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

1	//! Optional values.
2	//!
3	//! Type [`Option`] represents an optional value: every [`Option`]
4	//! is either [`Some`] and contains a value, or [`None`], and
5	//! does not. [`Option`] types are very common in Rust code, as
6	//! they have a number of uses:
7	//!
8	//! Initial values*
9	//! Return values for functions that are not defined*
10	//! over their entire input range (partial functions)
11	//! Return value for otherwise reporting simple errors, where* [`None`] is
12	//! returned on error
13	//! Optional struct fields*
14	//! Struct fields that can be loaned or "taken"*
15	//! Optional function arguments*
16	//! Nullable pointers*
17	//! Swapping things out of difficult situations*
18	//!
19	//! [`Option`]s are commonly paired with pattern matching to query the presence
20	//! of a value and take action, always accounting for the [`None`] case.
21	//!
22	//! ```
23	//! fn divide(numerator: f64, denominator: f64) -> Option<f64> {
24	//! if denominator == `0.0` {
25	//! None
26	//! } else {
27	//! Some(numerator / denominator)
28	//! }
29	//! }
30	//!
31	//! // The return value of the function is an option
32	//! let result = divide(`2.0`, `3.0`);
33	//!
34	//! // Pattern match to retrieve the value
35	//! match result {
36	//! // The division was valid
37	//! Some(x) => println!("Result: {x}"),
38	//! // The division was invalid
39	//! None => println!("Cannot divide by 0"),
40	//! }
41	//! ```
42	//!
43	//
44	// FIXME: Show how `Option` is used in practice, with lots of methods
45	//
46	//! # Options and pointers ("nullable" pointers)
47	//!
48	//! Rust's pointer types must always point to a valid location; there are
49	//! no "null" references. Instead, Rust has optional* pointers, like*
50	//! the optional owned box, <code>[Option]<[Box\<T>]></code>.
51	//!
52	//! [Box\<T>]: ../../std/boxed/struct.Box.html
53	//!
54	//! The following example uses [`Option`] to create an optional box of
55	//! [`i32`]. Notice that in order to use the inner [`i32`] value, the
56	//! `check_optional` function first needs to use pattern matching to
57	//! determine whether the box has a value (i.e., it is [`Some(...)`][`Some`]) or
58	//! not ([`None`]).
59	//!
60	//! ```
61	//! let optional = None;
62	//! check_optional(optional);
63	//!
64	//! let optional = Some(Box::new(`9000`));
65	//! check_optional(optional);
66	//!
67	//! fn check_optional(optional: Option<Box<i32>>) {
68	//! match optional {
69	//! Some(p) => println!("has value {p}"),
70	//! None => println!("has no value"),
71	//! }
72	//! }
73	//! ```
74	//!
75	//! # The question mark operator, `?`
76	//!
77	//! Similar to the [`Result`] type, when writing code that calls many functions that return the
78	//! [`Option`] type, handling `Some`/`None` can be tedious. The question mark
79	//! operator, [`?`], hides some of the boilerplate of propagating values
80	//! up the call stack.
81	//!
82	//! It replaces this:
83	//!
84	//! ```
85	//! # #![allow(dead_code)]
86	//! fn add_last_numbers(stack: &mut Vec<i32>) -> Option<i32> {
87	//! let a = stack.pop();
88	//! let b = stack.pop();
89	//!
90	//! match (a, b) {
91	//! (Some(x), Some(y)) => Some(x + y),
92	//! _ => None,
93	//! }
94	//! }
95	//!
96	//! ```
97	//!
98	//! With this:
99	//!
100	//! ```
101	//! # #![allow(dead_code)]
102	//! fn add_last_numbers(stack: &mut Vec<i32>) -> Option<i32> {
103	//! Some(stack.pop()? + stack.pop()?)
104	//! }
105	//! ```
106	//!
107	//! It's much nicer!
108	//!
109	//! Ending the expression with [`?`] will result in the [`Some`]'s unwrapped value, unless the
110	//! result is [`None`], in which case [`None`] is returned early from the enclosing function.
111	//!
112	//! [`?`] can be used in functions that return [`Option`] because of the
113	//! early return of [`None`] that it provides.
114	//!
115	//! [`?`]: crate::ops::Try
116	//! [`Some`]: Some
117	//! [`None`]: None
118	//!
119	//! # Representation
120	//!
121	//! Rust guarantees to optimize the following types `T` such that
122	//! [`Option<T>`] has the same size, alignment, and [function call ABI] as `T`. In some
123	//! of these cases, Rust further guarantees that
124	//! `transmute::<_, Option<T>>([0u8; size_of::<T>()])` is sound and
125	//! produces `Option::<T>::None`. These cases are identified by the
126	//! second column:
127	//!
128	//! \| `T` \| `transmute::<_, Option<T>>([0u8; size_of::<T>()])` sound? \|
129	//! \|---------------------------------------------------------------------\|----------------------------------------------------------------------\|
130	//! \| [`Box<U>`] (specifically, only `Box<U, Global>`) \| when `U: Sized` \|
131	//! \| `&U` \| when `U: Sized` \|
132	//! \| `&mut U` \| when `U: Sized` \|
133	//! \| `fn`, `extern "C" fn`[^extern_fn] \| always \|
134	//! \| [`num::NonZero`] \| always \|*
135	//! \| [`ptr::NonNull<U>`] \| when `U: Sized` \|
136	//! \| `#[repr(transparent)]` struct around one of the types in this list. \| when it holds for the inner type \|
137	//!
138	//! [^extern_fn]: this remains true for any argument/return types and any other ABI: `extern "abi" fn` (_e.g._, `extern "system" fn`)
139	//!
140	//! [`Box<U>`]: ../../std/boxed/struct.Box.html
141	//! [`num::NonZero`]: crate::num*
142	//! [`ptr::NonNull<U>`]: crate::ptr::NonNull
143	//! [function call ABI]: ../primitive.fn.html#abi-compatibility
144	//!
145	//! This is called the "null pointer optimization" or NPO.
146	//!
147	//! It is further guaranteed that, for the cases above, one can
148	//! [`mem::transmute`] from all valid values of `T` to `Option<T>` and
149	//! from `Some::<T>(_)` to `T` (but transmuting `None::<T>` to `T`
150	//! is undefined behaviour).
151	//!
152	//! # Method overview
153	//!
154	//! In addition to working with pattern matching, [`Option`] provides a wide
155	//! variety of different methods.
156	//!
157	//! ## Querying the variant
158	//!
159	//! The [`is_some`] and [`is_none`] methods return [`true`] if the [`Option`]
160	//! is [`Some`] or [`None`], respectively.
161	//!
162	//! [`is_none`]: Option::is_none
163	//! [`is_some`]: Option::is_some
164	//!
165	//! ## Adapters for working with references
166	//!
167	//! * [`as_ref`] converts from <code>[&][][Option]\<T></code> to <code>[Option]<[&]T></code>
168	//! * [`as_mut`] converts from <code>[&mut] [Option]\<T></code> to <code>[Option]<[&mut] T></code>
169	//! * [`as_deref`] converts from <code>[&][][Option]\<T></code> to
170	//! <code>[Option]<[&]T::[Target]></code>
171	//! * [`as_deref_mut`] converts from <code>[&mut] [Option]\<T></code> to
172	//! <code>[Option]<[&mut] T::[Target]></code>
173	//! * [`as_pin_ref`] converts from <code>[Pin]<[&][][Option]\<T>></code> to
174	//! <code>[Option]<[Pin]<[&]T>></code>
175	//! * [`as_pin_mut`] converts from <code>[Pin]<[&mut] [Option]\<T>></code> to
176	//! <code>[Option]<[Pin]<[&mut] T>></code>
177	//!
178	//! [&]: reference "shared reference"
179	//! [&mut]: reference "mutable reference"
180	//! [Target]: Deref::Target "ops::Deref::Target"
181	//! [`as_deref`]: Option::as_deref
182	//! [`as_deref_mut`]: Option::as_deref_mut
183	//! [`as_mut`]: Option::as_mut
184	//! [`as_pin_mut`]: Option::as_pin_mut
185	//! [`as_pin_ref`]: Option::as_pin_ref
186	//! [`as_ref`]: Option::as_ref
187	//!
188	//! ## Extracting the contained value
189	//!
190	//! These methods extract the contained value in an [`Option<T>`] when it
191	//! is the [`Some`] variant. If the [`Option`] is [`None`]:
192	//!
193	//! * [`expect`] panics with a provided custom message
194	//! * [`unwrap`] panics with a generic message
195	//! * [`unwrap_or`] returns the provided default value
196	//! * [`unwrap_or_default`] returns the default value of the type `T`
197	//! (which must implement the [`Default`] trait)
198	//! * [`unwrap_or_else`] returns the result of evaluating the provided
199	//! function
200	//!
201	//! [`expect`]: Option::expect
202	//! [`unwrap`]: Option::unwrap
203	//! [`unwrap_or`]: Option::unwrap_or
204	//! [`unwrap_or_default`]: Option::unwrap_or_default
205	//! [`unwrap_or_else`]: Option::unwrap_or_else
206	//!
207	//! ## Transforming contained values
208	//!
209	//! These methods transform [`Option`] to [`Result`]:
210	//!
211	//! * [`ok_or`] transforms [`Some(v)`] to [`Ok(v)`], and [`None`] to
212	//! [`Err(err)`] using the provided default `err` value
213	//! * [`ok_or_else`] transforms [`Some(v)`] to [`Ok(v)`], and [`None`] to
214	//! a value of [`Err`] using the provided function
215	//! * [`transpose`] transposes an [`Option`] of a [`Result`] into a
216	//! [`Result`] of an [`Option`]
217	//!
218	//! [`Err(err)`]: Err
219	//! [`Ok(v)`]: Ok
220	//! [`Some(v)`]: Some
221	//! [`ok_or`]: Option::ok_or
222	//! [`ok_or_else`]: Option::ok_or_else
223	//! [`transpose`]: Option::transpose
224	//!
225	//! These methods transform the [`Some`] variant:
226	//!
227	//! * [`filter`] calls the provided predicate function on the contained
228	//! value `t` if the [`Option`] is [`Some(t)`], and returns [`Some(t)`]
229	//! if the function returns `true`; otherwise, returns [`None`]
230	//! * [`flatten`] removes one level of nesting from an
231	//! [`Option<Option<T>>`]
232	//! * [`map`] transforms [`Option<T>`] to [`Option<U>`] by applying the
233	//! provided function to the contained value of [`Some`] and leaving
234	//! [`None`] values unchanged
235	//!
236	//! [`Some(t)`]: Some
237	//! [`filter`]: Option::filter
238	//! [`flatten`]: Option::flatten
239	//! [`map`]: Option::map
240	//!
241	//! These methods transform [`Option<T>`] to a value of a possibly
242	//! different type `U`:
243	//!
244	//! * [`map_or`] applies the provided function to the contained value of
245	//! [`Some`], or returns the provided default value if the [`Option`] is
246	//! [`None`]
247	//! * [`map_or_else`] applies the provided function to the contained value
248	//! of [`Some`], or returns the result of evaluating the provided
249	//! fallback function if the [`Option`] is [`None`]
250	//!
251	//! [`map_or`]: Option::map_or
252	//! [`map_or_else`]: Option::map_or_else
253	//!
254	//! These methods combine the [`Some`] variants of two [`Option`] values:
255	//!
256	//! * [`zip`] returns [`Some((s, o))`] if `self` is [`Some(s)`] and the
257	//! provided [`Option`] value is [`Some(o)`]; otherwise, returns [`None`]
258	//! * [`zip_with`] calls the provided function `f` and returns
259	//! [`Some(f(s, o))`] if `self` is [`Some(s)`] and the provided
260	//! [`Option`] value is [`Some(o)`]; otherwise, returns [`None`]
261	//!
262	//! [`Some(f(s, o))`]: Some
263	//! [`Some(o)`]: Some
264	//! [`Some(s)`]: Some
265	//! [`Some((s, o))`]: Some
266	//! [`zip`]: Option::zip
267	//! [`zip_with`]: Option::zip_with
268	//!
269	//! ## Boolean operators
270	//!
271	//! These methods treat the [`Option`] as a boolean value, where [`Some`]
272	//! acts like [`true`] and [`None`] acts like [`false`]. There are two
273	//! categories of these methods: ones that take an [`Option`] as input, and
274	//! ones that take a function as input (to be lazily evaluated).
275	//!
276	//! The [`and`], [`or`], and [`xor`] methods take another [`Option`] as
277	//! input, and produce an [`Option`] as output. Only the [`and`] method can
278	//! produce an [`Option<U>`] value having a different inner type `U` than
279	//! [`Option<T>`].
280	//!
281	//! \| method \| self \| input \| output \|
282	//! \|---------\|-----------\|-----------\|-----------\|
283	//! \| [`and`] \| `None` \| (ignored) \| `None` \|
284	//! \| [`and`] \| `Some(x)` \| `None` \| `None` \|
285	//! \| [`and`] \| `Some(x)` \| `Some(y)` \| `Some(y)` \|
286	//! \| [`or`] \| `None` \| `None` \| `None` \|
287	//! \| [`or`] \| `None` \| `Some(y)` \| `Some(y)` \|
288	//! \| [`or`] \| `Some(x)` \| (ignored) \| `Some(x)` \|
289	//! \| [`xor`] \| `None` \| `None` \| `None` \|
290	//! \| [`xor`] \| `None` \| `Some(y)` \| `Some(y)` \|
291	//! \| [`xor`] \| `Some(x)` \| `None` \| `Some(x)` \|
292	//! \| [`xor`] \| `Some(x)` \| `Some(y)` \| `None` \|
293	//!
294	//! [`and`]: Option::and
295	//! [`or`]: Option::or
296	//! [`xor`]: Option::xor
297	//!
298	//! The [`and_then`] and [`or_else`] methods take a function as input, and
299	//! only evaluate the function when they need to produce a new value. Only
300	//! the [`and_then`] method can produce an [`Option<U>`] value having a
301	//! different inner type `U` than [`Option<T>`].
302	//!
303	//! \| method \| self \| function input \| function result \| output \|
304	//! \|--------------\|-----------\|----------------\|-----------------\|-----------\|
305	//! \| [`and_then`] \| `None` \| (not provided) \| (not evaluated) \| `None` \|
306	//! \| [`and_then`] \| `Some(x)` \| `x` \| `None` \| `None` \|
307	//! \| [`and_then`] \| `Some(x)` \| `x` \| `Some(y)` \| `Some(y)` \|
308	//! \| [`or_else`] \| `None` \| (not provided) \| `None` \| `None` \|
309	//! \| [`or_else`] \| `None` \| (not provided) \| `Some(y)` \| `Some(y)` \|
310	//! \| [`or_else`] \| `Some(x)` \| (not provided) \| (not evaluated) \| `Some(x)` \|
311	//!
312	//! [`and_then`]: Option::and_then
313	//! [`or_else`]: Option::or_else
314	//!
315	//! This is an example of using methods like [`and_then`] and [`or`] in a
316	//! pipeline of method calls. Early stages of the pipeline pass failure
317	//! values ([`None`]) through unchanged, and continue processing on
318	//! success values ([`Some`]). Toward the end, [`or`] substitutes an error
319	//! message if it receives [`None`].
320	//!
321	//! ```
322	//! # use std::collections::BTreeMap;
323	//! let mut bt = BTreeMap::new();
324	//! bt.insert(`20u8`, "foo");
325	//! bt.insert(`42u8`, "bar");
326	//! let res = [`0u8`, `1`, `11`, `200`, `22`]
327	//! .into_iter()
328	//! .map(\|x\| {
329	//! // `checked_sub()` returns `None` on error
330	//! x.checked_sub(`1`)
331	//! // same with `checked_mul()`
332	//! .and_then(\|x\| x.checked_mul(`2`))
333	//! // `BTreeMap::get` returns `None` on error
334	//! .and_then(\|x\| bt.get(&x))
335	//! // Substitute an error message if we have `None` so far
336	//! .or(Some(&"error!"))
337	//! .copied()
338	//! // Won't panic because we unconditionally used `Some` above
339	//! .unwrap()
340	//! })
341	//! .collect::<Vec<_>>();
342	//! assert_eq!(res, ["error!", "error!", "foo", "error!", "bar"]);
343	//! ```
344	//!
345	//! ## Comparison operators
346	//!
347	//! If `T` implements [`PartialOrd`] then [`Option<T>`] will derive its
348	//! [`PartialOrd`] implementation. With this order, [`None`] compares as
349	//! less than any [`Some`], and two [`Some`] compare the same way as their
350	//! contained values would in `T`. If `T` also implements
351	//! [`Ord`], then so does [`Option<T>`].
352	//!
353	//! ```
354	//! assert!(None < Some(`0`));
355	//! assert!(Some(`0`) < Some(`1`));
356	//! ```
357	//!
358	//! ## Iterating over `Option`
359	//!
360	//! An [`Option`] can be iterated over. This can be helpful if you need an
361	//! iterator that is conditionally empty. The iterator will either produce
362	//! a single value (when the [`Option`] is [`Some`]), or produce no values
363	//! (when the [`Option`] is [`None`]). For example, [`into_iter`] acts like
364	//! [`once(v)`] if the [`Option`] is [`Some(v)`], and like [`empty()`] if
365	//! the [`Option`] is [`None`].
366	//!
367	//! [`Some(v)`]: Some
368	//! [`empty()`]: crate::iter::empty
369	//! [`once(v)`]: crate::iter::once
370	//!
371	//! Iterators over [`Option<T>`] come in three types:
372	//!
373	//! * [`into_iter`] consumes the [`Option`] and produces the contained
374	//! value
375	//! * [`iter`] produces an immutable reference of type `&T` to the
376	//! contained value
377	//! * [`iter_mut`] produces a mutable reference of type `&mut T` to the
378	//! contained value
379	//!
380	//! [`into_iter`]: Option::into_iter
381	//! [`iter`]: Option::iter
382	//! [`iter_mut`]: Option::iter_mut
383	//!
384	//! An iterator over [`Option`] can be useful when chaining iterators, for
385	//! example, to conditionally insert items. (It's not always necessary to
386	//! explicitly call an iterator constructor: many [`Iterator`] methods that
387	//! accept other iterators will also accept iterable types that implement
388	//! [`IntoIterator`], which includes [`Option`].)
389	//!
390	//! ```
391	//! let yep = Some(`42`);
392	//! let nope = None;
393	//! // chain() already calls into_iter(), so we don't have to do so
394	//! let nums: Vec<i32> = (`0`..`4`).chain(yep).chain(`4`..`8`).collect();
395	//! assert_eq!(nums, [`0`, `1`, `2`, `3`, `42`, `4`, `5`, `6`, `7`]);
396	//! let nums: Vec<i32> = (`0`..`4`).chain(nope).chain(`4`..`8`).collect();
397	//! assert_eq!(nums, [`0`, `1`, `2`, `3`, `4`, `5`, `6`, `7`]);
398	//! ```
399	//!
400	//! One reason to chain iterators in this way is that a function returning
401	//! `impl Iterator` must have all possible return values be of the same
402	//! concrete type. Chaining an iterated [`Option`] can help with that.
403	//!
404	//! ```
405	//! fn make_iter(do_insert: bool) -> impl Iterator<Item = i32> {
406	//! // Explicit returns to illustrate return types matching
407	//! match do_insert {
408	//! `true` => return (`0`..`4`).chain(Some(`42`)).chain(`4`..`8`),
409	//! `false` => return (`0`..`4`).chain(None).chain(`4`..`8`),
410	//! }
411	//! }
412	//! println!("{:?}", make_iter(`true`).collect::<Vec<_>>());
413	//! println!("{:?}", make_iter(`false`).collect::<Vec<_>>());
414	//! ```
415	//!
416	//! If we try to do the same thing, but using [`once()`] and [`empty()`],
417	//! we can't return `impl Iterator` anymore because the concrete types of
418	//! the return values differ.
419	//!
420	//! [`empty()`]: crate::iter::empty
421	//! [`once()`]: crate::iter::once
422	//!
423	//! ```compile_fail,E0308
424	//! # use std::iter::{empty, once};
425	//! // This won't compile because all possible returns from the function
426	//! // must have the same concrete type.
427	//! fn make_iter(do_insert: bool) -> impl Iterator<Item = i32> {
428	//! // Explicit returns to illustrate return types not matching
429	//! match do_insert {
430	//! `true` => return (`0`..`4`).chain(once(`42`)).chain(`4`..`8`),
431	//! `false` => return (`0`..`4`).chain(empty()).chain(`4`..`8`),
432	//! }
433	//! }
434	//! ```
435	//!
436	//! ## Collecting into `Option`
437	//!
438	//! [`Option`] implements the [`FromIterator`][impl-FromIterator] trait,
439	//! which allows an iterator over [`Option`] values to be collected into an
440	//! [`Option`] of a collection of each contained value of the original
441	//! [`Option`] values, or [`None`] if any of the elements was [`None`].
442	//!
443	//! [impl-FromIterator]: Option#impl-FromIterator%3COption%3CA%3E%3E-for-Option%3CV%3E
444	//!
445	//! ```
446	//! let v = [Some(`2`), Some(`4`), None, Some(`8`)];
447	//! let res: Option<Vec<_>> = v.into_iter().collect();
448	//! assert_eq!(res, None);
449	//! let v = [Some(`2`), Some(`4`), Some(`8`)];
450	//! let res: Option<Vec<_>> = v.into_iter().collect();
451	//! assert_eq!(res, Some(vec![`2`, `4`, `8`]));
452	//! ```
453	//!
454	//! [`Option`] also implements the [`Product`][impl-Product] and
455	//! [`Sum`][impl-Sum] traits, allowing an iterator over [`Option`] values
456	//! to provide the [`product`][Iterator::product] and
457	//! [`sum`][Iterator::sum] methods.
458	//!
459	//! [impl-Product]: Option#impl-Product%3COption%3CU%3E%3E-for-Option%3CT%3E
460	//! [impl-Sum]: Option#impl-Sum%3COption%3CU%3E%3E-for-Option%3CT%3E
461	//!
462	//! ```
463	//! let v = [None, Some(`1`), Some(`2`), Some(`3`)];
464	//! let res: Option<i32> = v.into_iter().sum();
465	//! assert_eq!(res, None);
466	//! let v = [Some(`1`), Some(`2`), Some(`21`)];
467	//! let res: Option<i32> = v.into_iter().product();
468	//! assert_eq!(res, Some(`42`));
469	//! ```
470	//!
471	//! ## Modifying an [`Option`] in-place
472	//!
473	//! These methods return a mutable reference to the contained value of an
474	//! [`Option<T>`]:
475	//!
476	//! * [`insert`] inserts a value, dropping any old contents
477	//! * [`get_or_insert`] gets the current value, inserting a provided
478	//! default value if it is [`None`]
479	//! * [`get_or_insert_default`] gets the current value, inserting the
480	//! default value of type `T` (which must implement [`Default`]) if it is
481	//! [`None`]
482	//! * [`get_or_insert_with`] gets the current value, inserting a default
483	//! computed by the provided function if it is [`None`]
484	//!
485	//! [`get_or_insert`]: Option::get_or_insert
486	//! [`get_or_insert_default`]: Option::get_or_insert_default
487	//! [`get_or_insert_with`]: Option::get_or_insert_with
488	//! [`insert`]: Option::insert
489	//!
490	//! These methods transfer ownership of the contained value of an
491	//! [`Option`]:
492	//!
493	//! * [`take`] takes ownership of the contained value of an [`Option`], if
494	//! any, replacing the [`Option`] with [`None`]
495	//! * [`replace`] takes ownership of the contained value of an [`Option`],
496	//! if any, replacing the [`Option`] with a [`Some`] containing the
497	//! provided value
498	//!
499	//! [`replace`]: Option::replace
500	//! [`take`]: Option::take
501	//!
502	//! # Examples
503	//!
504	//! Basic pattern matching on [`Option`]:
505	//!
506	//! ```
507	//! let msg = Some("howdy");
508	//!
509	//! // Take a reference to the contained string
510	//! if let Some(m) = &msg {
511	//! println!("{}", *m);
512	//! }
513	//!
514	//! // Remove the contained string, destroying the Option
515	//! let unwrapped_msg = msg.unwrap_or("default message");
516	//! ```
517	//!
518	//! Initialize a result to [`None`] before a loop:
519	//!
520	//! ```
521	//! enum Kingdom { Plant(u32, &'static str), Animal(u32, &'static str) }
522	//!
523	//! // A list of data to search through.
524	//! let all_the_big_things = [
525	//! Kingdom::Plant(`250`, "redwood"),
526	//! Kingdom::Plant(`230`, "noble fir"),
527	//! Kingdom::Plant(`229`, "sugar pine"),
528	//! Kingdom::Animal(`25`, "blue whale"),
529	//! Kingdom::Animal(`19`, "fin whale"),
530	//! Kingdom::Animal(`15`, "north pacific right whale"),
531	//! ];
532	//!
533	//! // We're going to search for the name of the biggest animal,
534	//! // but to start with we've just got `None`.
535	//! let mut name_of_biggest_animal = None;
536	//! let mut size_of_biggest_animal = `0`;
537	//! for big_thing in &all_the_big_things {
538	//! match *big_thing {
539	//! Kingdom::Animal(size, name) if size > size_of_biggest_animal => {
540	//! // Now we've found the name of some big animal
541	//! size_of_biggest_animal = size;
542	//! name_of_biggest_animal = Some(name);
543	//! }
544	//! Kingdom::Animal(..) \| Kingdom::Plant(..) => ()
545	//! }
546	//! }
547	//!
548	//! match name_of_biggest_animal {
549	//! Some(name) => println!("the biggest animal is {name}"),
550	//! None => println!("there are no animals :("),
551	//! }
552	//! ```
553
554	#![stable(feature = "rust1", since = "1.0.0")]
555
556	use crate::iter::{self, FusedIterator, TrustedLen};
557	use crate::panicking::{panic, panic_display};
558	use crate::pin::Pin;
559	use crate::{
560	cmp, convert, hint, mem,
561	ops::{self, ControlFlow, Deref, DerefMut},
562	slice,
563	};
564
565	/// The `Option` type. See [the module level documentation](self) for more.
566	#[derive(Copy, Eq, Debug, Hash)]
567	#[rustc_diagnostic_item = "Option"]
568	#[lang = "Option"]
569	#[stable(feature = "rust1", since = "1.0.0")]
570	#[allow(clippy::derived_hash_with_manual_eq)] // PartialEq is manually implemented equivalently
571	pub enum Option<T> {
572	/// No value.
573	#[lang = "None"]
574	#[stable(feature = "rust1", since = "1.0.0")]
575	None,
576	/// Some value of type `T`.
577	#[lang = "Some"]
578	#[stable(feature = "rust1", since = "1.0.0")]
579	Some(#[stable(feature = "rust1", since = "1.0.0")] T),
580	}
581
582	/////////////////////////////////////////////////////////////////////////////
583	// Type implementation
584	/////////////////////////////////////////////////////////////////////////////
585
586	impl<T> Option<T> {
587	/////////////////////////////////////////////////////////////////////////
588	// Querying the contained values
589	/////////////////////////////////////////////////////////////////////////
590
591	/// Returns `true` if the option is a [`Some`] value.
592	///
593	/// # Examples
594	///
595	/// ```
596	/// let x: Option<u32> = Some(`2`);
597	/// assert_eq!(x.is_some(), `true`);
598	///
599	/// let x: Option<u32> = None;
600	/// assert_eq!(x.is_some(), `false`);
601	/// ```
602	#[must_use = "if you intended to assert that this has a value, consider `.unwrap()` instead"]
603	#[inline]
604	#[stable(feature = "rust1", since = "1.0.0")]
605	#[rustc_const_stable(feature = "const_option_basics", since = "1.48.0")]
606	pub const fn is_some(&self) -> bool {
607	matches!(*self, Some(_))
608	}
609
610	/// Returns `true` if the option is a [`Some`] and the value inside of it matches a predicate.
611	///
612	/// # Examples
613	///
614	/// ```
615	/// let x: Option<u32> = Some(`2`);
616	/// assert_eq!(x.is_some_and(\|x\| x > `1`), `true`);
617	///
618	/// let x: Option<u32> = Some(`0`);
619	/// assert_eq!(x.is_some_and(\|x\| x > `1`), `false`);
620	///
621	/// let x: Option<u32> = None;
622	/// assert_eq!(x.is_some_and(\|x\| x > `1`), `false`);
623	/// ```
624	#[must_use]
625	#[inline]
626	#[stable(feature = "is_some_and", since = "1.70.0")]
627	pub fn is_some_and(self, f: impl FnOnce(T) -> bool) -> bool {
628	match self {
629	None => `false`,
630	Some(x) => f(x),
631	}
632	}
633
634	/// Returns `true` if the option is a [`None`] value.
635	///
636	/// # Examples
637	///
638	/// ```
639	/// let x: Option<u32> = Some(`2`);
640	/// assert_eq!(x.is_none(), `false`);
641	///
642	/// let x: Option<u32> = None;
643	/// assert_eq!(x.is_none(), `true`);
644	/// ```
645	#[must_use = "if you intended to assert that this doesn't have a value, consider \
646	wrapping this in an `assert!()` instead"]
647	#[inline]
648	#[stable(feature = "rust1", since = "1.0.0")]
649	#[rustc_const_stable(feature = "const_option_basics", since = "1.48.0")]
650	pub const fn is_none(&self) -> bool {
651	!self.is_some()
652	}
653
654	/////////////////////////////////////////////////////////////////////////
655	// Adapter for working with references
656	/////////////////////////////////////////////////////////////////////////
657
658	/// Converts from `&Option<T>` to `Option<&T>`.
659	///
660	/// # Examples
661	///
662	/// Calculates the length of an <code>Option<[String]></code> as an <code>Option<[usize]></code>
663	/// without moving the [`String`]. The [`map`] method takes the `self` argument by value,
664	/// consuming the original, so this technique uses `as_ref` to first take an `Option` to a
665	/// reference to the value inside the original.
666	///
667	/// [`map`]: Option::map
668	/// [String]: ../../std/string/struct.String.html "String"
669	/// [`String`]: ../../std/string/struct.String.html "String"
670	///
671	/// ```
672	/// let text: Option<String> = Some("Hello, world!".to_string());
673	/// // First, cast `Option<String>` to `Option<&String>` with `as_ref`,
674	/// // then consume that* with `map`, leaving `text` on the stack.*
675	/// let text_length: Option<usize> = text.as_ref().map(\|s\| s.len());
676	/// println!("still can print text: {text:?}");
677	/// ```
678	#[inline]
679	#[rustc_const_stable(feature = "const_option_basics", since = "1.48.0")]
680	#[stable(feature = "rust1", since = "1.0.0")]
681	pub const fn as_ref(&self) -> Option<&T> {
682	match *self {
683	Some(ref x) => Some(x),
684	None => None,
685	}
686	}
687
688	/// Converts from `&mut Option<T>` to `Option<&mut T>`.
689	///
690	/// # Examples
691	///
692	/// ```
693	/// let mut x = Some(`2`);
694	/// match x.as_mut() {
695	/// Some(v) => *v = `42`,
696	/// None => {},
697	/// }
698	/// assert_eq!(x, Some(`42`));
699	/// ```
700	#[inline]
701	#[stable(feature = "rust1", since = "1.0.0")]
702	#[rustc_const_unstable(feature = "const_option", issue = "67441")]
703	pub const fn as_mut(&mut self) -> Option<&mut T> {
704	match *self {
705	Some(ref mut x) => Some(x),
706	None => None,
707	}
708	}
709
710	/// Converts from <code>[Pin]<[&]Option\<T>></code> to <code>Option<[Pin]<[&]T>></code>.
711	///
712	/// [&]: reference "shared reference"
713	#[inline]
714	#[must_use]
715	#[stable(feature = "pin", since = "1.33.0")]
716	#[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
717	pub const fn as_pin_ref(self: Pin<&Self>) -> Option<Pin<&T>> {
718	match Pin::get_ref(self).as_ref() {
719	// SAFETY: `x` is guaranteed to be pinned because it comes from `self`
720	// which is pinned.
721	Some(x) => unsafe { Some(Pin::new_unchecked(x)) },
722	None => None,
723	}
724	}
725
726	/// Converts from <code>[Pin]<[&mut] Option\<T>></code> to <code>Option<[Pin]<[&mut] T>></code>.
727	///
728	/// [&mut]: reference "mutable reference"
729	#[inline]
730	#[must_use]
731	#[stable(feature = "pin", since = "1.33.0")]
732	#[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
733	pub const fn as_pin_mut(self: Pin<&mut Self>) -> Option<Pin<&mut T>> {
734	// SAFETY: `get_unchecked_mut` is never used to move the `Option` inside `self`.
735	// `x` is guaranteed to be pinned because it comes from `self` which is pinned.
736	unsafe {
737	match Pin::get_unchecked_mut(self).as_mut() {
738	Some(x) => Some(Pin::new_unchecked(x)),
739	None => None,
740	}
741	}
742	}
743
744	/// Returns a slice of the contained value, if any. If this is `None`, an
745	/// empty slice is returned. This can be useful to have a single type of
746	/// iterator over an `Option` or slice.
747	///
748	/// Note: Should you have an `Option<&T>` and wish to get a slice of `T`,
749	/// you can unpack it via `opt.map_or(&[], std::slice::from_ref)`.
750	///
751	/// # Examples
752	///
753	/// ```rust
754	/// assert_eq!(
755	/// [Some(`1234`).as_slice(), None.as_slice()],
756	/// [&[`1234`][..], &[][..]],
757	/// );
758	/// ```
759	///
760	/// The inverse of this function is (discounting
761	/// borrowing) [`[_]::first`](slice::first):
762	///
763	/// ```rust
764	/// for i in [Some(`1234_u16`), None] {
765	/// assert_eq!(i.as_ref(), i.as_slice().first());
766	/// }
767	/// ```
768	#[inline]
769	#[must_use]
770	#[stable(feature = "option_as_slice", since = "1.75.0")]
771	pub fn as_slice(&self) -> &[T] {
772	// SAFETY: When the `Option` is `Some`, we're using the actual pointer
773	// to the payload, with a length of 1, so this is equivalent to
774	// `slice::from_ref`, and thus is safe.
775	// When the `Option` is `None`, the length used is 0, so to be safe it
776	// just needs to be aligned, which it is because `&self` is aligned and
777	// the offset used is a multiple of alignment.
778	//
779	// In the new version, the intrinsic always returns a pointer to an
780	// in-bounds and correctly aligned position for a `T` (even if in the
781	// `None` case it's just padding).
782	unsafe {
783	slice::from_raw_parts(
784	(self as *const Self).byte_add(core::mem::offset_of!(Self, Some.0)).cast(),
785	usize::from(self.is_some()),
786	)
787	}
788	}
789
790	/// Returns a mutable slice of the contained value, if any. If this is
791	/// `None`, an empty slice is returned. This can be useful to have a
792	/// single type of iterator over an `Option` or slice.
793	///
794	/// Note: Should you have an `Option<&mut T>` instead of a
795	/// `&mut Option<T>`, which this method takes, you can obtain a mutable
796	/// slice via `opt.map_or(&mut [], std::slice::from_mut)`.
797	///
798	/// # Examples
799	///
800	/// ```rust
801	/// assert_eq!(
802	/// [Some(`1234`).as_mut_slice(), None.as_mut_slice()],
803	/// [&mut [`1234`][..], &mut [][..]],
804	/// );
805	/// ```
806	///
807	/// The result is a mutable slice of zero or one items that points into
808	/// our original `Option`:
809	///
810	/// ```rust
811	/// let mut x = Some(`1234`);
812	/// x.as_mut_slice()[`0`] += `1`;
813	/// assert_eq!(x, Some(`1235`));
814	/// ```
815	///
816	/// The inverse of this method (discounting borrowing)
817	/// is [`[_]::first_mut`](slice::first_mut):
818	///
819	/// ```rust
820	/// assert_eq!(Some(`123`).as_mut_slice().first_mut(), Some(&mut `123`))
821	/// ```
822	#[inline]
823	#[must_use]
824	#[stable(feature = "option_as_slice", since = "1.75.0")]
825	pub fn as_mut_slice(&mut self) -> &mut [T] {
826	// SAFETY: When the `Option` is `Some`, we're using the actual pointer
827	// to the payload, with a length of 1, so this is equivalent to
828	// `slice::from_mut`, and thus is safe.
829	// When the `Option` is `None`, the length used is 0, so to be safe it
830	// just needs to be aligned, which it is because `&self` is aligned and
831	// the offset used is a multiple of alignment.
832	//
833	// In the new version, the intrinsic creates a `const T` from a*
834	// mutable reference so it is safe to cast back to a mutable pointer
835	// here. As with `as_slice`, the intrinsic always returns a pointer to
836	// an in-bounds and correctly aligned position for a `T` (even if in
837	// the `None` case it's just padding).
838	unsafe {
839	slice::from_raw_parts_mut(
840	(self as *mut Self).byte_add(core::mem::offset_of!(Self, Some.0)).cast(),
841	usize::from(self.is_some()),
842	)
843	}
844	}
845
846	/////////////////////////////////////////////////////////////////////////
847	// Getting to contained values
848	/////////////////////////////////////////////////////////////////////////
849
850	/// Returns the contained [`Some`] value, consuming the `self` value.
851	///
852	/// # Panics
853	///
854	/// Panics if the value is a [`None`] with a custom panic message provided by
855	/// `msg`.
856	///
857	/// # Examples
858	///
859	/// ```
860	/// let x = Some("value");
861	/// assert_eq!(x.expect("fruits are healthy"), "value");
862	/// ```
863	///
864	/// ```should_panic
865	/// let x: Option<&str> = None;
866	/// x.expect("fruits are healthy"); // panics with `fruits are healthy`
867	/// ```
868	///
869	/// # Recommended Message Style
870	///
871	/// We recommend that `expect` messages are used to describe the reason you
872	/// _expect_ the `Option` should be `Some`.
873	///
874	/// ```should_panic
875	/// # let slice: &[u8] = &[];
876	/// let item = slice.get(`0`)
877	/// .expect("slice should not be empty");
878	/// ```
879	///
880	/// Hint: If you're having trouble remembering how to phrase expect
881	/// error messages remember to focus on the word "should" as in "env
882	/// variable should be set by blah" or "the given binary should be available
883	/// and executable by the current user".
884	///
885	/// For more detail on expect message styles and the reasoning behind our
886	/// recommendation please refer to the section on ["Common Message
887	/// Styles"](../../std/error/index.html#common-message-styles) in the [`std::error`](../../std/error/index.html) module docs.
888	#[inline]
889	#[track_caller]
890	#[stable(feature = "rust1", since = "1.0.0")]
891	#[rustc_const_unstable(feature = "const_option", issue = "67441")]
892	pub const fn expect(self, msg: &str) -> T {
893	match self {
894	Some(val) => val,
895	None => expect_failed(msg),
896	}
897	}
898
899	/// Returns the contained [`Some`] value, consuming the `self` value.
900	///
901	/// Because this function may panic, its use is generally discouraged.
902	/// Instead, prefer to use pattern matching and handle the [`None`]
903	/// case explicitly, or call [`unwrap_or`], [`unwrap_or_else`], or
904	/// [`unwrap_or_default`].
905	///
906	/// [`unwrap_or`]: Option::unwrap_or
907	/// [`unwrap_or_else`]: Option::unwrap_or_else
908	/// [`unwrap_or_default`]: Option::unwrap_or_default
909	///
910	/// # Panics
911	///
912	/// Panics if the self value equals [`None`].
913	///
914	/// # Examples
915	///
916	/// ```
917	/// let x = Some("air");
918	/// assert_eq!(x.unwrap(), "air");
919	/// ```
920	///
921	/// ```should_panic
922	/// let x: Option<&str> = None;
923	/// assert_eq!(x.unwrap(), "air"); // fails
924	/// ```
925	#[inline(always)]
926	#[track_caller]
927	#[stable(feature = "rust1", since = "1.0.0")]
928	#[rustc_const_unstable(feature = "const_option", issue = "67441")]
929	pub const fn unwrap(self) -> T {
930	match self {
931	Some(val) => val,
932	None => unwrap_failed(),
933	}
934	}
935
936	/// Returns the contained [`Some`] value or a provided default.
937	///
938	/// Arguments passed to `unwrap_or` are eagerly evaluated; if you are passing
939	/// the result of a function call, it is recommended to use [`unwrap_or_else`],
940	/// which is lazily evaluated.
941	///
942	/// [`unwrap_or_else`]: Option::unwrap_or_else
943	///
944	/// # Examples
945	///
946	/// ```
947	/// assert_eq!(Some("car").unwrap_or("bike"), "car");
948	/// assert_eq!(None.unwrap_or("bike"), "bike");
949	/// ```
950	#[inline]
951	#[stable(feature = "rust1", since = "1.0.0")]
952	pub fn unwrap_or(self, default: T) -> T {
953	match self {
954	Some(x) => x,
955	None => default,
956	}
957	}
958
959	/// Returns the contained [`Some`] value or computes it from a closure.
960	///
961	/// # Examples
962	///
963	/// ```
964	/// let k = `10`;
965	/// assert_eq!(Some(`4`).unwrap_or_else(\|\| `2` * k), `4`);
966	/// assert_eq!(None.unwrap_or_else(\|\| `2` * k), `20`);
967	/// ```
968	#[inline]
969	#[track_caller]
970	#[stable(feature = "rust1", since = "1.0.0")]
971	pub fn unwrap_or_else<F>(self, f: F) -> T
972	where
973	F: FnOnce() -> T,
974	{
975	match self {
976	Some(x) => x,
977	None => f(),
978	}
979	}
980
981	/// Returns the contained [`Some`] value or a default.
982	///
983	/// Consumes the `self` argument then, if [`Some`], returns the contained
984	/// value, otherwise if [`None`], returns the [default value] for that
985	/// type.
986	///
987	/// # Examples
988	///
989	/// ```
990	/// let x: Option<u32> = None;
991	/// let y: Option<u32> = Some(`12`);
992	///
993	/// assert_eq!(x.unwrap_or_default(), `0`);
994	/// assert_eq!(y.unwrap_or_default(), `12`);
995	/// ```
996	///
997	/// [default value]: Default::default
998	/// [`parse`]: str::parse
999	/// [`FromStr`]: crate::str::FromStr
1000	#[inline]
1001	#[stable(feature = "rust1", since = "1.0.0")]
1002	pub fn unwrap_or_default(self) -> T
1003	where
1004	T: Default,
1005	{
1006	match self {
1007	Some(x) => x,
1008	None => T::default(),
1009	}
1010	}
1011
1012	/// Returns the contained [`Some`] value, consuming the `self` value,
1013	/// without checking that the value is not [`None`].
1014	///
1015	/// # Safety
1016	///
1017	/// Calling this method on [`None`] is [undefined behavior].
1018	///
1019	/// [undefined behavior]: https://doc.rust-lang.org/reference/behavior-considered-undefined.html
1020	///
1021	/// # Examples
1022	///
1023	/// ```
1024	/// let x = Some("air");
1025	/// assert_eq!(unsafe { x.unwrap_unchecked() }, "air");
1026	/// ```
1027	///
1028	/// ```no_run
1029	/// let x: Option<&str> = None;
1030	/// assert_eq!(unsafe { x.unwrap_unchecked() }, "air"); // Undefined behavior!
1031	/// ```
1032	#[inline]
1033	#[track_caller]
1034	#[stable(feature = "option_result_unwrap_unchecked", since = "1.58.0")]
1035	#[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
1036	pub const unsafe fn unwrap_unchecked(self) -> T {
1037	match self {
1038	Some(val) => val,
1039	// SAFETY: the safety contract must be upheld by the caller.
1040	None => unsafe { hint::unreachable_unchecked() },
1041	}
1042	}
1043
1044	/////////////////////////////////////////////////////////////////////////
1045	// Transforming contained values
1046	/////////////////////////////////////////////////////////////////////////
1047
1048	/// Maps an `Option<T>` to `Option<U>` by applying a function to a contained value (if `Some`) or returns `None` (if `None`).
1049	///
1050	/// # Examples
1051	///
1052	/// Calculates the length of an <code>Option<[String]></code> as an
1053	/// <code>Option<[usize]></code>, consuming the original:
1054	///
1055	/// [String]: ../../std/string/struct.String.html "String"
1056	/// ```
1057	/// let maybe_some_string = Some(String::from("Hello, World!"));
1058	/// // `Option::map` takes self by value, consuming `maybe_some_string`
1059	/// let maybe_some_len = maybe_some_string.map(\|s\| s.len());
1060	/// assert_eq!(maybe_some_len, Some(`13`));
1061	///
1062	/// let x: Option<&str> = None;
1063	/// assert_eq!(x.map(\|s\| s.len()), None);
1064	/// ```
1065	#[inline]
1066	#[stable(feature = "rust1", since = "1.0.0")]
1067	pub fn map<U, F>(self, f: F) -> Option<U>
1068	where
1069	F: FnOnce(T) -> U,
1070	{
1071	match self {
1072	Some(x) => Some(f(x)),
1073	None => None,
1074	}
1075	}
1076
1077	/// Calls a function with a reference to the contained value if [`Some`].
1078	///
1079	/// Returns the original option.
1080	///
1081	/// # Examples
1082	///
1083	/// ```
1084	/// let list = vec![`1`, `2`, `3`];
1085	///
1086	/// // prints "got: 2"
1087	/// let x = list
1088	/// .get(`1`)
1089	/// .inspect(\|x\| println!("got: {x}"))
1090	/// .expect("list should be long enough");
1091	///
1092	/// // prints nothing
1093	/// list.get(`5`).inspect(\|x\| println!("got: {x}"));
1094	/// ```
1095	#[inline]
1096	#[stable(feature = "result_option_inspect", since = "1.76.0")]
1097	pub fn inspect<F: FnOnce(&T)>(self, f: F) -> Self {
1098	if let Some(ref x) = self {
1099	f(x);
1100	}
1101
1102	self
1103	}
1104
1105	/// Returns the provided default result (if none),
1106	/// or applies a function to the contained value (if any).
1107	///
1108	/// Arguments passed to `map_or` are eagerly evaluated; if you are passing
1109	/// the result of a function call, it is recommended to use [`map_or_else`],
1110	/// which is lazily evaluated.
1111	///
1112	/// [`map_or_else`]: Option::map_or_else
1113	///
1114	/// # Examples
1115	///
1116	/// ```
1117	/// let x = Some("foo");
1118	/// assert_eq!(x.map_or(`42`, \|v\| v.len()), `3`);
1119	///
1120	/// let x: Option<&str> = None;
1121	/// assert_eq!(x.map_or(`42`, \|v\| v.len()), `42`);
1122	/// ```
1123	#[inline]
1124	#[stable(feature = "rust1", since = "1.0.0")]
1125	#[must_use = "if you don't need the returned value, use `if let` instead"]
1126	pub fn map_or<U, F>(self, default: U, f: F) -> U
1127	where
1128	F: FnOnce(T) -> U,
1129	{
1130	match self {
1131	Some(t) => f(t),
1132	None => default,
1133	}
1134	}
1135
1136	/// Computes a default function result (if none), or
1137	/// applies a different function to the contained value (if any).
1138	///
1139	/// # Basic examples
1140	///
1141	/// ```
1142	/// let k = `21`;
1143	///
1144	/// let x = Some("foo");
1145	/// assert_eq!(x.map_or_else(\|\| `2` * k, \|v\| v.len()), `3`);
1146	///
1147	/// let x: Option<&str> = None;
1148	/// assert_eq!(x.map_or_else(\|\| `2` * k, \|v\| v.len()), `42`);
1149	/// ```
1150	///
1151	/// # Handling a Result-based fallback
1152	///
1153	/// A somewhat common occurrence when dealing with optional values
1154	/// in combination with [`Result<T, E>`] is the case where one wants to invoke
1155	/// a fallible fallback if the option is not present. This example
1156	/// parses a command line argument (if present), or the contents of a file to
1157	/// an integer. However, unlike accessing the command line argument, reading
1158	/// the file is fallible, so it must be wrapped with `Ok`.
1159	///
1160	/// ```no_run
1161	/// # fn main() -> Result<(), Box<dyn std::error::Error>> {
1162	/// let v: u64 = std::env::args()
1163	/// .nth(`1`)
1164	/// .map_or_else(\|\| std::fs::read_to_string("/etc/someconfig.conf"), Ok)?
1165	/// .parse()?;
1166	/// # Ok(())
1167	/// # }
1168	/// ```
1169	#[inline]
1170	#[stable(feature = "rust1", since = "1.0.0")]
1171	pub fn map_or_else<U, D, F>(self, default: D, f: F) -> U
1172	where
1173	D: FnOnce() -> U,
1174	F: FnOnce(T) -> U,
1175	{
1176	match self {
1177	Some(t) => f(t),
1178	None => default(),
1179	}
1180	}
1181
1182	/// Transforms the `Option<T>` into a [`Result<T, E>`], mapping [`Some(v)`] to
1183	/// [`Ok(v)`] and [`None`] to [`Err(err)`].
1184	///
1185	/// Arguments passed to `ok_or` are eagerly evaluated; if you are passing the
1186	/// result of a function call, it is recommended to use [`ok_or_else`], which is
1187	/// lazily evaluated.
1188	///
1189	/// [`Ok(v)`]: Ok
1190	/// [`Err(err)`]: Err
1191	/// [`Some(v)`]: Some
1192	/// [`ok_or_else`]: Option::ok_or_else
1193	///
1194	/// # Examples
1195	///
1196	/// ```
1197	/// let x = Some("foo");
1198	/// assert_eq!(x.ok_or(`0`), Ok("foo"));
1199	///
1200	/// let x: Option<&str> = None;
1201	/// assert_eq!(x.ok_or(`0`), Err(`0`));
1202	/// ```
1203	#[inline]
1204	#[stable(feature = "rust1", since = "1.0.0")]
1205	pub fn ok_or<E>(self, err: E) -> Result<T, E> {
1206	match self {
1207	Some(v) => Ok(v),
1208	None => Err(err),
1209	}
1210	}
1211
1212	/// Transforms the `Option<T>` into a [`Result<T, E>`], mapping [`Some(v)`] to
1213	/// [`Ok(v)`] and [`None`] to [`Err(err())`].
1214	///
1215	/// [`Ok(v)`]: Ok
1216	/// [`Err(err())`]: Err
1217	/// [`Some(v)`]: Some
1218	///
1219	/// # Examples
1220	///
1221	/// ```
1222	/// let x = Some("foo");
1223	/// assert_eq!(x.ok_or_else(\|\| `0`), Ok("foo"));
1224	///
1225	/// let x: Option<&str> = None;
1226	/// assert_eq!(x.ok_or_else(\|\| `0`), Err(`0`));
1227	/// ```
1228	#[inline]
1229	#[stable(feature = "rust1", since = "1.0.0")]
1230	pub fn ok_or_else<E, F>(self, err: F) -> Result<T, E>
1231	where
1232	F: FnOnce() -> E,
1233	{
1234	match self {
1235	Some(v) => Ok(v),
1236	None => Err(err()),
1237	}
1238	}
1239
1240	/// Converts from `Option<T>` (or `&Option<T>`) to `Option<&T::Target>`.
1241	///
1242	/// Leaves the original Option in-place, creating a new one with a reference
1243	/// to the original one, additionally coercing the contents via [`Deref`].
1244	///
1245	/// # Examples
1246	///
1247	/// ```
1248	/// let x: Option<String> = Some("hey".to_owned());
1249	/// assert_eq!(x.as_deref(), Some("hey"));
1250	///
1251	/// let x: Option<String> = None;
1252	/// assert_eq!(x.as_deref(), None);
1253	/// ```
1254	#[inline]
1255	#[stable(feature = "option_deref", since = "1.40.0")]
1256	pub fn as_deref(&self) -> Option<&T::Target>
1257	where
1258	T: Deref,
1259	{
1260	match self.as_ref() {
1261	Some(t) => Some(t.deref()),
1262	None => None,
1263	}
1264	}
1265
1266	/// Converts from `Option<T>` (or `&mut Option<T>`) to `Option<&mut T::Target>`.
1267	///
1268	/// Leaves the original `Option` in-place, creating a new one containing a mutable reference to
1269	/// the inner type's [`Deref::Target`] type.
1270	///
1271	/// # Examples
1272	///
1273	/// ```
1274	/// let mut x: Option<String> = Some("hey".to_owned());
1275	/// assert_eq!(x.as_deref_mut().map(\|x\| {
1276	/// x.make_ascii_uppercase();
1277	/// x
1278	/// }), Some("HEY".to_owned().as_mut_str()));
1279	/// ```
1280	#[inline]
1281	#[stable(feature = "option_deref", since = "1.40.0")]
1282	pub fn as_deref_mut(&mut self) -> Option<&mut T::Target>
1283	where
1284	T: DerefMut,
1285	{
1286	match self.as_mut() {
1287	Some(t) => Some(t.deref_mut()),
1288	None => None,
1289	}
1290	}
1291
1292	/////////////////////////////////////////////////////////////////////////
1293	// Iterator constructors
1294	/////////////////////////////////////////////////////////////////////////
1295
1296	/// Returns an iterator over the possibly contained value.
1297	///
1298	/// # Examples
1299	///
1300	/// ```
1301	/// let x = Some(`4`);
1302	/// assert_eq!(x.iter().next(), Some(&`4`));
1303	///
1304	/// let x: Option<u32> = None;
1305	/// assert_eq!(x.iter().next(), None);
1306	/// ```
1307	#[inline]
1308	#[rustc_const_unstable(feature = "const_option", issue = "67441")]
1309	#[stable(feature = "rust1", since = "1.0.0")]
1310	pub const fn iter(&self) -> Iter<'_, T> {
1311	Iter { inner: Item { opt: self.as_ref() } }
1312	}
1313
1314	/// Returns a mutable iterator over the possibly contained value.
1315	///
1316	/// # Examples
1317	///
1318	/// ```
1319	/// let mut x = Some(`4`);
1320	/// match x.iter_mut().next() {
1321	/// Some(v) => *v = `42`,
1322	/// None => {},
1323	/// }
1324	/// assert_eq!(x, Some(`42`));
1325	///
1326	/// let mut x: Option<u32> = None;
1327	/// assert_eq!(x.iter_mut().next(), None);
1328	/// ```
1329	#[inline]
1330	#[stable(feature = "rust1", since = "1.0.0")]
1331	pub fn iter_mut(&mut self) -> IterMut<'_, T> {
1332	IterMut { inner: Item { opt: self.as_mut() } }
1333	}
1334
1335	/////////////////////////////////////////////////////////////////////////
1336	// Boolean operations on the values, eager and lazy
1337	/////////////////////////////////////////////////////////////////////////
1338
1339	/// Returns [`None`] if the option is [`None`], otherwise returns `optb`.
1340	///
1341	/// Arguments passed to `and` are eagerly evaluated; if you are passing the
1342	/// result of a function call, it is recommended to use [`and_then`], which is
1343	/// lazily evaluated.
1344	///
1345	/// [`and_then`]: Option::and_then
1346	///
1347	/// # Examples
1348	///
1349	/// ```
1350	/// let x = Some(`2`);
1351	/// let y: Option<&str> = None;
1352	/// assert_eq!(x.and(y), None);
1353	///
1354	/// let x: Option<u32> = None;
1355	/// let y = Some("foo");
1356	/// assert_eq!(x.and(y), None);
1357	///
1358	/// let x = Some(`2`);
1359	/// let y = Some("foo");
1360	/// assert_eq!(x.and(y), Some("foo"));
1361	///
1362	/// let x: Option<u32> = None;
1363	/// let y: Option<&str> = None;
1364	/// assert_eq!(x.and(y), None);
1365	/// ```
1366	#[inline]
1367	#[stable(feature = "rust1", since = "1.0.0")]
1368	pub fn and<U>(self, optb: Option<U>) -> Option<U> {
1369	match self {
1370	Some(_) => optb,
1371	None => None,
1372	}
1373	}
1374
1375	/// Returns [`None`] if the option is [`None`], otherwise calls `f` with the
1376	/// wrapped value and returns the result.
1377	///
1378	/// Some languages call this operation flatmap.
1379	///
1380	/// # Examples
1381	///
1382	/// ```
1383	/// fn sq_then_to_string(x: u32) -> Option<String> {
1384	/// x.checked_mul(x).map(\|sq\| sq.to_string())
1385	/// }
1386	///
1387	/// assert_eq!(Some(`2`).and_then(sq_then_to_string), Some(`4`.to_string()));
1388	/// assert_eq!(Some(`1_000_000`).and_then(sq_then_to_string), None); // overflowed!
1389	/// assert_eq!(None.and_then(sq_then_to_string), None);
1390	/// ```
1391	///
1392	/// Often used to chain fallible operations that may return [`None`].
1393	///
1394	/// ```
1395	/// let arr_2d = [["A0", "A1"], ["B0", "B1"]];
1396	///
1397	/// let item_0_1 = arr_2d.get(`0`).and_then(\|row\| row.get(`1`));
1398	/// assert_eq!(item_0_1, Some(&"A1"));
1399	///
1400	/// let item_2_0 = arr_2d.get(`2`).and_then(\|row\| row.get(`0`));
1401	/// assert_eq!(item_2_0, None);
1402	/// ```
1403	#[doc(alias = "flatmap")]
1404	#[inline]
1405	#[stable(feature = "rust1", since = "1.0.0")]
1406	#[rustc_confusables("flat_map", "flatmap")]
1407	pub fn and_then<U, F>(self, f: F) -> Option<U>
1408	where
1409	F: FnOnce(T) -> Option<U>,
1410	{
1411	match self {
1412	Some(x) => f(x),
1413	None => None,
1414	}
1415	}
1416
1417	/// Returns [`None`] if the option is [`None`], otherwise calls `predicate`
1418	/// with the wrapped value and returns:
1419	///
1420	/// - [`Some(t)`] if `predicate` returns `true` (where `t` is the wrapped
1421	/// value), and
1422	/// - [`None`] if `predicate` returns `false`.
1423	///
1424	/// This function works similar to [`Iterator::filter()`]. You can imagine
1425	/// the `Option<T>` being an iterator over one or zero elements. `filter()`
1426	/// lets you decide which elements to keep.
1427	///
1428	/// # Examples
1429	///
1430	/// ```rust
1431	/// fn is_even(n: &i32) -> bool {
1432	/// n % `2` == `0`
1433	/// }
1434	///
1435	/// assert_eq!(None.filter(is_even), None);
1436	/// assert_eq!(Some(`3`).filter(is_even), None);
1437	/// assert_eq!(Some(`4`).filter(is_even), Some(`4`));
1438	/// ```
1439	///
1440	/// [`Some(t)`]: Some
1441	#[inline]
1442	#[stable(feature = "option_filter", since = "1.27.0")]
1443	pub fn filter<P>(self, predicate: P) -> Self
1444	where
1445	P: FnOnce(&T) -> bool,
1446	{
1447	if let Some(x) = self {
1448	if predicate(&x) {
1449	return Some(x);
1450	}
1451	}
1452	None
1453	}
1454
1455	/// Returns the option if it contains a value, otherwise returns `optb`.
1456	///
1457	/// Arguments passed to `or` are eagerly evaluated; if you are passing the
1458	/// result of a function call, it is recommended to use [`or_else`], which is
1459	/// lazily evaluated.
1460	///
1461	/// [`or_else`]: Option::or_else
1462	///
1463	/// # Examples
1464	///
1465	/// ```
1466	/// let x = Some(`2`);
1467	/// let y = None;
1468	/// assert_eq!(x.or(y), Some(`2`));
1469	///
1470	/// let x = None;
1471	/// let y = Some(`100`);
1472	/// assert_eq!(x.or(y), Some(`100`));
1473	///
1474	/// let x = Some(`2`);
1475	/// let y = Some(`100`);
1476	/// assert_eq!(x.or(y), Some(`2`));
1477	///
1478	/// let x: Option<u32> = None;
1479	/// let y = None;
1480	/// assert_eq!(x.or(y), None);
1481	/// ```
1482	#[inline]
1483	#[stable(feature = "rust1", since = "1.0.0")]
1484	pub fn or(self, optb: Option<T>) -> Option<T> {
1485	match self {
1486	x @ Some(_) => x,
1487	None => optb,
1488	}
1489	}
1490
1491	/// Returns the option if it contains a value, otherwise calls `f` and
1492	/// returns the result.
1493	///
1494	/// # Examples
1495	///
1496	/// ```
1497	/// fn nobody() -> Option<&'static str> { None }
1498	/// fn vikings() -> Option<&'static str> { Some("vikings") }
1499	///
1500	/// assert_eq!(Some("barbarians").or_else(vikings), Some("barbarians"));
1501	/// assert_eq!(None.or_else(vikings), Some("vikings"));
1502	/// assert_eq!(None.or_else(nobody), None);
1503	/// ```
1504	#[inline]
1505	#[stable(feature = "rust1", since = "1.0.0")]
1506	pub fn or_else<F>(self, f: F) -> Option<T>
1507	where
1508	F: FnOnce() -> Option<T>,
1509	{
1510	match self {
1511	x @ Some(_) => x,
1512	None => f(),
1513	}
1514	}
1515
1516	/// Returns [`Some`] if exactly one of `self`, `optb` is [`Some`], otherwise returns [`None`].
1517	///
1518	/// # Examples
1519	///
1520	/// ```
1521	/// let x = Some(`2`);
1522	/// let y: Option<u32> = None;
1523	/// assert_eq!(x.xor(y), Some(`2`));
1524	///
1525	/// let x: Option<u32> = None;
1526	/// let y = Some(`2`);
1527	/// assert_eq!(x.xor(y), Some(`2`));
1528	///
1529	/// let x = Some(`2`);
1530	/// let y = Some(`2`);
1531	/// assert_eq!(x.xor(y), None);
1532	///
1533	/// let x: Option<u32> = None;
1534	/// let y: Option<u32> = None;
1535	/// assert_eq!(x.xor(y), None);
1536	/// ```
1537	#[inline]
1538	#[stable(feature = "option_xor", since = "1.37.0")]
1539	pub fn xor(self, optb: Option<T>) -> Option<T> {
1540	match (self, optb) {
1541	(a @ Some(_), None) => a,
1542	(None, b @ Some(_)) => b,
1543	_ => None,
1544	}
1545	}
1546
1547	/////////////////////////////////////////////////////////////////////////
1548	// Entry-like operations to insert a value and return a reference
1549	/////////////////////////////////////////////////////////////////////////
1550
1551	/// Inserts `value` into the option, then returns a mutable reference to it.
1552	///
1553	/// If the option already contains a value, the old value is dropped.
1554	///
1555	/// See also [`Option::get_or_insert`], which doesn't update the value if
1556	/// the option already contains [`Some`].
1557	///
1558	/// # Example
1559	///
1560	/// ```
1561	/// let mut opt = None;
1562	/// let val = opt.insert(`1`);
1563	/// assert_eq!(*val, `1`);
1564	/// assert_eq!(opt.unwrap(), `1`);
1565	/// let val = opt.insert(`2`);
1566	/// assert_eq!(*val, `2`);
1567	/// *val = `3`;
1568	/// assert_eq!(opt.unwrap(), `3`);
1569	/// ```
1570	#[must_use = "if you intended to set a value, consider assignment instead"]
1571	#[inline]
1572	#[stable(feature = "option_insert", since = "1.53.0")]
1573	pub fn insert(&mut self, value: T) -> &mut T {
1574	*self = Some(value);
1575
1576	// SAFETY: the code above just filled the option
1577	unsafe { self.as_mut().unwrap_unchecked() }
1578	}
1579
1580	/// Inserts `value` into the option if it is [`None`], then
1581	/// returns a mutable reference to the contained value.
1582	///
1583	/// See also [`Option::insert`], which updates the value even if
1584	/// the option already contains [`Some`].
1585	///
1586	/// # Examples
1587	///
1588	/// ```
1589	/// let mut x = None;
1590	///
1591	/// {
1592	/// let y: &mut u32 = x.get_or_insert(`5`);
1593	/// assert_eq!(y, &`5`);
1594	///
1595	/// *y = `7`;
1596	/// }
1597	///
1598	/// assert_eq!(x, Some(`7`));
1599	/// ```
1600	#[inline]
1601	#[stable(feature = "option_entry", since = "1.20.0")]
1602	pub fn get_or_insert(&mut self, value: T) -> &mut T {
1603	if let None = *self {
1604	*self = Some(value);
1605	}
1606
1607	// SAFETY: a `None` variant for `self` would have been replaced by a `Some`
1608	// variant in the code above.
1609	unsafe { self.as_mut().unwrap_unchecked() }
1610	}
1611
1612	/// Inserts the default value into the option if it is [`None`], then
1613	/// returns a mutable reference to the contained value.
1614	///
1615	/// # Examples
1616	///
1617	/// ```
1618	/// #![feature(option_get_or_insert_default)]
1619	///
1620	/// let mut x = None;
1621	///
1622	/// {
1623	/// let y: &mut u32 = x.get_or_insert_default();
1624	/// assert_eq!(y, &`0`);
1625	///
1626	/// *y = `7`;
1627	/// }
1628	///
1629	/// assert_eq!(x, Some(`7`));
1630	/// ```
1631	#[inline]
1632	#[unstable(feature = "option_get_or_insert_default", issue = "82901")]
1633	pub fn get_or_insert_default(&mut self) -> &mut T
1634	where
1635	T: Default,
1636	{
1637	self.get_or_insert_with(T::default)
1638	}
1639
1640	/// Inserts a value computed from `f` into the option if it is [`None`],
1641	/// then returns a mutable reference to the contained value.
1642	///
1643	/// # Examples
1644	///
1645	/// ```
1646	/// let mut x = None;
1647	///
1648	/// {
1649	/// let y: &mut u32 = x.get_or_insert_with(\|\| `5`);
1650	/// assert_eq!(y, &`5`);
1651	///
1652	/// *y = `7`;
1653	/// }
1654	///
1655	/// assert_eq!(x, Some(`7`));
1656	/// ```
1657	#[inline]
1658	#[stable(feature = "option_entry", since = "1.20.0")]
1659	pub fn get_or_insert_with<F>(&mut self, f: F) -> &mut T
1660	where
1661	F: FnOnce() -> T,
1662	{
1663	if let None = self {
1664	*self = Some(f());
1665	}
1666
1667	// SAFETY: a `None` variant for `self` would have been replaced by a `Some`
1668	// variant in the code above.
1669	unsafe { self.as_mut().unwrap_unchecked() }
1670	}
1671
1672	/////////////////////////////////////////////////////////////////////////
1673	// Misc
1674	/////////////////////////////////////////////////////////////////////////
1675
1676	/// Takes the value out of the option, leaving a [`None`] in its place.
1677	///
1678	/// # Examples
1679	///
1680	/// ```
1681	/// let mut x = Some(`2`);
1682	/// let y = x.take();
1683	/// assert_eq!(x, None);
1684	/// assert_eq!(y, Some(`2`));
1685	///
1686	/// let mut x: Option<u32> = None;
1687	/// let y = x.take();
1688	/// assert_eq!(x, None);
1689	/// assert_eq!(y, None);
1690	/// ```
1691	#[inline]
1692	#[stable(feature = "rust1", since = "1.0.0")]
1693	#[rustc_const_unstable(feature = "const_option", issue = "67441")]
1694	pub const fn take(&mut self) -> Option<T> {
1695	// FIXME replace `mem::replace` by `mem::take` when the latter is const ready
1696	mem::replace(self, None)
1697	}
1698
1699	/// Takes the value out of the option, but only if the predicate evaluates to
1700	/// `true` on a mutable reference to the value.
1701	///
1702	/// In other words, replaces `self` with `None` if the predicate returns `true`.
1703	/// This method operates similar to [`Option::take`] but conditional.
1704	///
1705	/// # Examples
1706	///
1707	/// ```
1708	/// #![feature(option_take_if)]
1709	///
1710	/// let mut x = Some(`42`);
1711	///
1712	/// let prev = x.take_if(\|v\| if *v == `42` {
1713	/// *v += `1`;
1714	/// `false`
1715	/// } else {
1716	/// `false`
1717	/// });
1718	/// assert_eq!(x, Some(`43`));
1719	/// assert_eq!(prev, None);
1720	///
1721	/// let prev = x.take_if(\|v\| *v == `43`);
1722	/// assert_eq!(x, None);
1723	/// assert_eq!(prev, Some(`43`));
1724	/// ```
1725	#[inline]
1726	#[unstable(feature = "option_take_if", issue = "98934")]
1727	pub fn take_if<P>(&mut self, predicate: P) -> Option<T>
1728	where
1729	P: FnOnce(&mut T) -> bool,
1730	{
1731	if self.as_mut().map_or(`false`, predicate) { self.take() } else { None }
1732	}
1733
1734	/// Replaces the actual value in the option by the value given in parameter,
1735	/// returning the old value if present,
1736	/// leaving a [`Some`] in its place without deinitializing either one.
1737	///
1738	/// # Examples
1739	///
1740	/// ```
1741	/// let mut x = Some(`2`);
1742	/// let old = x.replace(`5`);
1743	/// assert_eq!(x, Some(`5`));
1744	/// assert_eq!(old, Some(`2`));
1745	///
1746	/// let mut x = None;
1747	/// let old = x.replace(`3`);
1748	/// assert_eq!(x, Some(`3`));
1749	/// assert_eq!(old, None);
1750	/// ```
1751	#[inline]
1752	#[rustc_const_unstable(feature = "const_option", issue = "67441")]
1753	#[stable(feature = "option_replace", since = "1.31.0")]
1754	pub const fn replace(&mut self, value: T) -> Option<T> {
1755	mem::replace(self, Some(value))
1756	}
1757
1758	/// Zips `self` with another `Option`.
1759	///
1760	/// If `self` is `Some(s)` and `other` is `Some(o)`, this method returns `Some((s, o))`.
1761	/// Otherwise, `None` is returned.
1762	///
1763	/// # Examples
1764	///
1765	/// ```
1766	/// let x = Some(`1`);
1767	/// let y = Some("hi");
1768	/// let z = None::<u8>;
1769	///
1770	/// assert_eq!(x.zip(y), Some((`1`, "hi")));
1771	/// assert_eq!(x.zip(z), None);
1772	/// ```
1773	#[stable(feature = "option_zip_option", since = "1.46.0")]
1774	pub fn zip<U>(self, other: Option<U>) -> Option<(T, U)> {
1775	match (self, other) {
1776	(Some(a), Some(b)) => Some((a, b)),
1777	_ => None,
1778	}
1779	}
1780
1781	/// Zips `self` and another `Option` with function `f`.
1782	///
1783	/// If `self` is `Some(s)` and `other` is `Some(o)`, this method returns `Some(f(s, o))`.
1784	/// Otherwise, `None` is returned.
1785	///
1786	/// # Examples
1787	///
1788	/// ```
1789	/// #![feature(option_zip)]
1790	///
1791	/// #[derive(Debug, PartialEq)]
1792	/// struct Point {
1793	/// x: f64,
1794	/// y: f64,
1795	/// }
1796	///
1797	/// impl Point {
1798	/// fn new(x: f64, y: f64) -> Self {
1799	/// Self { x, y }
1800	/// }
1801	/// }
1802	///
1803	/// let x = Some(`17.5`);
1804	/// let y = Some(`42.7`);
1805	///
1806	/// assert_eq!(x.zip_with(y, Point::new), Some(Point { x: `17.5`, y: `42.7` }));
1807	/// assert_eq!(x.zip_with(None, Point::new), None);
1808	/// ```
1809	#[unstable(feature = "option_zip", issue = "70086")]
1810	pub fn zip_with<U, F, R>(self, other: Option<U>, f: F) -> Option<R>
1811	where
1812	F: FnOnce(T, U) -> R,
1813	{
1814	match (self, other) {
1815	(Some(a), Some(b)) => Some(f(a, b)),
1816	_ => None,
1817	}
1818	}
1819	}
1820
1821	impl<T, U> Option<(T, U)> {
1822	/// Unzips an option containing a tuple of two options.
1823	///
1824	/// If `self` is `Some((a, b))` this method returns `(Some(a), Some(b))`.
1825	/// Otherwise, `(None, None)` is returned.
1826	///
1827	/// # Examples
1828	///
1829	/// ```
1830	/// let x = Some((`1`, "hi"));
1831	/// let y = None::<(u8, u32)>;
1832	///
1833	/// assert_eq!(x.unzip(), (Some(`1`), Some("hi")));
1834	/// assert_eq!(y.unzip(), (None, None));
1835	/// ```
1836	#[inline]
1837	#[stable(feature = "unzip_option", since = "1.66.0")]
1838	pub fn unzip(self) -> (Option<T>, Option<U>) {
1839	match self {
1840	Some((a: T, b: U)) => (Some(a), Some(b)),
1841	None => (None, None),
1842	}
1843	}
1844	}
1845
1846	impl<T> Option<&T> {
1847	/// Maps an `Option<&T>` to an `Option<T>` by copying the contents of the
1848	/// option.
1849	///
1850	/// # Examples
1851	///
1852	/// ```
1853	/// let x = `12`;
1854	/// let opt_x = Some(&x);
1855	/// assert_eq!(opt_x, Some(&`12`));
1856	/// let copied = opt_x.copied();
1857	/// assert_eq!(copied, Some(`12`));
1858	/// ```
1859	#[must_use = "`self` will be dropped if the result is not used"]
1860	#[stable(feature = "copied", since = "1.35.0")]
1861	#[rustc_const_unstable(feature = "const_option", issue = "67441")]
1862	pub const fn copied(self) -> Option<T>
1863	where
1864	T: Copy,
1865	{
1866	// FIXME: this implementation, which sidesteps using `Option::map` since it's not const
1867	// ready yet, should be reverted when possible to avoid code repetition
1868	match self {
1869	Some(&v) => Some(v),
1870	None => None,
1871	}
1872	}
1873
1874	/// Maps an `Option<&T>` to an `Option<T>` by cloning the contents of the
1875	/// option.
1876	///
1877	/// # Examples
1878	///
1879	/// ```
1880	/// let x = `12`;
1881	/// let opt_x = Some(&x);
1882	/// assert_eq!(opt_x, Some(&`12`));
1883	/// let cloned = opt_x.cloned();
1884	/// assert_eq!(cloned, Some(`12`));
1885	/// ```
1886	#[must_use = "`self` will be dropped if the result is not used"]
1887	#[stable(feature = "rust1", since = "1.0.0")]
1888	pub fn cloned(self) -> Option<T>
1889	where
1890	T: Clone,
1891	{
1892	match self {
1893	Some(t) => Some(t.clone()),
1894	None => None,
1895	}
1896	}
1897	}
1898
1899	impl<T> Option<&mut T> {
1900	/// Maps an `Option<&mut T>` to an `Option<T>` by copying the contents of the
1901	/// option.
1902	///
1903	/// # Examples
1904	///
1905	/// ```
1906	/// let mut x = `12`;
1907	/// let opt_x = Some(&mut x);
1908	/// assert_eq!(opt_x, Some(&mut `12`));
1909	/// let copied = opt_x.copied();
1910	/// assert_eq!(copied, Some(`12`));
1911	/// ```
1912	#[must_use = "`self` will be dropped if the result is not used"]
1913	#[stable(feature = "copied", since = "1.35.0")]
1914	#[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
1915	pub const fn copied(self) -> Option<T>
1916	where
1917	T: Copy,
1918	{
1919	match self {
1920	Some(&mut t) => Some(t),
1921	None => None,
1922	}
1923	}
1924
1925	/// Maps an `Option<&mut T>` to an `Option<T>` by cloning the contents of the
1926	/// option.
1927	///
1928	/// # Examples
1929	///
1930	/// ```
1931	/// let mut x = `12`;
1932	/// let opt_x = Some(&mut x);
1933	/// assert_eq!(opt_x, Some(&mut `12`));
1934	/// let cloned = opt_x.cloned();
1935	/// assert_eq!(cloned, Some(`12`));
1936	/// ```
1937	#[must_use = "`self` will be dropped if the result is not used"]
1938	#[stable(since = "1.26.0", feature = "option_ref_mut_cloned")]
1939	pub fn cloned(self) -> Option<T>
1940	where
1941	T: Clone,
1942	{
1943	match self {
1944	Some(t) => Some(t.clone()),
1945	None => None,
1946	}
1947	}
1948	}
1949
1950	impl<T, E> Option<Result<T, E>> {
1951	/// Transposes an `Option` of a [`Result`] into a [`Result`] of an `Option`.
1952	///
1953	/// [`None`] will be mapped to <code>[Ok]\([None])</code>.
1954	/// <code>[Some]\([Ok]\(\_))</code> and <code>[Some]\([Err]\(\_))</code> will be mapped to
1955	/// <code>[Ok]\([Some]\(\_))</code> and <code>[Err]\(\_)</code>.
1956	///
1957	/// # Examples
1958	///
1959	/// ```
1960	/// #[derive(Debug, Eq, PartialEq)]
1961	/// struct SomeErr;
1962	///
1963	/// let x: Result<Option<i32>, SomeErr> = Ok(Some(`5`));
1964	/// let y: Option<Result<i32, SomeErr>> = Some(Ok(`5`));
1965	/// assert_eq!(x, y.transpose());
1966	/// ```
1967	#[inline]
1968	#[stable(feature = "transpose_result", since = "1.33.0")]
1969	#[rustc_const_unstable(feature = "const_option", issue = "67441")]
1970	pub const fn transpose(self) -> Result<Option<T>, E> {
1971	match self {
1972	Some(Ok(x)) => Ok(Some(x)),
1973	Some(Err(e)) => Err(e),
1974	None => Ok(None),
1975	}
1976	}
1977	}
1978
1979	#[cfg_attr(not(feature = "panic_immediate_abort"), inline(never))]
1980	#[cfg_attr(feature = "panic_immediate_abort", inline)]
1981	#[cold]
1982	#[track_caller]
1983	const fn unwrap_failed() -> ! {
1984	panic(expr:"called `Option::unwrap()` on a `None` value")
1985	}
1986
1987	// This is a separate function to reduce the code size of .expect() itself.
1988	#[cfg_attr(not(feature = "panic_immediate_abort"), inline(never))]
1989	#[cfg_attr(feature = "panic_immediate_abort", inline)]
1990	#[cold]
1991	#[track_caller]
1992	#[rustc_const_unstable(feature = "const_option", issue = "67441")]
1993	const fn expect_failed(msg: &str) -> ! {
1994	panic_display(&msg)
1995	}
1996
1997	/////////////////////////////////////////////////////////////////////////////
1998	// Trait implementations
1999	/////////////////////////////////////////////////////////////////////////////
2000
2001	#[stable(feature = "rust1", since = "1.0.0")]
2002	impl<T> Clone for Option<T>
2003	where
2004	T: Clone,
2005	{
2006	#[inline]
2007	fn clone(&self) -> Self {
2008	match self {
2009	Some(x: &T) => Some(x.clone()),
2010	None => None,
2011	}
2012	}
2013
2014	#[inline]
2015	fn clone_from(&mut self, source: &Self) {
2016	match (self, source) {
2017	(Some(to: &mut T), Some(from: &T)) => to.clone_from(source:from),
2018	(to: &mut Option, from: &Option) => *to = from.clone(),
2019	}
2020	}
2021	}
2022
2023	#[stable(feature = "rust1", since = "1.0.0")]
2024	impl<T> Default for Option<T> {
2025	/// Returns [`None`][Option::None].
2026	///
2027	/// # Examples
2028	///
2029	/// ```
2030	/// let opt: Option<u32> = Option::default();
2031	/// assert!(opt.is_none());
2032	/// ```
2033	#[inline]
2034	fn default() -> Option<T> {
2035	None
2036	}
2037	}
2038
2039	#[stable(feature = "rust1", since = "1.0.0")]
2040	impl<T> IntoIterator for Option<T> {
2041	type Item = T;
2042	type IntoIter = IntoIter<T>;
2043
2044	/// Returns a consuming iterator over the possibly contained value.
2045	///
2046	/// # Examples
2047	///
2048	/// ```
2049	/// let x = Some("string");
2050	/// let v: Vec<&str> = x.into_iter().collect();
2051	/// assert_eq!(v, ["string"]);
2052	///
2053	/// let x = None;
2054	/// let v: Vec<&str> = x.into_iter().collect();
2055	/// assert!(v.is_empty());
2056	/// ```
2057	#[inline]
2058	fn into_iter(self) -> IntoIter<T> {
2059	IntoIter { inner: Item { opt: self } }
2060	}
2061	}
2062
2063	#[stable(since = "1.4.0", feature = "option_iter")]
2064	impl<'a, T> IntoIterator for &'a Option<T> {
2065	type Item = &'a T;
2066	type IntoIter = Iter<'a, T>;
2067
2068	fn into_iter(self) -> Iter<'a, T> {
2069	self.iter()
2070	}
2071	}
2072
2073	#[stable(since = "1.4.0", feature = "option_iter")]
2074	impl<'a, T> IntoIterator for &'a mut Option<T> {
2075	type Item = &'a mut T;
2076	type IntoIter = IterMut<'a, T>;
2077
2078	fn into_iter(self) -> IterMut<'a, T> {
2079	self.iter_mut()
2080	}
2081	}
2082
2083	#[stable(since = "1.12.0", feature = "option_from")]
2084	impl<T> From<T> for Option<T> {
2085	/// Moves `val` into a new [`Some`].
2086	///
2087	/// # Examples
2088	///
2089	/// ```
2090	/// let o: Option<u8> = Option::from(`67`);
2091	///
2092	/// assert_eq!(Some(`67`), o);
2093	/// ```
2094	fn from(val: T) -> Option<T> {
2095	Some(val)
2096	}
2097	}
2098
2099	#[stable(feature = "option_ref_from_ref_option", since = "1.30.0")]
2100	impl<'a, T> From<&'a Option<T>> for Option<&'a T> {
2101	/// Converts from `&Option<T>` to `Option<&T>`.
2102	///
2103	/// # Examples
2104	///
2105	/// Converts an <code>[Option]<[String]></code> into an <code>[Option]<[usize]></code>, preserving
2106	/// the original. The [`map`] method takes the `self` argument by value, consuming the original,
2107	/// so this technique uses `from` to first take an [`Option`] to a reference
2108	/// to the value inside the original.
2109	///
2110	/// [`map`]: Option::map
2111	/// [String]: ../../std/string/struct.String.html "String"
2112	///
2113	/// ```
2114	/// let s: Option<String> = Some(String::from("Hello, Rustaceans!"));
2115	/// let o: Option<usize> = Option::from(&s).map(\|ss: &String\| ss.len());
2116	///
2117	/// println!("Can still print s: {s:?}");
2118	///
2119	/// assert_eq!(o, Some(`18`));
2120	/// ```
2121	fn from(o: &'a Option<T>) -> Option<&'a T> {
2122	o.as_ref()
2123	}
2124	}
2125
2126	#[stable(feature = "option_ref_from_ref_option", since = "1.30.0")]
2127	impl<'a, T> From<&'a mut Option<T>> for Option<&'a mut T> {
2128	/// Converts from `&mut Option<T>` to `Option<&mut T>`
2129	///
2130	/// # Examples
2131	///
2132	/// ```
2133	/// let mut s = Some(String::from("Hello"));
2134	/// let o: Option<&mut String> = Option::from(&mut s);
2135	///
2136	/// match o {
2137	/// Some(t) => *t = String::from("Hello, Rustaceans!"),
2138	/// None => (),
2139	/// }
2140	///
2141	/// assert_eq!(s, Some(String::from("Hello, Rustaceans!")));
2142	/// ```
2143	fn from(o: &'a mut Option<T>) -> Option<&'a mut T> {
2144	o.as_mut()
2145	}
2146	}
2147
2148	// Ideally, LLVM should be able to optimize our derive code to this.
2149	// Once https://github.com/llvm/llvm-project/issues/52622 is fixed, we can
2150	// go back to deriving `PartialEq`.
2151	#[stable(feature = "rust1", since = "1.0.0")]
2152	impl<T> crate::marker::StructuralPartialEq for Option<T> {}
2153	#[stable(feature = "rust1", since = "1.0.0")]
2154	impl<T: PartialEq> PartialEq for Option<T> {
2155	#[inline]
2156	fn eq(&self, other: &Self) -> bool {
2157	// Spelling out the cases explicitly optimizes better than
2158	// `_ => false`
2159	match (self, other) {
2160	(Some(l: &T), Some(r: &T)) => l == r,
2161	(Some(_), None) => `false`,
2162	(None, Some(_)) => `false`,
2163	(None, None) => `true`,
2164	}
2165	}
2166	}
2167
2168	// Manually implementing here somewhat improves codegen for
2169	// https://github.com/rust-lang/rust/issues/49892, although still
2170	// not optimal.
2171	#[stable(feature = "rust1", since = "1.0.0")]
2172	impl<T: PartialOrd> PartialOrd for Option<T> {
2173	#[inline]
2174	fn partial_cmp(&self, other: &Self) -> Option<cmp::Ordering> {
2175	match (self, other) {
2176	(Some(l: &T), Some(r: &T)) => l.partial_cmp(r),
2177	(Some(_), None) => Some(cmp::Ordering::Greater),
2178	(None, Some(_)) => Some(cmp::Ordering::Less),
2179	(None, None) => Some(cmp::Ordering::Equal),
2180	}
2181	}
2182	}
2183
2184	#[stable(feature = "rust1", since = "1.0.0")]
2185	impl<T: Ord> Ord for Option<T> {
2186	#[inline]
2187	fn cmp(&self, other: &Self) -> cmp::Ordering {
2188	match (self, other) {
2189	(Some(l: &T), Some(r: &T)) => l.cmp(r),
2190	(Some(_), None) => cmp::Ordering::Greater,
2191	(None, Some(_)) => cmp::Ordering::Less,
2192	(None, None) => cmp::Ordering::Equal,
2193	}
2194	}
2195	}
2196
2197	/////////////////////////////////////////////////////////////////////////////
2198	// The Option Iterators
2199	/////////////////////////////////////////////////////////////////////////////
2200
2201	#[derive(Clone, Debug)]
2202	struct Item<A> {
2203	opt: Option<A>,
2204	}
2205
2206	impl<A> Iterator for Item<A> {
2207	type Item = A;
2208
2209	#[inline]
2210	fn next(&mut self) -> Option<A> {
2211	self.opt.take()
2212	}
2213
2214	#[inline]
2215	fn size_hint(&self) -> (usize, Option<usize>) {
2216	match self.opt {
2217	Some(_) => (`1`, Some(`1`)),
2218	None => (`0`, Some(`0`)),
2219	}
2220	}
2221	}
2222
2223	impl<A> DoubleEndedIterator for Item<A> {
2224	#[inline]
2225	fn next_back(&mut self) -> Option<A> {
2226	self.opt.take()
2227	}
2228	}
2229
2230	impl<A> ExactSizeIterator for Item<A> {}
2231	impl<A> FusedIterator for Item<A> {}
2232	unsafe impl<A> TrustedLen for Item<A> {}
2233
2234	/// An iterator over a reference to the [`Some`] variant of an [`Option`].
2235	///
2236	/// The iterator yields one value if the [`Option`] is a [`Some`], otherwise none.
2237	///
2238	/// This `struct` is created by the [`Option::iter`] function.
2239	#[stable(feature = "rust1", since = "1.0.0")]
2240	#[derive(Debug)]
2241	pub struct Iter<'a, A: 'a> {
2242	inner: Item<&'a A>,
2243	}
2244
2245	#[stable(feature = "rust1", since = "1.0.0")]
2246	impl<'a, A> Iterator for Iter<'a, A> {
2247	type Item = &'a A;
2248
2249	#[inline]
2250	fn next(&mut self) -> Option<&'a A> {
2251	self.inner.next()
2252	}
2253	#[inline]
2254	fn size_hint(&self) -> (usize, Option<usize>) {
2255	self.inner.size_hint()
2256	}
2257	}
2258
2259	#[stable(feature = "rust1", since = "1.0.0")]
2260	impl<'a, A> DoubleEndedIterator for Iter<'a, A> {
2261	#[inline]
2262	fn next_back(&mut self) -> Option<&'a A> {
2263	self.inner.next_back()
2264	}
2265	}
2266
2267	#[stable(feature = "rust1", since = "1.0.0")]
2268	impl<A> ExactSizeIterator for Iter<'_, A> {}
2269
2270	#[stable(feature = "fused", since = "1.26.0")]
2271	impl<A> FusedIterator for Iter<'_, A> {}
2272
2273	#[unstable(feature = "trusted_len", issue = "37572")]
2274	unsafe impl<A> TrustedLen for Iter<'_, A> {}
2275
2276	#[stable(feature = "rust1", since = "1.0.0")]
2277	impl<A> Clone for Iter<'_, A> {
2278	#[inline]
2279	fn clone(&self) -> Self {
2280	Iter { inner: self.inner.clone() }
2281	}
2282	}
2283
2284	/// An iterator over a mutable reference to the [`Some`] variant of an [`Option`].
2285	///
2286	/// The iterator yields one value if the [`Option`] is a [`Some`], otherwise none.
2287	///
2288	/// This `struct` is created by the [`Option::iter_mut`] function.
2289	#[stable(feature = "rust1", since = "1.0.0")]
2290	#[derive(Debug)]
2291	pub struct IterMut<'a, A: 'a> {
2292	inner: Item<&'a mut A>,
2293	}
2294
2295	#[stable(feature = "rust1", since = "1.0.0")]
2296	impl<'a, A> Iterator for IterMut<'a, A> {
2297	type Item = &'a mut A;
2298
2299	#[inline]
2300	fn next(&mut self) -> Option<&'a mut A> {
2301	self.inner.next()
2302	}
2303	#[inline]
2304	fn size_hint(&self) -> (usize, Option<usize>) {
2305	self.inner.size_hint()
2306	}
2307	}
2308
2309	#[stable(feature = "rust1", since = "1.0.0")]
2310	impl<'a, A> DoubleEndedIterator for IterMut<'a, A> {
2311	#[inline]
2312	fn next_back(&mut self) -> Option<&'a mut A> {
2313	self.inner.next_back()
2314	}
2315	}
2316
2317	#[stable(feature = "rust1", since = "1.0.0")]
2318	impl<A> ExactSizeIterator for IterMut<'_, A> {}
2319
2320	#[stable(feature = "fused", since = "1.26.0")]
2321	impl<A> FusedIterator for IterMut<'_, A> {}
2322	#[unstable(feature = "trusted_len", issue = "37572")]
2323	unsafe impl<A> TrustedLen for IterMut<'_, A> {}
2324
2325	/// An iterator over the value in [`Some`] variant of an [`Option`].
2326	///
2327	/// The iterator yields one value if the [`Option`] is a [`Some`], otherwise none.
2328	///
2329	/// This `struct` is created by the [`Option::into_iter`] function.
2330	#[derive(Clone, Debug)]
2331	#[stable(feature = "rust1", since = "1.0.0")]
2332	pub struct IntoIter<A> {
2333	inner: Item<A>,
2334	}
2335
2336	#[stable(feature = "rust1", since = "1.0.0")]
2337	impl<A> Iterator for IntoIter<A> {
2338	type Item = A;
2339
2340	#[inline]
2341	fn next(&mut self) -> Option<A> {
2342	self.inner.next()
2343	}
2344	#[inline]
2345	fn size_hint(&self) -> (usize, Option<usize>) {
2346	self.inner.size_hint()
2347	}
2348	}
2349
2350	#[stable(feature = "rust1", since = "1.0.0")]
2351	impl<A> DoubleEndedIterator for IntoIter<A> {
2352	#[inline]
2353	fn next_back(&mut self) -> Option<A> {
2354	self.inner.next_back()
2355	}
2356	}
2357
2358	#[stable(feature = "rust1", since = "1.0.0")]
2359	impl<A> ExactSizeIterator for IntoIter<A> {}
2360
2361	#[stable(feature = "fused", since = "1.26.0")]
2362	impl<A> FusedIterator for IntoIter<A> {}
2363
2364	#[unstable(feature = "trusted_len", issue = "37572")]
2365	unsafe impl<A> TrustedLen for IntoIter<A> {}
2366
2367	/////////////////////////////////////////////////////////////////////////////
2368	// FromIterator
2369	/////////////////////////////////////////////////////////////////////////////
2370
2371	#[stable(feature = "rust1", since = "1.0.0")]
2372	impl<A, V: FromIterator<A>> FromIterator<Option<A>> for Option<V> {
2373	/// Takes each element in the [`Iterator`]: if it is [`None`][Option::None],
2374	/// no further elements are taken, and the [`None`][Option::None] is
2375	/// returned. Should no [`None`][Option::None] occur, a container of type
2376	/// `V` containing the values of each [`Option`] is returned.
2377	///
2378	/// # Examples
2379	///
2380	/// Here is an example which increments every integer in a vector.
2381	/// We use the checked variant of `add` that returns `None` when the
2382	/// calculation would result in an overflow.
2383	///
2384	/// ```
2385	/// let items = vec![`0_u16`, `1`, `2`];
2386	///
2387	/// let res: Option<Vec<u16>> = items
2388	/// .iter()
2389	/// .map(\|x\| x.checked_add(`1`))
2390	/// .collect();
2391	///
2392	/// assert_eq!(res, Some(vec![`1`, `2`, `3`]));
2393	/// ```
2394	///
2395	/// As you can see, this will return the expected, valid items.
2396	///
2397	/// Here is another example that tries to subtract one from another list
2398	/// of integers, this time checking for underflow:
2399	///
2400	/// ```
2401	/// let items = vec![`2_u16`, `1`, `0`];
2402	///
2403	/// let res: Option<Vec<u16>> = items
2404	/// .iter()
2405	/// .map(\|x\| x.checked_sub(`1`))
2406	/// .collect();
2407	///
2408	/// assert_eq!(res, None);
2409	/// ```
2410	///
2411	/// Since the last element is zero, it would underflow. Thus, the resulting
2412	/// value is `None`.
2413	///
2414	/// Here is a variation on the previous example, showing that no
2415	/// further elements are taken from `iter` after the first `None`.
2416	///
2417	/// ```
2418	/// let items = vec![`3_u16`, `2`, `1`, `10`];
2419	///
2420	/// let mut shared = `0`;
2421	///
2422	/// let res: Option<Vec<u16>> = items
2423	/// .iter()
2424	/// .map(\|x\| { shared += x; x.checked_sub(`2`) })
2425	/// .collect();
2426	///
2427	/// assert_eq!(res, None);
2428	/// assert_eq!(shared, `6`);
2429	/// ```
2430	///
2431	/// Since the third element caused an underflow, no further elements were taken,
2432	/// so the final value of `shared` is 6 (= `3 + 2 + 1`), not 16.
2433	#[inline]
2434	fn from_iter<I: IntoIterator<Item = Option<A>>>(iter: I) -> Option<V> {
2435	// FIXME(#11084): This could be replaced with Iterator::scan when this
2436	// performance bug is closed.
2437
2438	iter::try_process(iter.into_iter(), \|i\| i.collect())
2439	}
2440	}
2441
2442	#[unstable(feature = "try_trait_v2", issue = "84277")]
2443	impl<T> ops::Try for Option<T> {
2444	type Output = T;
2445	type Residual = Option<convert::Infallible>;
2446
2447	#[inline]
2448	fn from_output(output: Self::Output) -> Self {
2449	Some(output)
2450	}
2451
2452	#[inline]
2453	fn branch(self) -> ControlFlow<Self::Residual, Self::Output> {
2454	match self {
2455	Some(v: T) => ControlFlow::Continue(v),
2456	None => ControlFlow::Break(None),
2457	}
2458	}
2459	}
2460
2461	#[unstable(feature = "try_trait_v2", issue = "84277")]
2462	impl<T> ops::FromResidual for Option<T> {
2463	#[inline]
2464	fn from_residual(residual: Option<convert::Infallible>) -> Self {
2465	match residual {
2466	None => None,
2467	}
2468	}
2469	}
2470
2471	#[unstable(feature = "try_trait_v2_yeet", issue = "96374")]
2472	impl<T> ops::FromResidual<ops::Yeet<()>> for Option<T> {
2473	#[inline]
2474	fn from_residual(ops::Yeet(()): ops::Yeet<()>) -> Self {
2475	None
2476	}
2477	}
2478
2479	#[unstable(feature = "try_trait_v2_residual", issue = "91285")]
2480	impl<T> ops::Residual<T> for Option<convert::Infallible> {
2481	type TryType = Option<T>;
2482	}
2483
2484	impl<T> Option<Option<T>> {
2485	/// Converts from `Option<Option<T>>` to `Option<T>`.
2486	///
2487	/// # Examples
2488	///
2489	/// Basic usage:
2490	///
2491	/// ```
2492	/// let x: Option<Option<u32>> = Some(Some(`6`));
2493	/// assert_eq!(Some(`6`), x.flatten());
2494	///
2495	/// let x: Option<Option<u32>> = Some(None);
2496	/// assert_eq!(None, x.flatten());
2497	///
2498	/// let x: Option<Option<u32>> = None;
2499	/// assert_eq!(None, x.flatten());
2500	/// ```
2501	///
2502	/// Flattening only removes one level of nesting at a time:
2503	///
2504	/// ```
2505	/// let x: Option<Option<Option<u32>>> = Some(Some(Some(`6`)));
2506	/// assert_eq!(Some(Some(`6`)), x.flatten());
2507	/// assert_eq!(Some(`6`), x.flatten().flatten());
2508	/// ```
2509	#[inline]
2510	#[stable(feature = "option_flattening", since = "1.40.0")]
2511	#[rustc_const_unstable(feature = "const_option", issue = "67441")]
2512	pub const fn flatten(self) -> Option<T> {
2513	match self {
2514	Some(inner) => inner,
2515	None => None,
2516	}
2517	}
2518	}
2519