAFETY: When the `Option` is `Some`, we're using the actual pointer
835	// to the payload, with a length of 1, so this is equivalent to
836	// `slice::from_ref`, and thus is safe.
837	// When the `Option` is `None`, the length used is 0, so to be safe it
838	// just needs to be aligned, which it is because `&self` is aligned and
839	// the offset used is a multiple of alignment.
840	//
841	// In the new version, the intrinsic always returns a pointer to an
842	// in-bounds and correctly aligned position for a `T` (even if in the
843	// `None` case it's just padding).
844	unsafe {
845	slice::from_raw_parts(
846	(self as *const Self).byte_add(core::mem::offset_of!(Self, Some.0)).cast(),
847	self.len(),
848	)
849	}
850	}
851
852	/// Returns a mutable slice of the contained value, if any. If this is
853	/// `None`, an empty slice is returned. This can be useful to have a
854	/// single type of iterator over an `Option` or slice.
855	///
856	/// Note: Should you have an `Option<&mut T>` instead of a
857	/// `&mut Option<T>`, which this method takes, you can obtain a mutable
858	/// slice via `opt.map_or(&mut [], std::slice::from_mut)`.
859	///
860	/// # Examples
861	///
862	/// ```rust
863	/// assert_eq!(
864	/// [Some(`1234`).as_mut_slice(), None.as_mut_slice()],
865	/// [&mut [`1234`][..], &mut [][..]],
866	/// );
867	/// ```
868	///
869	/// The result is a mutable slice of zero or one items that points into
870	/// our original `Option`:
871	///
872	/// ```rust
873	/// let mut x = Some(`1234`);
874	/// x.as_mut_slice()[`0`] += `1`;
875	/// assert_eq!(x, Some(`1235`));
876	/// ```
877	///
878	/// The inverse of this method (discounting borrowing)
879	/// is [`[_]::first_mut`](slice::first_mut):
880	///
881	/// ```rust
882	/// assert_eq!(Some(`123`).as_mut_slice().first_mut(), Some(&mut `123`))
883	/// ```
884	#[inline]
885	#[must_use]
886	#[stable(feature = "option_as_slice", since = "1.75.0")]
887	#[rustc_const_stable(feature = "const_option_ext", since = "1.84.0")]
888	pub const fn as_mut_slice(&mut self) -> &mut [T] {
889	// SAFETY: When the `Option` is `Some`, we're using the actual pointer
890	// to the payload, with a length of 1, so this is equivalent to
891	// `slice::from_mut`, and thus is safe.
892	// When the `Option` is `None`, the length used is 0, so to be safe it
893	// just needs to be aligned, which it is because `&self` is aligned and
894	// the offset used is a multiple of alignment.
895	//
896	// In the new version, the intrinsic creates a `const T` from a*
897	// mutable reference so it is safe to cast back to a mutable pointer
898	// here. As with `as_slice`, the intrinsic always returns a pointer to
899	// an in-bounds and correctly aligned position for a `T` (even if in
900	// the `None` case it's just padding).
901	unsafe {
902	slice::from_raw_parts_mut(
903	(self as *mut Self).byte_add(core::mem::offset_of!(Self, Some.0)).cast(),
904	self.len(),
905	)
906	}
907	}
908
909	/////////////////////////////////////////////////////////////////////////
910	// Getting to contained values
911	/////////////////////////////////////////////////////////////////////////
912
913	/// Returns the contained [`Some`] value, consuming the `self` value.
914	///
915	/// # Panics
916	///
917	/// Panics if the value is a [`None`] with a custom panic message provided by
918	/// `msg`.
919	///
920	/// # Examples
921	///
922	/// ```
923	/// let x = Some("value");
924	/// assert_eq!(x.expect("fruits are healthy"), "value");
925	/// ```
926	///
927	/// ```should_panic
928	/// let x: Option<&str> = None;
929	/// x.expect("fruits are healthy"); // panics with `fruits are healthy`
930	/// ```
931	///
932	/// # Recommended Message Style
933	///
934	/// We recommend that `expect` messages are used to describe the reason you
935	/// _expect_ the `Option` should be `Some`.
936	///
937	/// ```should_panic
938	/// # let slice: &[u8] = &[];
939	/// let item = slice.get(`0`)
940	/// .expect("slice should not be empty");
941	/// ```
942	///
943	/// Hint: If you're having trouble remembering how to phrase expect
944	/// error messages remember to focus on the word "should" as in "env
945	/// variable should be set by blah" or "the given binary should be available
946	/// and executable by the current user".
947	///
948	/// For more detail on expect message styles and the reasoning behind our
949	/// recommendation please refer to the section on ["Common Message
950	/// Styles"](../../std/error/index.html#common-message-styles) in the [`std::error`](../../std/error/index.html) module docs.
951	#[inline]
952	#[track_caller]
953	#[stable(feature = "rust1", since = "1.0.0")]
954	#[rustc_diagnostic_item = "option_expect"]
955	#[rustc_allow_const_fn_unstable(const_precise_live_drops)]
956	#[rustc_const_stable(feature = "const_option", since = "1.83.0")]
957	pub const fn expect(self, msg: &str) -> T {
958	match self {
959	Some(val) => val,
960	None => expect_failed(msg),
961	}
962	}
963
964	/// Returns the contained [`Some`] value, consuming the `self` value.
965	///
966	/// Because this function may panic, its use is generally discouraged.
967	/// Panics are meant for unrecoverable errors, and
968	/// [may abort the entire program][panic-abort].
969	///
970	/// Instead, prefer to use pattern matching and handle the [`None`]
971	/// case explicitly, or call [`unwrap_or`], [`unwrap_or_else`], or
972	/// [`unwrap_or_default`]. In functions returning `Option`, you can use
973	/// [the `?` (try) operator][try-option].
974	///
975	/// [panic-abort]: https://doc.rust-lang.org/book/ch09-01-unrecoverable-errors-with-panic.html
976	/// [try-option]: https://doc.rust-lang.org/book/ch09-02-recoverable-errors-with-result.html#where-the--operator-can-be-used
977	/// [`unwrap_or`]: Option::unwrap_or
978	/// [`unwrap_or_else`]: Option::unwrap_or_else
979	/// [`unwrap_or_default`]: Option::unwrap_or_default
980	///
981	/// # Panics
982	///
983	/// Panics if the self value equals [`None`].
984	///
985	/// # Examples
986	///
987	/// ```
988	/// let x = Some("air");
989	/// assert_eq!(x.unwrap(), "air");
990	/// ```
991	///
992	/// ```should_panic
993	/// let x: Option<&str> = None;
994	/// assert_eq!(x.unwrap(), "air"); // fails
995	/// ```
996	#[inline(always)]
997	#[track_caller]
998	#[stable(feature = "rust1", since = "1.0.0")]
999	#[rustc_diagnostic_item = "option_unwrap"]
1000	#[rustc_allow_const_fn_unstable(const_precise_live_drops)]
1001	#[rustc_const_stable(feature = "const_option", since = "1.83.0")]
1002	pub const fn unwrap(self) -> T {
1003	match self {
1004	Some(val) => val,
1005	None => unwrap_failed(),
1006	}
1007	}
1008
1009	/// Returns the contained [`Some`] value or a provided default.
1010	///
1011	/// Arguments passed to `unwrap_or` are eagerly evaluated; if you are passing
1012	/// the result of a function call, it is recommended to use [`unwrap_or_else`],
1013	/// which is lazily evaluated.
1014	///
1015	/// [`unwrap_or_else`]: Option::unwrap_or_else
1016	///
1017	/// # Examples
1018	///
1019	/// ```
1020	/// assert_eq!(Some("car").unwrap_or("bike"), "car");
1021	/// assert_eq!(None.unwrap_or("bike"), "bike");
1022	/// ```
1023	#[inline]
1024	#[stable(feature = "rust1", since = "1.0.0")]
1025	pub fn unwrap_or(self, default: T) -> T {
1026	match self {
1027	Some(x) => x,
1028	None => default,
1029	}
1030	}
1031
1032	/// Returns the contained [`Some`] value or computes it from a closure.
1033	///
1034	/// # Examples
1035	///
1036	/// ```
1037	/// let k = `10`;
1038	/// assert_eq!(Some(`4`).unwrap_or_else(\|\| `2` * k), `4`);
1039	/// assert_eq!(None.unwrap_or_else(\|\| `2` * k), `20`);
1040	/// ```
1041	#[inline]
1042	#[track_caller]
1043	#[stable(feature = "rust1", since = "1.0.0")]
1044	pub fn unwrap_or_else<F>(self, f: F) -> T
1045	where
1046	F: FnOnce() -> T,
1047	{
1048	match self {
1049	Some(x) => x,
1050	None => f(),
1051	}
1052	}
1053
1054	/// Returns the contained [`Some`] value or a default.
1055	///
1056	/// Consumes the `self` argument then, if [`Some`], returns the contained
1057	/// value, otherwise if [`None`], returns the [default value] for that
1058	/// type.
1059	///
1060	/// # Examples
1061	///
1062	/// ```
1063	/// let x: Option<u32> = None;
1064	/// let y: Option<u32> = Some(`12`);
1065	///
1066	/// assert_eq!(x.unwrap_or_default(), `0`);
1067	/// assert_eq!(y.unwrap_or_default(), `12`);
1068	/// ```
1069	///
1070	/// [default value]: Default::default
1071	/// [`parse`]: str::parse
1072	/// [`FromStr`]: crate::str::FromStr
1073	#[inline]
1074	#[stable(feature = "rust1", since = "1.0.0")]
1075	pub fn unwrap_or_default(self) -> T
1076	where
1077	T: Default,
1078	{
1079	match self {
1080	Some(x) => x,
1081	None => T::default(),
1082	}
1083	}
1084
1085	/// Returns the contained [`Some`] value, consuming the `self` value,
1086	/// without checking that the value is not [`None`].
1087	///
1088	/// # Safety
1089	///
1090	/// Calling this method on [`None`] is [undefined behavior].
1091	///
1092	/// [undefined behavior]: https://doc.rust-lang.org/reference/behavior-considered-undefined.html
1093	///
1094	/// # Examples
1095	///
1096	/// ```
1097	/// let x = Some("air");
1098	/// assert_eq!(unsafe { x.unwrap_unchecked() }, "air");
1099	/// ```
1100	///
1101	/// ```no_run
1102	/// let x: Option<&str> = None;
1103	/// assert_eq!(unsafe { x.unwrap_unchecked() }, "air"); // Undefined behavior!
1104	/// ```
1105	#[inline]
1106	#[track_caller]
1107	#[stable(feature = "option_result_unwrap_unchecked", since = "1.58.0")]
1108	#[rustc_allow_const_fn_unstable(const_precise_live_drops)]
1109	#[rustc_const_stable(feature = "const_option", since = "1.83.0")]
1110	pub const unsafe fn unwrap_unchecked(self) -> T {
1111	match self {
1112	Some(val) => val,
1113	// SAFETY: the safety contract must be upheld by the caller.
1114	None => unsafe { hint::unreachable_unchecked() },
1115	}
1116	}
1117
1118	/////////////////////////////////////////////////////////////////////////
1119	// Transforming contained values
1120	/////////////////////////////////////////////////////////////////////////
1121
1122	/// Maps an `Option<T>` to `Option<U>` by applying a function to a contained value (if `Some`) or returns `None` (if `None`).
1123	///
1124	/// # Examples
1125	///
1126	/// Calculates the length of an <code>Option<[String]></code> as an
1127	/// <code>Option<[usize]></code>, consuming the original:
1128	///
1129	/// [String]: ../../std/string/struct.String.html "String"
1130	/// ```
1131	/// let maybe_some_string = Some(String::from("Hello, World!"));
1132	/// // `Option::map` takes self by value, consuming `maybe_some_string`
1133	/// let maybe_some_len = maybe_some_string.map(\|s\| s.len());
1134	/// assert_eq!(maybe_some_len, Some(`13`));
1135	///
1136	/// let x: Option<&str> = None;
1137	/// assert_eq!(x.map(\|s\| s.len()), None);
1138	/// ```
1139	#[inline]
1140	#[stable(feature = "rust1", since = "1.0.0")]
1141	pub fn map<U, F>(self, f: F) -> Option<U>
1142	where
1143	F: FnOnce(T) -> U,
1144	{
1145	match self {
1146	Some(x) => Some(f(x)),
1147	None => None,
1148	}
1149	}
1150
1151	/// Calls a function with a reference to the contained value if [`Some`].
1152	///
1153	/// Returns the original option.
1154	///
1155	/// # Examples
1156	///
1157	/// ```
1158	/// let list = vec![`1`, `2`, `3`];
1159	///
1160	/// // prints "got: 2"
1161	/// let x = list
1162	/// .get(`1`)
1163	/// .inspect(\|x\| println!("got: {x}"))
1164	/// .expect("list should be long enough");
1165	///
1166	/// // prints nothing
1167	/// list.get(`5`).inspect(\|x\| println!("got: {x}"));
1168	/// ```
1169	#[inline]
1170	#[stable(feature = "result_option_inspect", since = "1.76.0")]
1171	pub fn inspect<F: FnOnce(&T)>(self, f: F) -> Self {
1172	if let Some(ref x) = self {
1173	f(x);
1174	}
1175
1176	self
1177	}
1178
1179	/// Returns the provided default result (if none),
1180	/// or applies a function to the contained value (if any).
1181	///
1182	/// Arguments passed to `map_or` are eagerly evaluated; if you are passing
1183	/// the result of a function call, it is recommended to use [`map_or_else`],
1184	/// which is lazily evaluated.
1185	///
1186	/// [`map_or_else`]: Option::map_or_else
1187	///
1188	/// # Examples
1189	///
1190	/// ```
1191	/// let x = Some("foo");
1192	/// assert_eq!(x.map_or(`42`, \|v\| v.len()), `3`);
1193	///
1194	/// let x: Option<&str> = None;
1195	/// assert_eq!(x.map_or(`42`, \|v\| v.len()), `42`);
1196	/// ```
1197	#[inline]
1198	#[stable(feature = "rust1", since = "1.0.0")]
1199	#[must_use = "if you don't need the returned value, use `if let` instead"]
1200	pub fn map_or<U, F>(self, default: U, f: F) -> U
1201	where
1202	F: FnOnce(T) -> U,
1203	{
1204	match self {
1205	Some(t) => f(t),
1206	None => default,
1207	}
1208	}
1209
1210	/// Computes a default function result (if none), or
1211	/// applies a different function to the contained value (if any).
1212	///
1213	/// # Basic examples
1214	///
1215	/// ```
1216	/// let k = `21`;
1217	///
1218	/// let x = Some("foo");
1219	/// assert_eq!(x.map_or_else(\|\| `2` * k, \|v\| v.len()), `3`);
1220	///
1221	/// let x: Option<&str> = None;
1222	/// assert_eq!(x.map_or_else(\|\| `2` * k, \|v\| v.len()), `42`);
1223	/// ```
1224	///
1225	/// # Handling a Result-based fallback
1226	///
1227	/// A somewhat common occurrence when dealing with optional values
1228	/// in combination with [`Result<T, E>`] is the case where one wants to invoke
1229	/// a fallible fallback if the option is not present. This example
1230	/// parses a command line argument (if present), or the contents of a file to
1231	/// an integer. However, unlike accessing the command line argument, reading
1232	/// the file is fallible, so it must be wrapped with `Ok`.
1233	///
1234	/// ```no_run
1235	/// # fn main() -> Result<(), Box<dyn std::error::Error>> {
1236	/// let v: u64 = std::env::args()
1237	/// .nth(`1`)
1238	/// .map_or_else(\|\| std::fs::read_to_string("/etc/someconfig.conf"), Ok)?
1239	/// .parse()?;
1240	/// # Ok(())
1241	/// # }
1242	/// ```
1243	#[inline]
1244	#[stable(feature = "rust1", since = "1.0.0")]
1245	pub fn map_or_else<U, D, F>(self, default: D, f: F) -> U
1246	where
1247	D: FnOnce() -> U,
1248	F: FnOnce(T) -> U,
1249	{
1250	match self {
1251	Some(t) => f(t),
1252	None => default(),
1253	}
1254	}
1255
1256	/// Maps an `Option<T>` to a `U` by applying function `f` to the contained
1257	/// value if the option is [`Some`], otherwise if [`None`], returns the
1258	/// [default value] for the type `U`.
1259	///
1260	/// # Examples
1261	///
1262	/// ```
1263	/// #![feature(result_option_map_or_default)]
1264	///
1265	/// let x: Option<&str> = Some("hi");
1266	/// let y: Option<&str> = None;
1267	///
1268	/// assert_eq!(x.map_or_default(\|x\| x.len()), `2`);
1269	/// assert_eq!(y.map_or_default(\|y\| y.len()), `0`);
1270	/// ```
1271	///
1272	/// [default value]: Default::default
1273	#[inline]
1274	#[unstable(feature = "result_option_map_or_default", issue = "138099")]
1275	pub fn map_or_default<U, F>(self, f: F) -> U
1276	where
1277	U: Default,
1278	F: FnOnce(T) -> U,
1279	{
1280	match self {
1281	Some(t) => f(t),
1282	None => U::default(),
1283	}
1284	}
1285
1286	/// Transforms the `Option<T>` into a [`Result<T, E>`], mapping [`Some(v)`] to
1287	/// [`Ok(v)`] and [`None`] to [`Err(err)`].
1288	///
1289	/// Arguments passed to `ok_or` are eagerly evaluated; if you are passing the
1290	/// result of a function call, it is recommended to use [`ok_or_else`], which is
1291	/// lazily evaluated.
1292	///
1293	/// [`Ok(v)`]: Ok
1294	/// [`Err(err)`]: Err
1295	/// [`Some(v)`]: Some
1296	/// [`ok_or_else`]: Option::ok_or_else
1297	///
1298	/// # Examples
1299	///
1300	/// ```
1301	/// let x = Some("foo");
1302	/// assert_eq!(x.ok_or(`0`), Ok("foo"));
1303	///
1304	/// let x: Option<&str> = None;
1305	/// assert_eq!(x.ok_or(`0`), Err(`0`));
1306	/// ```
1307	#[inline]
1308	#[stable(feature = "rust1", since = "1.0.0")]
1309	pub fn ok_or<E>(self, err: E) -> Result<T, E> {
1310	match self {
1311	Some(v) => Ok(v),
1312	None => Err(err),
1313	}
1314	}
1315
1316	/// Transforms the `Option<T>` into a [`Result<T, E>`], mapping [`Some(v)`] to
1317	/// [`Ok(v)`] and [`None`] to [`Err(err())`].
1318	///
1319	/// [`Ok(v)`]: Ok
1320	/// [`Err(err())`]: Err
1321	/// [`Some(v)`]: Some
1322	///
1323	/// # Examples
1324	///
1325	/// ```
1326	/// let x = Some("foo");
1327	/// assert_eq!(x.ok_or_else(\|\| `0`), Ok("foo"));
1328	///
1329	/// let x: Option<&str> = None;
1330	/// assert_eq!(x.ok_or_else(\|\| `0`), Err(`0`));
1331	/// ```
1332	#[inline]
1333	#[stable(feature = "rust1", since = "1.0.0")]
1334	pub fn ok_or_else<E, F>(self, err: F) -> Result<T, E>
1335	where
1336	F: FnOnce() -> E,
1337	{
1338	match self {
1339	Some(v) => Ok(v),
1340	None => Err(err()),
1341	}
1342	}
1343
1344	/// Converts from `Option<T>` (or `&Option<T>`) to `Option<&T::Target>`.
1345	///
1346	/// Leaves the original Option in-place, creating a new one with a reference
1347	/// to the original one, additionally coercing the contents via [`Deref`].
1348	///
1349	/// # Examples
1350	///
1351	/// ```
1352	/// let x: Option<String> = Some("hey".to_owned());
1353	/// assert_eq!(x.as_deref(), Some("hey"));
1354	///
1355	/// let x: Option<String> = None;
1356	/// assert_eq!(x.as_deref(), None);
1357	/// ```
1358	#[inline]
1359	#[stable(feature = "option_deref", since = "1.40.0")]
1360	pub fn as_deref(&self) -> Option<&T::Target>
1361	where
1362	T: Deref,
1363	{
1364	self.as_ref().map(\|t\| t.deref())
1365	}
1366
1367	/// Converts from `Option<T>` (or `&mut Option<T>`) to `Option<&mut T::Target>`.
1368	///
1369	/// Leaves the original `Option` in-place, creating a new one containing a mutable reference to
1370	/// the inner type's [`Deref::Target`] type.
1371	///
1372	/// # Examples
1373	///
1374	/// ```
1375	/// let mut x: Option<String> = Some("hey".to_owned());
1376	/// assert_eq!(x.as_deref_mut().map(\|x\| {
1377	/// x.make_ascii_uppercase();
1378	/// x
1379	/// }), Some("HEY".to_owned().as_mut_str()));
1380	/// ```
1381	#[inline]
1382	#[stable(feature = "option_deref", since = "1.40.0")]
1383	pub fn as_deref_mut(&mut self) -> Option<&mut T::Target>
1384	where
1385	T: DerefMut,
1386	{
1387	self.as_mut().map(\|t\| t.deref_mut())
1388	}
1389
1390	/////////////////////////////////////////////////////////////////////////
1391	// Iterator constructors
1392	/////////////////////////////////////////////////////////////////////////
1393
1394	/// Returns an iterator over the possibly contained value.
1395	///
1396	/// # Examples
1397	///
1398	/// ```
1399	/// let x = Some(`4`);
1400	/// assert_eq!(x.iter().next(), Some(&`4`));
1401	///
1402	/// let x: Option<u32> = None;
1403	/// assert_eq!(x.iter().next(), None);
1404	/// ```
1405	#[inline]
1406	#[stable(feature = "rust1", since = "1.0.0")]
1407	pub fn iter(&self) -> Iter<'_, T> {
1408	Iter { inner: Item { opt: self.as_ref() } }
1409	}
1410
1411	/// Returns a mutable iterator over the possibly contained value.
1412	///
1413	/// # Examples
1414	///
1415	/// ```
1416	/// let mut x = Some(`4`);
1417	/// match x.iter_mut().next() {
1418	/// Some(v) => *v = `42`,
1419	/// None => {},
1420	/// }
1421	/// assert_eq!(x, Some(`42`));
1422	///
1423	/// let mut x: Option<u32> = None;
1424	/// assert_eq!(x.iter_mut().next(), None);
1425	/// ```
1426	#[inline]
1427	#[stable(feature = "rust1", since = "1.0.0")]
1428	pub fn iter_mut(&mut self) -> IterMut<'_, T> {
1429	IterMut { inner: Item { opt: self.as_mut() } }
1430	}
1431
1432	/////////////////////////////////////////////////////////////////////////
1433	// Boolean operations on the values, eager and lazy
1434	/////////////////////////////////////////////////////////////////////////
1435
1436	/// Returns [`None`] if the option is [`None`], otherwise returns `optb`.
1437	///
1438	/// Arguments passed to `and` are eagerly evaluated; if you are passing the
1439	/// result of a function call, it is recommended to use [`and_then`], which is
1440	/// lazily evaluated.
1441	///
1442	/// [`and_then`]: Option::and_then
1443	///
1444	/// # Examples
1445	///
1446	/// ```
1447	/// let x = Some(`2`);
1448	/// let y: Option<&str> = None;
1449	/// assert_eq!(x.and(y), None);
1450	///
1451	/// let x: Option<u32> = None;
1452	/// let y = Some("foo");
1453	/// assert_eq!(x.and(y), None);
1454	///
1455	/// let x = Some(`2`);
1456	/// let y = Some("foo");
1457	/// assert_eq!(x.and(y), Some("foo"));
1458	///
1459	/// let x: Option<u32> = None;
1460	/// let y: Option<&str> = None;
1461	/// assert_eq!(x.and(y), None);
1462	/// ```
1463	#[inline]
1464	#[stable(feature = "rust1", since = "1.0.0")]
1465	pub fn and<U>(self, optb: Option<U>) -> Option<U> {
1466	match self {
1467	Some(_) => optb,
1468	None => None,
1469	}
1470	}
1471
1472	/// Returns [`None`] if the option is [`None`], otherwise calls `f` with the
1473	/// wrapped value and returns the result.
1474	///
1475	/// Some languages call this operation flatmap.
1476	///
1477	/// # Examples
1478	///
1479	/// ```
1480	/// fn sq_then_to_string(x: u32) -> Option<String> {
1481	/// x.checked_mul(x).map(\|sq\| sq.to_string())
1482	/// }
1483	///
1484	/// assert_eq!(Some(`2`).and_then(sq_then_to_string), Some(`4`.to_string()));
1485	/// assert_eq!(Some(`1_000_000`).and_then(sq_then_to_string), None); // overflowed!
1486	/// assert_eq!(None.and_then(sq_then_to_string), None);
1487	/// ```
1488	///
1489	/// Often used to chain fallible operations that may return [`None`].
1490	///
1491	/// ```
1492	/// let arr_2d = [["A0", "A1"], ["B0", "B1"]];
1493	///
1494	/// let item_0_1 = arr_2d.get(`0`).and_then(\|row\| row.get(`1`));
1495	/// assert_eq!(item_0_1, Some(&"A1"));
1496	///
1497	/// let item_2_0 = arr_2d.get(`2`).and_then(\|row\| row.get(`0`));
1498	/// assert_eq!(item_2_0, None);
1499	/// ```
1500	#[doc(alias = "flatmap")]
1501	#[inline]
1502	#[stable(feature = "rust1", since = "1.0.0")]
1503	#[rustc_confusables("flat_map", "flatmap")]
1504	pub fn and_then<U, F>(self, f: F) -> Option<U>
1505	where
1506	F: FnOnce(T) -> Option<U>,
1507	{
1508	match self {
1509	Some(x) => f(x),
1510	None => None,
1511	}
1512	}
1513
1514	/// Returns [`None`] if the option is [`None`], otherwise calls `predicate`
1515	/// with the wrapped value and returns:
1516	///
1517	/// - [`Some(t)`] if `predicate` returns `true` (where `t` is the wrapped
1518	/// value), and
1519	/// - [`None`] if `predicate` returns `false`.
1520	///
1521	/// This function works similar to [`Iterator::filter()`]. You can imagine
1522	/// the `Option<T>` being an iterator over one or zero elements. `filter()`
1523	/// lets you decide which elements to keep.
1524	///
1525	/// # Examples
1526	///
1527	/// ```rust
1528	/// fn is_even(n: &i32) -> bool {
1529	/// n % `2` == `0`
1530	/// }
1531	///
1532	/// assert_eq!(None.filter(is_even), None);
1533	/// assert_eq!(Some(`3`).filter(is_even), None);
1534	/// assert_eq!(Some(`4`).filter(is_even), Some(`4`));
1535	/// ```
1536	///
1537	/// [`Some(t)`]: Some
1538	#[inline]
1539	#[stable(feature = "option_filter", since = "1.27.0")]
1540	pub fn filter<P>(self, predicate: P) -> Self
1541	where
1542	P: FnOnce(&T) -> bool,
1543	{
1544	if let Some(x) = self {
1545	if predicate(&x) {
1546	return Some(x);
1547	}
1548	}
1549	None
1550	}
1551
1552	/// Returns the option if it contains a value, otherwise returns `optb`.
1553	///
1554	/// Arguments passed to `or` are eagerly evaluated; if you are passing the
1555	/// result of a function call, it is recommended to use [`or_else`], which is
1556	/// lazily evaluated.
1557	///
1558	/// [`or_else`]: Option::or_else
1559	///
1560	/// # Examples
1561	///
1562	/// ```
1563	/// let x = Some(`2`);
1564	/// let y = None;
1565	/// assert_eq!(x.or(y), Some(`2`));
1566	///
1567	/// let x = None;
1568	/// let y = Some(`100`);
1569	/// assert_eq!(x.or(y), Some(`100`));
1570	///
1571	/// let x = Some(`2`);
1572	/// let y = Some(`100`);
1573	/// assert_eq!(x.or(y), Some(`2`));
1574	///
1575	/// let x: Option<u32> = None;
1576	/// let y = None;
1577	/// assert_eq!(x.or(y), None);
1578	/// ```
1579	#[inline]
1580	#[stable(feature = "rust1", since = "1.0.0")]
1581	pub fn or(self, optb: Option<T>) -> Option<T> {
1582	match self {
1583	x @ Some(_) => x,
1584	None => optb,
1585	}
1586	}
1587
1588	/// Returns the option if it contains a value, otherwise calls `f` and
1589	/// returns the result.
1590	///
1591	/// # Examples
1592	///
1593	/// ```
1594	/// fn nobody() -> Option<&'static str> { None }
1595	/// fn vikings() -> Option<&'static str> { Some("vikings") }
1596	///
1597	/// assert_eq!(Some("barbarians").or_else(vikings), Some("barbarians"));
1598	/// assert_eq!(None.or_else(vikings), Some("vikings"));
1599	/// assert_eq!(None.or_else(nobody), None);
1600	/// ```
1601	#[inline]
1602	#[stable(feature = "rust1", since = "1.0.0")]
1603	pub fn or_else<F>(self, f: F) -> Option<T>
1604	where
1605	F: FnOnce() -> Option<T>,
1606	{
1607	match self {
1608	x @ Some(_) => x,
1609	None => f(),
1610	}
1611	}
1612
1613	/// Returns [`Some`] if exactly one of `self`, `optb` is [`Some`], otherwise returns [`None`].
1614	///
1615	/// # Examples
1616	///
1617	/// ```
1618	/// let x = Some(`2`);
1619	/// let y: Option<u32> = None;
1620	/// assert_eq!(x.xor(y), Some(`2`));
1621	///
1622	/// let x: Option<u32> = None;
1623	/// let y = Some(`2`);
1624	/// assert_eq!(x.xor(y), Some(`2`));
1625	///
1626	/// let x = Some(`2`);
1627	/// let y = Some(`2`);
1628	/// assert_eq!(x.xor(y), None);
1629	///
1630	/// let x: Option<u32> = None;
1631	/// let y: Option<u32> = None;
1632	/// assert_eq!(x.xor(y), None);
1633	/// ```
1634	#[inline]
1635	#[stable(feature = "option_xor", since = "1.37.0")]
1636	pub fn xor(self, optb: Option<T>) -> Option<T> {
1637	match (self, optb) {
1638	(a @ Some(_), None) => a,
1639	(None, b @ Some(_)) => b,
1640	_ => None,
1641	}
1642	}
1643
1644	/////////////////////////////////////////////////////////////////////////
1645	// Entry-like operations to insert a value and return a reference
1646	/////////////////////////////////////////////////////////////////////////
1647
1648	/// Inserts `value` into the option, then returns a mutable reference to it.
1649	///
1650	/// If the option already contains a value, the old value is dropped.
1651	///
1652	/// See also [`Option::get_or_insert`], which doesn't update the value if
1653	/// the option already contains [`Some`].
1654	///
1655	/// # Example
1656	///
1657	/// ```
1658	/// let mut opt = None;
1659	/// let val = opt.insert(`1`);
1660	/// assert_eq!(*val, `1`);
1661	/// assert_eq!(opt.unwrap(), `1`);
1662	/// let val = opt.insert(`2`);
1663	/// assert_eq!(*val, `2`);
1664	/// *val = `3`;
1665	/// assert_eq!(opt.unwrap(), `3`);
1666	/// ```
1667	#[must_use = "if you intended to set a value, consider assignment instead"]
1668	#[inline]
1669	#[stable(feature = "option_insert", since = "1.53.0")]
1670	pub fn insert(&mut self, value: T) -> &mut T {
1671	*self = Some(value);
1672
1673	// SAFETY: the code above just filled the option
1674	unsafe { self.as_mut().unwrap_unchecked() }
1675	}
1676
1677	/// Inserts `value` into the option if it is [`None`], then
1678	/// returns a mutable reference to the contained value.
1679	///
1680	/// See also [`Option::insert`], which updates the value even if
1681	/// the option already contains [`Some`].
1682	///
1683	/// # Examples
1684	///
1685	/// ```
1686	/// let mut x = None;
1687	///
1688	/// {
1689	/// let y: &mut u32 = x.get_or_insert(`5`);
1690	/// assert_eq!(y, &`5`);
1691	///
1692	/// *y = `7`;
1693	/// }
1694	///
1695	/// assert_eq!(x, Some(`7`));
1696	/// ```
1697	#[inline]
1698	#[stable(feature = "option_entry", since = "1.20.0")]
1699	pub fn get_or_insert(&mut self, value: T) -> &mut T {
1700	self.get_or_insert_with(\|\| value)
1701	}
1702
1703	/// Inserts the default value into the option if it is [`None`], then
1704	/// returns a mutable reference to the contained value.
1705	///
1706	/// # Examples
1707	///
1708	/// ```
1709	/// let mut x = None;
1710	///
1711	/// {
1712	/// let y: &mut u32 = x.get_or_insert_default();
1713	/// assert_eq!(y, &`0`);
1714	///
1715	/// *y = `7`;
1716	/// }
1717	///
1718	/// assert_eq!(x, Some(`7`));
1719	/// ```
1720	#[inline]
1721	#[stable(feature = "option_get_or_insert_default", since = "1.83.0")]
1722	pub fn get_or_insert_default(&mut self) -> &mut T
1723	where
1724	T: Default,
1725	{
1726	self.get_or_insert_with(T::default)
1727	}
1728
1729	/// Inserts a value computed from `f` into the option if it is [`None`],
1730	/// then returns a mutable reference to the contained value.
1731	///
1732	/// # Examples
1733	///
1734	/// ```
1735	/// let mut x = None;
1736	///
1737	/// {
1738	/// let y: &mut u32 = x.get_or_insert_with(\|\| `5`);
1739	/// assert_eq!(y, &`5`);
1740	///
1741	/// *y = `7`;
1742	/// }
1743	///
1744	/// assert_eq!(x, Some(`7`));
1745	/// ```
1746	#[inline]
1747	#[stable(feature = "option_entry", since = "1.20.0")]
1748	pub fn get_or_insert_with<F>(&mut self, f: F) -> &mut T
1749	where
1750	F: FnOnce() -> T,
1751	{
1752	if let None = self {
1753	*self = Some(f());
1754	}
1755
1756	// SAFETY: a `None` variant for `self` would have been replaced by a `Some`
1757	// variant in the code above.
1758	unsafe { self.as_mut().unwrap_unchecked() }
1759	}
1760
1761	/////////////////////////////////////////////////////////////////////////
1762	// Misc
1763	/////////////////////////////////////////////////////////////////////////
1764
1765	/// Takes the value out of the option, leaving a [`None`] in its place.
1766	///
1767	/// # Examples
1768	///
1769	/// ```
1770	/// let mut x = Some(`2`);
1771	/// let y = x.take();
1772	/// assert_eq!(x, None);
1773	/// assert_eq!(y, Some(`2`));
1774	///
1775	/// let mut x: Option<u32> = None;
1776	/// let y = x.take();
1777	/// assert_eq!(x, None);
1778	/// assert_eq!(y, None);
1779	/// ```
1780	#[inline]
1781	#[stable(feature = "rust1", since = "1.0.0")]
1782	#[rustc_const_stable(feature = "const_option", since = "1.83.0")]
1783	pub const fn take(&mut self) -> Option<T> {
1784	// FIXME(const-hack) replace `mem::replace` by `mem::take` when the latter is const ready
1785	mem::replace(self, None)
1786	}
1787
1788	/// Takes the value out of the option, but only if the predicate evaluates to
1789	/// `true` on a mutable reference to the value.
1790	///
1791	/// In other words, replaces `self` with `None` if the predicate returns `true`.
1792	/// This method operates similar to [`Option::take`] but conditional.
1793	///
1794	/// # Examples
1795	///
1796	/// ```
1797	/// let mut x = Some(`42`);
1798	///
1799	/// let prev = x.take_if(\|v\| if *v == `42` {
1800	/// *v += `1`;
1801	/// `false`
1802	/// } else {
1803	/// `false`
1804	/// });
1805	/// assert_eq!(x, Some(`43`));
1806	/// assert_eq!(prev, None);
1807	///
1808	/// let prev = x.take_if(\|v\| *v == `43`);
1809	/// assert_eq!(x, None);
1810	/// assert_eq!(prev, Some(`43`));
1811	/// ```
1812	#[inline]
1813	#[stable(feature = "option_take_if", since = "1.80.0")]
1814	pub fn take_if<P>(&mut self, predicate: P) -> Option<T>
1815	where
1816	P: FnOnce(&mut T) -> bool,
1817	{
1818	if self.as_mut().map_or(`false`, predicate) { self.take() } else { None }
1819	}
1820
1821	/// Replaces the actual value in the option by the value given in parameter,
1822	/// returning the old value if present,
1823	/// leaving a [`Some`] in its place without deinitializing either one.
1824	///
1825	/// # Examples
1826	///
1827	/// ```
1828	/// let mut x = Some(`2`);
1829	/// let old = x.replace(`5`);
1830	/// assert_eq!(x, Some(`5`));
1831	/// assert_eq!(old, Some(`2`));
1832	///
1833	/// let mut x = None;
1834	/// let old = x.replace(`3`);
1835	/// assert_eq!(x, Some(`3`));
1836	/// assert_eq!(old, None);
1837	/// ```
1838	#[inline]
1839	#[stable(feature = "option_replace", since = "1.31.0")]
1840	#[rustc_const_stable(feature = "const_option", since = "1.83.0")]
1841	pub const fn replace(&mut self, value: T) -> Option<T> {
1842	mem::replace(self, Some(value))
1843	}
1844
1845	/// Zips `self` with another `Option`.
1846	///
1847	/// If `self` is `Some(s)` and `other` is `Some(o)`, this method returns `Some((s, o))`.
1848	/// Otherwise, `None` is returned.
1849	///
1850	/// # Examples
1851	///
1852	/// ```
1853	/// let x = Some(`1`);
1854	/// let y = Some("hi");
1855	/// let z = None::<u8>;
1856	///
1857	/// assert_eq!(x.zip(y), Some((`1`, "hi")));
1858	/// assert_eq!(x.zip(z), None);
1859	/// ```
1860	#[stable(feature = "option_zip_option", since = "1.46.0")]
1861	pub fn zip<U>(self, other: Option<U>) -> Option<(T, U)> {
1862	match (self, other) {
1863	(Some(a), Some(b)) => Some((a, b)),
1864	_ => None,
1865	}
1866	}
1867
1868	/// Zips `self` and another `Option` with function `f`.
1869	///
1870	/// If `self` is `Some(s)` and `other` is `Some(o)`, this method returns `Some(f(s, o))`.
1871	/// Otherwise, `None` is returned.
1872	///
1873	/// # Examples
1874	///
1875	/// ```
1876	/// #![feature(option_zip)]
1877	///
1878	/// #[derive(Debug, PartialEq)]
1879	/// struct Point {
1880	/// x: f64,
1881	/// y: f64,
1882	/// }
1883	///
1884	/// impl Point {
1885	/// fn new(x: f64, y: f64) -> Self {
1886	/// Self { x, y }
1887	/// }
1888	/// }
1889	///
1890	/// let x = Some(`17.5`);
1891	/// let y = Some(`42.7`);
1892	///
1893	/// assert_eq!(x.zip_with(y, Point::new), Some(Point { x: `17.5`, y: `42.7` }));
1894	/// assert_eq!(x.zip_with(None, Point::new), None);
1895	/// ```
1896	#[unstable(feature = "option_zip", issue = "70086")]
1897	pub fn zip_with<U, F, R>(self, other: Option<U>, f: F) -> Option<R>
1898	where
1899	F: FnOnce(T, U) -> R,
1900	{
1901	match (self, other) {
1902	(Some(a), Some(b)) => Some(f(a, b)),
1903	_ => None,
1904	}
1905	}
1906	}
1907
1908	impl<T, U> Option<(T, U)> {
1909	/// Unzips an option containing a tuple of two options.
1910	///
1911	/// If `self` is `Some((a, b))` this method returns `(Some(a), Some(b))`.
1912	/// Otherwise, `(None, None)` is returned.
1913	///
1914	/// # Examples
1915	///
1916	/// ```
1917	/// let x = Some((`1`, "hi"));
1918	/// let y = None::<(u8, u32)>;
1919	///
1920	/// assert_eq!(x.unzip(), (Some(`1`), Some("hi")));
1921	/// assert_eq!(y.unzip(), (None, None));
1922	/// ```
1923	#[inline]
1924	#[stable(feature = "unzip_option", since = "1.66.0")]
1925	pub fn unzip(self) -> (Option<T>, Option<U>) {
1926	match self {
1927	Some((a: T, b: U)) => (Some(a), Some(b)),
1928	None => (None, None),
1929	}
1930	}
1931	}
1932
1933	impl<T> Option<&T> {
1934	/// Maps an `Option<&T>` to an `Option<T>` by copying the contents of the
1935	/// option.
1936	///
1937	/// # Examples
1938	///
1939	/// ```
1940	/// let x = `12`;
1941	/// let opt_x = Some(&x);
1942	/// assert_eq!(opt_x, Some(&`12`));
1943	/// let copied = opt_x.copied();
1944	/// assert_eq!(copied, Some(`12`));
1945	/// ```
1946	#[must_use = "`self` will be dropped if the result is not used"]
1947	#[stable(feature = "copied", since = "1.35.0")]
1948	#[rustc_const_stable(feature = "const_option", since = "1.83.0")]
1949	pub const fn copied(self) -> Option<T>
1950	where
1951	T: Copy,
1952	{
1953	// FIXME(const-hack): this implementation, which sidesteps using `Option::map` since it's not const
1954	// ready yet, should be reverted when possible to avoid code repetition
1955	match self {
1956	Some(&v) => Some(v),
1957	None => None,
1958	}
1959	}
1960
1961	/// Maps an `Option<&T>` to an `Option<T>` by cloning the contents of the
1962	/// option.
1963	///
1964	/// # Examples
1965	///
1966	/// ```
1967	/// let x = `12`;
1968	/// let opt_x = Some(&x);
1969	/// assert_eq!(opt_x, Some(&`12`));
1970	/// let cloned = opt_x.cloned();
1971	/// assert_eq!(cloned, Some(`12`));
1972	/// ```
1973	#[must_use = "`self` will be dropped if the result is not used"]
1974	#[stable(feature = "rust1", since = "1.0.0")]
1975	pub fn cloned(self) -> Option<T>
1976	where
1977	T: Clone,
1978	{
1979	match self {
1980	Some(t) => Some(t.clone()),
1981	None => None,
1982	}
1983	}
1984	}
1985
1986	impl<T> Option<&mut T> {
1987	/// Maps an `Option<&mut T>` to an `Option<T>` by copying the contents of the
1988	/// option.
1989	///
1990	/// # Examples
1991	///
1992	/// ```
1993	/// let mut x = `12`;
1994	/// let opt_x = Some(&mut x);
1995	/// assert_eq!(opt_x, Some(&mut `12`));
1996	/// let copied = opt_x.copied();
1997	/// assert_eq!(copied, Some(`12`));
1998	/// ```
1999	#[must_use = "`self` will be dropped if the result is not used"]
2000	#[stable(feature = "copied", since = "1.35.0")]
2001	#[rustc_const_stable(feature = "const_option", since = "1.83.0")]
2002	pub const fn copied(self) -> Option<T>
2003	where
2004	T: Copy,
2005	{
2006	match self {
2007	Some(&mut t) => Some(t),
2008	None => None,
2009	}
2010	}
2011
2012	/// Maps an `Option<&mut T>` to an `Option<T>` by cloning the contents of the
2013	/// option.
2014	///
2015	/// # Examples
2016	///
2017	/// ```
2018	/// let mut x = `12`;
2019	/// let opt_x = Some(&mut x);
2020	/// assert_eq!(opt_x, Some(&mut `12`));
2021	/// let cloned = opt_x.cloned();
2022	/// assert_eq!(cloned, Some(`12`));
2023	/// ```
2024	#[must_use = "`self` will be dropped if the result is not used"]
2025	#[stable(since = "1.26.0", feature = "option_ref_mut_cloned")]
2026	pub fn cloned(self) -> Option<T>
2027	where
2028	T: Clone,
2029	{
2030	match self {
2031	Some(t) => Some(t.clone()),
2032	None => None,
2033	}
2034	}
2035	}
2036
2037	impl<T, E> Option<Result<T, E>> {
2038	/// Transposes an `Option` of a [`Result`] into a [`Result`] of an `Option`.
2039	///
2040	/// [`None`] will be mapped to <code>[Ok]\([None])</code>.
2041	/// <code>[Some]\([Ok]\(\_))</code> and <code>[Some]\([Err]\(\_))</code> will be mapped to
2042	/// <code>[Ok]\([Some]\(\_))</code> and <code>[Err]\(\_)</code>.
2043	///
2044	/// # Examples
2045	///
2046	/// ```
2047	/// #[derive(Debug, Eq, PartialEq)]
2048	/// struct SomeErr;
2049	///
2050	/// let x: Result<Option<i32>, SomeErr> = Ok(Some(`5`));
2051	/// let y: Option<Result<i32, SomeErr>> = Some(Ok(`5`));
2052	/// assert_eq!(x, y.transpose());
2053	/// ```
2054	#[inline]
2055	#[stable(feature = "transpose_result", since = "1.33.0")]
2056	#[rustc_allow_const_fn_unstable(const_precise_live_drops)]
2057	#[rustc_const_stable(feature = "const_option", since = "1.83.0")]
2058	pub const fn transpose(self) -> Result<Option<T>, E> {
2059	match self {
2060	Some(Ok(x)) => Ok(Some(x)),
2061	Some(Err(e)) => Err(e),
2062	None => Ok(None),
2063	}
2064	}
2065	}
2066
2067	#[cfg_attr(not(feature = "panic_immediate_abort"), inline(never))]
2068	#[cfg_attr(feature = "panic_immediate_abort", inline)]
2069	#[cold]
2070	#[track_caller]
2071	const fn unwrap_failed() -> ! {
2072	panic(expr:"called `Option::unwrap()` on a `None` value")
2073	}
2074
2075	// This is a separate function to reduce the code size of .expect() itself.
2076	#[cfg_attr(not(feature = "panic_immediate_abort"), inline(never))]
2077	#[cfg_attr(feature = "panic_immediate_abort", inline)]
2078	#[cold]
2079	#[track_caller]
2080	const fn expect_failed(msg: &str) -> ! {
2081	panic_display(&msg)
2082	}
2083
2084	/////////////////////////////////////////////////////////////////////////////
2085	// Trait implementations
2086	/////////////////////////////////////////////////////////////////////////////
2087
2088	#[stable(feature = "rust1", since = "1.0.0")]
2089	impl<T> Clone for Option<T>
2090	where
2091	T: Clone,
2092	{
2093	#[inline]
2094	fn clone(&self) -> Self {
2095	match self {
2096	Some(x: &T) => Some(x.clone()),
2097	None => None,
2098	}
2099	}
2100
2101	#[inline]
2102	fn clone_from(&mut self, source: &Self) {
2103	match (self, source) {
2104	(Some(to: &mut T), Some(from: &T)) => to.clone_from(source:from),
2105	(to: &mut Option, from: &Option) => *to = from.clone(),
2106	}
2107	}
2108	}
2109
2110	#[unstable(feature = "ergonomic_clones", issue = "132290")]
2111	impl<T> crate::clone::UseCloned for Option<T> where T: crate::clone::UseCloned {}
2112
2113	#[stable(feature = "rust1", since = "1.0.0")]
2114	impl<T> Default for Option<T> {
2115	/// Returns [`None`][Option::None].
2116	///
2117	/// # Examples
2118	///
2119	/// ```
2120	/// let opt: Option<u32> = Option::default();
2121	/// assert!(opt.is_none());
2122	/// ```
2123	#[inline]
2124	fn default() -> Option<T> {
2125	None
2126	}
2127	}
2128
2129	#[stable(feature = "rust1", since = "1.0.0")]
2130	impl<T> IntoIterator for Option<T> {
2131	type Item = T;
2132	type IntoIter = IntoIter<T>;
2133
2134	/// Returns a consuming iterator over the possibly contained value.
2135	///
2136	/// # Examples
2137	///
2138	/// ```
2139	/// let x = Some("string");
2140	/// let v: Vec<&str> = x.into_iter().collect();
2141	/// assert_eq!(v, ["string"]);
2142	///
2143	/// let x = None;
2144	/// let v: Vec<&str> = x.into_iter().collect();
2145	/// assert!(v.is_empty());
2146	/// ```
2147	#[inline]
2148	fn into_iter(self) -> IntoIter<T> {
2149	IntoIter { inner: Item { opt: self } }
2150	}
2151	}
2152
2153	#[stable(since = "1.4.0", feature = "option_iter")]
2154	impl<'a, T> IntoIterator for &'a Option<T> {
2155	type Item = &'a T;
2156	type IntoIter = Iter<'a, T>;
2157
2158	fn into_iter(self) -> Iter<'a, T> {
2159	self.iter()
2160	}
2161	}
2162
2163	#[stable(since = "1.4.0", feature = "option_iter")]
2164	impl<'a, T> IntoIterator for &'a mut Option<T> {
2165	type Item = &'a mut T;
2166	type IntoIter = IterMut<'a, T>;
2167
2168	fn into_iter(self) -> IterMut<'a, T> {
2169	self.iter_mut()
2170	}
2171	}
2172
2173	#[stable(since = "1.12.0", feature = "option_from")]
2174	impl<T> From<T> for Option<T> {
2175	/// Moves `val` into a new [`Some`].
2176	///
2177	/// # Examples
2178	///
2179	/// ```
2180	/// let o: Option<u8> = Option::from(`67`);
2181	///
2182	/// assert_eq!(Some(`67`), o);
2183	/// ```
2184	fn from(val: T) -> Option<T> {
2185	Some(val)
2186	}
2187	}
2188
2189	#[stable(feature = "option_ref_from_ref_option", since = "1.30.0")]
2190	impl<'a, T> From<&'a Option<T>> for Option<&'a T> {
2191	/// Converts from `&Option<T>` to `Option<&T>`.
2192	///
2193	/// # Examples
2194	///
2195	/// Converts an <code>[Option]<[String]></code> into an <code>[Option]<[usize]></code>, preserving
2196	/// the original. The [`map`] method takes the `self` argument by value, consuming the original,
2197	/// so this technique uses `from` to first take an [`Option`] to a reference
2198	/// to the value inside the original.
2199	///
2200	/// [`map`]: Option::map
2201	/// [String]: ../../std/string/struct.String.html "String"
2202	///
2203	/// ```
2204	/// let s: Option<String> = Some(String::from("Hello, Rustaceans!"));
2205	/// let o: Option<usize> = Option::from(&s).map(\|ss: &String\| ss.len());
2206	///
2207	/// println!("Can still print s: {s:?}");
2208	///
2209	/// assert_eq!(o, Some(`18`));
2210	/// ```
2211	fn from(o: &'a Option<T>) -> Option<&'a T> {
2212	o.as_ref()
2213	}
2214	}
2215
2216	#[stable(feature = "option_ref_from_ref_option", since = "1.30.0")]
2217	impl<'a, T> From<&'a mut Option<T>> for Option<&'a mut T> {
2218	/// Converts from `&mut Option<T>` to `Option<&mut T>`
2219	///
2220	/// # Examples
2221	///
2222	/// ```
2223	/// let mut s = Some(String::from("Hello"));
2224	/// let o: Option<&mut String> = Option::from(&mut s);
2225	///
2226	/// match o {
2227	/// Some(t) => *t = String::from("Hello, Rustaceans!"),
2228	/// None => (),
2229	/// }
2230	///
2231	/// assert_eq!(s, Some(String::from("Hello, Rustaceans!")));
2232	/// ```
2233	fn from(o: &'a mut Option<T>) -> Option<&'a mut T> {
2234	o.as_mut()
2235	}
2236	}
2237
2238	// Ideally, LLVM should be able to optimize our derive code to this.
2239	// Once https://github.com/llvm/llvm-project/issues/52622 is fixed, we can
2240	// go back to deriving `PartialEq`.
2241	#[stable(feature = "rust1", since = "1.0.0")]
2242	impl<T> crate::marker::StructuralPartialEq for Option<T> {}
2243	#[stable(feature = "rust1", since = "1.0.0")]
2244	impl<T: PartialEq> PartialEq for Option<T> {
2245	#[inline]
2246	fn eq(&self, other: &Self) -> bool {
2247	// Spelling out the cases explicitly optimizes better than
2248	// `_ => false`
2249	match (self, other) {
2250	(Some(l: &T), Some(r: &T)) => l == r,
2251	(Some(_), None) => `false`,
2252	(None, Some(_)) => `false`,
2253	(None, None) => `true`,
2254	}
2255	}
2256	}
2257
2258	// Manually implementing here somewhat improves codegen for
2259	// https://github.com/rust-lang/rust/issues/49892, although still
2260	// not optimal.
2261	#[stable(feature = "rust1", since = "1.0.0")]
2262	impl<T: PartialOrd> PartialOrd for Option<T> {
2263	#[inline]
2264	fn partial_cmp(&self, other: &Self) -> Option<cmp::Ordering> {
2265	match (self, other) {
2266	(Some(l: &T), Some(r: &T)) => l.partial_cmp(r),
2267	(Some(_), None) => Some(cmp::Ordering::Greater),
2268	(None, Some(_)) => Some(cmp::Ordering::Less),
2269	(None, None) => Some(cmp::Ordering::Equal),
2270	}
2271	}
2272	}
2273
2274	#[stable(feature = "rust1", since = "1.0.0")]
2275	impl<T: Ord> Ord for Option<T> {
2276	#[inline]
2277	fn cmp(&self, other: &Self) -> cmp::Ordering {
2278	match (self, other) {
2279	(Some(l: &T), Some(r: &T)) => l.cmp(r),
2280	(Some(_), None) => cmp::Ordering::Greater,
2281	(None, Some(_)) => cmp::Ordering::Less,
2282	(None, None) => cmp::Ordering::Equal,
2283	}
2284	}
2285	}
2286
2287	/////////////////////////////////////////////////////////////////////////////
2288	// The Option Iterators
2289	/////////////////////////////////////////////////////////////////////////////
2290
2291	#[derive(Clone, Debug)]
2292	struct Item<A> {
2293	opt: Option<A>,
2294	}
2295
2296	impl<A> Iterator for Item<A> {
2297	type Item = A;
2298
2299	#[inline]
2300	fn next(&mut self) -> Option<A> {
2301	self.opt.take()
2302	}
2303
2304	#[inline]
2305	fn size_hint(&self) -> (usize, Option<usize>) {
2306	let len: usize = self.len();
2307	(len, Some(len))
2308	}
2309	}
2310
2311	impl<A> DoubleEndedIterator for Item<A> {
2312	#[inline]
2313	fn next_back(&mut self) -> Option<A> {
2314	self.opt.take()
2315	}
2316	}
2317
2318	impl<A> ExactSizeIterator for Item<A> {
2319	#[inline]
2320	fn len(&self) -> usize {
2321	self.opt.len()
2322	}
2323	}
2324	impl<A> FusedIterator for Item<A> {}
2325	unsafe impl<A> TrustedLen for Item<A> {}
2326
2327	/// An iterator over a reference to the [`Some`] variant of an [`Option`].
2328	///
2329	/// The iterator yields one value if the [`Option`] is a [`Some`], otherwise none.
2330	///
2331	/// This `struct` is created by the [`Option::iter`] function.
2332	#[stable(feature = "rust1", since = "1.0.0")]
2333	#[derive(Debug)]
2334	pub struct Iter<'a, A: 'a> {
2335	inner: Item<&'a A>,
2336	}
2337
2338	#[stable(feature = "rust1", since = "1.0.0")]
2339	impl<'a, A> Iterator for Iter<'a, A> {
2340	type Item = &'a A;
2341
2342	#[inline]
2343	fn next(&mut self) -> Option<&'a A> {
2344	self.inner.next()
2345	}
2346	#[inline]
2347	fn size_hint(&self) -> (usize, Option<usize>) {
2348	self.inner.size_hint()
2349	}
2350	}
2351
2352	#[stable(feature = "rust1", since = "1.0.0")]
2353	impl<'a, A> DoubleEndedIterator for Iter<'a, A> {
2354	#[inline]
2355	fn next_back(&mut self) -> Option<&'a A> {
2356	self.inner.next_back()
2357	}
2358	}
2359
2360	#[stable(feature = "rust1", since = "1.0.0")]
2361	impl<A> ExactSizeIterator for Iter<'_, A> {}
2362
2363	#[stable(feature = "fused", since = "1.26.0")]
2364	impl<A> FusedIterator for Iter<'_, A> {}
2365
2366	#[unstable(feature = "trusted_len", issue = "37572")]
2367	unsafe impl<A> TrustedLen for Iter<'_, A> {}
2368
2369	#[stable(feature = "rust1", since = "1.0.0")]
2370	impl<A> Clone for Iter<'_, A> {
2371	#[inline]
2372	fn clone(&self) -> Self {
2373	Iter { inner: self.inner.clone() }
2374	}
2375	}
2376
2377	/// An iterator over a mutable reference to the [`Some`] variant of an [`Option`].
2378	///
2379	/// The iterator yields one value if the [`Option`] is a [`Some`], otherwise none.
2380	///
2381	/// This `struct` is created by the [`Option::iter_mut`] function.
2382	#[stable(feature = "rust1", since = "1.0.0")]
2383	#[derive(Debug)]
2384	pub struct IterMut<'a, A: 'a> {
2385	inner: Item<&'a mut A>,
2386	}
2387
2388	#[stable(feature = "rust1", since = "1.0.0")]
2389	impl<'a, A> Iterator for IterMut<'a, A> {
2390	type Item = &'a mut A;
2391
2392	#[inline]
2393	fn next(&mut self) -> Option<&'a mut A> {
2394	self.inner.next()
2395	}
2396	#[inline]
2397	fn size_hint(&self) -> (usize, Option<usize>) {
2398	self.inner.size_hint()
2399	}
2400	}
2401
2402	#[stable(feature = "rust1", since = "1.0.0")]
2403	impl<'a, A> DoubleEndedIterator for IterMut<'a, A> {
2404	#[inline]
2405	fn next_back(&mut self) -> Option<&'a mut A> {
2406	self.inner.next_back()
2407	}
2408	}
2409
2410	#[stable(feature = "rust1", since = "1.0.0")]
2411	impl<A> ExactSizeIterator for IterMut<'_, A> {}
2412
2413	#[stable(feature = "fused", since = "1.26.0")]
2414	impl<A> FusedIterator for IterMut<'_, A> {}
2415	#[unstable(feature = "trusted_len", issue = "37572")]
2416	unsafe impl<A> TrustedLen for IterMut<'_, A> {}
2417
2418	/// An iterator over the value in [`Some`] variant of an [`Option`].
2419	///
2420	/// The iterator yields one value if the [`Option`] is a [`Some`], otherwise none.
2421	///
2422	/// This `struct` is created by the [`Option::into_iter`] function.
2423	#[derive(Clone, Debug)]
2424	#[stable(feature = "rust1", since = "1.0.0")]
2425	pub struct IntoIter<A> {
2426	inner: Item<A>,
2427	}
2428
2429	#[stable(feature = "rust1", since = "1.0.0")]
2430	impl<A> Iterator for IntoIter<A> {
2431	type Item = A;
2432
2433	#[inline]
2434	fn next(&mut self) -> Option<A> {
2435	self.inner.next()
2436	}
2437	#[inline]
2438	fn size_hint(&self) -> (usize, Option<usize>) {
2439	self.inner.size_hint()
2440	}
2441	}
2442
2443	#[stable(feature = "rust1", since = "1.0.0")]
2444	impl<A> DoubleEndedIterator for IntoIter<A> {
2445	#[inline]
2446	fn next_back(&mut self) -> Option<A> {
2447	self.inner.next_back()
2448	}
2449	}
2450
2451	#[stable(feature = "rust1", since = "1.0.0")]
2452	impl<A> ExactSizeIterator for IntoIter<A> {}
2453
2454	#[stable(feature = "fused", since = "1.26.0")]
2455	impl<A> FusedIterator for IntoIter<A> {}
2456
2457	#[unstable(feature = "trusted_len", issue = "37572")]
2458	unsafe impl<A> TrustedLen for IntoIter<A> {}
2459
2460	/////////////////////////////////////////////////////////////////////////////
2461	// FromIterator
2462	/////////////////////////////////////////////////////////////////////////////
2463
2464	#[stable(feature = "rust1", since = "1.0.0")]
2465	impl<A, V: FromIterator<A>> FromIterator<Option<A>> for Option<V> {
2466	/// Takes each element in the [`Iterator`]: if it is [`None`][Option::None],
2467	/// no further elements are taken, and the [`None`][Option::None] is
2468	/// returned. Should no [`None`][Option::None] occur, a container of type
2469	/// `V` containing the values of each [`Option`] is returned.
2470	///
2471	/// # Examples
2472	///
2473	/// Here is an example which increments every integer in a vector.
2474	/// We use the checked variant of `add` that returns `None` when the
2475	/// calculation would result in an overflow.
2476	///
2477	/// ```
2478	/// let items = vec![`0_u16`, `1`, `2`];
2479	///
2480	/// let res: Option<Vec<u16>> = items
2481	/// .iter()
2482	/// .map(\|x\| x.checked_add(`1`))
2483	/// .collect();
2484	///
2485	/// assert_eq!(res, Some(vec![`1`, `2`, `3`]));
2486	/// ```
2487	///
2488	/// As you can see, this will return the expected, valid items.
2489	///
2490	/// Here is another example that tries to subtract one from another list
2491	/// of integers, this time checking for underflow:
2492	///
2493	/// ```
2494	/// let items = vec![`2_u16`, `1`, `0`];
2495	///
2496	/// let res: Option<Vec<u16>> = items
2497	/// .iter()
2498	/// .map(\|x\| x.checked_sub(`1`))
2499	/// .collect();
2500	///
2501	/// assert_eq!(res, None);
2502	/// ```
2503	///
2504	/// Since the last element is zero, it would underflow. Thus, the resulting
2505	/// value is `None`.
2506	///
2507	/// Here is a variation on the previous example, showing that no
2508	/// further elements are taken from `iter` after the first `None`.
2509	///
2510	/// ```
2511	/// let items = vec![`3_u16`, `2`, `1`, `10`];
2512	///
2513	/// let mut shared = `0`;
2514	///
2515	/// let res: Option<Vec<u16>> = items
2516	/// .iter()
2517	/// .map(\|x\| { shared += x; x.checked_sub(`2`) })
2518	/// .collect();
2519	///
2520	/// assert_eq!(res, None);
2521	/// assert_eq!(shared, `6`);
2522	/// ```
2523	///
2524	/// Since the third element caused an underflow, no further elements were taken,
2525	/// so the final value of `shared` is 6 (= `3 + 2 + 1`), not 16.
2526	#[inline]
2527	fn from_iter<I: IntoIterator<Item = Option<A>>>(iter: I) -> Option<V> {
2528	// FIXME(#11084): This could be replaced with Iterator::scan when this
2529	// performance bug is closed.
2530
2531	iter::try_process(iter.into_iter(), \|i\| i.collect())
2532	}
2533	}
2534
2535	#[unstable(feature = "try_trait_v2", issue = "84277")]
2536	impl<T> ops::Try for Option<T> {
2537	type Output = T;
2538	type Residual = Option<convert::Infallible>;
2539
2540	#[inline]
2541	fn from_output(output: Self::Output) -> Self {
2542	Some(output)
2543	}
2544
2545	#[inline]
2546	fn branch(self) -> ControlFlow<Self::Residual, Self::Output> {
2547	match self {
2548	Some(v: T) => ControlFlow::Continue(v),
2549	None => ControlFlow::Break(None),
2550	}
2551	}
2552	}
2553
2554	#[unstable(feature = "try_trait_v2", issue = "84277")]
2555	// Note: manually specifying the residual type instead of using the default to work around
2556	// https://github.com/rust-lang/rust/issues/99940
2557	impl<T> ops::FromResidual<Option<convert::Infallible>> for Option<T> {
2558	#[inline]
2559	fn from_residual(residual: Option<convert::Infallible>) -> Self {
2560	match residual {
2561	None => None,
2562	}
2563	}
2564	}
2565
2566	#[diagnostic::do_not_recommend]
2567	#[unstable(feature = "try_trait_v2_yeet", issue = "96374")]
2568	impl<T> ops::FromResidual<ops::Yeet<()>> for Option<T> {
2569	#[inline]
2570	fn from_residual(ops::Yeet(()): ops::Yeet<()>) -> Self {
2571	None
2572	}
2573	}
2574
2575	#[unstable(feature = "try_trait_v2_residual", issue = "91285")]
2576	impl<T> ops::Residual<T> for Option<convert::Infallible> {
2577	type TryType = Option<T>;
2578	}
2579
2580	impl<T> Option<Option<T>> {
2581	/// Converts from `Option<Option<T>>` to `Option<T>`.
2582	///
2583	/// # Examples
2584	///
2585	/// Basic usage:
2586	///
2587	/// ```
2588	/// let x: Option<Option<u32>> = Some(Some(`6`));
2589	/// assert_eq!(Some(`6`), x.flatten());
2590	///
2591	/// let x: Option<Option<u32>> = Some(None);
2592	/// assert_eq!(None, x.flatten());
2593	///
2594	/// let x: Option<Option<u32>> = None;
2595	/// assert_eq!(None, x.flatten());
2596	/// ```
2597	///
2598	/// Flattening only removes one level of nesting at a time:
2599	///
2600	/// ```
2601	/// let x: Option<Option<Option<u32>>> = Some(Some(Some(`6`)));
2602	/// assert_eq!(Some(Some(`6`)), x.flatten());
2603	/// assert_eq!(Some(`6`), x.flatten().flatten());
2604	/// ```
2605	#[inline]
2606	#[stable(feature = "option_flattening", since = "1.40.0")]
2607	#[rustc_allow_const_fn_unstable(const_precise_live_drops)]
2608	#[rustc_const_stable(feature = "const_option", since = "1.83.0")]
2609	pub const fn flatten(self) -> Option<T> {
2610	// FIXME(const-hack): could be written with `and_then`
2611	match self {
2612	Some(inner) => inner,
2613	None => None,
2614	}
2615	}
2616	}
2617
2618	impl<T, const N: usize> [Option<T>; N] {
2619	/// Transposes a `[Option<T>; N]` into a `Option<[T; N]>`.
2620	///
2621	/// # Examples
2622	///
2623	/// ```
2624	/// #![feature(option_array_transpose)]
2625	/// # use std::option::Option;
2626	///
2627	/// let data = [Some(`0`); `1000`];
2628	/// let data: Option<[u8; `1000`]> = data.transpose();
2629	/// assert_eq!(data, Some([`0`; `1000`]));
2630	///
2631	/// let data = [Some(`0`), None];
2632	/// let data: Option<[u8; `2`]> = data.transpose();
2633	/// assert_eq!(data, None);
2634	/// ```
2635	#[inline]
2636	#[unstable(feature = "option_array_transpose", issue = "130828")]
2637	pub fn transpose(self) -> Option<[T; N]> {
2638	self.try_map(core::convert::identity)
2639	}
2640	}
2641