core/
result.rs

1//! Error handling with the `Result` type.
2//!
3//! [`Result<T, E>`][`Result`] is the type used for returning and propagating
4//! errors. It is an enum with the variants, [`Ok(T)`], representing
5//! success and containing a value, and [`Err(E)`], representing error
6//! and containing an error value.
7//!
8//! ```
9//! # #[allow(dead_code)]
10//! enum Result<T, E> {
11//!    Ok(T),
12//!    Err(E),
13//! }
14//! ```
15//!
16//! Functions return [`Result`] whenever errors are expected and
17//! recoverable. In the `std` crate, [`Result`] is most prominently used
18//! for [I/O](../../std/io/index.html).
19//!
20//! A simple function returning [`Result`] might be
21//! defined and used like so:
22//!
23//! ```
24//! #[derive(Debug)]
25//! enum Version { Version1, Version2 }
26//!
27//! fn parse_version(header: &[u8]) -> Result<Version, &'static str> {
28//!     match header.get(0) {
29//!         None => Err("invalid header length"),
30//!         Some(&1) => Ok(Version::Version1),
31//!         Some(&2) => Ok(Version::Version2),
32//!         Some(_) => Err("invalid version"),
33//!     }
34//! }
35//!
36//! let version = parse_version(&[1, 2, 3, 4]);
37//! match version {
38//!     Ok(v) => println!("working with version: {v:?}"),
39//!     Err(e) => println!("error parsing header: {e:?}"),
40//! }
41//! ```
42//!
43//! Pattern matching on [`Result`]s is clear and straightforward for
44//! simple cases, but [`Result`] comes with some convenience methods
45//! that make working with it more succinct.
46//!
47//! ```
48//! // The `is_ok` and `is_err` methods do what they say.
49//! let good_result: Result<i32, i32> = Ok(10);
50//! let bad_result: Result<i32, i32> = Err(10);
51//! assert!(good_result.is_ok() && !good_result.is_err());
52//! assert!(bad_result.is_err() && !bad_result.is_ok());
53//!
54//! // `map` and `map_err` consume the `Result` and produce another.
55//! let good_result: Result<i32, i32> = good_result.map(|i| i + 1);
56//! let bad_result: Result<i32, i32> = bad_result.map_err(|i| i - 1);
57//! assert_eq!(good_result, Ok(11));
58//! assert_eq!(bad_result, Err(9));
59//!
60//! // Use `and_then` to continue the computation.
61//! let good_result: Result<bool, i32> = good_result.and_then(|i| Ok(i == 11));
62//! assert_eq!(good_result, Ok(true));
63//!
64//! // Use `or_else` to handle the error.
65//! let bad_result: Result<i32, i32> = bad_result.or_else(|i| Ok(i + 20));
66//! assert_eq!(bad_result, Ok(29));
67//!
68//! // Consume the result and return the contents with `unwrap`.
69//! let final_awesome_result = good_result.unwrap();
70//! assert!(final_awesome_result)
71//! ```
72//!
73//! # Results must be used
74//!
75//! A common problem with using return values to indicate errors is
76//! that it is easy to ignore the return value, thus failing to handle
77//! the error. [`Result`] is annotated with the `#[must_use]` attribute,
78//! which will cause the compiler to issue a warning when a Result
79//! value is ignored. This makes [`Result`] especially useful with
80//! functions that may encounter errors but don't otherwise return a
81//! useful value.
82//!
83//! Consider the [`write_all`] method defined for I/O types
84//! by the [`Write`] trait:
85//!
86//! ```
87//! use std::io;
88//!
89//! trait Write {
90//!     fn write_all(&mut self, bytes: &[u8]) -> Result<(), io::Error>;
91//! }
92//! ```
93//!
94//! *Note: The actual definition of [`Write`] uses [`io::Result`], which
95//! is just a synonym for <code>[Result]<T, [io::Error]></code>.*
96//!
97//! This method doesn't produce a value, but the write may
98//! fail. It's crucial to handle the error case, and *not* write
99//! something like this:
100//!
101//! ```no_run
102//! # #![allow(unused_must_use)] // \o/
103//! use std::fs::File;
104//! use std::io::prelude::*;
105//!
106//! let mut file = File::create("valuable_data.txt").unwrap();
107//! // If `write_all` errors, then we'll never know, because the return
108//! // value is ignored.
109//! file.write_all(b"important message");
110//! ```
111//!
112//! If you *do* write that in Rust, the compiler will give you a
113//! warning (by default, controlled by the `unused_must_use` lint).
114//!
115//! You might instead, if you don't want to handle the error, simply
116//! assert success with [`expect`]. This will panic if the
117//! write fails, providing a marginally useful message indicating why:
118//!
119//! ```no_run
120//! use std::fs::File;
121//! use std::io::prelude::*;
122//!
123//! let mut file = File::create("valuable_data.txt").unwrap();
124//! file.write_all(b"important message").expect("failed to write message");
125//! ```
126//!
127//! You might also simply assert success:
128//!
129//! ```no_run
130//! # use std::fs::File;
131//! # use std::io::prelude::*;
132//! # let mut file = File::create("valuable_data.txt").unwrap();
133//! assert!(file.write_all(b"important message").is_ok());
134//! ```
135//!
136//! Or propagate the error up the call stack with [`?`]:
137//!
138//! ```
139//! # use std::fs::File;
140//! # use std::io::prelude::*;
141//! # use std::io;
142//! # #[allow(dead_code)]
143//! fn write_message() -> io::Result<()> {
144//!     let mut file = File::create("valuable_data.txt")?;
145//!     file.write_all(b"important message")?;
146//!     Ok(())
147//! }
148//! ```
149//!
150//! # The question mark operator, `?`
151//!
152//! When writing code that calls many functions that return the
153//! [`Result`] type, the error handling can be tedious. The question mark
154//! operator, [`?`], hides some of the boilerplate of propagating errors
155//! up the call stack.
156//!
157//! It replaces this:
158//!
159//! ```
160//! # #![allow(dead_code)]
161//! use std::fs::File;
162//! use std::io::prelude::*;
163//! use std::io;
164//!
165//! struct Info {
166//!     name: String,
167//!     age: i32,
168//!     rating: i32,
169//! }
170//!
171//! fn write_info(info: &Info) -> io::Result<()> {
172//!     // Early return on error
173//!     let mut file = match File::create("my_best_friends.txt") {
174//!            Err(e) => return Err(e),
175//!            Ok(f) => f,
176//!     };
177//!     if let Err(e) = file.write_all(format!("name: {}\n", info.name).as_bytes()) {
178//!         return Err(e)
179//!     }
180//!     if let Err(e) = file.write_all(format!("age: {}\n", info.age).as_bytes()) {
181//!         return Err(e)
182//!     }
183//!     if let Err(e) = file.write_all(format!("rating: {}\n", info.rating).as_bytes()) {
184//!         return Err(e)
185//!     }
186//!     Ok(())
187//! }
188//! ```
189//!
190//! With this:
191//!
192//! ```
193//! # #![allow(dead_code)]
194//! use std::fs::File;
195//! use std::io::prelude::*;
196//! use std::io;
197//!
198//! struct Info {
199//!     name: String,
200//!     age: i32,
201//!     rating: i32,
202//! }
203//!
204//! fn write_info(info: &Info) -> io::Result<()> {
205//!     let mut file = File::create("my_best_friends.txt")?;
206//!     // Early return on error
207//!     file.write_all(format!("name: {}\n", info.name).as_bytes())?;
208//!     file.write_all(format!("age: {}\n", info.age).as_bytes())?;
209//!     file.write_all(format!("rating: {}\n", info.rating).as_bytes())?;
210//!     Ok(())
211//! }
212//! ```
213//!
214//! *It's much nicer!*
215//!
216//! Ending the expression with [`?`] will result in the [`Ok`]'s unwrapped value, unless the result
217//! is [`Err`], in which case [`Err`] is returned early from the enclosing function.
218//!
219//! [`?`] can be used in functions that return [`Result`] because of the
220//! early return of [`Err`] that it provides.
221//!
222//! [`expect`]: Result::expect
223//! [`Write`]: ../../std/io/trait.Write.html "io::Write"
224//! [`write_all`]: ../../std/io/trait.Write.html#method.write_all "io::Write::write_all"
225//! [`io::Result`]: ../../std/io/type.Result.html "io::Result"
226//! [`?`]: crate::ops::Try
227//! [`Ok(T)`]: Ok
228//! [`Err(E)`]: Err
229//! [io::Error]: ../../std/io/struct.Error.html "io::Error"
230//!
231//! # Representation
232//!
233//! In some cases, [`Result<T, E>`] will gain the same size, alignment, and ABI
234//! guarantees as [`Option<U>`] has. One of either the `T` or `E` type must be a
235//! type that qualifies for the `Option` [representation guarantees][opt-rep],
236//! and the *other* type must meet all of the following conditions:
237//! * Is a zero-sized type with alignment 1 (a "1-ZST").
238//! * Has no fields.
239//! * Does not have the `#[non_exhaustive]` attribute.
240//!
241//! For example, `NonZeroI32` qualifies for the `Option` representation
242//! guarantees, and `()` is a zero-sized type with alignment 1, no fields, and
243//! it isn't `non_exhaustive`. This means that both `Result<NonZeroI32, ()>` and
244//! `Result<(), NonZeroI32>` have the same size, alignment, and ABI guarantees
245//! as `Option<NonZeroI32>`. The only difference is the implied semantics:
246//! * `Option<NonZeroI32>` is "a non-zero i32 might be present"
247//! * `Result<NonZeroI32, ()>` is "a non-zero i32 success result, if any"
248//! * `Result<(), NonZeroI32>` is "a non-zero i32 error result, if any"
249//!
250//! [opt-rep]: ../option/index.html#representation "Option Representation"
251//!
252//! # Method overview
253//!
254//! In addition to working with pattern matching, [`Result`] provides a
255//! wide variety of different methods.
256//!
257//! ## Querying the variant
258//!
259//! The [`is_ok`] and [`is_err`] methods return [`true`] if the [`Result`]
260//! is [`Ok`] or [`Err`], respectively.
261//!
262//! The [`is_ok_and`] and [`is_err_and`] methods apply the provided function
263//! to the contents of the [`Result`] to produce a boolean value. If the [`Result`] does not have the expected variant
264//! then [`false`] is returned instead without executing the function.
265//!
266//! [`is_err`]: Result::is_err
267//! [`is_ok`]: Result::is_ok
268//! [`is_ok_and`]: Result::is_ok_and
269//! [`is_err_and`]: Result::is_err_and
270//!
271//! ## Adapters for working with references
272//!
273//! * [`as_ref`] converts from `&Result<T, E>` to `Result<&T, &E>`
274//! * [`as_mut`] converts from `&mut Result<T, E>` to `Result<&mut T, &mut E>`
275//! * [`as_deref`] converts from `&Result<T, E>` to `Result<&T::Target, &E>`
276//! * [`as_deref_mut`] converts from `&mut Result<T, E>` to
277//!   `Result<&mut T::Target, &mut E>`
278//!
279//! [`as_deref`]: Result::as_deref
280//! [`as_deref_mut`]: Result::as_deref_mut
281//! [`as_mut`]: Result::as_mut
282//! [`as_ref`]: Result::as_ref
283//!
284//! ## Extracting contained values
285//!
286//! These methods extract the contained value in a [`Result<T, E>`] when it
287//! is the [`Ok`] variant. If the [`Result`] is [`Err`]:
288//!
289//! * [`expect`] panics with a provided custom message
290//! * [`unwrap`] panics with a generic message
291//! * [`unwrap_or`] returns the provided default value
292//! * [`unwrap_or_default`] returns the default value of the type `T`
293//!   (which must implement the [`Default`] trait)
294//! * [`unwrap_or_else`] returns the result of evaluating the provided
295//!   function
296//! * [`unwrap_unchecked`] produces *[undefined behavior]*
297//!
298//! The panicking methods [`expect`] and [`unwrap`] require `E` to
299//! implement the [`Debug`] trait.
300//!
301//! [`Debug`]: crate::fmt::Debug
302//! [`expect`]: Result::expect
303//! [`unwrap`]: Result::unwrap
304//! [`unwrap_or`]: Result::unwrap_or
305//! [`unwrap_or_default`]: Result::unwrap_or_default
306//! [`unwrap_or_else`]: Result::unwrap_or_else
307//! [`unwrap_unchecked`]: Result::unwrap_unchecked
308//! [undefined behavior]: https://doc.rust-lang.org/reference/behavior-considered-undefined.html
309//!
310//! These methods extract the contained value in a [`Result<T, E>`] when it
311//! is the [`Err`] variant. They require `T` to implement the [`Debug`]
312//! trait. If the [`Result`] is [`Ok`]:
313//!
314//! * [`expect_err`] panics with a provided custom message
315//! * [`unwrap_err`] panics with a generic message
316//! * [`unwrap_err_unchecked`] produces *[undefined behavior]*
317//!
318//! [`Debug`]: crate::fmt::Debug
319//! [`expect_err`]: Result::expect_err
320//! [`unwrap_err`]: Result::unwrap_err
321//! [`unwrap_err_unchecked`]: Result::unwrap_err_unchecked
322//! [undefined behavior]: https://doc.rust-lang.org/reference/behavior-considered-undefined.html
323//!
324//! ## Transforming contained values
325//!
326//! These methods transform [`Result`] to [`Option`]:
327//!
328//! * [`err`][Result::err] transforms [`Result<T, E>`] into [`Option<E>`],
329//!   mapping [`Err(e)`] to [`Some(e)`] and [`Ok(v)`] to [`None`]
330//! * [`ok`][Result::ok] transforms [`Result<T, E>`] into [`Option<T>`],
331//!   mapping [`Ok(v)`] to [`Some(v)`] and [`Err(e)`] to [`None`]
332//! * [`transpose`] transposes a [`Result`] of an [`Option`] into an
333//!   [`Option`] of a [`Result`]
334//!
335// Do NOT add link reference definitions for `err` or `ok`, because they
336// will generate numerous incorrect URLs for `Err` and `Ok` elsewhere, due
337// to case folding.
338//!
339//! [`Err(e)`]: Err
340//! [`Ok(v)`]: Ok
341//! [`Some(e)`]: Option::Some
342//! [`Some(v)`]: Option::Some
343//! [`transpose`]: Result::transpose
344//!
345//! These methods transform the contained value of the [`Ok`] variant:
346//!
347//! * [`map`] transforms [`Result<T, E>`] into [`Result<U, E>`] by applying
348//!   the provided function to the contained value of [`Ok`] and leaving
349//!   [`Err`] values unchanged
350//! * [`inspect`] takes ownership of the [`Result`], applies the
351//!   provided function to the contained value by reference,
352//!   and then returns the [`Result`]
353//!
354//! [`map`]: Result::map
355//! [`inspect`]: Result::inspect
356//!
357//! These methods transform the contained value of the [`Err`] variant:
358//!
359//! * [`map_err`] transforms [`Result<T, E>`] into [`Result<T, F>`] by
360//!   applying the provided function to the contained value of [`Err`] and
361//!   leaving [`Ok`] values unchanged
362//! * [`inspect_err`] takes ownership of the [`Result`], applies the
363//!   provided function to the contained value of [`Err`] by reference,
364//!   and then returns the [`Result`]
365//!
366//! [`map_err`]: Result::map_err
367//! [`inspect_err`]: Result::inspect_err
368//!
369//! These methods transform a [`Result<T, E>`] into a value of a possibly
370//! different type `U`:
371//!
372//! * [`map_or`] applies the provided function to the contained value of
373//!   [`Ok`], or returns the provided default value if the [`Result`] is
374//!   [`Err`]
375//! * [`map_or_else`] applies the provided function to the contained value
376//!   of [`Ok`], or applies the provided default fallback function to the
377//!   contained value of [`Err`]
378//!
379//! [`map_or`]: Result::map_or
380//! [`map_or_else`]: Result::map_or_else
381//!
382//! ## Boolean operators
383//!
384//! These methods treat the [`Result`] as a boolean value, where [`Ok`]
385//! acts like [`true`] and [`Err`] acts like [`false`]. There are two
386//! categories of these methods: ones that take a [`Result`] as input, and
387//! ones that take a function as input (to be lazily evaluated).
388//!
389//! The [`and`] and [`or`] methods take another [`Result`] as input, and
390//! produce a [`Result`] as output. The [`and`] method can produce a
391//! [`Result<U, E>`] value having a different inner type `U` than
392//! [`Result<T, E>`]. The [`or`] method can produce a [`Result<T, F>`]
393//! value having a different error type `F` than [`Result<T, E>`].
394//!
395//! | method  | self     | input     | output   |
396//! |---------|----------|-----------|----------|
397//! | [`and`] | `Err(e)` | (ignored) | `Err(e)` |
398//! | [`and`] | `Ok(x)`  | `Err(d)`  | `Err(d)` |
399//! | [`and`] | `Ok(x)`  | `Ok(y)`   | `Ok(y)`  |
400//! | [`or`]  | `Err(e)` | `Err(d)`  | `Err(d)` |
401//! | [`or`]  | `Err(e)` | `Ok(y)`   | `Ok(y)`  |
402//! | [`or`]  | `Ok(x)`  | (ignored) | `Ok(x)`  |
403//!
404//! [`and`]: Result::and
405//! [`or`]: Result::or
406//!
407//! The [`and_then`] and [`or_else`] methods take a function as input, and
408//! only evaluate the function when they need to produce a new value. The
409//! [`and_then`] method can produce a [`Result<U, E>`] value having a
410//! different inner type `U` than [`Result<T, E>`]. The [`or_else`] method
411//! can produce a [`Result<T, F>`] value having a different error type `F`
412//! than [`Result<T, E>`].
413//!
414//! | method       | self     | function input | function result | output   |
415//! |--------------|----------|----------------|-----------------|----------|
416//! | [`and_then`] | `Err(e)` | (not provided) | (not evaluated) | `Err(e)` |
417//! | [`and_then`] | `Ok(x)`  | `x`            | `Err(d)`        | `Err(d)` |
418//! | [`and_then`] | `Ok(x)`  | `x`            | `Ok(y)`         | `Ok(y)`  |
419//! | [`or_else`]  | `Err(e)` | `e`            | `Err(d)`        | `Err(d)` |
420//! | [`or_else`]  | `Err(e)` | `e`            | `Ok(y)`         | `Ok(y)`  |
421//! | [`or_else`]  | `Ok(x)`  | (not provided) | (not evaluated) | `Ok(x)`  |
422//!
423//! [`and_then`]: Result::and_then
424//! [`or_else`]: Result::or_else
425//!
426//! ## Comparison operators
427//!
428//! If `T` and `E` both implement [`PartialOrd`] then [`Result<T, E>`] will
429//! derive its [`PartialOrd`] implementation.  With this order, an [`Ok`]
430//! compares as less than any [`Err`], while two [`Ok`] or two [`Err`]
431//! compare as their contained values would in `T` or `E` respectively.  If `T`
432//! and `E` both also implement [`Ord`], then so does [`Result<T, E>`].
433//!
434//! ```
435//! assert!(Ok(1) < Err(0));
436//! let x: Result<i32, ()> = Ok(0);
437//! let y = Ok(1);
438//! assert!(x < y);
439//! let x: Result<(), i32> = Err(0);
440//! let y = Err(1);
441//! assert!(x < y);
442//! ```
443//!
444//! ## Iterating over `Result`
445//!
446//! A [`Result`] can be iterated over. This can be helpful if you need an
447//! iterator that is conditionally empty. The iterator will either produce
448//! a single value (when the [`Result`] is [`Ok`]), or produce no values
449//! (when the [`Result`] is [`Err`]). For example, [`into_iter`] acts like
450//! [`once(v)`] if the [`Result`] is [`Ok(v)`], and like [`empty()`] if the
451//! [`Result`] is [`Err`].
452//!
453//! [`Ok(v)`]: Ok
454//! [`empty()`]: crate::iter::empty
455//! [`once(v)`]: crate::iter::once
456//!
457//! Iterators over [`Result<T, E>`] come in three types:
458//!
459//! * [`into_iter`] consumes the [`Result`] and produces the contained
460//!   value
461//! * [`iter`] produces an immutable reference of type `&T` to the
462//!   contained value
463//! * [`iter_mut`] produces a mutable reference of type `&mut T` to the
464//!   contained value
465//!
466//! See [Iterating over `Option`] for examples of how this can be useful.
467//!
468//! [Iterating over `Option`]: crate::option#iterating-over-option
469//! [`into_iter`]: Result::into_iter
470//! [`iter`]: Result::iter
471//! [`iter_mut`]: Result::iter_mut
472//!
473//! You might want to use an iterator chain to do multiple instances of an
474//! operation that can fail, but would like to ignore failures while
475//! continuing to process the successful results. In this example, we take
476//! advantage of the iterable nature of [`Result`] to select only the
477//! [`Ok`] values using [`flatten`][Iterator::flatten].
478//!
479//! ```
480//! # use std::str::FromStr;
481//! let mut results = vec![];
482//! let mut errs = vec![];
483//! let nums: Vec<_> = ["17", "not a number", "99", "-27", "768"]
484//!    .into_iter()
485//!    .map(u8::from_str)
486//!    // Save clones of the raw `Result` values to inspect
487//!    .inspect(|x| results.push(x.clone()))
488//!    // Challenge: explain how this captures only the `Err` values
489//!    .inspect(|x| errs.extend(x.clone().err()))
490//!    .flatten()
491//!    .collect();
492//! assert_eq!(errs.len(), 3);
493//! assert_eq!(nums, [17, 99]);
494//! println!("results {results:?}");
495//! println!("errs {errs:?}");
496//! println!("nums {nums:?}");
497//! ```
498//!
499//! ## Collecting into `Result`
500//!
501//! [`Result`] implements the [`FromIterator`][impl-FromIterator] trait,
502//! which allows an iterator over [`Result`] values to be collected into a
503//! [`Result`] of a collection of each contained value of the original
504//! [`Result`] values, or [`Err`] if any of the elements was [`Err`].
505//!
506//! [impl-FromIterator]: Result#impl-FromIterator%3CResult%3CA,+E%3E%3E-for-Result%3CV,+E%3E
507//!
508//! ```
509//! let v = [Ok(2), Ok(4), Err("err!"), Ok(8)];
510//! let res: Result<Vec<_>, &str> = v.into_iter().collect();
511//! assert_eq!(res, Err("err!"));
512//! let v = [Ok(2), Ok(4), Ok(8)];
513//! let res: Result<Vec<_>, &str> = v.into_iter().collect();
514//! assert_eq!(res, Ok(vec![2, 4, 8]));
515//! ```
516//!
517//! [`Result`] also implements the [`Product`][impl-Product] and
518//! [`Sum`][impl-Sum] traits, allowing an iterator over [`Result`] values
519//! to provide the [`product`][Iterator::product] and
520//! [`sum`][Iterator::sum] methods.
521//!
522//! [impl-Product]: Result#impl-Product%3CResult%3CU,+E%3E%3E-for-Result%3CT,+E%3E
523//! [impl-Sum]: Result#impl-Sum%3CResult%3CU,+E%3E%3E-for-Result%3CT,+E%3E
524//!
525//! ```
526//! let v = [Err("error!"), Ok(1), Ok(2), Ok(3), Err("foo")];
527//! let res: Result<i32, &str> = v.into_iter().sum();
528//! assert_eq!(res, Err("error!"));
529//! let v = [Ok(1), Ok(2), Ok(21)];
530//! let res: Result<i32, &str> = v.into_iter().product();
531//! assert_eq!(res, Ok(42));
532//! ```
533
534#![stable(feature = "rust1", since = "1.0.0")]
535
536#[cfg(not(feature = "ferrocene_certified"))]
537use crate::iter::{self, FusedIterator, TrustedLen};
538use crate::marker::Destruct;
539#[cfg(not(feature = "ferrocene_certified"))]
540use crate::ops::{self, ControlFlow, Deref, DerefMut};
541#[cfg(feature = "ferrocene_certified")]
542use crate::ops::{Deref, DerefMut};
543#[cfg(not(feature = "ferrocene_certified"))]
544use crate::{convert, fmt, hint};
545
546/// `Result` is a type that represents either success ([`Ok`]) or failure ([`Err`]).
547///
548/// See the [module documentation](self) for details.
549#[doc(search_unbox)]
550#[cfg_attr(not(feature = "ferrocene_certified"), derive(Copy, Debug, Hash))]
551#[cfg_attr(not(feature = "ferrocene_certified"), derive_const(PartialEq, PartialOrd, Eq, Ord,))]
552#[must_use = "this `Result` may be an `Err` variant, which should be handled"]
553#[rustc_diagnostic_item = "Result"]
554#[stable(feature = "rust1", since = "1.0.0")]
555pub enum Result<T, E> {
556    /// Contains the success value
557    #[lang = "Ok"]
558    #[stable(feature = "rust1", since = "1.0.0")]
559    Ok(#[stable(feature = "rust1", since = "1.0.0")] T),
560
561    /// Contains the error value
562    #[lang = "Err"]
563    #[stable(feature = "rust1", since = "1.0.0")]
564    Err(#[stable(feature = "rust1", since = "1.0.0")] E),
565}
566
567/////////////////////////////////////////////////////////////////////////////
568// Type implementation
569/////////////////////////////////////////////////////////////////////////////
570
571impl<T, E> Result<T, E> {
572    /////////////////////////////////////////////////////////////////////////
573    // Querying the contained values
574    /////////////////////////////////////////////////////////////////////////
575
576    /// Returns `true` if the result is [`Ok`].
577    ///
578    /// # Examples
579    ///
580    /// ```
581    /// let x: Result<i32, &str> = Ok(-3);
582    /// assert_eq!(x.is_ok(), true);
583    ///
584    /// let x: Result<i32, &str> = Err("Some error message");
585    /// assert_eq!(x.is_ok(), false);
586    /// ```
587    #[must_use = "if you intended to assert that this is ok, consider `.unwrap()` instead"]
588    #[rustc_const_stable(feature = "const_result_basics", since = "1.48.0")]
589    #[inline]
590    #[stable(feature = "rust1", since = "1.0.0")]
591    pub const fn is_ok(&self) -> bool {
592        matches!(*self, Ok(_))
593    }
594
595    /// Returns `true` if the result is [`Ok`] and the value inside of it matches a predicate.
596    ///
597    /// # Examples
598    ///
599    /// ```
600    /// let x: Result<u32, &str> = Ok(2);
601    /// assert_eq!(x.is_ok_and(|x| x > 1), true);
602    ///
603    /// let x: Result<u32, &str> = Ok(0);
604    /// assert_eq!(x.is_ok_and(|x| x > 1), false);
605    ///
606    /// let x: Result<u32, &str> = Err("hey");
607    /// assert_eq!(x.is_ok_and(|x| x > 1), false);
608    ///
609    /// let x: Result<String, &str> = Ok("ownership".to_string());
610    /// assert_eq!(x.as_ref().is_ok_and(|x| x.len() > 1), true);
611    /// println!("still alive {:?}", x);
612    /// ```
613    #[must_use]
614    #[inline]
615    #[stable(feature = "is_some_and", since = "1.70.0")]
616    #[rustc_const_unstable(feature = "const_result_trait_fn", issue = "144211")]
617    pub const fn is_ok_and<F>(self, f: F) -> bool
618    where
619        F: [const] FnOnce(T) -> bool + [const] Destruct,
620        T: [const] Destruct,
621        E: [const] Destruct,
622    {
623        match self {
624            Err(_) => false,
625            Ok(x) => f(x),
626        }
627    }
628
629    /// Returns `true` if the result is [`Err`].
630    ///
631    /// # Examples
632    ///
633    /// ```
634    /// let x: Result<i32, &str> = Ok(-3);
635    /// assert_eq!(x.is_err(), false);
636    ///
637    /// let x: Result<i32, &str> = Err("Some error message");
638    /// assert_eq!(x.is_err(), true);
639    /// ```
640    #[must_use = "if you intended to assert that this is err, consider `.unwrap_err()` instead"]
641    #[rustc_const_stable(feature = "const_result_basics", since = "1.48.0")]
642    #[inline]
643    #[stable(feature = "rust1", since = "1.0.0")]
644    pub const fn is_err(&self) -> bool {
645        !self.is_ok()
646    }
647
648    /// Returns `true` if the result is [`Err`] and the value inside of it matches a predicate.
649    ///
650    /// # Examples
651    ///
652    /// ```
653    /// use std::io::{Error, ErrorKind};
654    ///
655    /// let x: Result<u32, Error> = Err(Error::new(ErrorKind::NotFound, "!"));
656    /// assert_eq!(x.is_err_and(|x| x.kind() == ErrorKind::NotFound), true);
657    ///
658    /// let x: Result<u32, Error> = Err(Error::new(ErrorKind::PermissionDenied, "!"));
659    /// assert_eq!(x.is_err_and(|x| x.kind() == ErrorKind::NotFound), false);
660    ///
661    /// let x: Result<u32, Error> = Ok(123);
662    /// assert_eq!(x.is_err_and(|x| x.kind() == ErrorKind::NotFound), false);
663    ///
664    /// let x: Result<u32, String> = Err("ownership".to_string());
665    /// assert_eq!(x.as_ref().is_err_and(|x| x.len() > 1), true);
666    /// println!("still alive {:?}", x);
667    /// ```
668    #[must_use]
669    #[inline]
670    #[stable(feature = "is_some_and", since = "1.70.0")]
671    #[rustc_const_unstable(feature = "const_result_trait_fn", issue = "144211")]
672    pub const fn is_err_and<F>(self, f: F) -> bool
673    where
674        F: [const] FnOnce(E) -> bool + [const] Destruct,
675        E: [const] Destruct,
676        T: [const] Destruct,
677    {
678        match self {
679            Ok(_) => false,
680            Err(e) => f(e),
681        }
682    }
683
684    /////////////////////////////////////////////////////////////////////////
685    // Adapter for each variant
686    /////////////////////////////////////////////////////////////////////////
687
688    /// Converts from `Result<T, E>` to [`Option<T>`].
689    ///
690    /// Converts `self` into an [`Option<T>`], consuming `self`,
691    /// and discarding the error, if any.
692    ///
693    /// # Examples
694    ///
695    /// ```
696    /// let x: Result<u32, &str> = Ok(2);
697    /// assert_eq!(x.ok(), Some(2));
698    ///
699    /// let x: Result<u32, &str> = Err("Nothing here");
700    /// assert_eq!(x.ok(), None);
701    /// ```
702    #[inline]
703    #[stable(feature = "rust1", since = "1.0.0")]
704    #[rustc_const_unstable(feature = "const_result_trait_fn", issue = "144211")]
705    #[rustc_diagnostic_item = "result_ok_method"]
706    pub const fn ok(self) -> Option<T>
707    where
708        T: [const] Destruct,
709        E: [const] Destruct,
710    {
711        match self {
712            Ok(x) => Some(x),
713            Err(_) => None,
714        }
715    }
716
717    /// Converts from `Result<T, E>` to [`Option<E>`].
718    ///
719    /// Converts `self` into an [`Option<E>`], consuming `self`,
720    /// and discarding the success value, if any.
721    ///
722    /// # Examples
723    ///
724    /// ```
725    /// let x: Result<u32, &str> = Ok(2);
726    /// assert_eq!(x.err(), None);
727    ///
728    /// let x: Result<u32, &str> = Err("Nothing here");
729    /// assert_eq!(x.err(), Some("Nothing here"));
730    /// ```
731    #[inline]
732    #[stable(feature = "rust1", since = "1.0.0")]
733    #[rustc_const_unstable(feature = "const_result_trait_fn", issue = "144211")]
734    pub const fn err(self) -> Option<E>
735    where
736        T: [const] Destruct,
737        E: [const] Destruct,
738    {
739        match self {
740            Ok(_) => None,
741            Err(x) => Some(x),
742        }
743    }
744
745    /////////////////////////////////////////////////////////////////////////
746    // Adapter for working with references
747    /////////////////////////////////////////////////////////////////////////
748
749    /// Converts from `&Result<T, E>` to `Result<&T, &E>`.
750    ///
751    /// Produces a new `Result`, containing a reference
752    /// into the original, leaving the original in place.
753    ///
754    /// # Examples
755    ///
756    /// ```
757    /// let x: Result<u32, &str> = Ok(2);
758    /// assert_eq!(x.as_ref(), Ok(&2));
759    ///
760    /// let x: Result<u32, &str> = Err("Error");
761    /// assert_eq!(x.as_ref(), Err(&"Error"));
762    /// ```
763    #[inline]
764    #[rustc_const_stable(feature = "const_result_basics", since = "1.48.0")]
765    #[stable(feature = "rust1", since = "1.0.0")]
766    pub const fn as_ref(&self) -> Result<&T, &E> {
767        match *self {
768            Ok(ref x) => Ok(x),
769            Err(ref x) => Err(x),
770        }
771    }
772
773    /// Converts from `&mut Result<T, E>` to `Result<&mut T, &mut E>`.
774    ///
775    /// # Examples
776    ///
777    /// ```
778    /// fn mutate(r: &mut Result<i32, i32>) {
779    ///     match r.as_mut() {
780    ///         Ok(v) => *v = 42,
781    ///         Err(e) => *e = 0,
782    ///     }
783    /// }
784    ///
785    /// let mut x: Result<i32, i32> = Ok(2);
786    /// mutate(&mut x);
787    /// assert_eq!(x.unwrap(), 42);
788    ///
789    /// let mut x: Result<i32, i32> = Err(13);
790    /// mutate(&mut x);
791    /// assert_eq!(x.unwrap_err(), 0);
792    /// ```
793    #[inline]
794    #[stable(feature = "rust1", since = "1.0.0")]
795    #[rustc_const_stable(feature = "const_result", since = "1.83.0")]
796    pub const fn as_mut(&mut self) -> Result<&mut T, &mut E> {
797        match *self {
798            Ok(ref mut x) => Ok(x),
799            Err(ref mut x) => Err(x),
800        }
801    }
802
803    /////////////////////////////////////////////////////////////////////////
804    // Transforming contained values
805    /////////////////////////////////////////////////////////////////////////
806
807    /// Maps a `Result<T, E>` to `Result<U, E>` by applying a function to a
808    /// contained [`Ok`] value, leaving an [`Err`] value untouched.
809    ///
810    /// This function can be used to compose the results of two functions.
811    ///
812    /// # Examples
813    ///
814    /// Print the numbers on each line of a string multiplied by two.
815    ///
816    /// ```
817    /// let line = "1\n2\n3\n4\n";
818    ///
819    /// for num in line.lines() {
820    ///     match num.parse::<i32>().map(|i| i * 2) {
821    ///         Ok(n) => println!("{n}"),
822    ///         Err(..) => {}
823    ///     }
824    /// }
825    /// ```
826    #[inline]
827    #[stable(feature = "rust1", since = "1.0.0")]
828    #[rustc_const_unstable(feature = "const_result_trait_fn", issue = "144211")]
829    pub const fn map<U, F>(self, op: F) -> Result<U, E>
830    where
831        F: [const] FnOnce(T) -> U + [const] Destruct,
832    {
833        match self {
834            Ok(t) => Ok(op(t)),
835            Err(e) => Err(e),
836        }
837    }
838
839    /// Returns the provided default (if [`Err`]), or
840    /// applies a function to the contained value (if [`Ok`]).
841    ///
842    /// Arguments passed to `map_or` are eagerly evaluated; if you are passing
843    /// the result of a function call, it is recommended to use [`map_or_else`],
844    /// which is lazily evaluated.
845    ///
846    /// [`map_or_else`]: Result::map_or_else
847    ///
848    /// # Examples
849    ///
850    /// ```
851    /// let x: Result<_, &str> = Ok("foo");
852    /// assert_eq!(x.map_or(42, |v| v.len()), 3);
853    ///
854    /// let x: Result<&str, _> = Err("bar");
855    /// assert_eq!(x.map_or(42, |v| v.len()), 42);
856    /// ```
857    #[inline]
858    #[stable(feature = "result_map_or", since = "1.41.0")]
859    #[rustc_const_unstable(feature = "const_result_trait_fn", issue = "144211")]
860    #[must_use = "if you don't need the returned value, use `if let` instead"]
861    pub const fn map_or<U, F>(self, default: U, f: F) -> U
862    where
863        F: [const] FnOnce(T) -> U + [const] Destruct,
864        T: [const] Destruct,
865        E: [const] Destruct,
866        U: [const] Destruct,
867    {
868        match self {
869            Ok(t) => f(t),
870            Err(_) => default,
871        }
872    }
873
874    /// Maps a `Result<T, E>` to `U` by applying fallback function `default` to
875    /// a contained [`Err`] value, or function `f` to a contained [`Ok`] value.
876    ///
877    /// This function can be used to unpack a successful result
878    /// while handling an error.
879    ///
880    ///
881    /// # Examples
882    ///
883    /// ```
884    /// let k = 21;
885    ///
886    /// let x : Result<_, &str> = Ok("foo");
887    /// assert_eq!(x.map_or_else(|e| k * 2, |v| v.len()), 3);
888    ///
889    /// let x : Result<&str, _> = Err("bar");
890    /// assert_eq!(x.map_or_else(|e| k * 2, |v| v.len()), 42);
891    /// ```
892    #[inline]
893    #[stable(feature = "result_map_or_else", since = "1.41.0")]
894    #[rustc_const_unstable(feature = "const_result_trait_fn", issue = "144211")]
895    pub const fn map_or_else<U, D, F>(self, default: D, f: F) -> U
896    where
897        D: [const] FnOnce(E) -> U + [const] Destruct,
898        F: [const] FnOnce(T) -> U + [const] Destruct,
899    {
900        match self {
901            Ok(t) => f(t),
902            Err(e) => default(e),
903        }
904    }
905
906    /// Maps a `Result<T, E>` to a `U` by applying function `f` to the contained
907    /// value if the result is [`Ok`], otherwise if [`Err`], returns the
908    /// [default value] for the type `U`.
909    ///
910    /// # Examples
911    ///
912    /// ```
913    /// #![feature(result_option_map_or_default)]
914    ///
915    /// let x: Result<_, &str> = Ok("foo");
916    /// let y: Result<&str, _> = Err("bar");
917    ///
918    /// assert_eq!(x.map_or_default(|x| x.len()), 3);
919    /// assert_eq!(y.map_or_default(|y| y.len()), 0);
920    /// ```
921    ///
922    /// [default value]: Default::default
923    #[inline]
924    #[unstable(feature = "result_option_map_or_default", issue = "138099")]
925    #[rustc_const_unstable(feature = "const_result_trait_fn", issue = "144211")]
926    pub const fn map_or_default<U, F>(self, f: F) -> U
927    where
928        F: [const] FnOnce(T) -> U + [const] Destruct,
929        U: [const] Default,
930        T: [const] Destruct,
931        E: [const] Destruct,
932    {
933        match self {
934            Ok(t) => f(t),
935            Err(_) => U::default(),
936        }
937    }
938
939    /// Maps a `Result<T, E>` to `Result<T, F>` by applying a function to a
940    /// contained [`Err`] value, leaving an [`Ok`] value untouched.
941    ///
942    /// This function can be used to pass through a successful result while handling
943    /// an error.
944    ///
945    ///
946    /// # Examples
947    ///
948    /// ```
949    /// fn stringify(x: u32) -> String { format!("error code: {x}") }
950    ///
951    /// let x: Result<u32, u32> = Ok(2);
952    /// assert_eq!(x.map_err(stringify), Ok(2));
953    ///
954    /// let x: Result<u32, u32> = Err(13);
955    /// assert_eq!(x.map_err(stringify), Err("error code: 13".to_string()));
956    /// ```
957    #[inline]
958    #[stable(feature = "rust1", since = "1.0.0")]
959    #[rustc_const_unstable(feature = "const_result_trait_fn", issue = "144211")]
960    pub const fn map_err<F, O>(self, op: O) -> Result<T, F>
961    where
962        O: [const] FnOnce(E) -> F + [const] Destruct,
963    {
964        match self {
965            Ok(t) => Ok(t),
966            Err(e) => Err(op(e)),
967        }
968    }
969
970    /// Calls a function with a reference to the contained value if [`Ok`].
971    ///
972    /// Returns the original result.
973    ///
974    /// # Examples
975    ///
976    /// ```
977    /// let x: u8 = "4"
978    ///     .parse::<u8>()
979    ///     .inspect(|x| println!("original: {x}"))
980    ///     .map(|x| x.pow(3))
981    ///     .expect("failed to parse number");
982    /// ```
983    #[inline]
984    #[stable(feature = "result_option_inspect", since = "1.76.0")]
985    #[rustc_const_unstable(feature = "const_result_trait_fn", issue = "144211")]
986    pub const fn inspect<F>(self, f: F) -> Self
987    where
988        F: [const] FnOnce(&T) + [const] Destruct,
989    {
990        if let Ok(ref t) = self {
991            f(t);
992        }
993
994        self
995    }
996
997    /// Calls a function with a reference to the contained value if [`Err`].
998    ///
999    /// Returns the original result.
1000    ///
1001    /// # Examples
1002    ///
1003    /// ```
1004    /// use std::{fs, io};
1005    ///
1006    /// fn read() -> io::Result<String> {
1007    ///     fs::read_to_string("address.txt")
1008    ///         .inspect_err(|e| eprintln!("failed to read file: {e}"))
1009    /// }
1010    /// ```
1011    #[inline]
1012    #[stable(feature = "result_option_inspect", since = "1.76.0")]
1013    #[rustc_const_unstable(feature = "const_result_trait_fn", issue = "144211")]
1014    pub const fn inspect_err<F>(self, f: F) -> Self
1015    where
1016        F: [const] FnOnce(&E) + [const] Destruct,
1017    {
1018        if let Err(ref e) = self {
1019            f(e);
1020        }
1021
1022        self
1023    }
1024
1025    /// Converts from `Result<T, E>` (or `&Result<T, E>`) to `Result<&<T as Deref>::Target, &E>`.
1026    ///
1027    /// Coerces the [`Ok`] variant of the original [`Result`] via [`Deref`](crate::ops::Deref)
1028    /// and returns the new [`Result`].
1029    ///
1030    /// # Examples
1031    ///
1032    /// ```
1033    /// let x: Result<String, u32> = Ok("hello".to_string());
1034    /// let y: Result<&str, &u32> = Ok("hello");
1035    /// assert_eq!(x.as_deref(), y);
1036    ///
1037    /// let x: Result<String, u32> = Err(42);
1038    /// let y: Result<&str, &u32> = Err(&42);
1039    /// assert_eq!(x.as_deref(), y);
1040    /// ```
1041    #[inline]
1042    #[stable(feature = "inner_deref", since = "1.47.0")]
1043    #[rustc_const_unstable(feature = "const_convert", issue = "143773")]
1044    pub const fn as_deref(&self) -> Result<&T::Target, &E>
1045    where
1046        T: [const] Deref,
1047    {
1048        self.as_ref().map(Deref::deref)
1049    }
1050
1051    /// Converts from `Result<T, E>` (or `&mut Result<T, E>`) to `Result<&mut <T as DerefMut>::Target, &mut E>`.
1052    ///
1053    /// Coerces the [`Ok`] variant of the original [`Result`] via [`DerefMut`](crate::ops::DerefMut)
1054    /// and returns the new [`Result`].
1055    ///
1056    /// # Examples
1057    ///
1058    /// ```
1059    /// let mut s = "HELLO".to_string();
1060    /// let mut x: Result<String, u32> = Ok("hello".to_string());
1061    /// let y: Result<&mut str, &mut u32> = Ok(&mut s);
1062    /// assert_eq!(x.as_deref_mut().map(|x| { x.make_ascii_uppercase(); x }), y);
1063    ///
1064    /// let mut i = 42;
1065    /// let mut x: Result<String, u32> = Err(42);
1066    /// let y: Result<&mut str, &mut u32> = Err(&mut i);
1067    /// assert_eq!(x.as_deref_mut().map(|x| { x.make_ascii_uppercase(); x }), y);
1068    /// ```
1069    #[inline]
1070    #[stable(feature = "inner_deref", since = "1.47.0")]
1071    #[rustc_const_unstable(feature = "const_convert", issue = "143773")]
1072    pub const fn as_deref_mut(&mut self) -> Result<&mut T::Target, &mut E>
1073    where
1074        T: [const] DerefMut,
1075    {
1076        self.as_mut().map(DerefMut::deref_mut)
1077    }
1078
1079    /////////////////////////////////////////////////////////////////////////
1080    // Iterator constructors
1081    /////////////////////////////////////////////////////////////////////////
1082
1083    /// Returns an iterator over the possibly contained value.
1084    ///
1085    /// The iterator yields one value if the result is [`Result::Ok`], otherwise none.
1086    ///
1087    /// # Examples
1088    ///
1089    /// ```
1090    /// let x: Result<u32, &str> = Ok(7);
1091    /// assert_eq!(x.iter().next(), Some(&7));
1092    ///
1093    /// let x: Result<u32, &str> = Err("nothing!");
1094    /// assert_eq!(x.iter().next(), None);
1095    /// ```
1096    #[inline]
1097    #[stable(feature = "rust1", since = "1.0.0")]
1098    #[rustc_const_unstable(feature = "const_result_trait_fn", issue = "144211")]
1099    // blocked on Iterator
1100    #[cfg(not(feature = "ferrocene_certified"))]
1101    pub const fn iter(&self) -> Iter<'_, T> {
1102        Iter { inner: self.as_ref().ok() }
1103    }
1104
1105    /// Returns a mutable iterator over the possibly contained value.
1106    ///
1107    /// The iterator yields one value if the result is [`Result::Ok`], otherwise none.
1108    ///
1109    /// # Examples
1110    ///
1111    /// ```
1112    /// let mut x: Result<u32, &str> = Ok(7);
1113    /// match x.iter_mut().next() {
1114    ///     Some(v) => *v = 40,
1115    ///     None => {},
1116    /// }
1117    /// assert_eq!(x, Ok(40));
1118    ///
1119    /// let mut x: Result<u32, &str> = Err("nothing!");
1120    /// assert_eq!(x.iter_mut().next(), None);
1121    /// ```
1122    #[inline]
1123    #[stable(feature = "rust1", since = "1.0.0")]
1124    #[rustc_const_unstable(feature = "const_result_trait_fn", issue = "144211")]
1125    // blocked on Iterator
1126    #[cfg(not(feature = "ferrocene_certified"))]
1127    pub const fn iter_mut(&mut self) -> IterMut<'_, T> {
1128        IterMut { inner: self.as_mut().ok() }
1129    }
1130
1131    /////////////////////////////////////////////////////////////////////////
1132    // Extract a value
1133    /////////////////////////////////////////////////////////////////////////
1134
1135    /// Returns the contained [`Ok`] value, consuming the `self` value.
1136    ///
1137    /// Because this function may panic, its use is generally discouraged.
1138    /// Instead, prefer to use pattern matching and handle the [`Err`]
1139    /// case explicitly, or call [`unwrap_or`], [`unwrap_or_else`], or
1140    /// [`unwrap_or_default`].
1141    ///
1142    /// [`unwrap_or`]: Result::unwrap_or
1143    /// [`unwrap_or_else`]: Result::unwrap_or_else
1144    /// [`unwrap_or_default`]: Result::unwrap_or_default
1145    ///
1146    /// # Panics
1147    ///
1148    /// Panics if the value is an [`Err`], with a panic message including the
1149    /// passed message, and the content of the [`Err`].
1150    ///
1151    ///
1152    /// # Examples
1153    ///
1154    /// ```should_panic
1155    /// let x: Result<u32, &str> = Err("emergency failure");
1156    /// x.expect("Testing expect"); // panics with `Testing expect: emergency failure`
1157    /// ```
1158    ///
1159    /// # Recommended Message Style
1160    ///
1161    /// We recommend that `expect` messages are used to describe the reason you
1162    /// _expect_ the `Result` should be `Ok`.
1163    ///
1164    /// ```should_panic
1165    /// let path = std::env::var("IMPORTANT_PATH")
1166    ///     .expect("env variable `IMPORTANT_PATH` should be set by `wrapper_script.sh`");
1167    /// ```
1168    ///
1169    /// **Hint**: If you're having trouble remembering how to phrase expect
1170    /// error messages remember to focus on the word "should" as in "env
1171    /// variable should be set by blah" or "the given binary should be available
1172    /// and executable by the current user".
1173    ///
1174    /// For more detail on expect message styles and the reasoning behind our recommendation please
1175    /// refer to the section on ["Common Message
1176    /// Styles"](../../std/error/index.html#common-message-styles) in the
1177    /// [`std::error`](../../std/error/index.html) module docs.
1178    #[inline]
1179    #[track_caller]
1180    #[stable(feature = "result_expect", since = "1.4.0")]
1181    // blocked on Debug
1182    #[cfg(not(feature = "ferrocene_certified"))]
1183    pub fn expect(self, msg: &str) -> T
1184    where
1185        E: fmt::Debug,
1186    {
1187        match self {
1188            Ok(t) => t,
1189            Err(e) => unwrap_failed(msg, &e),
1190        }
1191    }
1192
1193    /// Returns the contained [`Ok`] value, consuming the `self` value.
1194    ///
1195    /// Because this function may panic, its use is generally discouraged.
1196    /// Panics are meant for unrecoverable errors, and
1197    /// [may abort the entire program][panic-abort].
1198    ///
1199    /// Instead, prefer to use [the `?` (try) operator][try-operator], or pattern matching
1200    /// to handle the [`Err`] case explicitly, or call [`unwrap_or`],
1201    /// [`unwrap_or_else`], or [`unwrap_or_default`].
1202    ///
1203    /// [panic-abort]: https://doc.rust-lang.org/book/ch09-01-unrecoverable-errors-with-panic.html
1204    /// [try-operator]: https://doc.rust-lang.org/book/ch09-02-recoverable-errors-with-result.html#a-shortcut-for-propagating-errors-the--operator
1205    /// [`unwrap_or`]: Result::unwrap_or
1206    /// [`unwrap_or_else`]: Result::unwrap_or_else
1207    /// [`unwrap_or_default`]: Result::unwrap_or_default
1208    ///
1209    /// # Panics
1210    ///
1211    /// Panics if the value is an [`Err`], with a panic message provided by the
1212    /// [`Err`]'s value.
1213    ///
1214    ///
1215    /// # Examples
1216    ///
1217    /// Basic usage:
1218    ///
1219    /// ```
1220    /// let x: Result<u32, &str> = Ok(2);
1221    /// assert_eq!(x.unwrap(), 2);
1222    /// ```
1223    ///
1224    /// ```should_panic
1225    /// let x: Result<u32, &str> = Err("emergency failure");
1226    /// x.unwrap(); // panics with `emergency failure`
1227    /// ```
1228    #[inline(always)]
1229    #[track_caller]
1230    #[stable(feature = "rust1", since = "1.0.0")]
1231    // blocked on Debug
1232    #[cfg(not(feature = "ferrocene_certified"))]
1233    pub fn unwrap(self) -> T
1234    where
1235        E: fmt::Debug,
1236    {
1237        match self {
1238            Ok(t) => t,
1239            Err(e) => unwrap_failed("called `Result::unwrap()` on an `Err` value", &e),
1240        }
1241    }
1242
1243    /// Returns the contained [`Ok`] value or a default
1244    ///
1245    /// Consumes the `self` argument then, if [`Ok`], returns the contained
1246    /// value, otherwise if [`Err`], returns the default value for that
1247    /// type.
1248    ///
1249    /// # Examples
1250    ///
1251    /// Converts a string to an integer, turning poorly-formed strings
1252    /// into 0 (the default value for integers). [`parse`] converts
1253    /// a string to any other type that implements [`FromStr`], returning an
1254    /// [`Err`] on error.
1255    ///
1256    /// ```
1257    /// let good_year_from_input = "1909";
1258    /// let bad_year_from_input = "190blarg";
1259    /// let good_year = good_year_from_input.parse().unwrap_or_default();
1260    /// let bad_year = bad_year_from_input.parse().unwrap_or_default();
1261    ///
1262    /// assert_eq!(1909, good_year);
1263    /// assert_eq!(0, bad_year);
1264    /// ```
1265    ///
1266    /// [`parse`]: str::parse
1267    /// [`FromStr`]: crate::str::FromStr
1268    #[inline]
1269    #[stable(feature = "result_unwrap_or_default", since = "1.16.0")]
1270    #[rustc_const_unstable(feature = "const_result_trait_fn", issue = "144211")]
1271    pub const fn unwrap_or_default(self) -> T
1272    where
1273        T: [const] Default + [const] Destruct,
1274        E: [const] Destruct,
1275    {
1276        match self {
1277            Ok(x) => x,
1278            Err(_) => Default::default(),
1279        }
1280    }
1281
1282    /// Returns the contained [`Err`] value, consuming the `self` value.
1283    ///
1284    /// # Panics
1285    ///
1286    /// Panics if the value is an [`Ok`], with a panic message including the
1287    /// passed message, and the content of the [`Ok`].
1288    ///
1289    ///
1290    /// # Examples
1291    ///
1292    /// ```should_panic
1293    /// let x: Result<u32, &str> = Ok(10);
1294    /// x.expect_err("Testing expect_err"); // panics with `Testing expect_err: 10`
1295    /// ```
1296    #[inline]
1297    #[track_caller]
1298    #[stable(feature = "result_expect_err", since = "1.17.0")]
1299    // blocked on Debug
1300    #[cfg(not(feature = "ferrocene_certified"))]
1301    pub fn expect_err(self, msg: &str) -> E
1302    where
1303        T: fmt::Debug,
1304    {
1305        match self {
1306            Ok(t) => unwrap_failed(msg, &t),
1307            Err(e) => e,
1308        }
1309    }
1310
1311    /// Returns the contained [`Err`] value, consuming the `self` value.
1312    ///
1313    /// # Panics
1314    ///
1315    /// Panics if the value is an [`Ok`], with a custom panic message provided
1316    /// by the [`Ok`]'s value.
1317    ///
1318    /// # Examples
1319    ///
1320    /// ```should_panic
1321    /// let x: Result<u32, &str> = Ok(2);
1322    /// x.unwrap_err(); // panics with `2`
1323    /// ```
1324    ///
1325    /// ```
1326    /// let x: Result<u32, &str> = Err("emergency failure");
1327    /// assert_eq!(x.unwrap_err(), "emergency failure");
1328    /// ```
1329    #[inline]
1330    #[track_caller]
1331    #[stable(feature = "rust1", since = "1.0.0")]
1332    // blocked on Debug
1333    #[cfg(not(feature = "ferrocene_certified"))]
1334    pub fn unwrap_err(self) -> E
1335    where
1336        T: fmt::Debug,
1337    {
1338        match self {
1339            Ok(t) => unwrap_failed("called `Result::unwrap_err()` on an `Ok` value", &t),
1340            Err(e) => e,
1341        }
1342    }
1343
1344    /// Returns the contained [`Ok`] value, but never panics.
1345    ///
1346    /// Unlike [`unwrap`], this method is known to never panic on the
1347    /// result types it is implemented for. Therefore, it can be used
1348    /// instead of `unwrap` as a maintainability safeguard that will fail
1349    /// to compile if the error type of the `Result` is later changed
1350    /// to an error that can actually occur.
1351    ///
1352    /// [`unwrap`]: Result::unwrap
1353    ///
1354    /// # Examples
1355    ///
1356    /// ```
1357    /// # #![feature(never_type)]
1358    /// # #![feature(unwrap_infallible)]
1359    ///
1360    /// fn only_good_news() -> Result<String, !> {
1361    ///     Ok("this is fine".into())
1362    /// }
1363    ///
1364    /// let s: String = only_good_news().into_ok();
1365    /// println!("{s}");
1366    /// ```
1367    #[unstable(feature = "unwrap_infallible", reason = "newly added", issue = "61695")]
1368    #[inline]
1369    #[rustc_allow_const_fn_unstable(const_precise_live_drops)]
1370    #[rustc_const_unstable(feature = "const_convert", issue = "143773")]
1371    // blocked on !
1372    #[cfg(not(feature = "ferrocene_certified"))]
1373    pub const fn into_ok(self) -> T
1374    where
1375        E: [const] Into<!>,
1376    {
1377        match self {
1378            Ok(x) => x,
1379            Err(e) => e.into(),
1380        }
1381    }
1382
1383    /// Returns the contained [`Err`] value, but never panics.
1384    ///
1385    /// Unlike [`unwrap_err`], this method is known to never panic on the
1386    /// result types it is implemented for. Therefore, it can be used
1387    /// instead of `unwrap_err` as a maintainability safeguard that will fail
1388    /// to compile if the ok type of the `Result` is later changed
1389    /// to a type that can actually occur.
1390    ///
1391    /// [`unwrap_err`]: Result::unwrap_err
1392    ///
1393    /// # Examples
1394    ///
1395    /// ```
1396    /// # #![feature(never_type)]
1397    /// # #![feature(unwrap_infallible)]
1398    ///
1399    /// fn only_bad_news() -> Result<!, String> {
1400    ///     Err("Oops, it failed".into())
1401    /// }
1402    ///
1403    /// let error: String = only_bad_news().into_err();
1404    /// println!("{error}");
1405    /// ```
1406    #[unstable(feature = "unwrap_infallible", reason = "newly added", issue = "61695")]
1407    #[inline]
1408    #[rustc_allow_const_fn_unstable(const_precise_live_drops)]
1409    #[rustc_const_unstable(feature = "const_convert", issue = "143773")]
1410    // blocked on !
1411    #[cfg(not(feature = "ferrocene_certified"))]
1412    pub const fn into_err(self) -> E
1413    where
1414        T: [const] Into<!>,
1415    {
1416        match self {
1417            Ok(x) => x.into(),
1418            Err(e) => e,
1419        }
1420    }
1421
1422    ////////////////////////////////////////////////////////////////////////
1423    // Boolean operations on the values, eager and lazy
1424    /////////////////////////////////////////////////////////////////////////
1425
1426    /// Returns `res` if the result is [`Ok`], otherwise returns the [`Err`] value of `self`.
1427    ///
1428    /// Arguments passed to `and` are eagerly evaluated; if you are passing the
1429    /// result of a function call, it is recommended to use [`and_then`], which is
1430    /// lazily evaluated.
1431    ///
1432    /// [`and_then`]: Result::and_then
1433    ///
1434    /// # Examples
1435    ///
1436    /// ```
1437    /// let x: Result<u32, &str> = Ok(2);
1438    /// let y: Result<&str, &str> = Err("late error");
1439    /// assert_eq!(x.and(y), Err("late error"));
1440    ///
1441    /// let x: Result<u32, &str> = Err("early error");
1442    /// let y: Result<&str, &str> = Ok("foo");
1443    /// assert_eq!(x.and(y), Err("early error"));
1444    ///
1445    /// let x: Result<u32, &str> = Err("not a 2");
1446    /// let y: Result<&str, &str> = Err("late error");
1447    /// assert_eq!(x.and(y), Err("not a 2"));
1448    ///
1449    /// let x: Result<u32, &str> = Ok(2);
1450    /// let y: Result<&str, &str> = Ok("different result type");
1451    /// assert_eq!(x.and(y), Ok("different result type"));
1452    /// ```
1453    #[inline]
1454    #[stable(feature = "rust1", since = "1.0.0")]
1455    #[rustc_const_unstable(feature = "const_result_trait_fn", issue = "144211")]
1456    pub const fn and<U>(self, res: Result<U, E>) -> Result<U, E>
1457    where
1458        T: [const] Destruct,
1459        E: [const] Destruct,
1460        U: [const] Destruct,
1461    {
1462        match self {
1463            Ok(_) => res,
1464            Err(e) => Err(e),
1465        }
1466    }
1467
1468    /// Calls `op` if the result is [`Ok`], otherwise returns the [`Err`] value of `self`.
1469    ///
1470    ///
1471    /// This function can be used for control flow based on `Result` values.
1472    ///
1473    /// # Examples
1474    ///
1475    /// ```
1476    /// fn sq_then_to_string(x: u32) -> Result<String, &'static str> {
1477    ///     x.checked_mul(x).map(|sq| sq.to_string()).ok_or("overflowed")
1478    /// }
1479    ///
1480    /// assert_eq!(Ok(2).and_then(sq_then_to_string), Ok(4.to_string()));
1481    /// assert_eq!(Ok(1_000_000).and_then(sq_then_to_string), Err("overflowed"));
1482    /// assert_eq!(Err("not a number").and_then(sq_then_to_string), Err("not a number"));
1483    /// ```
1484    ///
1485    /// Often used to chain fallible operations that may return [`Err`].
1486    ///
1487    /// ```
1488    /// use std::{io::ErrorKind, path::Path};
1489    ///
1490    /// // Note: on Windows "/" maps to "C:\"
1491    /// let root_modified_time = Path::new("/").metadata().and_then(|md| md.modified());
1492    /// assert!(root_modified_time.is_ok());
1493    ///
1494    /// let should_fail = Path::new("/bad/path").metadata().and_then(|md| md.modified());
1495    /// assert!(should_fail.is_err());
1496    /// assert_eq!(should_fail.unwrap_err().kind(), ErrorKind::NotFound);
1497    /// ```
1498    #[inline]
1499    #[stable(feature = "rust1", since = "1.0.0")]
1500    #[rustc_const_unstable(feature = "const_result_trait_fn", issue = "144211")]
1501    #[rustc_confusables("flat_map", "flatmap")]
1502    pub const fn and_then<U, F>(self, op: F) -> Result<U, E>
1503    where
1504        F: [const] FnOnce(T) -> Result<U, E> + [const] Destruct,
1505    {
1506        match self {
1507            Ok(t) => op(t),
1508            Err(e) => Err(e),
1509        }
1510    }
1511
1512    /// Returns `res` if the result is [`Err`], otherwise returns the [`Ok`] value of `self`.
1513    ///
1514    /// Arguments passed to `or` are eagerly evaluated; if you are passing the
1515    /// result of a function call, it is recommended to use [`or_else`], which is
1516    /// lazily evaluated.
1517    ///
1518    /// [`or_else`]: Result::or_else
1519    ///
1520    /// # Examples
1521    ///
1522    /// ```
1523    /// let x: Result<u32, &str> = Ok(2);
1524    /// let y: Result<u32, &str> = Err("late error");
1525    /// assert_eq!(x.or(y), Ok(2));
1526    ///
1527    /// let x: Result<u32, &str> = Err("early error");
1528    /// let y: Result<u32, &str> = Ok(2);
1529    /// assert_eq!(x.or(y), Ok(2));
1530    ///
1531    /// let x: Result<u32, &str> = Err("not a 2");
1532    /// let y: Result<u32, &str> = Err("late error");
1533    /// assert_eq!(x.or(y), Err("late error"));
1534    ///
1535    /// let x: Result<u32, &str> = Ok(2);
1536    /// let y: Result<u32, &str> = Ok(100);
1537    /// assert_eq!(x.or(y), Ok(2));
1538    /// ```
1539    #[inline]
1540    #[stable(feature = "rust1", since = "1.0.0")]
1541    #[rustc_const_unstable(feature = "const_result_trait_fn", issue = "144211")]
1542    pub const fn or<F>(self, res: Result<T, F>) -> Result<T, F>
1543    where
1544        T: [const] Destruct,
1545        E: [const] Destruct,
1546        F: [const] Destruct,
1547    {
1548        match self {
1549            Ok(v) => Ok(v),
1550            Err(_) => res,
1551        }
1552    }
1553
1554    /// Calls `op` if the result is [`Err`], otherwise returns the [`Ok`] value of `self`.
1555    ///
1556    /// This function can be used for control flow based on result values.
1557    ///
1558    ///
1559    /// # Examples
1560    ///
1561    /// ```
1562    /// fn sq(x: u32) -> Result<u32, u32> { Ok(x * x) }
1563    /// fn err(x: u32) -> Result<u32, u32> { Err(x) }
1564    ///
1565    /// assert_eq!(Ok(2).or_else(sq).or_else(sq), Ok(2));
1566    /// assert_eq!(Ok(2).or_else(err).or_else(sq), Ok(2));
1567    /// assert_eq!(Err(3).or_else(sq).or_else(err), Ok(9));
1568    /// assert_eq!(Err(3).or_else(err).or_else(err), Err(3));
1569    /// ```
1570    #[inline]
1571    #[stable(feature = "rust1", since = "1.0.0")]
1572    #[rustc_const_unstable(feature = "const_result_trait_fn", issue = "144211")]
1573    pub const fn or_else<F, O>(self, op: O) -> Result<T, F>
1574    where
1575        O: [const] FnOnce(E) -> Result<T, F> + [const] Destruct,
1576    {
1577        match self {
1578            Ok(t) => Ok(t),
1579            Err(e) => op(e),
1580        }
1581    }
1582
1583    /// Returns the contained [`Ok`] value or a provided default.
1584    ///
1585    /// Arguments passed to `unwrap_or` are eagerly evaluated; if you are passing
1586    /// the result of a function call, it is recommended to use [`unwrap_or_else`],
1587    /// which is lazily evaluated.
1588    ///
1589    /// [`unwrap_or_else`]: Result::unwrap_or_else
1590    ///
1591    /// # Examples
1592    ///
1593    /// ```
1594    /// let default = 2;
1595    /// let x: Result<u32, &str> = Ok(9);
1596    /// assert_eq!(x.unwrap_or(default), 9);
1597    ///
1598    /// let x: Result<u32, &str> = Err("error");
1599    /// assert_eq!(x.unwrap_or(default), default);
1600    /// ```
1601    #[inline]
1602    #[stable(feature = "rust1", since = "1.0.0")]
1603    #[rustc_const_unstable(feature = "const_result_trait_fn", issue = "144211")]
1604    pub const fn unwrap_or(self, default: T) -> T
1605    where
1606        T: [const] Destruct,
1607        E: [const] Destruct,
1608    {
1609        match self {
1610            Ok(t) => t,
1611            Err(_) => default,
1612        }
1613    }
1614
1615    /// Returns the contained [`Ok`] value or computes it from a closure.
1616    ///
1617    ///
1618    /// # Examples
1619    ///
1620    /// ```
1621    /// fn count(x: &str) -> usize { x.len() }
1622    ///
1623    /// assert_eq!(Ok(2).unwrap_or_else(count), 2);
1624    /// assert_eq!(Err("foo").unwrap_or_else(count), 3);
1625    /// ```
1626    #[inline]
1627    #[track_caller]
1628    #[stable(feature = "rust1", since = "1.0.0")]
1629    #[rustc_const_unstable(feature = "const_result_trait_fn", issue = "144211")]
1630    pub const fn unwrap_or_else<F>(self, op: F) -> T
1631    where
1632        F: [const] FnOnce(E) -> T + [const] Destruct,
1633    {
1634        match self {
1635            Ok(t) => t,
1636            Err(e) => op(e),
1637        }
1638    }
1639
1640    /// Returns the contained [`Ok`] value, consuming the `self` value,
1641    /// without checking that the value is not an [`Err`].
1642    ///
1643    /// # Safety
1644    ///
1645    /// Calling this method on an [`Err`] is *[undefined behavior]*.
1646    ///
1647    /// [undefined behavior]: https://doc.rust-lang.org/reference/behavior-considered-undefined.html
1648    ///
1649    /// # Examples
1650    ///
1651    /// ```
1652    /// let x: Result<u32, &str> = Ok(2);
1653    /// assert_eq!(unsafe { x.unwrap_unchecked() }, 2);
1654    /// ```
1655    ///
1656    /// ```no_run
1657    /// let x: Result<u32, &str> = Err("emergency failure");
1658    /// unsafe { x.unwrap_unchecked() }; // Undefined behavior!
1659    /// ```
1660    #[inline]
1661    #[track_caller]
1662    #[stable(feature = "option_result_unwrap_unchecked", since = "1.58.0")]
1663    // blocked on hint
1664    #[cfg(not(feature = "ferrocene_certified"))]
1665    pub unsafe fn unwrap_unchecked(self) -> T {
1666        match self {
1667            Ok(t) => t,
1668            // SAFETY: the safety contract must be upheld by the caller.
1669            Err(_) => unsafe { hint::unreachable_unchecked() },
1670        }
1671    }
1672
1673    /// Returns the contained [`Err`] value, consuming the `self` value,
1674    /// without checking that the value is not an [`Ok`].
1675    ///
1676    /// # Safety
1677    ///
1678    /// Calling this method on an [`Ok`] is *[undefined behavior]*.
1679    ///
1680    /// [undefined behavior]: https://doc.rust-lang.org/reference/behavior-considered-undefined.html
1681    ///
1682    /// # Examples
1683    ///
1684    /// ```no_run
1685    /// let x: Result<u32, &str> = Ok(2);
1686    /// unsafe { x.unwrap_err_unchecked() }; // Undefined behavior!
1687    /// ```
1688    ///
1689    /// ```
1690    /// let x: Result<u32, &str> = Err("emergency failure");
1691    /// assert_eq!(unsafe { x.unwrap_err_unchecked() }, "emergency failure");
1692    /// ```
1693    #[inline]
1694    #[track_caller]
1695    #[stable(feature = "option_result_unwrap_unchecked", since = "1.58.0")]
1696    // blocked on hint
1697    #[cfg(not(feature = "ferrocene_certified"))]
1698    pub unsafe fn unwrap_err_unchecked(self) -> E {
1699        match self {
1700            // SAFETY: the safety contract must be upheld by the caller.
1701            Ok(_) => unsafe { hint::unreachable_unchecked() },
1702            Err(e) => e,
1703        }
1704    }
1705}
1706
1707impl<T, E> Result<&T, E> {
1708    /// Maps a `Result<&T, E>` to a `Result<T, E>` by copying the contents of the
1709    /// `Ok` part.
1710    ///
1711    /// # Examples
1712    ///
1713    /// ```
1714    /// let val = 12;
1715    /// let x: Result<&i32, i32> = Ok(&val);
1716    /// assert_eq!(x, Ok(&12));
1717    /// let copied = x.copied();
1718    /// assert_eq!(copied, Ok(12));
1719    /// ```
1720    #[inline]
1721    #[stable(feature = "result_copied", since = "1.59.0")]
1722    #[rustc_const_stable(feature = "const_result", since = "1.83.0")]
1723    #[rustc_allow_const_fn_unstable(const_precise_live_drops)]
1724    pub const fn copied(self) -> Result<T, E>
1725    where
1726        T: Copy,
1727    {
1728        // FIXME(const-hack): this implementation, which sidesteps using `Result::map` since it's not const
1729        // ready yet, should be reverted when possible to avoid code repetition
1730        match self {
1731            Ok(&v) => Ok(v),
1732            Err(e) => Err(e),
1733        }
1734    }
1735
1736    /// Maps a `Result<&T, E>` to a `Result<T, E>` by cloning the contents of the
1737    /// `Ok` part.
1738    ///
1739    /// # Examples
1740    ///
1741    /// ```
1742    /// let val = 12;
1743    /// let x: Result<&i32, i32> = Ok(&val);
1744    /// assert_eq!(x, Ok(&12));
1745    /// let cloned = x.cloned();
1746    /// assert_eq!(cloned, Ok(12));
1747    /// ```
1748    #[inline]
1749    #[stable(feature = "result_cloned", since = "1.59.0")]
1750    pub fn cloned(self) -> Result<T, E>
1751    where
1752        T: Clone,
1753    {
1754        self.map(|t| t.clone())
1755    }
1756}
1757
1758impl<T, E> Result<&mut T, E> {
1759    /// Maps a `Result<&mut T, E>` to a `Result<T, E>` by copying the contents of the
1760    /// `Ok` part.
1761    ///
1762    /// # Examples
1763    ///
1764    /// ```
1765    /// let mut val = 12;
1766    /// let x: Result<&mut i32, i32> = Ok(&mut val);
1767    /// assert_eq!(x, Ok(&mut 12));
1768    /// let copied = x.copied();
1769    /// assert_eq!(copied, Ok(12));
1770    /// ```
1771    #[inline]
1772    #[stable(feature = "result_copied", since = "1.59.0")]
1773    #[rustc_const_stable(feature = "const_result", since = "1.83.0")]
1774    #[rustc_allow_const_fn_unstable(const_precise_live_drops)]
1775    pub const fn copied(self) -> Result<T, E>
1776    where
1777        T: Copy,
1778    {
1779        // FIXME(const-hack): this implementation, which sidesteps using `Result::map` since it's not const
1780        // ready yet, should be reverted when possible to avoid code repetition
1781        match self {
1782            Ok(&mut v) => Ok(v),
1783            Err(e) => Err(e),
1784        }
1785    }
1786
1787    /// Maps a `Result<&mut T, E>` to a `Result<T, E>` by cloning the contents of the
1788    /// `Ok` part.
1789    ///
1790    /// # Examples
1791    ///
1792    /// ```
1793    /// let mut val = 12;
1794    /// let x: Result<&mut i32, i32> = Ok(&mut val);
1795    /// assert_eq!(x, Ok(&mut 12));
1796    /// let cloned = x.cloned();
1797    /// assert_eq!(cloned, Ok(12));
1798    /// ```
1799    #[inline]
1800    #[stable(feature = "result_cloned", since = "1.59.0")]
1801    pub fn cloned(self) -> Result<T, E>
1802    where
1803        T: Clone,
1804    {
1805        self.map(|t| t.clone())
1806    }
1807}
1808
1809impl<T, E> Result<Option<T>, E> {
1810    /// Transposes a `Result` of an `Option` into an `Option` of a `Result`.
1811    ///
1812    /// `Ok(None)` will be mapped to `None`.
1813    /// `Ok(Some(_))` and `Err(_)` will be mapped to `Some(Ok(_))` and `Some(Err(_))`.
1814    ///
1815    /// # Examples
1816    ///
1817    /// ```
1818    /// #[derive(Debug, Eq, PartialEq)]
1819    /// struct SomeErr;
1820    ///
1821    /// let x: Result<Option<i32>, SomeErr> = Ok(Some(5));
1822    /// let y: Option<Result<i32, SomeErr>> = Some(Ok(5));
1823    /// assert_eq!(x.transpose(), y);
1824    /// ```
1825    #[inline]
1826    #[stable(feature = "transpose_result", since = "1.33.0")]
1827    #[rustc_const_stable(feature = "const_result", since = "1.83.0")]
1828    #[rustc_allow_const_fn_unstable(const_precise_live_drops)]
1829    pub const fn transpose(self) -> Option<Result<T, E>> {
1830        match self {
1831            Ok(Some(x)) => Some(Ok(x)),
1832            Ok(None) => None,
1833            Err(e) => Some(Err(e)),
1834        }
1835    }
1836}
1837
1838impl<T, E> Result<Result<T, E>, E> {
1839    /// Converts from `Result<Result<T, E>, E>` to `Result<T, E>`
1840    ///
1841    /// # Examples
1842    ///
1843    /// ```
1844    /// let x: Result<Result<&'static str, u32>, u32> = Ok(Ok("hello"));
1845    /// assert_eq!(Ok("hello"), x.flatten());
1846    ///
1847    /// let x: Result<Result<&'static str, u32>, u32> = Ok(Err(6));
1848    /// assert_eq!(Err(6), x.flatten());
1849    ///
1850    /// let x: Result<Result<&'static str, u32>, u32> = Err(6);
1851    /// assert_eq!(Err(6), x.flatten());
1852    /// ```
1853    ///
1854    /// Flattening only removes one level of nesting at a time:
1855    ///
1856    /// ```
1857    /// let x: Result<Result<Result<&'static str, u32>, u32>, u32> = Ok(Ok(Ok("hello")));
1858    /// assert_eq!(Ok(Ok("hello")), x.flatten());
1859    /// assert_eq!(Ok("hello"), x.flatten().flatten());
1860    /// ```
1861    #[inline]
1862    #[stable(feature = "result_flattening", since = "1.89.0")]
1863    #[rustc_allow_const_fn_unstable(const_precise_live_drops)]
1864    #[rustc_const_stable(feature = "result_flattening", since = "1.89.0")]
1865    // blocked on const impl Drop for Result<Result<T, E>>
1866    #[cfg(not(feature = "ferrocene_certified"))]
1867    pub const fn flatten(self) -> Result<T, E> {
1868        // FIXME(const-hack): could be written with `and_then`
1869        match self {
1870            Ok(inner) => inner,
1871            Err(e) => Err(e),
1872        }
1873    }
1874}
1875
1876// This is a separate function to reduce the code size of the methods
1877#[cfg(not(feature = "panic_immediate_abort"))]
1878#[inline(never)]
1879#[cold]
1880#[track_caller]
1881// blocked on Debug
1882#[cfg(not(feature = "ferrocene_certified"))]
1883fn unwrap_failed(msg: &str, error: &dyn fmt::Debug) -> ! {
1884    panic!("{msg}: {error:?}");
1885}
1886
1887// This is a separate function to avoid constructing a `dyn Debug`
1888// that gets immediately thrown away, since vtables don't get cleaned up
1889// by dead code elimination if a trait object is constructed even if it goes
1890// unused
1891#[cfg(feature = "panic_immediate_abort")]
1892#[inline]
1893#[cold]
1894#[track_caller]
1895// blocked on Debug
1896#[cfg(not(feature = "ferrocene_certified"))]
1897const fn unwrap_failed<T>(_msg: &str, _error: &T) -> ! {
1898    panic!()
1899}
1900
1901/////////////////////////////////////////////////////////////////////////////
1902// Trait implementations
1903/////////////////////////////////////////////////////////////////////////////
1904
1905#[stable(feature = "rust1", since = "1.0.0")]
1906#[cfg(not(feature = "ferrocene_certified"))]
1907impl<T, E> Clone for Result<T, E>
1908where
1909    T: Clone,
1910    E: Clone,
1911{
1912    #[inline]
1913    fn clone(&self) -> Self {
1914        match self {
1915            Ok(x) => Ok(x.clone()),
1916            Err(x) => Err(x.clone()),
1917        }
1918    }
1919
1920    #[inline]
1921    fn clone_from(&mut self, source: &Self) {
1922        match (self, source) {
1923            (Ok(to), Ok(from)) => to.clone_from(from),
1924            (Err(to), Err(from)) => to.clone_from(from),
1925            (to, from) => *to = from.clone(),
1926        }
1927    }
1928}
1929
1930#[unstable(feature = "ergonomic_clones", issue = "132290")]
1931#[cfg(not(feature = "ferrocene_certified"))]
1932impl<T, E> crate::clone::UseCloned for Result<T, E>
1933where
1934    T: crate::clone::UseCloned,
1935    E: crate::clone::UseCloned,
1936{
1937}
1938
1939#[stable(feature = "rust1", since = "1.0.0")]
1940#[cfg(not(feature = "ferrocene_certified"))]
1941impl<T, E> IntoIterator for Result<T, E> {
1942    type Item = T;
1943    type IntoIter = IntoIter<T>;
1944
1945    /// Returns a consuming iterator over the possibly contained value.
1946    ///
1947    /// The iterator yields one value if the result is [`Result::Ok`], otherwise none.
1948    ///
1949    /// # Examples
1950    ///
1951    /// ```
1952    /// let x: Result<u32, &str> = Ok(5);
1953    /// let v: Vec<u32> = x.into_iter().collect();
1954    /// assert_eq!(v, [5]);
1955    ///
1956    /// let x: Result<u32, &str> = Err("nothing!");
1957    /// let v: Vec<u32> = x.into_iter().collect();
1958    /// assert_eq!(v, []);
1959    /// ```
1960    #[inline]
1961    fn into_iter(self) -> IntoIter<T> {
1962        IntoIter { inner: self.ok() }
1963    }
1964}
1965
1966#[stable(since = "1.4.0", feature = "result_iter")]
1967#[cfg(not(feature = "ferrocene_certified"))]
1968impl<'a, T, E> IntoIterator for &'a Result<T, E> {
1969    type Item = &'a T;
1970    type IntoIter = Iter<'a, T>;
1971
1972    fn into_iter(self) -> Iter<'a, T> {
1973        self.iter()
1974    }
1975}
1976
1977#[stable(since = "1.4.0", feature = "result_iter")]
1978#[cfg(not(feature = "ferrocene_certified"))]
1979impl<'a, T, E> IntoIterator for &'a mut Result<T, E> {
1980    type Item = &'a mut T;
1981    type IntoIter = IterMut<'a, T>;
1982
1983    fn into_iter(self) -> IterMut<'a, T> {
1984        self.iter_mut()
1985    }
1986}
1987
1988/////////////////////////////////////////////////////////////////////////////
1989// The Result Iterators
1990/////////////////////////////////////////////////////////////////////////////
1991
1992/// An iterator over a reference to the [`Ok`] variant of a [`Result`].
1993///
1994/// The iterator yields one value if the result is [`Ok`], otherwise none.
1995///
1996/// Created by [`Result::iter`].
1997#[derive(Debug)]
1998#[stable(feature = "rust1", since = "1.0.0")]
1999#[cfg(not(feature = "ferrocene_certified"))]
2000pub struct Iter<'a, T: 'a> {
2001    inner: Option<&'a T>,
2002}
2003
2004#[stable(feature = "rust1", since = "1.0.0")]
2005#[cfg(not(feature = "ferrocene_certified"))]
2006impl<'a, T> Iterator for Iter<'a, T> {
2007    type Item = &'a T;
2008
2009    #[inline]
2010    fn next(&mut self) -> Option<&'a T> {
2011        self.inner.take()
2012    }
2013    #[inline]
2014    fn size_hint(&self) -> (usize, Option<usize>) {
2015        let n = if self.inner.is_some() { 1 } else { 0 };
2016        (n, Some(n))
2017    }
2018}
2019
2020#[stable(feature = "rust1", since = "1.0.0")]
2021#[cfg(not(feature = "ferrocene_certified"))]
2022impl<'a, T> DoubleEndedIterator for Iter<'a, T> {
2023    #[inline]
2024    fn next_back(&mut self) -> Option<&'a T> {
2025        self.inner.take()
2026    }
2027}
2028
2029#[stable(feature = "rust1", since = "1.0.0")]
2030#[cfg(not(feature = "ferrocene_certified"))]
2031impl<T> ExactSizeIterator for Iter<'_, T> {}
2032
2033#[stable(feature = "fused", since = "1.26.0")]
2034#[cfg(not(feature = "ferrocene_certified"))]
2035impl<T> FusedIterator for Iter<'_, T> {}
2036
2037#[unstable(feature = "trusted_len", issue = "37572")]
2038#[cfg(not(feature = "ferrocene_certified"))]
2039unsafe impl<A> TrustedLen for Iter<'_, A> {}
2040
2041#[stable(feature = "rust1", since = "1.0.0")]
2042#[cfg(not(feature = "ferrocene_certified"))]
2043impl<T> Clone for Iter<'_, T> {
2044    #[inline]
2045    fn clone(&self) -> Self {
2046        Iter { inner: self.inner }
2047    }
2048}
2049
2050/// An iterator over a mutable reference to the [`Ok`] variant of a [`Result`].
2051///
2052/// Created by [`Result::iter_mut`].
2053#[derive(Debug)]
2054#[stable(feature = "rust1", since = "1.0.0")]
2055#[cfg(not(feature = "ferrocene_certified"))]
2056pub struct IterMut<'a, T: 'a> {
2057    inner: Option<&'a mut T>,
2058}
2059
2060#[stable(feature = "rust1", since = "1.0.0")]
2061#[cfg(not(feature = "ferrocene_certified"))]
2062impl<'a, T> Iterator for IterMut<'a, T> {
2063    type Item = &'a mut T;
2064
2065    #[inline]
2066    fn next(&mut self) -> Option<&'a mut T> {
2067        self.inner.take()
2068    }
2069    #[inline]
2070    fn size_hint(&self) -> (usize, Option<usize>) {
2071        let n = if self.inner.is_some() { 1 } else { 0 };
2072        (n, Some(n))
2073    }
2074}
2075
2076#[stable(feature = "rust1", since = "1.0.0")]
2077#[cfg(not(feature = "ferrocene_certified"))]
2078impl<'a, T> DoubleEndedIterator for IterMut<'a, T> {
2079    #[inline]
2080    fn next_back(&mut self) -> Option<&'a mut T> {
2081        self.inner.take()
2082    }
2083}
2084
2085#[stable(feature = "rust1", since = "1.0.0")]
2086#[cfg(not(feature = "ferrocene_certified"))]
2087impl<T> ExactSizeIterator for IterMut<'_, T> {}
2088
2089#[stable(feature = "fused", since = "1.26.0")]
2090#[cfg(not(feature = "ferrocene_certified"))]
2091impl<T> FusedIterator for IterMut<'_, T> {}
2092
2093#[unstable(feature = "trusted_len", issue = "37572")]
2094#[cfg(not(feature = "ferrocene_certified"))]
2095unsafe impl<A> TrustedLen for IterMut<'_, A> {}
2096
2097/// An iterator over the value in a [`Ok`] variant of a [`Result`].
2098///
2099/// The iterator yields one value if the result is [`Ok`], otherwise none.
2100///
2101/// This struct is created by the [`into_iter`] method on
2102/// [`Result`] (provided by the [`IntoIterator`] trait).
2103///
2104/// [`into_iter`]: IntoIterator::into_iter
2105#[derive(Clone, Debug)]
2106#[stable(feature = "rust1", since = "1.0.0")]
2107#[cfg(not(feature = "ferrocene_certified"))]
2108pub struct IntoIter<T> {
2109    inner: Option<T>,
2110}
2111
2112#[stable(feature = "rust1", since = "1.0.0")]
2113#[cfg(not(feature = "ferrocene_certified"))]
2114impl<T> Iterator for IntoIter<T> {
2115    type Item = T;
2116
2117    #[inline]
2118    fn next(&mut self) -> Option<T> {
2119        self.inner.take()
2120    }
2121    #[inline]
2122    fn size_hint(&self) -> (usize, Option<usize>) {
2123        let n = if self.inner.is_some() { 1 } else { 0 };
2124        (n, Some(n))
2125    }
2126}
2127
2128#[stable(feature = "rust1", since = "1.0.0")]
2129#[cfg(not(feature = "ferrocene_certified"))]
2130impl<T> DoubleEndedIterator for IntoIter<T> {
2131    #[inline]
2132    fn next_back(&mut self) -> Option<T> {
2133        self.inner.take()
2134    }
2135}
2136
2137#[stable(feature = "rust1", since = "1.0.0")]
2138#[cfg(not(feature = "ferrocene_certified"))]
2139impl<T> ExactSizeIterator for IntoIter<T> {}
2140
2141#[stable(feature = "fused", since = "1.26.0")]
2142#[cfg(not(feature = "ferrocene_certified"))]
2143impl<T> FusedIterator for IntoIter<T> {}
2144
2145#[unstable(feature = "trusted_len", issue = "37572")]
2146#[cfg(not(feature = "ferrocene_certified"))]
2147unsafe impl<A> TrustedLen for IntoIter<A> {}
2148
2149/////////////////////////////////////////////////////////////////////////////
2150// FromIterator
2151/////////////////////////////////////////////////////////////////////////////
2152
2153#[stable(feature = "rust1", since = "1.0.0")]
2154#[cfg(not(feature = "ferrocene_certified"))]
2155impl<A, E, V: FromIterator<A>> FromIterator<Result<A, E>> for Result<V, E> {
2156    /// Takes each element in the `Iterator`: if it is an `Err`, no further
2157    /// elements are taken, and the `Err` is returned. Should no `Err` occur, a
2158    /// container with the values of each `Result` is returned.
2159    ///
2160    /// Here is an example which increments every integer in a vector,
2161    /// checking for overflow:
2162    ///
2163    /// ```
2164    /// let v = vec![1, 2];
2165    /// let res: Result<Vec<u32>, &'static str> = v.iter().map(|x: &u32|
2166    ///     x.checked_add(1).ok_or("Overflow!")
2167    /// ).collect();
2168    /// assert_eq!(res, Ok(vec![2, 3]));
2169    /// ```
2170    ///
2171    /// Here is another example that tries to subtract one from another list
2172    /// of integers, this time checking for underflow:
2173    ///
2174    /// ```
2175    /// let v = vec![1, 2, 0];
2176    /// let res: Result<Vec<u32>, &'static str> = v.iter().map(|x: &u32|
2177    ///     x.checked_sub(1).ok_or("Underflow!")
2178    /// ).collect();
2179    /// assert_eq!(res, Err("Underflow!"));
2180    /// ```
2181    ///
2182    /// Here is a variation on the previous example, showing that no
2183    /// further elements are taken from `iter` after the first `Err`.
2184    ///
2185    /// ```
2186    /// let v = vec![3, 2, 1, 10];
2187    /// let mut shared = 0;
2188    /// let res: Result<Vec<u32>, &'static str> = v.iter().map(|x: &u32| {
2189    ///     shared += x;
2190    ///     x.checked_sub(2).ok_or("Underflow!")
2191    /// }).collect();
2192    /// assert_eq!(res, Err("Underflow!"));
2193    /// assert_eq!(shared, 6);
2194    /// ```
2195    ///
2196    /// Since the third element caused an underflow, no further elements were taken,
2197    /// so the final value of `shared` is 6 (= `3 + 2 + 1`), not 16.
2198    #[inline]
2199    fn from_iter<I: IntoIterator<Item = Result<A, E>>>(iter: I) -> Result<V, E> {
2200        iter::try_process(iter.into_iter(), |i| i.collect())
2201    }
2202}
2203
2204#[unstable(feature = "try_trait_v2", issue = "84277", old_name = "try_trait")]
2205#[rustc_const_unstable(feature = "const_try", issue = "74935")]
2206#[cfg(not(feature = "ferrocene_certified"))]
2207impl<T, E> const ops::Try for Result<T, E> {
2208    type Output = T;
2209    type Residual = Result<convert::Infallible, E>;
2210
2211    #[inline]
2212    fn from_output(output: Self::Output) -> Self {
2213        Ok(output)
2214    }
2215
2216    #[inline]
2217    fn branch(self) -> ControlFlow<Self::Residual, Self::Output> {
2218        match self {
2219            Ok(v) => ControlFlow::Continue(v),
2220            Err(e) => ControlFlow::Break(Err(e)),
2221        }
2222    }
2223}
2224
2225#[unstable(feature = "try_trait_v2", issue = "84277", old_name = "try_trait")]
2226#[rustc_const_unstable(feature = "const_try", issue = "74935")]
2227#[cfg(not(feature = "ferrocene_certified"))]
2228impl<T, E, F: [const] From<E>> const ops::FromResidual<Result<convert::Infallible, E>>
2229    for Result<T, F>
2230{
2231    #[inline]
2232    #[track_caller]
2233    fn from_residual(residual: Result<convert::Infallible, E>) -> Self {
2234        match residual {
2235            Err(e) => Err(From::from(e)),
2236        }
2237    }
2238}
2239#[diagnostic::do_not_recommend]
2240#[unstable(feature = "try_trait_v2_yeet", issue = "96374")]
2241#[rustc_const_unstable(feature = "const_try", issue = "74935")]
2242#[cfg(not(feature = "ferrocene_certified"))]
2243impl<T, E, F: [const] From<E>> const ops::FromResidual<ops::Yeet<E>> for Result<T, F> {
2244    #[inline]
2245    fn from_residual(ops::Yeet(e): ops::Yeet<E>) -> Self {
2246        Err(From::from(e))
2247    }
2248}
2249
2250#[unstable(feature = "try_trait_v2_residual", issue = "91285")]
2251#[rustc_const_unstable(feature = "const_try", issue = "74935")]
2252#[cfg(not(feature = "ferrocene_certified"))]
2253impl<T, E> const ops::Residual<T> for Result<convert::Infallible, E> {
2254    type TryType = Result<T, E>;
2255}