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}