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//! let good_result: Result<i32, i32> = Ok(10);
49//! let bad_result: Result<i32, i32> = Err(10);
50//!
51//! // The `is_ok` and `is_err` methods do what they say.
52//! assert!(good_result.is_ok() && !good_result.is_err());
53//! assert!(bad_result.is_err() && !bad_result.is_ok());
54//!
55//! // `map` consumes the `Result` and produces another.
56//! let good_result: Result<i32, i32> = good_result.map(|i| i + 1);
57//! let bad_result: Result<i32, i32> = bad_result.map(|i| i - 1);
58//!
59//! // Use `and_then` to continue the computation.
60//! let good_result: Result<bool, i32> = good_result.and_then(|i| Ok(i == 11));
61//!
62//! // Use `or_else` to handle the error.
63//! let bad_result: Result<i32, i32> = bad_result.or_else(|i| Ok(i + 20));
64//!
65//! // Consume the result and return the contents with `unwrap`.
66//! let final_awesome_result = good_result.unwrap();
67//! ```
68//!
69//! # Results must be used
70//!
71//! A common problem with using return values to indicate errors is
72//! that it is easy to ignore the return value, thus failing to handle
73//! the error. [`Result`] is annotated with the `#[must_use]` attribute,
74//! which will cause the compiler to issue a warning when a Result
75//! value is ignored. This makes [`Result`] especially useful with
76//! functions that may encounter errors but don't otherwise return a
77//! useful value.
78//!
79//! Consider the [`write_all`] method defined for I/O types
80//! by the [`Write`] trait:
81//!
82//! ```
83//! use std::io;
84//!
85//! trait Write {
86//! fn write_all(&mut self, bytes: &[u8]) -> Result<(), io::Error>;
87//! }
88//! ```
89//!
90//! *Note: The actual definition of [`Write`] uses [`io::Result`], which
91//! is just a synonym for <code>[Result]<T, [io::Error]></code>.*
92//!
93//! This method doesn't produce a value, but the write may
94//! fail. It's crucial to handle the error case, and *not* write
95//! something like this:
96//!
97//! ```no_run
98//! # #![allow(unused_must_use)] // \o/
99//! use std::fs::File;
100//! use std::io::prelude::*;
101//!
102//! let mut file = File::create("valuable_data.txt").unwrap();
103//! // If `write_all` errors, then we'll never know, because the return
104//! // value is ignored.
105//! file.write_all(b"important message");
106//! ```
107//!
108//! If you *do* write that in Rust, the compiler will give you a
109//! warning (by default, controlled by the `unused_must_use` lint).
110//!
111//! You might instead, if you don't want to handle the error, simply
112//! assert success with [`expect`]. This will panic if the
113//! write fails, providing a marginally useful message indicating why:
114//!
115//! ```no_run
116//! use std::fs::File;
117//! use std::io::prelude::*;
118//!
119//! let mut file = File::create("valuable_data.txt").unwrap();
120//! file.write_all(b"important message").expect("failed to write message");
121//! ```
122//!
123//! You might also simply assert success:
124//!
125//! ```no_run
126//! # use std::fs::File;
127//! # use std::io::prelude::*;
128//! # let mut file = File::create("valuable_data.txt").unwrap();
129//! assert!(file.write_all(b"important message").is_ok());
130//! ```
131//!
132//! Or propagate the error up the call stack with [`?`]:
133//!
134//! ```
135//! # use std::fs::File;
136//! # use std::io::prelude::*;
137//! # use std::io;
138//! # #[allow(dead_code)]
139//! fn write_message() -> io::Result<()> {
140//! let mut file = File::create("valuable_data.txt")?;
141//! file.write_all(b"important message")?;
142//! Ok(())
143//! }
144//! ```
145//!
146//! # The question mark operator, `?`
147//!
148//! When writing code that calls many functions that return the
149//! [`Result`] type, the error handling can be tedious. The question mark
150//! operator, [`?`], hides some of the boilerplate of propagating errors
151//! up the call stack.
152//!
153//! It replaces this:
154//!
155//! ```
156//! # #![allow(dead_code)]
157//! use std::fs::File;
158//! use std::io::prelude::*;
159//! use std::io;
160//!
161//! struct Info {
162//! name: String,
163//! age: i32,
164//! rating: i32,
165//! }
166//!
167//! fn write_info(info: &Info) -> io::Result<()> {
168//! // Early return on error
169//! let mut file = match File::create("my_best_friends.txt") {
170//! Err(e) => return Err(e),
171//! Ok(f) => f,
172//! };
173//! if let Err(e) = file.write_all(format!("name: {}\n", info.name).as_bytes()) {
174//! return Err(e)
175//! }
176//! if let Err(e) = file.write_all(format!("age: {}\n", info.age).as_bytes()) {
177//! return Err(e)
178//! }
179//! if let Err(e) = file.write_all(format!("rating: {}\n", info.rating).as_bytes()) {
180//! return Err(e)
181//! }
182//! Ok(())
183//! }
184//! ```
185//!
186//! With this:
187//!
188//! ```
189//! # #![allow(dead_code)]
190//! use std::fs::File;
191//! use std::io::prelude::*;
192//! use std::io;
193//!
194//! struct Info {
195//! name: String,
196//! age: i32,
197//! rating: i32,
198//! }
199//!
200//! fn write_info(info: &Info) -> io::Result<()> {
201//! let mut file = File::create("my_best_friends.txt")?;
202//! // Early return on error
203//! file.write_all(format!("name: {}\n", info.name).as_bytes())?;
204//! file.write_all(format!("age: {}\n", info.age).as_bytes())?;
205//! file.write_all(format!("rating: {}\n", info.rating).as_bytes())?;
206//! Ok(())
207//! }
208//! ```
209//!
210//! *It's much nicer!*
211//!
212//! Ending the expression with [`?`] will result in the [`Ok`]'s unwrapped value, unless the result
213//! is [`Err`], in which case [`Err`] is returned early from the enclosing function.
214//!
215//! [`?`] can be used in functions that return [`Result`] because of the
216//! early return of [`Err`] that it provides.
217//!
218//! [`expect`]: Result::expect
219//! [`Write`]: ../../std/io/trait.Write.html "io::Write"
220//! [`write_all`]: ../../std/io/trait.Write.html#method.write_all "io::Write::write_all"
221//! [`io::Result`]: ../../std/io/type.Result.html "io::Result"
222//! [`?`]: crate::ops::Try
223//! [`Ok(T)`]: Ok
224//! [`Err(E)`]: Err
225//! [io::Error]: ../../std/io/struct.Error.html "io::Error"
226//!
227//! # Method overview
228//!
229//! In addition to working with pattern matching, [`Result`] provides a
230//! wide variety of different methods.
231//!
232//! ## Querying the variant
233//!
234//! The [`is_ok`] and [`is_err`] methods return [`true`] if the [`Result`]
235//! is [`Ok`] or [`Err`], respectively.
236//!
237//! [`is_err`]: Result::is_err
238//! [`is_ok`]: Result::is_ok
239//!
240//! ## Adapters for working with references
241//!
242//! * [`as_ref`] converts from `&Result<T, E>` to `Result<&T, &E>`
243//! * [`as_mut`] converts from `&mut Result<T, E>` to `Result<&mut T, &mut E>`
244//! * [`as_deref`] converts from `&Result<T, E>` to `Result<&T::Target, &E>`
245//! * [`as_deref_mut`] converts from `&mut Result<T, E>` to
246//! `Result<&mut T::Target, &mut E>`
247//!
248//! [`as_deref`]: Result::as_deref
249//! [`as_deref_mut`]: Result::as_deref_mut
250//! [`as_mut`]: Result::as_mut
251//! [`as_ref`]: Result::as_ref
252//!
253//! ## Extracting contained values
254//!
255//! These methods extract the contained value in a [`Result<T, E>`] when it
256//! is the [`Ok`] variant. If the [`Result`] is [`Err`]:
257//!
258//! * [`expect`] panics with a provided custom message
259//! * [`unwrap`] panics with a generic message
260//! * [`unwrap_or`] returns the provided default value
261//! * [`unwrap_or_default`] returns the default value of the type `T`
262//! (which must implement the [`Default`] trait)
263//! * [`unwrap_or_else`] returns the result of evaluating the provided
264//! function
265//!
266//! The panicking methods [`expect`] and [`unwrap`] require `E` to
267//! implement the [`Debug`] trait.
268//!
269//! [`Debug`]: crate::fmt::Debug
270//! [`expect`]: Result::expect
271//! [`unwrap`]: Result::unwrap
272//! [`unwrap_or`]: Result::unwrap_or
273//! [`unwrap_or_default`]: Result::unwrap_or_default
274//! [`unwrap_or_else`]: Result::unwrap_or_else
275//!
276//! These methods extract the contained value in a [`Result<T, E>`] when it
277//! is the [`Err`] variant. They require `T` to implement the [`Debug`]
278//! trait. If the [`Result`] is [`Ok`]:
279//!
280//! * [`expect_err`] panics with a provided custom message
281//! * [`unwrap_err`] panics with a generic message
282//!
283//! [`Debug`]: crate::fmt::Debug
284//! [`expect_err`]: Result::expect_err
285//! [`unwrap_err`]: Result::unwrap_err
286//!
287//! ## Transforming contained values
288//!
289//! These methods transform [`Result`] to [`Option`]:
290//!
291//! * [`err`][Result::err] transforms [`Result<T, E>`] into [`Option<E>`],
292//! mapping [`Err(e)`] to [`Some(e)`] and [`Ok(v)`] to [`None`]
293//! * [`ok`][Result::ok] transforms [`Result<T, E>`] into [`Option<T>`],
294//! mapping [`Ok(v)`] to [`Some(v)`] and [`Err(e)`] to [`None`]
295//! * [`transpose`] transposes a [`Result`] of an [`Option`] into an
296//! [`Option`] of a [`Result`]
297//!
298// Do NOT add link reference definitions for `err` or `ok`, because they
299// will generate numerous incorrect URLs for `Err` and `Ok` elsewhere, due
300// to case folding.
301//!
302//! [`Err(e)`]: Err
303//! [`Ok(v)`]: Ok
304//! [`Some(e)`]: Option::Some
305//! [`Some(v)`]: Option::Some
306//! [`transpose`]: Result::transpose
307//!
308//! This method transforms the contained value of the [`Ok`] variant:
309//!
310//! * [`map`] transforms [`Result<T, E>`] into [`Result<U, E>`] by applying
311//! the provided function to the contained value of [`Ok`] and leaving
312//! [`Err`] values unchanged
313//!
314//! [`map`]: Result::map
315//!
316//! This method transforms the contained value of the [`Err`] variant:
317//!
318//! * [`map_err`] transforms [`Result<T, E>`] into [`Result<T, F>`] by
319//! applying the provided function to the contained value of [`Err`] and
320//! leaving [`Ok`] values unchanged
321//!
322//! [`map_err`]: Result::map_err
323//!
324//! These methods transform a [`Result<T, E>`] into a value of a possibly
325//! different type `U`:
326//!
327//! * [`map_or`] applies the provided function to the contained value of
328//! [`Ok`], or returns the provided default value if the [`Result`] is
329//! [`Err`]
330//! * [`map_or_else`] applies the provided function to the contained value
331//! of [`Ok`], or applies the provided default fallback function to the
332//! contained value of [`Err`]
333//!
334//! [`map_or`]: Result::map_or
335//! [`map_or_else`]: Result::map_or_else
336//!
337//! ## Boolean operators
338//!
339//! These methods treat the [`Result`] as a boolean value, where [`Ok`]
340//! acts like [`true`] and [`Err`] acts like [`false`]. There are two
341//! categories of these methods: ones that take a [`Result`] as input, and
342//! ones that take a function as input (to be lazily evaluated).
343//!
344//! The [`and`] and [`or`] methods take another [`Result`] as input, and
345//! produce a [`Result`] as output. The [`and`] method can produce a
346//! [`Result<U, E>`] value having a different inner type `U` than
347//! [`Result<T, E>`]. The [`or`] method can produce a [`Result<T, F>`]
348//! value having a different error type `F` than [`Result<T, E>`].
349//!
350//! | method | self | input | output |
351//! |---------|----------|-----------|----------|
352//! | [`and`] | `Err(e)` | (ignored) | `Err(e)` |
353//! | [`and`] | `Ok(x)` | `Err(d)` | `Err(d)` |
354//! | [`and`] | `Ok(x)` | `Ok(y)` | `Ok(y)` |
355//! | [`or`] | `Err(e)` | `Err(d)` | `Err(d)` |
356//! | [`or`] | `Err(e)` | `Ok(y)` | `Ok(y)` |
357//! | [`or`] | `Ok(x)` | (ignored) | `Ok(x)` |
358//!
359//! [`and`]: Result::and
360//! [`or`]: Result::or
361//!
362//! The [`and_then`] and [`or_else`] methods take a function as input, and
363//! only evaluate the function when they need to produce a new value. The
364//! [`and_then`] method can produce a [`Result<U, E>`] value having a
365//! different inner type `U` than [`Result<T, E>`]. The [`or_else`] method
366//! can produce a [`Result<T, F>`] value having a different error type `F`
367//! than [`Result<T, E>`].
368//!
369//! | method | self | function input | function result | output |
370//! |--------------|----------|----------------|-----------------|----------|
371//! | [`and_then`] | `Err(e)` | (not provided) | (not evaluated) | `Err(e)` |
372//! | [`and_then`] | `Ok(x)` | `x` | `Err(d)` | `Err(d)` |
373//! | [`and_then`] | `Ok(x)` | `x` | `Ok(y)` | `Ok(y)` |
374//! | [`or_else`] | `Err(e)` | `e` | `Err(d)` | `Err(d)` |
375//! | [`or_else`] | `Err(e)` | `e` | `Ok(y)` | `Ok(y)` |
376//! | [`or_else`] | `Ok(x)` | (not provided) | (not evaluated) | `Ok(x)` |
377//!
378//! [`and_then`]: Result::and_then
379//! [`or_else`]: Result::or_else
380//!
381//! ## Comparison operators
382//!
383//! If `T` and `E` both implement [`PartialOrd`] then [`Result<T, E>`] will
384//! derive its [`PartialOrd`] implementation. With this order, an [`Ok`]
385//! compares as less than any [`Err`], while two [`Ok`] or two [`Err`]
386//! compare as their contained values would in `T` or `E` respectively. If `T`
387//! and `E` both also implement [`Ord`], then so does [`Result<T, E>`].
388//!
389//! ```
390//! assert!(Ok(1) < Err(0));
391//! let x: Result<i32, ()> = Ok(0);
392//! let y = Ok(1);
393//! assert!(x < y);
394//! let x: Result<(), i32> = Err(0);
395//! let y = Err(1);
396//! assert!(x < y);
397//! ```
398//!
399//! ## Iterating over `Result`
400//!
401//! A [`Result`] can be iterated over. This can be helpful if you need an
402//! iterator that is conditionally empty. The iterator will either produce
403//! a single value (when the [`Result`] is [`Ok`]), or produce no values
404//! (when the [`Result`] is [`Err`]). For example, [`into_iter`] acts like
405//! [`once(v)`] if the [`Result`] is [`Ok(v)`], and like [`empty()`] if the
406//! [`Result`] is [`Err`].
407//!
408//! [`Ok(v)`]: Ok
409//! [`empty()`]: crate::iter::empty
410//! [`once(v)`]: crate::iter::once
411//!
412//! Iterators over [`Result<T, E>`] come in three types:
413//!
414//! * [`into_iter`] consumes the [`Result`] and produces the contained
415//! value
416//! * [`iter`] produces an immutable reference of type `&T` to the
417//! contained value
418//! * [`iter_mut`] produces a mutable reference of type `&mut T` to the
419//! contained value
420//!
421//! See [Iterating over `Option`] for examples of how this can be useful.
422//!
423//! [Iterating over `Option`]: crate::option#iterating-over-option
424//! [`into_iter`]: Result::into_iter
425//! [`iter`]: Result::iter
426//! [`iter_mut`]: Result::iter_mut
427//!
428//! You might want to use an iterator chain to do multiple instances of an
429//! operation that can fail, but would like to ignore failures while
430//! continuing to process the successful results. In this example, we take
431//! advantage of the iterable nature of [`Result`] to select only the
432//! [`Ok`] values using [`flatten`][Iterator::flatten].
433//!
434//! ```
435//! # use std::str::FromStr;
436//! let mut results = vec![];
437//! let mut errs = vec![];
438//! let nums: Vec<_> = ["17", "not a number", "99", "-27", "768"]
439//! .into_iter()
440//! .map(u8::from_str)
441//! // Save clones of the raw `Result` values to inspect
442//! .inspect(|x| results.push(x.clone()))
443//! // Challenge: explain how this captures only the `Err` values
444//! .inspect(|x| errs.extend(x.clone().err()))
445//! .flatten()
446//! .collect();
447//! assert_eq!(errs.len(), 3);
448//! assert_eq!(nums, [17, 99]);
449//! println!("results {results:?}");
450//! println!("errs {errs:?}");
451//! println!("nums {nums:?}");
452//! ```
453//!
454//! ## Collecting into `Result`
455//!
456//! [`Result`] implements the [`FromIterator`][impl-FromIterator] trait,
457//! which allows an iterator over [`Result`] values to be collected into a
458//! [`Result`] of a collection of each contained value of the original
459//! [`Result`] values, or [`Err`] if any of the elements was [`Err`].
460//!
461//! [impl-FromIterator]: Result#impl-FromIterator%3CResult%3CA,+E%3E%3E-for-Result%3CV,+E%3E
462//!
463//! ```
464//! let v = [Ok(2), Ok(4), Err("err!"), Ok(8)];
465//! let res: Result<Vec<_>, &str> = v.into_iter().collect();
466//! assert_eq!(res, Err("err!"));
467//! let v = [Ok(2), Ok(4), Ok(8)];
468//! let res: Result<Vec<_>, &str> = v.into_iter().collect();
469//! assert_eq!(res, Ok(vec![2, 4, 8]));
470//! ```
471//!
472//! [`Result`] also implements the [`Product`][impl-Product] and
473//! [`Sum`][impl-Sum] traits, allowing an iterator over [`Result`] values
474//! to provide the [`product`][Iterator::product] and
475//! [`sum`][Iterator::sum] methods.
476//!
477//! [impl-Product]: Result#impl-Product%3CResult%3CU,+E%3E%3E-for-Result%3CT,+E%3E
478//! [impl-Sum]: Result#impl-Sum%3CResult%3CU,+E%3E%3E-for-Result%3CT,+E%3E
479//!
480//! ```
481//! let v = [Err("error!"), Ok(1), Ok(2), Ok(3), Err("foo")];
482//! let res: Result<i32, &str> = v.into_iter().sum();
483//! assert_eq!(res, Err("error!"));
484//! let v = [Ok(1), Ok(2), Ok(21)];
485//! let res: Result<i32, &str> = v.into_iter().product();
486//! assert_eq!(res, Ok(42));
487//! ```
488
489#![stable(feature = "rust1", since = "1.0.0")]
490
491use crate::iter::{self, FusedIterator, TrustedLen};
492use crate::ops::{self, ControlFlow, Deref, DerefMut};
493use crate::{convert, fmt, hint};
494
495/// `Result` is a type that represents either success ([`Ok`]) or failure ([`Err`]).
496///
497/// See the [module documentation](self) for details.
498#[derive(Copy, PartialEq, PartialOrd, Eq, Ord, Debug, Hash)]
499#[must_use = "this `Result` may be an `Err` variant, which should be handled"]
500#[rustc_diagnostic_item = "Result"]
501#[stable(feature = "rust1", since = "1.0.0")]
502pub enum Result<T, E> {
503 /// Contains the success value
504 #[lang = "Ok"]
505 #[stable(feature = "rust1", since = "1.0.0")]
506 Ok(#[stable(feature = "rust1", since = "1.0.0")] T),
507
508 /// Contains the error value
509 #[lang = "Err"]
510 #[stable(feature = "rust1", since = "1.0.0")]
511 Err(#[stable(feature = "rust1", since = "1.0.0")] E),
512}
513
514/////////////////////////////////////////////////////////////////////////////
515// Type implementation
516/////////////////////////////////////////////////////////////////////////////
517
518impl<T, E> Result<T, E> {
519 /////////////////////////////////////////////////////////////////////////
520 // Querying the contained values
521 /////////////////////////////////////////////////////////////////////////
522
523 /// Returns `true` if the result is [`Ok`].
524 ///
525 /// # Examples
526 ///
527 /// ```
528 /// let x: Result<i32, &str> = Ok(-3);
529 /// assert_eq!(x.is_ok(), true);
530 ///
531 /// let x: Result<i32, &str> = Err("Some error message");
532 /// assert_eq!(x.is_ok(), false);
533 /// ```
534 #[must_use = "if you intended to assert that this is ok, consider `.unwrap()` instead"]
535 #[rustc_const_stable(feature = "const_result_basics", since = "1.48.0")]
536 #[inline]
537 #[stable(feature = "rust1", since = "1.0.0")]
538 pub const fn is_ok(&self) -> bool {
539 matches!(*self, Ok(_))
540 }
541
542 /// Returns `true` if the result is [`Ok`] and the value inside of it matches a predicate.
543 ///
544 /// # Examples
545 ///
546 /// ```
547 /// let x: Result<u32, &str> = Ok(2);
548 /// assert_eq!(x.is_ok_and(|x| x > 1), true);
549 ///
550 /// let x: Result<u32, &str> = Ok(0);
551 /// assert_eq!(x.is_ok_and(|x| x > 1), false);
552 ///
553 /// let x: Result<u32, &str> = Err("hey");
554 /// assert_eq!(x.is_ok_and(|x| x > 1), false);
555 /// ```
556 #[must_use]
557 #[inline]
558 #[stable(feature = "is_some_and", since = "1.70.0")]
559 pub fn is_ok_and(self, f: impl FnOnce(T) -> bool) -> bool {
560 match self {
561 Err(_) => false,
562 Ok(x) => f(x),
563 }
564 }
565
566 /// Returns `true` if the result is [`Err`].
567 ///
568 /// # Examples
569 ///
570 /// ```
571 /// let x: Result<i32, &str> = Ok(-3);
572 /// assert_eq!(x.is_err(), false);
573 ///
574 /// let x: Result<i32, &str> = Err("Some error message");
575 /// assert_eq!(x.is_err(), true);
576 /// ```
577 #[must_use = "if you intended to assert that this is err, consider `.unwrap_err()` instead"]
578 #[rustc_const_stable(feature = "const_result_basics", since = "1.48.0")]
579 #[inline]
580 #[stable(feature = "rust1", since = "1.0.0")]
581 pub const fn is_err(&self) -> bool {
582 !self.is_ok()
583 }
584
585 /// Returns `true` if the result is [`Err`] and the value inside of it matches a predicate.
586 ///
587 /// # Examples
588 ///
589 /// ```
590 /// use std::io::{Error, ErrorKind};
591 ///
592 /// let x: Result<u32, Error> = Err(Error::new(ErrorKind::NotFound, "!"));
593 /// assert_eq!(x.is_err_and(|x| x.kind() == ErrorKind::NotFound), true);
594 ///
595 /// let x: Result<u32, Error> = Err(Error::new(ErrorKind::PermissionDenied, "!"));
596 /// assert_eq!(x.is_err_and(|x| x.kind() == ErrorKind::NotFound), false);
597 ///
598 /// let x: Result<u32, Error> = Ok(123);
599 /// assert_eq!(x.is_err_and(|x| x.kind() == ErrorKind::NotFound), false);
600 /// ```
601 #[must_use]
602 #[inline]
603 #[stable(feature = "is_some_and", since = "1.70.0")]
604 pub fn is_err_and(self, f: impl FnOnce(E) -> bool) -> bool {
605 match self {
606 Ok(_) => false,
607 Err(e) => f(e),
608 }
609 }
610
611 /////////////////////////////////////////////////////////////////////////
612 // Adapter for each variant
613 /////////////////////////////////////////////////////////////////////////
614
615 /// Converts from `Result<T, E>` to [`Option<T>`].
616 ///
617 /// Converts `self` into an [`Option<T>`], consuming `self`,
618 /// and discarding the error, if any.
619 ///
620 /// # Examples
621 ///
622 /// ```
623 /// let x: Result<u32, &str> = Ok(2);
624 /// assert_eq!(x.ok(), Some(2));
625 ///
626 /// let x: Result<u32, &str> = Err("Nothing here");
627 /// assert_eq!(x.ok(), None);
628 /// ```
629 #[inline]
630 #[stable(feature = "rust1", since = "1.0.0")]
631 pub fn ok(self) -> Option<T> {
632 match self {
633 Ok(x) => Some(x),
634 Err(_) => None,
635 }
636 }
637
638 /// Converts from `Result<T, E>` to [`Option<E>`].
639 ///
640 /// Converts `self` into an [`Option<E>`], consuming `self`,
641 /// and discarding the success value, if any.
642 ///
643 /// # Examples
644 ///
645 /// ```
646 /// let x: Result<u32, &str> = Ok(2);
647 /// assert_eq!(x.err(), None);
648 ///
649 /// let x: Result<u32, &str> = Err("Nothing here");
650 /// assert_eq!(x.err(), Some("Nothing here"));
651 /// ```
652 #[inline]
653 #[stable(feature = "rust1", since = "1.0.0")]
654 pub fn err(self) -> Option<E> {
655 match self {
656 Ok(_) => None,
657 Err(x) => Some(x),
658 }
659 }
660
661 /////////////////////////////////////////////////////////////////////////
662 // Adapter for working with references
663 /////////////////////////////////////////////////////////////////////////
664
665 /// Converts from `&Result<T, E>` to `Result<&T, &E>`.
666 ///
667 /// Produces a new `Result`, containing a reference
668 /// into the original, leaving the original in place.
669 ///
670 /// # Examples
671 ///
672 /// ```
673 /// let x: Result<u32, &str> = Ok(2);
674 /// assert_eq!(x.as_ref(), Ok(&2));
675 ///
676 /// let x: Result<u32, &str> = Err("Error");
677 /// assert_eq!(x.as_ref(), Err(&"Error"));
678 /// ```
679 #[inline]
680 #[rustc_const_stable(feature = "const_result_basics", since = "1.48.0")]
681 #[stable(feature = "rust1", since = "1.0.0")]
682 pub const fn as_ref(&self) -> Result<&T, &E> {
683 match *self {
684 Ok(ref x) => Ok(x),
685 Err(ref x) => Err(x),
686 }
687 }
688
689 /// Converts from `&mut Result<T, E>` to `Result<&mut T, &mut E>`.
690 ///
691 /// # Examples
692 ///
693 /// ```
694 /// fn mutate(r: &mut Result<i32, i32>) {
695 /// match r.as_mut() {
696 /// Ok(v) => *v = 42,
697 /// Err(e) => *e = 0,
698 /// }
699 /// }
700 ///
701 /// let mut x: Result<i32, i32> = Ok(2);
702 /// mutate(&mut x);
703 /// assert_eq!(x.unwrap(), 42);
704 ///
705 /// let mut x: Result<i32, i32> = Err(13);
706 /// mutate(&mut x);
707 /// assert_eq!(x.unwrap_err(), 0);
708 /// ```
709 #[inline]
710 #[stable(feature = "rust1", since = "1.0.0")]
711 #[rustc_const_unstable(feature = "const_result", issue = "82814")]
712 pub const fn as_mut(&mut self) -> Result<&mut T, &mut E> {
713 match *self {
714 Ok(ref mut x) => Ok(x),
715 Err(ref mut x) => Err(x),
716 }
717 }
718
719 /////////////////////////////////////////////////////////////////////////
720 // Transforming contained values
721 /////////////////////////////////////////////////////////////////////////
722
723 /// Maps a `Result<T, E>` to `Result<U, E>` by applying a function to a
724 /// contained [`Ok`] value, leaving an [`Err`] value untouched.
725 ///
726 /// This function can be used to compose the results of two functions.
727 ///
728 /// # Examples
729 ///
730 /// Print the numbers on each line of a string multiplied by two.
731 ///
732 /// ```
733 /// let line = "1\n2\n3\n4\n";
734 ///
735 /// for num in line.lines() {
736 /// match num.parse::<i32>().map(|i| i * 2) {
737 /// Ok(n) => println!("{n}"),
738 /// Err(..) => {}
739 /// }
740 /// }
741 /// ```
742 #[inline]
743 #[stable(feature = "rust1", since = "1.0.0")]
744 pub fn map<U, F: FnOnce(T) -> U>(self, op: F) -> Result<U, E> {
745 match self {
746 Ok(t) => Ok(op(t)),
747 Err(e) => Err(e),
748 }
749 }
750
751 /// Returns the provided default (if [`Err`]), or
752 /// applies a function to the contained value (if [`Ok`]).
753 ///
754 /// Arguments passed to `map_or` are eagerly evaluated; if you are passing
755 /// the result of a function call, it is recommended to use [`map_or_else`],
756 /// which is lazily evaluated.
757 ///
758 /// [`map_or_else`]: Result::map_or_else
759 ///
760 /// # Examples
761 ///
762 /// ```
763 /// let x: Result<_, &str> = Ok("foo");
764 /// assert_eq!(x.map_or(42, |v| v.len()), 3);
765 ///
766 /// let x: Result<&str, _> = Err("bar");
767 /// assert_eq!(x.map_or(42, |v| v.len()), 42);
768 /// ```
769 #[inline]
770 #[stable(feature = "result_map_or", since = "1.41.0")]
771 #[must_use = "if you don't need the returned value, use `if let` instead"]
772 pub fn map_or<U, F: FnOnce(T) -> U>(self, default: U, f: F) -> U {
773 match self {
774 Ok(t) => f(t),
775 Err(_) => default,
776 }
777 }
778
779 /// Maps a `Result<T, E>` to `U` by applying fallback function `default` to
780 /// a contained [`Err`] value, or function `f` to a contained [`Ok`] value.
781 ///
782 /// This function can be used to unpack a successful result
783 /// while handling an error.
784 ///
785 ///
786 /// # Examples
787 ///
788 /// ```
789 /// let k = 21;
790 ///
791 /// let x : Result<_, &str> = Ok("foo");
792 /// assert_eq!(x.map_or_else(|e| k * 2, |v| v.len()), 3);
793 ///
794 /// let x : Result<&str, _> = Err("bar");
795 /// assert_eq!(x.map_or_else(|e| k * 2, |v| v.len()), 42);
796 /// ```
797 #[inline]
798 #[stable(feature = "result_map_or_else", since = "1.41.0")]
799 pub fn map_or_else<U, D: FnOnce(E) -> U, F: FnOnce(T) -> U>(self, default: D, f: F) -> U {
800 match self {
801 Ok(t) => f(t),
802 Err(e) => default(e),
803 }
804 }
805
806 /// Maps a `Result<T, E>` to `Result<T, F>` by applying a function to a
807 /// contained [`Err`] value, leaving an [`Ok`] value untouched.
808 ///
809 /// This function can be used to pass through a successful result while handling
810 /// an error.
811 ///
812 ///
813 /// # Examples
814 ///
815 /// ```
816 /// fn stringify(x: u32) -> String { format!("error code: {x}") }
817 ///
818 /// let x: Result<u32, u32> = Ok(2);
819 /// assert_eq!(x.map_err(stringify), Ok(2));
820 ///
821 /// let x: Result<u32, u32> = Err(13);
822 /// assert_eq!(x.map_err(stringify), Err("error code: 13".to_string()));
823 /// ```
824 #[inline]
825 #[stable(feature = "rust1", since = "1.0.0")]
826 pub fn map_err<F, O: FnOnce(E) -> F>(self, op: O) -> Result<T, F> {
827 match self {
828 Ok(t) => Ok(t),
829 Err(e) => Err(op(e)),
830 }
831 }
832
833 /// Calls a function with a reference to the contained value if [`Ok`].
834 ///
835 /// Returns the original result.
836 ///
837 /// # Examples
838 ///
839 /// ```
840 /// let x: u8 = "4"
841 /// .parse::<u8>()
842 /// .inspect(|x| println!("original: {x}"))
843 /// .map(|x| x.pow(3))
844 /// .expect("failed to parse number");
845 /// ```
846 #[inline]
847 #[stable(feature = "result_option_inspect", since = "1.76.0")]
848 pub fn inspect<F: FnOnce(&T)>(self, f: F) -> Self {
849 if let Ok(ref t) = self {
850 f(t);
851 }
852
853 self
854 }
855
856 /// Calls a function with a reference to the contained value if [`Err`].
857 ///
858 /// Returns the original result.
859 ///
860 /// # Examples
861 ///
862 /// ```
863 /// use std::{fs, io};
864 ///
865 /// fn read() -> io::Result<String> {
866 /// fs::read_to_string("address.txt")
867 /// .inspect_err(|e| eprintln!("failed to read file: {e}"))
868 /// }
869 /// ```
870 #[inline]
871 #[stable(feature = "result_option_inspect", since = "1.76.0")]
872 pub fn inspect_err<F: FnOnce(&E)>(self, f: F) -> Self {
873 if let Err(ref e) = self {
874 f(e);
875 }
876
877 self
878 }
879
880 /// Converts from `Result<T, E>` (or `&Result<T, E>`) to `Result<&<T as Deref>::Target, &E>`.
881 ///
882 /// Coerces the [`Ok`] variant of the original [`Result`] via [`Deref`](crate::ops::Deref)
883 /// and returns the new [`Result`].
884 ///
885 /// # Examples
886 ///
887 /// ```
888 /// let x: Result<String, u32> = Ok("hello".to_string());
889 /// let y: Result<&str, &u32> = Ok("hello");
890 /// assert_eq!(x.as_deref(), y);
891 ///
892 /// let x: Result<String, u32> = Err(42);
893 /// let y: Result<&str, &u32> = Err(&42);
894 /// assert_eq!(x.as_deref(), y);
895 /// ```
896 #[inline]
897 #[stable(feature = "inner_deref", since = "1.47.0")]
898 pub fn as_deref(&self) -> Result<&T::Target, &E>
899 where
900 T: Deref,
901 {
902 self.as_ref().map(|t| t.deref())
903 }
904
905 /// Converts from `Result<T, E>` (or `&mut Result<T, E>`) to `Result<&mut <T as DerefMut>::Target, &mut E>`.
906 ///
907 /// Coerces the [`Ok`] variant of the original [`Result`] via [`DerefMut`](crate::ops::DerefMut)
908 /// and returns the new [`Result`].
909 ///
910 /// # Examples
911 ///
912 /// ```
913 /// let mut s = "HELLO".to_string();
914 /// let mut x: Result<String, u32> = Ok("hello".to_string());
915 /// let y: Result<&mut str, &mut u32> = Ok(&mut s);
916 /// assert_eq!(x.as_deref_mut().map(|x| { x.make_ascii_uppercase(); x }), y);
917 ///
918 /// let mut i = 42;
919 /// let mut x: Result<String, u32> = Err(42);
920 /// let y: Result<&mut str, &mut u32> = Err(&mut i);
921 /// assert_eq!(x.as_deref_mut().map(|x| { x.make_ascii_uppercase(); x }), y);
922 /// ```
923 #[inline]
924 #[stable(feature = "inner_deref", since = "1.47.0")]
925 pub fn as_deref_mut(&mut self) -> Result<&mut T::Target, &mut E>
926 where
927 T: DerefMut,
928 {
929 self.as_mut().map(|t| t.deref_mut())
930 }
931
932 /////////////////////////////////////////////////////////////////////////
933 // Iterator constructors
934 /////////////////////////////////////////////////////////////////////////
935
936 /// Returns an iterator over the possibly contained value.
937 ///
938 /// The iterator yields one value if the result is [`Result::Ok`], otherwise none.
939 ///
940 /// # Examples
941 ///
942 /// ```
943 /// let x: Result<u32, &str> = Ok(7);
944 /// assert_eq!(x.iter().next(), Some(&7));
945 ///
946 /// let x: Result<u32, &str> = Err("nothing!");
947 /// assert_eq!(x.iter().next(), None);
948 /// ```
949 #[inline]
950 #[stable(feature = "rust1", since = "1.0.0")]
951 pub fn iter(&self) -> Iter<'_, T> {
952 Iter { inner: self.as_ref().ok() }
953 }
954
955 /// Returns a mutable iterator over the possibly contained value.
956 ///
957 /// The iterator yields one value if the result is [`Result::Ok`], otherwise none.
958 ///
959 /// # Examples
960 ///
961 /// ```
962 /// let mut x: Result<u32, &str> = Ok(7);
963 /// match x.iter_mut().next() {
964 /// Some(v) => *v = 40,
965 /// None => {},
966 /// }
967 /// assert_eq!(x, Ok(40));
968 ///
969 /// let mut x: Result<u32, &str> = Err("nothing!");
970 /// assert_eq!(x.iter_mut().next(), None);
971 /// ```
972 #[inline]
973 #[stable(feature = "rust1", since = "1.0.0")]
974 pub fn iter_mut(&mut self) -> IterMut<'_, T> {
975 IterMut { inner: self.as_mut().ok() }
976 }
977
978 /////////////////////////////////////////////////////////////////////////
979 // Extract a value
980 /////////////////////////////////////////////////////////////////////////
981
982 /// Returns the contained [`Ok`] value, consuming the `self` value.
983 ///
984 /// Because this function may panic, its use is generally discouraged.
985 /// Instead, prefer to use pattern matching and handle the [`Err`]
986 /// case explicitly, or call [`unwrap_or`], [`unwrap_or_else`], or
987 /// [`unwrap_or_default`].
988 ///
989 /// [`unwrap_or`]: Result::unwrap_or
990 /// [`unwrap_or_else`]: Result::unwrap_or_else
991 /// [`unwrap_or_default`]: Result::unwrap_or_default
992 ///
993 /// # Panics
994 ///
995 /// Panics if the value is an [`Err`], with a panic message including the
996 /// passed message, and the content of the [`Err`].
997 ///
998 ///
999 /// # Examples
1000 ///
1001 /// ```should_panic
1002 /// let x: Result<u32, &str> = Err("emergency failure");
1003 /// x.expect("Testing expect"); // panics with `Testing expect: emergency failure`
1004 /// ```
1005 ///
1006 /// # Recommended Message Style
1007 ///
1008 /// We recommend that `expect` messages are used to describe the reason you
1009 /// _expect_ the `Result` should be `Ok`.
1010 ///
1011 /// ```should_panic
1012 /// let path = std::env::var("IMPORTANT_PATH")
1013 /// .expect("env variable `IMPORTANT_PATH` should be set by `wrapper_script.sh`");
1014 /// ```
1015 ///
1016 /// **Hint**: If you're having trouble remembering how to phrase expect
1017 /// error messages remember to focus on the word "should" as in "env
1018 /// variable should be set by blah" or "the given binary should be available
1019 /// and executable by the current user".
1020 ///
1021 /// For more detail on expect message styles and the reasoning behind our recommendation please
1022 /// refer to the section on ["Common Message
1023 /// Styles"](../../std/error/index.html#common-message-styles) in the
1024 /// [`std::error`](../../std/error/index.html) module docs.
1025 #[inline]
1026 #[track_caller]
1027 #[stable(feature = "result_expect", since = "1.4.0")]
1028 pub fn expect(self, msg: &str) -> T
1029 where
1030 E: fmt::Debug,
1031 {
1032 match self {
1033 Ok(t) => t,
1034 Err(e) => unwrap_failed(msg, &e),
1035 }
1036 }
1037
1038 /// Returns the contained [`Ok`] value, consuming the `self` value.
1039 ///
1040 /// Because this function may panic, its use is generally discouraged.
1041 /// Instead, prefer to use pattern matching and handle the [`Err`]
1042 /// case explicitly, or call [`unwrap_or`], [`unwrap_or_else`], or
1043 /// [`unwrap_or_default`].
1044 ///
1045 /// [`unwrap_or`]: Result::unwrap_or
1046 /// [`unwrap_or_else`]: Result::unwrap_or_else
1047 /// [`unwrap_or_default`]: Result::unwrap_or_default
1048 ///
1049 /// # Panics
1050 ///
1051 /// Panics if the value is an [`Err`], with a panic message provided by the
1052 /// [`Err`]'s value.
1053 ///
1054 ///
1055 /// # Examples
1056 ///
1057 /// Basic usage:
1058 ///
1059 /// ```
1060 /// let x: Result<u32, &str> = Ok(2);
1061 /// assert_eq!(x.unwrap(), 2);
1062 /// ```
1063 ///
1064 /// ```should_panic
1065 /// let x: Result<u32, &str> = Err("emergency failure");
1066 /// x.unwrap(); // panics with `emergency failure`
1067 /// ```
1068 #[inline(always)]
1069 #[track_caller]
1070 #[stable(feature = "rust1", since = "1.0.0")]
1071 pub fn unwrap(self) -> T
1072 where
1073 E: fmt::Debug,
1074 {
1075 match self {
1076 Ok(t) => t,
1077 Err(e) => unwrap_failed("called `Result::unwrap()` on an `Err` value", &e),
1078 }
1079 }
1080
1081 /// Returns the contained [`Ok`] value or a default
1082 ///
1083 /// Consumes the `self` argument then, if [`Ok`], returns the contained
1084 /// value, otherwise if [`Err`], returns the default value for that
1085 /// type.
1086 ///
1087 /// # Examples
1088 ///
1089 /// Converts a string to an integer, turning poorly-formed strings
1090 /// into 0 (the default value for integers). [`parse`] converts
1091 /// a string to any other type that implements [`FromStr`], returning an
1092 /// [`Err`] on error.
1093 ///
1094 /// ```
1095 /// let good_year_from_input = "1909";
1096 /// let bad_year_from_input = "190blarg";
1097 /// let good_year = good_year_from_input.parse().unwrap_or_default();
1098 /// let bad_year = bad_year_from_input.parse().unwrap_or_default();
1099 ///
1100 /// assert_eq!(1909, good_year);
1101 /// assert_eq!(0, bad_year);
1102 /// ```
1103 ///
1104 /// [`parse`]: str::parse
1105 /// [`FromStr`]: crate::str::FromStr
1106 #[inline]
1107 #[stable(feature = "result_unwrap_or_default", since = "1.16.0")]
1108 pub fn unwrap_or_default(self) -> T
1109 where
1110 T: Default,
1111 {
1112 match self {
1113 Ok(x) => x,
1114 Err(_) => Default::default(),
1115 }
1116 }
1117
1118 /// Returns the contained [`Err`] value, consuming the `self` value.
1119 ///
1120 /// # Panics
1121 ///
1122 /// Panics if the value is an [`Ok`], with a panic message including the
1123 /// passed message, and the content of the [`Ok`].
1124 ///
1125 ///
1126 /// # Examples
1127 ///
1128 /// ```should_panic
1129 /// let x: Result<u32, &str> = Ok(10);
1130 /// x.expect_err("Testing expect_err"); // panics with `Testing expect_err: 10`
1131 /// ```
1132 #[inline]
1133 #[track_caller]
1134 #[stable(feature = "result_expect_err", since = "1.17.0")]
1135 pub fn expect_err(self, msg: &str) -> E
1136 where
1137 T: fmt::Debug,
1138 {
1139 match self {
1140 Ok(t) => unwrap_failed(msg, &t),
1141 Err(e) => e,
1142 }
1143 }
1144
1145 /// Returns the contained [`Err`] value, consuming the `self` value.
1146 ///
1147 /// # Panics
1148 ///
1149 /// Panics if the value is an [`Ok`], with a custom panic message provided
1150 /// by the [`Ok`]'s value.
1151 ///
1152 /// # Examples
1153 ///
1154 /// ```should_panic
1155 /// let x: Result<u32, &str> = Ok(2);
1156 /// x.unwrap_err(); // panics with `2`
1157 /// ```
1158 ///
1159 /// ```
1160 /// let x: Result<u32, &str> = Err("emergency failure");
1161 /// assert_eq!(x.unwrap_err(), "emergency failure");
1162 /// ```
1163 #[inline]
1164 #[track_caller]
1165 #[stable(feature = "rust1", since = "1.0.0")]
1166 pub fn unwrap_err(self) -> E
1167 where
1168 T: fmt::Debug,
1169 {
1170 match self {
1171 Ok(t) => unwrap_failed("called `Result::unwrap_err()` on an `Ok` value", &t),
1172 Err(e) => e,
1173 }
1174 }
1175
1176 /// Returns the contained [`Ok`] value, but never panics.
1177 ///
1178 /// Unlike [`unwrap`], this method is known to never panic on the
1179 /// result types it is implemented for. Therefore, it can be used
1180 /// instead of `unwrap` as a maintainability safeguard that will fail
1181 /// to compile if the error type of the `Result` is later changed
1182 /// to an error that can actually occur.
1183 ///
1184 /// [`unwrap`]: Result::unwrap
1185 ///
1186 /// # Examples
1187 ///
1188 /// ```
1189 /// # #![feature(never_type)]
1190 /// # #![feature(unwrap_infallible)]
1191 ///
1192 /// fn only_good_news() -> Result<String, !> {
1193 /// Ok("this is fine".into())
1194 /// }
1195 ///
1196 /// let s: String = only_good_news().into_ok();
1197 /// println!("{s}");
1198 /// ```
1199 #[unstable(feature = "unwrap_infallible", reason = "newly added", issue = "61695")]
1200 #[inline]
1201 pub fn into_ok(self) -> T
1202 where
1203 E: Into<!>,
1204 {
1205 match self {
1206 Ok(x) => x,
1207 Err(e) => e.into(),
1208 }
1209 }
1210
1211 /// Returns the contained [`Err`] value, but never panics.
1212 ///
1213 /// Unlike [`unwrap_err`], this method is known to never panic on the
1214 /// result types it is implemented for. Therefore, it can be used
1215 /// instead of `unwrap_err` as a maintainability safeguard that will fail
1216 /// to compile if the ok type of the `Result` is later changed
1217 /// to a type that can actually occur.
1218 ///
1219 /// [`unwrap_err`]: Result::unwrap_err
1220 ///
1221 /// # Examples
1222 ///
1223 /// ```
1224 /// # #![feature(never_type)]
1225 /// # #![feature(unwrap_infallible)]
1226 ///
1227 /// fn only_bad_news() -> Result<!, String> {
1228 /// Err("Oops, it failed".into())
1229 /// }
1230 ///
1231 /// let error: String = only_bad_news().into_err();
1232 /// println!("{error}");
1233 /// ```
1234 #[unstable(feature = "unwrap_infallible", reason = "newly added", issue = "61695")]
1235 #[inline]
1236 pub fn into_err(self) -> E
1237 where
1238 T: Into<!>,
1239 {
1240 match self {
1241 Ok(x) => x.into(),
1242 Err(e) => e,
1243 }
1244 }
1245
1246 ////////////////////////////////////////////////////////////////////////
1247 // Boolean operations on the values, eager and lazy
1248 /////////////////////////////////////////////////////////////////////////
1249
1250 /// Returns `res` if the result is [`Ok`], otherwise returns the [`Err`] value of `self`.
1251 ///
1252 /// Arguments passed to `and` are eagerly evaluated; if you are passing the
1253 /// result of a function call, it is recommended to use [`and_then`], which is
1254 /// lazily evaluated.
1255 ///
1256 /// [`and_then`]: Result::and_then
1257 ///
1258 /// # Examples
1259 ///
1260 /// ```
1261 /// let x: Result<u32, &str> = Ok(2);
1262 /// let y: Result<&str, &str> = Err("late error");
1263 /// assert_eq!(x.and(y), Err("late error"));
1264 ///
1265 /// let x: Result<u32, &str> = Err("early error");
1266 /// let y: Result<&str, &str> = Ok("foo");
1267 /// assert_eq!(x.and(y), Err("early error"));
1268 ///
1269 /// let x: Result<u32, &str> = Err("not a 2");
1270 /// let y: Result<&str, &str> = Err("late error");
1271 /// assert_eq!(x.and(y), Err("not a 2"));
1272 ///
1273 /// let x: Result<u32, &str> = Ok(2);
1274 /// let y: Result<&str, &str> = Ok("different result type");
1275 /// assert_eq!(x.and(y), Ok("different result type"));
1276 /// ```
1277 #[inline]
1278 #[stable(feature = "rust1", since = "1.0.0")]
1279 pub fn and<U>(self, res: Result<U, E>) -> Result<U, E> {
1280 match self {
1281 Ok(_) => res,
1282 Err(e) => Err(e),
1283 }
1284 }
1285
1286 /// Calls `op` if the result is [`Ok`], otherwise returns the [`Err`] value of `self`.
1287 ///
1288 ///
1289 /// This function can be used for control flow based on `Result` values.
1290 ///
1291 /// # Examples
1292 ///
1293 /// ```
1294 /// fn sq_then_to_string(x: u32) -> Result<String, &'static str> {
1295 /// x.checked_mul(x).map(|sq| sq.to_string()).ok_or("overflowed")
1296 /// }
1297 ///
1298 /// assert_eq!(Ok(2).and_then(sq_then_to_string), Ok(4.to_string()));
1299 /// assert_eq!(Ok(1_000_000).and_then(sq_then_to_string), Err("overflowed"));
1300 /// assert_eq!(Err("not a number").and_then(sq_then_to_string), Err("not a number"));
1301 /// ```
1302 ///
1303 /// Often used to chain fallible operations that may return [`Err`].
1304 ///
1305 /// ```
1306 /// use std::{io::ErrorKind, path::Path};
1307 ///
1308 /// // Note: on Windows "/" maps to "C:\"
1309 /// let root_modified_time = Path::new("/").metadata().and_then(|md| md.modified());
1310 /// assert!(root_modified_time.is_ok());
1311 ///
1312 /// let should_fail = Path::new("/bad/path").metadata().and_then(|md| md.modified());
1313 /// assert!(should_fail.is_err());
1314 /// assert_eq!(should_fail.unwrap_err().kind(), ErrorKind::NotFound);
1315 /// ```
1316 #[inline]
1317 #[stable(feature = "rust1", since = "1.0.0")]
1318 #[rustc_confusables("flat_map", "flatmap")]
1319 pub fn and_then<U, F: FnOnce(T) -> Result<U, E>>(self, op: F) -> Result<U, E> {
1320 match self {
1321 Ok(t) => op(t),
1322 Err(e) => Err(e),
1323 }
1324 }
1325
1326 /// Returns `res` if the result is [`Err`], otherwise returns the [`Ok`] value of `self`.
1327 ///
1328 /// Arguments passed to `or` are eagerly evaluated; if you are passing the
1329 /// result of a function call, it is recommended to use [`or_else`], which is
1330 /// lazily evaluated.
1331 ///
1332 /// [`or_else`]: Result::or_else
1333 ///
1334 /// # Examples
1335 ///
1336 /// ```
1337 /// let x: Result<u32, &str> = Ok(2);
1338 /// let y: Result<u32, &str> = Err("late error");
1339 /// assert_eq!(x.or(y), Ok(2));
1340 ///
1341 /// let x: Result<u32, &str> = Err("early error");
1342 /// let y: Result<u32, &str> = Ok(2);
1343 /// assert_eq!(x.or(y), Ok(2));
1344 ///
1345 /// let x: Result<u32, &str> = Err("not a 2");
1346 /// let y: Result<u32, &str> = Err("late error");
1347 /// assert_eq!(x.or(y), Err("late error"));
1348 ///
1349 /// let x: Result<u32, &str> = Ok(2);
1350 /// let y: Result<u32, &str> = Ok(100);
1351 /// assert_eq!(x.or(y), Ok(2));
1352 /// ```
1353 #[inline]
1354 #[stable(feature = "rust1", since = "1.0.0")]
1355 pub fn or<F>(self, res: Result<T, F>) -> Result<T, F> {
1356 match self {
1357 Ok(v) => Ok(v),
1358 Err(_) => res,
1359 }
1360 }
1361
1362 /// Calls `op` if the result is [`Err`], otherwise returns the [`Ok`] value of `self`.
1363 ///
1364 /// This function can be used for control flow based on result values.
1365 ///
1366 ///
1367 /// # Examples
1368 ///
1369 /// ```
1370 /// fn sq(x: u32) -> Result<u32, u32> { Ok(x * x) }
1371 /// fn err(x: u32) -> Result<u32, u32> { Err(x) }
1372 ///
1373 /// assert_eq!(Ok(2).or_else(sq).or_else(sq), Ok(2));
1374 /// assert_eq!(Ok(2).or_else(err).or_else(sq), Ok(2));
1375 /// assert_eq!(Err(3).or_else(sq).or_else(err), Ok(9));
1376 /// assert_eq!(Err(3).or_else(err).or_else(err), Err(3));
1377 /// ```
1378 #[inline]
1379 #[stable(feature = "rust1", since = "1.0.0")]
1380 pub fn or_else<F, O: FnOnce(E) -> Result<T, F>>(self, op: O) -> Result<T, F> {
1381 match self {
1382 Ok(t) => Ok(t),
1383 Err(e) => op(e),
1384 }
1385 }
1386
1387 /// Returns the contained [`Ok`] value or a provided default.
1388 ///
1389 /// Arguments passed to `unwrap_or` are eagerly evaluated; if you are passing
1390 /// the result of a function call, it is recommended to use [`unwrap_or_else`],
1391 /// which is lazily evaluated.
1392 ///
1393 /// [`unwrap_or_else`]: Result::unwrap_or_else
1394 ///
1395 /// # Examples
1396 ///
1397 /// ```
1398 /// let default = 2;
1399 /// let x: Result<u32, &str> = Ok(9);
1400 /// assert_eq!(x.unwrap_or(default), 9);
1401 ///
1402 /// let x: Result<u32, &str> = Err("error");
1403 /// assert_eq!(x.unwrap_or(default), default);
1404 /// ```
1405 #[inline]
1406 #[stable(feature = "rust1", since = "1.0.0")]
1407 pub fn unwrap_or(self, default: T) -> T {
1408 match self {
1409 Ok(t) => t,
1410 Err(_) => default,
1411 }
1412 }
1413
1414 /// Returns the contained [`Ok`] value or computes it from a closure.
1415 ///
1416 ///
1417 /// # Examples
1418 ///
1419 /// ```
1420 /// fn count(x: &str) -> usize { x.len() }
1421 ///
1422 /// assert_eq!(Ok(2).unwrap_or_else(count), 2);
1423 /// assert_eq!(Err("foo").unwrap_or_else(count), 3);
1424 /// ```
1425 #[inline]
1426 #[track_caller]
1427 #[stable(feature = "rust1", since = "1.0.0")]
1428 pub fn unwrap_or_else<F: FnOnce(E) -> T>(self, op: F) -> T {
1429 match self {
1430 Ok(t) => t,
1431 Err(e) => op(e),
1432 }
1433 }
1434
1435 /// Returns the contained [`Ok`] value, consuming the `self` value,
1436 /// without checking that the value is not an [`Err`].
1437 ///
1438 /// # Safety
1439 ///
1440 /// Calling this method on an [`Err`] is *[undefined behavior]*.
1441 ///
1442 /// [undefined behavior]: https://doc.rust-lang.org/reference/behavior-considered-undefined.html
1443 ///
1444 /// # Examples
1445 ///
1446 /// ```
1447 /// let x: Result<u32, &str> = Ok(2);
1448 /// assert_eq!(unsafe { x.unwrap_unchecked() }, 2);
1449 /// ```
1450 ///
1451 /// ```no_run
1452 /// let x: Result<u32, &str> = Err("emergency failure");
1453 /// unsafe { x.unwrap_unchecked(); } // Undefined behavior!
1454 /// ```
1455 #[inline]
1456 #[track_caller]
1457 #[stable(feature = "option_result_unwrap_unchecked", since = "1.58.0")]
1458 pub unsafe fn unwrap_unchecked(self) -> T {
1459 debug_assert!(self.is_ok());
1460 match self {
1461 Ok(t) => t,
1462 // SAFETY: the safety contract must be upheld by the caller.
1463 Err(_) => unsafe { hint::unreachable_unchecked() },
1464 }
1465 }
1466
1467 /// Returns the contained [`Err`] value, consuming the `self` value,
1468 /// without checking that the value is not an [`Ok`].
1469 ///
1470 /// # Safety
1471 ///
1472 /// Calling this method on an [`Ok`] is *[undefined behavior]*.
1473 ///
1474 /// [undefined behavior]: https://doc.rust-lang.org/reference/behavior-considered-undefined.html
1475 ///
1476 /// # Examples
1477 ///
1478 /// ```no_run
1479 /// let x: Result<u32, &str> = Ok(2);
1480 /// unsafe { x.unwrap_err_unchecked() }; // Undefined behavior!
1481 /// ```
1482 ///
1483 /// ```
1484 /// let x: Result<u32, &str> = Err("emergency failure");
1485 /// assert_eq!(unsafe { x.unwrap_err_unchecked() }, "emergency failure");
1486 /// ```
1487 #[inline]
1488 #[track_caller]
1489 #[stable(feature = "option_result_unwrap_unchecked", since = "1.58.0")]
1490 pub unsafe fn unwrap_err_unchecked(self) -> E {
1491 debug_assert!(self.is_err());
1492 match self {
1493 // SAFETY: the safety contract must be upheld by the caller.
1494 Ok(_) => unsafe { hint::unreachable_unchecked() },
1495 Err(e) => e,
1496 }
1497 }
1498}
1499
1500impl<T, E> Result<&T, E> {
1501 /// Maps a `Result<&T, E>` to a `Result<T, E>` by copying the contents of the
1502 /// `Ok` part.
1503 ///
1504 /// # Examples
1505 ///
1506 /// ```
1507 /// let val = 12;
1508 /// let x: Result<&i32, i32> = Ok(&val);
1509 /// assert_eq!(x, Ok(&12));
1510 /// let copied = x.copied();
1511 /// assert_eq!(copied, Ok(12));
1512 /// ```
1513 #[inline]
1514 #[stable(feature = "result_copied", since = "1.59.0")]
1515 pub fn copied(self) -> Result<T, E>
1516 where
1517 T: Copy,
1518 {
1519 self.map(|&t| t)
1520 }
1521
1522 /// Maps a `Result<&T, E>` to a `Result<T, E>` by cloning the contents of the
1523 /// `Ok` part.
1524 ///
1525 /// # Examples
1526 ///
1527 /// ```
1528 /// let val = 12;
1529 /// let x: Result<&i32, i32> = Ok(&val);
1530 /// assert_eq!(x, Ok(&12));
1531 /// let cloned = x.cloned();
1532 /// assert_eq!(cloned, Ok(12));
1533 /// ```
1534 #[inline]
1535 #[stable(feature = "result_cloned", since = "1.59.0")]
1536 pub fn cloned(self) -> Result<T, E>
1537 where
1538 T: Clone,
1539 {
1540 self.map(|t| t.clone())
1541 }
1542}
1543
1544impl<T, E> Result<&mut T, E> {
1545 /// Maps a `Result<&mut T, E>` to a `Result<T, E>` by copying the contents of the
1546 /// `Ok` part.
1547 ///
1548 /// # Examples
1549 ///
1550 /// ```
1551 /// let mut val = 12;
1552 /// let x: Result<&mut i32, i32> = Ok(&mut val);
1553 /// assert_eq!(x, Ok(&mut 12));
1554 /// let copied = x.copied();
1555 /// assert_eq!(copied, Ok(12));
1556 /// ```
1557 #[inline]
1558 #[stable(feature = "result_copied", since = "1.59.0")]
1559 pub fn copied(self) -> Result<T, E>
1560 where
1561 T: Copy,
1562 {
1563 self.map(|&mut t| t)
1564 }
1565
1566 /// Maps a `Result<&mut T, E>` to a `Result<T, E>` by cloning the contents of the
1567 /// `Ok` part.
1568 ///
1569 /// # Examples
1570 ///
1571 /// ```
1572 /// let mut val = 12;
1573 /// let x: Result<&mut i32, i32> = Ok(&mut val);
1574 /// assert_eq!(x, Ok(&mut 12));
1575 /// let cloned = x.cloned();
1576 /// assert_eq!(cloned, Ok(12));
1577 /// ```
1578 #[inline]
1579 #[stable(feature = "result_cloned", since = "1.59.0")]
1580 pub fn cloned(self) -> Result<T, E>
1581 where
1582 T: Clone,
1583 {
1584 self.map(|t| t.clone())
1585 }
1586}
1587
1588impl<T, E> Result<Option<T>, E> {
1589 /// Transposes a `Result` of an `Option` into an `Option` of a `Result`.
1590 ///
1591 /// `Ok(None)` will be mapped to `None`.
1592 /// `Ok(Some(_))` and `Err(_)` will be mapped to `Some(Ok(_))` and `Some(Err(_))`.
1593 ///
1594 /// # Examples
1595 ///
1596 /// ```
1597 /// #[derive(Debug, Eq, PartialEq)]
1598 /// struct SomeErr;
1599 ///
1600 /// let x: Result<Option<i32>, SomeErr> = Ok(Some(5));
1601 /// let y: Option<Result<i32, SomeErr>> = Some(Ok(5));
1602 /// assert_eq!(x.transpose(), y);
1603 /// ```
1604 #[inline]
1605 #[stable(feature = "transpose_result", since = "1.33.0")]
1606 #[rustc_const_unstable(feature = "const_result", issue = "82814")]
1607 pub const fn transpose(self) -> Option<Result<T, E>> {
1608 match self {
1609 Ok(Some(x)) => Some(Ok(x)),
1610 Ok(None) => None,
1611 Err(e) => Some(Err(e)),
1612 }
1613 }
1614}
1615
1616impl<T, E> Result<Result<T, E>, E> {
1617 /// Converts from `Result<Result<T, E>, E>` to `Result<T, E>`
1618 ///
1619 /// # Examples
1620 ///
1621 /// ```
1622 /// #![feature(result_flattening)]
1623 /// let x: Result<Result<&'static str, u32>, u32> = Ok(Ok("hello"));
1624 /// assert_eq!(Ok("hello"), x.flatten());
1625 ///
1626 /// let x: Result<Result<&'static str, u32>, u32> = Ok(Err(6));
1627 /// assert_eq!(Err(6), x.flatten());
1628 ///
1629 /// let x: Result<Result<&'static str, u32>, u32> = Err(6);
1630 /// assert_eq!(Err(6), x.flatten());
1631 /// ```
1632 ///
1633 /// Flattening only removes one level of nesting at a time:
1634 ///
1635 /// ```
1636 /// #![feature(result_flattening)]
1637 /// let x: Result<Result<Result<&'static str, u32>, u32>, u32> = Ok(Ok(Ok("hello")));
1638 /// assert_eq!(Ok(Ok("hello")), x.flatten());
1639 /// assert_eq!(Ok("hello"), x.flatten().flatten());
1640 /// ```
1641 #[inline]
1642 #[unstable(feature = "result_flattening", issue = "70142")]
1643 pub fn flatten(self) -> Result<T, E> {
1644 self.and_then(convert::identity)
1645 }
1646}
1647
1648// This is a separate function to reduce the code size of the methods
1649#[cfg(not(feature = "panic_immediate_abort"))]
1650#[inline(never)]
1651#[cold]
1652#[track_caller]
1653fn unwrap_failed(msg: &str, error: &dyn fmt::Debug) -> ! {
1654 panic!("{msg}: {error:?}")
1655}
1656
1657// This is a separate function to avoid constructing a `dyn Debug`
1658// that gets immediately thrown away, since vtables don't get cleaned up
1659// by dead code elimination if a trait object is constructed even if it goes
1660// unused
1661#[cfg(feature = "panic_immediate_abort")]
1662#[inline]
1663#[cold]
1664#[track_caller]
1665fn unwrap_failed<T>(_msg: &str, _error: &T) -> ! {
1666 panic!()
1667}
1668
1669/////////////////////////////////////////////////////////////////////////////
1670// Trait implementations
1671/////////////////////////////////////////////////////////////////////////////
1672
1673#[stable(feature = "rust1", since = "1.0.0")]
1674impl<T, E> Clone for Result<T, E>
1675where
1676 T: Clone,
1677 E: Clone,
1678{
1679 #[inline]
1680 fn clone(&self) -> Self {
1681 match self {
1682 Ok(x: &T) => Ok(x.clone()),
1683 Err(x: &E) => Err(x.clone()),
1684 }
1685 }
1686
1687 #[inline]
1688 fn clone_from(&mut self, source: &Self) {
1689 match (self, source) {
1690 (Ok(to: &mut T), Ok(from: &T)) => to.clone_from(source:from),
1691 (Err(to: &mut E), Err(from: &E)) => to.clone_from(source:from),
1692 (to: &mut Result, from: &Result) => *to = from.clone(),
1693 }
1694 }
1695}
1696
1697#[stable(feature = "rust1", since = "1.0.0")]
1698impl<T, E> IntoIterator for Result<T, E> {
1699 type Item = T;
1700 type IntoIter = IntoIter<T>;
1701
1702 /// Returns a consuming iterator over the possibly contained value.
1703 ///
1704 /// The iterator yields one value if the result is [`Result::Ok`], otherwise none.
1705 ///
1706 /// # Examples
1707 ///
1708 /// ```
1709 /// let x: Result<u32, &str> = Ok(5);
1710 /// let v: Vec<u32> = x.into_iter().collect();
1711 /// assert_eq!(v, [5]);
1712 ///
1713 /// let x: Result<u32, &str> = Err("nothing!");
1714 /// let v: Vec<u32> = x.into_iter().collect();
1715 /// assert_eq!(v, []);
1716 /// ```
1717 #[inline]
1718 fn into_iter(self) -> IntoIter<T> {
1719 IntoIter { inner: self.ok() }
1720 }
1721}
1722
1723#[stable(since = "1.4.0", feature = "result_iter")]
1724impl<'a, T, E> IntoIterator for &'a Result<T, E> {
1725 type Item = &'a T;
1726 type IntoIter = Iter<'a, T>;
1727
1728 fn into_iter(self) -> Iter<'a, T> {
1729 self.iter()
1730 }
1731}
1732
1733#[stable(since = "1.4.0", feature = "result_iter")]
1734impl<'a, T, E> IntoIterator for &'a mut Result<T, E> {
1735 type Item = &'a mut T;
1736 type IntoIter = IterMut<'a, T>;
1737
1738 fn into_iter(self) -> IterMut<'a, T> {
1739 self.iter_mut()
1740 }
1741}
1742
1743/////////////////////////////////////////////////////////////////////////////
1744// The Result Iterators
1745/////////////////////////////////////////////////////////////////////////////
1746
1747/// An iterator over a reference to the [`Ok`] variant of a [`Result`].
1748///
1749/// The iterator yields one value if the result is [`Ok`], otherwise none.
1750///
1751/// Created by [`Result::iter`].
1752#[derive(Debug)]
1753#[stable(feature = "rust1", since = "1.0.0")]
1754pub struct Iter<'a, T: 'a> {
1755 inner: Option<&'a T>,
1756}
1757
1758#[stable(feature = "rust1", since = "1.0.0")]
1759impl<'a, T> Iterator for Iter<'a, T> {
1760 type Item = &'a T;
1761
1762 #[inline]
1763 fn next(&mut self) -> Option<&'a T> {
1764 self.inner.take()
1765 }
1766 #[inline]
1767 fn size_hint(&self) -> (usize, Option<usize>) {
1768 let n: usize = if self.inner.is_some() { 1 } else { 0 };
1769 (n, Some(n))
1770 }
1771}
1772
1773#[stable(feature = "rust1", since = "1.0.0")]
1774impl<'a, T> DoubleEndedIterator for Iter<'a, T> {
1775 #[inline]
1776 fn next_back(&mut self) -> Option<&'a T> {
1777 self.inner.take()
1778 }
1779}
1780
1781#[stable(feature = "rust1", since = "1.0.0")]
1782impl<T> ExactSizeIterator for Iter<'_, T> {}
1783
1784#[stable(feature = "fused", since = "1.26.0")]
1785impl<T> FusedIterator for Iter<'_, T> {}
1786
1787#[unstable(feature = "trusted_len", issue = "37572")]
1788unsafe impl<A> TrustedLen for Iter<'_, A> {}
1789
1790#[stable(feature = "rust1", since = "1.0.0")]
1791impl<T> Clone for Iter<'_, T> {
1792 #[inline]
1793 fn clone(&self) -> Self {
1794 Iter { inner: self.inner }
1795 }
1796}
1797
1798/// An iterator over a mutable reference to the [`Ok`] variant of a [`Result`].
1799///
1800/// Created by [`Result::iter_mut`].
1801#[derive(Debug)]
1802#[stable(feature = "rust1", since = "1.0.0")]
1803pub struct IterMut<'a, T: 'a> {
1804 inner: Option<&'a mut T>,
1805}
1806
1807#[stable(feature = "rust1", since = "1.0.0")]
1808impl<'a, T> Iterator for IterMut<'a, T> {
1809 type Item = &'a mut T;
1810
1811 #[inline]
1812 fn next(&mut self) -> Option<&'a mut T> {
1813 self.inner.take()
1814 }
1815 #[inline]
1816 fn size_hint(&self) -> (usize, Option<usize>) {
1817 let n: usize = if self.inner.is_some() { 1 } else { 0 };
1818 (n, Some(n))
1819 }
1820}
1821
1822#[stable(feature = "rust1", since = "1.0.0")]
1823impl<'a, T> DoubleEndedIterator for IterMut<'a, T> {
1824 #[inline]
1825 fn next_back(&mut self) -> Option<&'a mut T> {
1826 self.inner.take()
1827 }
1828}
1829
1830#[stable(feature = "rust1", since = "1.0.0")]
1831impl<T> ExactSizeIterator for IterMut<'_, T> {}
1832
1833#[stable(feature = "fused", since = "1.26.0")]
1834impl<T> FusedIterator for IterMut<'_, T> {}
1835
1836#[unstable(feature = "trusted_len", issue = "37572")]
1837unsafe impl<A> TrustedLen for IterMut<'_, A> {}
1838
1839/// An iterator over the value in a [`Ok`] variant of a [`Result`].
1840///
1841/// The iterator yields one value if the result is [`Ok`], otherwise none.
1842///
1843/// This struct is created by the [`into_iter`] method on
1844/// [`Result`] (provided by the [`IntoIterator`] trait).
1845///
1846/// [`into_iter`]: IntoIterator::into_iter
1847#[derive(Clone, Debug)]
1848#[stable(feature = "rust1", since = "1.0.0")]
1849pub struct IntoIter<T> {
1850 inner: Option<T>,
1851}
1852
1853#[stable(feature = "rust1", since = "1.0.0")]
1854impl<T> Iterator for IntoIter<T> {
1855 type Item = T;
1856
1857 #[inline]
1858 fn next(&mut self) -> Option<T> {
1859 self.inner.take()
1860 }
1861 #[inline]
1862 fn size_hint(&self) -> (usize, Option<usize>) {
1863 let n: usize = if self.inner.is_some() { 1 } else { 0 };
1864 (n, Some(n))
1865 }
1866}
1867
1868#[stable(feature = "rust1", since = "1.0.0")]
1869impl<T> DoubleEndedIterator for IntoIter<T> {
1870 #[inline]
1871 fn next_back(&mut self) -> Option<T> {
1872 self.inner.take()
1873 }
1874}
1875
1876#[stable(feature = "rust1", since = "1.0.0")]
1877impl<T> ExactSizeIterator for IntoIter<T> {}
1878
1879#[stable(feature = "fused", since = "1.26.0")]
1880impl<T> FusedIterator for IntoIter<T> {}
1881
1882#[unstable(feature = "trusted_len", issue = "37572")]
1883unsafe impl<A> TrustedLen for IntoIter<A> {}
1884
1885/////////////////////////////////////////////////////////////////////////////
1886// FromIterator
1887/////////////////////////////////////////////////////////////////////////////
1888
1889#[stable(feature = "rust1", since = "1.0.0")]
1890impl<A, E, V: FromIterator<A>> FromIterator<Result<A, E>> for Result<V, E> {
1891 /// Takes each element in the `Iterator`: if it is an `Err`, no further
1892 /// elements are taken, and the `Err` is returned. Should no `Err` occur, a
1893 /// container with the values of each `Result` is returned.
1894 ///
1895 /// Here is an example which increments every integer in a vector,
1896 /// checking for overflow:
1897 ///
1898 /// ```
1899 /// let v = vec![1, 2];
1900 /// let res: Result<Vec<u32>, &'static str> = v.iter().map(|x: &u32|
1901 /// x.checked_add(1).ok_or("Overflow!")
1902 /// ).collect();
1903 /// assert_eq!(res, Ok(vec![2, 3]));
1904 /// ```
1905 ///
1906 /// Here is another example that tries to subtract one from another list
1907 /// of integers, this time checking for underflow:
1908 ///
1909 /// ```
1910 /// let v = vec![1, 2, 0];
1911 /// let res: Result<Vec<u32>, &'static str> = v.iter().map(|x: &u32|
1912 /// x.checked_sub(1).ok_or("Underflow!")
1913 /// ).collect();
1914 /// assert_eq!(res, Err("Underflow!"));
1915 /// ```
1916 ///
1917 /// Here is a variation on the previous example, showing that no
1918 /// further elements are taken from `iter` after the first `Err`.
1919 ///
1920 /// ```
1921 /// let v = vec![3, 2, 1, 10];
1922 /// let mut shared = 0;
1923 /// let res: Result<Vec<u32>, &'static str> = v.iter().map(|x: &u32| {
1924 /// shared += x;
1925 /// x.checked_sub(2).ok_or("Underflow!")
1926 /// }).collect();
1927 /// assert_eq!(res, Err("Underflow!"));
1928 /// assert_eq!(shared, 6);
1929 /// ```
1930 ///
1931 /// Since the third element caused an underflow, no further elements were taken,
1932 /// so the final value of `shared` is 6 (= `3 + 2 + 1`), not 16.
1933 #[inline]
1934 fn from_iter<I: IntoIterator<Item = Result<A, E>>>(iter: I) -> Result<V, E> {
1935 iter::try_process(iter.into_iter(), |i| i.collect())
1936 }
1937}
1938
1939#[unstable(feature = "try_trait_v2", issue = "84277")]
1940impl<T, E> ops::Try for Result<T, E> {
1941 type Output = T;
1942 type Residual = Result<convert::Infallible, E>;
1943
1944 #[inline]
1945 fn from_output(output: Self::Output) -> Self {
1946 Ok(output)
1947 }
1948
1949 #[inline]
1950 fn branch(self) -> ControlFlow<Self::Residual, Self::Output> {
1951 match self {
1952 Ok(v: T) => ControlFlow::Continue(v),
1953 Err(e: E) => ControlFlow::Break(Err(e)),
1954 }
1955 }
1956}
1957
1958#[unstable(feature = "try_trait_v2", issue = "84277")]
1959impl<T, E, F: From<E>> ops::FromResidual<Result<convert::Infallible, E>> for Result<T, F> {
1960 #[inline]
1961 #[track_caller]
1962 fn from_residual(residual: Result<convert::Infallible, E>) -> Self {
1963 match residual {
1964 Err(e: E) => Err(From::from(e)),
1965 }
1966 }
1967}
1968
1969#[unstable(feature = "try_trait_v2_yeet", issue = "96374")]
1970impl<T, E, F: From<E>> ops::FromResidual<ops::Yeet<E>> for Result<T, F> {
1971 #[inline]
1972 fn from_residual(ops::Yeet(e: E): ops::Yeet<E>) -> Self {
1973 Err(From::from(e))
1974 }
1975}
1976
1977#[unstable(feature = "try_trait_v2_residual", issue = "91285")]
1978impl<T, E> ops::Residual<T> for Result<convert::Infallible, E> {
1979 type TryType = Result<T, E>;
1980}
1981