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 unwrapped
213 //! success ([`Ok`]) value, unless the result is [`Err`], in which case
214 //! [`Err`] is returned early from the enclosing function.
215 //!
216 //! [`?`] can only be used in functions that return [`Result`] because of the
217 //! early return of [`Err`] that it provides.
218 //!
219 //! [`expect`]: Result::expect
220 //! [`Write`]: ../../std/io/trait.Write.html "io::Write"
221 //! [`write_all`]: ../../std/io/trait.Write.html#method.write_all "io::Write::write_all"
222 //! [`io::Result`]: ../../std/io/type.Result.html "io::Result"
223 //! [`?`]: crate::ops::Try
224 //! [`Ok(T)`]: Ok
225 //! [`Err(E)`]: Err
226 //! [io::Error]: ../../std/io/struct.Error.html "io::Error"
227 //!
228 //! # Method overview
229 //!
230 //! In addition to working with pattern matching, [`Result`] provides a
231 //! wide variety of different methods.
232 //!
233 //! ## Querying the variant
234 //!
235 //! The [`is_ok`] and [`is_err`] methods return [`true`] if the [`Result`]
236 //! is [`Ok`] or [`Err`], respectively.
237 //!
238 //! [`is_err`]: Result::is_err
239 //! [`is_ok`]: Result::is_ok
240 //!
241 //! ## Adapters for working with references
242 //!
243 //! * [`as_ref`] converts from `&Result<T, E>` to `Result<&T, &E>`
244 //! * [`as_mut`] converts from `&mut Result<T, E>` to `Result<&mut T, &mut E>`
245 //! * [`as_deref`] converts from `&Result<T, E>` to `Result<&T::Target, &E>`
246 //! * [`as_deref_mut`] converts from `&mut Result<T, E>` to
247 //! `Result<&mut T::Target, &mut E>`
248 //!
249 //! [`as_deref`]: Result::as_deref
250 //! [`as_deref_mut`]: Result::as_deref_mut
251 //! [`as_mut`]: Result::as_mut
252 //! [`as_ref`]: Result::as_ref
253 //!
254 //! ## Extracting contained values
255 //!
256 //! These methods extract the contained value in a [`Result<T, E>`] when it
257 //! is the [`Ok`] variant. If the [`Result`] is [`Err`]:
258 //!
259 //! * [`expect`] panics with a provided custom message
260 //! * [`unwrap`] panics with a generic message
261 //! * [`unwrap_or`] returns the provided default value
262 //! * [`unwrap_or_default`] returns the default value of the type `T`
263 //! (which must implement the [`Default`] trait)
264 //! * [`unwrap_or_else`] returns the result of evaluating the provided
265 //! function
266 //!
267 //! The panicking methods [`expect`] and [`unwrap`] require `E` to
268 //! implement the [`Debug`] trait.
269 //!
270 //! [`Debug`]: crate::fmt::Debug
271 //! [`expect`]: Result::expect
272 //! [`unwrap`]: Result::unwrap
273 //! [`unwrap_or`]: Result::unwrap_or
274 //! [`unwrap_or_default`]: Result::unwrap_or_default
275 //! [`unwrap_or_else`]: Result::unwrap_or_else
276 //!
277 //! These methods extract the contained value in a [`Result<T, E>`] when it
278 //! is the [`Err`] variant. They require `T` to implement the [`Debug`]
279 //! trait. If the [`Result`] is [`Ok`]:
280 //!
281 //! * [`expect_err`] panics with a provided custom message
282 //! * [`unwrap_err`] panics with a generic message
283 //!
284 //! [`Debug`]: crate::fmt::Debug
285 //! [`expect_err`]: Result::expect_err
286 //! [`unwrap_err`]: Result::unwrap_err
287 //!
288 //! ## Transforming contained values
289 //!
290 //! These methods transform [`Result`] to [`Option`]:
291 //!
292 //! * [`err`][Result::err] transforms [`Result<T, E>`] into [`Option<E>`],
293 //! mapping [`Err(e)`] to [`Some(e)`] and [`Ok(v)`] to [`None`]
294 //! * [`ok`][Result::ok] transforms [`Result<T, E>`] into [`Option<T>`],
295 //! mapping [`Ok(v)`] to [`Some(v)`] and [`Err(e)`] to [`None`]
296 //! * [`transpose`] transposes a [`Result`] of an [`Option`] into an
297 //! [`Option`] of a [`Result`]
298 //!
299 // Do NOT add link reference definitions for `err` or `ok`, because they
300 // will generate numerous incorrect URLs for `Err` and `Ok` elsewhere, due
301 // to case folding.
302 //!
303 //! [`Err(e)`]: Err
304 //! [`Ok(v)`]: Ok
305 //! [`Some(e)`]: Option::Some
306 //! [`Some(v)`]: Option::Some
307 //! [`transpose`]: Result::transpose
308 //!
309 //! This method transforms the contained value of the [`Ok`] variant:
310 //!
311 //! * [`map`] transforms [`Result<T, E>`] into [`Result<U, E>`] by applying
312 //! the provided function to the contained value of [`Ok`] and leaving
313 //! [`Err`] values unchanged
314 //!
315 //! [`map`]: Result::map
316 //!
317 //! This method transforms the contained value of the [`Err`] variant:
318 //!
319 //! * [`map_err`] transforms [`Result<T, E>`] into [`Result<T, F>`] by
320 //! applying the provided function to the contained value of [`Err`] and
321 //! leaving [`Ok`] values unchanged
322 //!
323 //! [`map_err`]: Result::map_err
324 //!
325 //! These methods transform a [`Result<T, E>`] into a value of a possibly
326 //! different type `U`:
327 //!
328 //! * [`map_or`] applies the provided function to the contained value of
329 //! [`Ok`], or returns the provided default value if the [`Result`] is
330 //! [`Err`]
331 //! * [`map_or_else`] applies the provided function to the contained value
332 //! of [`Ok`], or applies the provided default fallback function to the
333 //! contained value of [`Err`]
334 //!
335 //! [`map_or`]: Result::map_or
336 //! [`map_or_else`]: Result::map_or_else
337 //!
338 //! ## Boolean operators
339 //!
340 //! These methods treat the [`Result`] as a boolean value, where [`Ok`]
341 //! acts like [`true`] and [`Err`] acts like [`false`]. There are two
342 //! categories of these methods: ones that take a [`Result`] as input, and
343 //! ones that take a function as input (to be lazily evaluated).
344 //!
345 //! The [`and`] and [`or`] methods take another [`Result`] as input, and
346 //! produce a [`Result`] as output. The [`and`] method can produce a
347 //! [`Result<U, E>`] value having a different inner type `U` than
348 //! [`Result<T, E>`]. The [`or`] method can produce a [`Result<T, F>`]
349 //! value having a different error type `F` than [`Result<T, E>`].
350 //!
351 //! | method | self | input | output |
352 //! |---------|----------|-----------|----------|
353 //! | [`and`] | `Err(e)` | (ignored) | `Err(e)` |
354 //! | [`and`] | `Ok(x)` | `Err(d)` | `Err(d)` |
355 //! | [`and`] | `Ok(x)` | `Ok(y)` | `Ok(y)` |
356 //! | [`or`] | `Err(e)` | `Err(d)` | `Err(d)` |
357 //! | [`or`] | `Err(e)` | `Ok(y)` | `Ok(y)` |
358 //! | [`or`] | `Ok(x)` | (ignored) | `Ok(x)` |
359 //!
360 //! [`and`]: Result::and
361 //! [`or`]: Result::or
362 //!
363 //! The [`and_then`] and [`or_else`] methods take a function as input, and
364 //! only evaluate the function when they need to produce a new value. The
365 //! [`and_then`] method can produce a [`Result<U, E>`] value having a
366 //! different inner type `U` than [`Result<T, E>`]. The [`or_else`] method
367 //! can produce a [`Result<T, F>`] value having a different error type `F`
368 //! than [`Result<T, E>`].
369 //!
370 //! | method | self | function input | function result | output |
371 //! |--------------|----------|----------------|-----------------|----------|
372 //! | [`and_then`] | `Err(e)` | (not provided) | (not evaluated) | `Err(e)` |
373 //! | [`and_then`] | `Ok(x)` | `x` | `Err(d)` | `Err(d)` |
374 //! | [`and_then`] | `Ok(x)` | `x` | `Ok(y)` | `Ok(y)` |
375 //! | [`or_else`] | `Err(e)` | `e` | `Err(d)` | `Err(d)` |
376 //! | [`or_else`] | `Err(e)` | `e` | `Ok(y)` | `Ok(y)` |
377 //! | [`or_else`] | `Ok(x)` | (not provided) | (not evaluated) | `Ok(x)` |
378 //!
379 //! [`and_then`]: Result::and_then
380 //! [`or_else`]: Result::or_else
381 //!
382 //! ## Comparison operators
383 //!
384 //! If `T` and `E` both implement [`PartialOrd`] then [`Result<T, E>`] will
385 //! derive its [`PartialOrd`] implementation. With this order, an [`Ok`]
386 //! compares as less than any [`Err`], while two [`Ok`] or two [`Err`]
387 //! compare as their contained values would in `T` or `E` respectively. If `T`
388 //! and `E` both also implement [`Ord`], then so does [`Result<T, E>`].
389 //!
390 //! ```
391 //! assert!(Ok(1) < Err(0));
392 //! let x: Result<i32, ()> = Ok(0);
393 //! let y = Ok(1);
394 //! assert!(x < y);
395 //! let x: Result<(), i32> = Err(0);
396 //! let y = Err(1);
397 //! assert!(x < y);
398 //! ```
399 //!
400 //! ## Iterating over `Result`
401 //!
402 //! A [`Result`] can be iterated over. This can be helpful if you need an
403 //! iterator that is conditionally empty. The iterator will either produce
404 //! a single value (when the [`Result`] is [`Ok`]), or produce no values
405 //! (when the [`Result`] is [`Err`]). For example, [`into_iter`] acts like
406 //! [`once(v)`] if the [`Result`] is [`Ok(v)`], and like [`empty()`] if the
407 //! [`Result`] is [`Err`].
408 //!
409 //! [`Ok(v)`]: Ok
410 //! [`empty()`]: crate::iter::empty
411 //! [`once(v)`]: crate::iter::once
412 //!
413 //! Iterators over [`Result<T, E>`] come in three types:
414 //!
415 //! * [`into_iter`] consumes the [`Result`] and produces the contained
416 //! value
417 //! * [`iter`] produces an immutable reference of type `&T` to the
418 //! contained value
419 //! * [`iter_mut`] produces a mutable reference of type `&mut T` to the
420 //! contained value
421 //!
422 //! See [Iterating over `Option`] for examples of how this can be useful.
423 //!
424 //! [Iterating over `Option`]: crate::option#iterating-over-option
425 //! [`into_iter`]: Result::into_iter
426 //! [`iter`]: Result::iter
427 //! [`iter_mut`]: Result::iter_mut
428 //!
429 //! You might want to use an iterator chain to do multiple instances of an
430 //! operation that can fail, but would like to ignore failures while
431 //! continuing to process the successful results. In this example, we take
432 //! advantage of the iterable nature of [`Result`] to select only the
433 //! [`Ok`] values using [`flatten`][Iterator::flatten].
434 //!
435 //! ```
436 //! # use std::str::FromStr;
437 //! let mut results = vec![];
438 //! let mut errs = vec![];
439 //! let nums: Vec<_> = vec!["17", "not a number", "99", "-27", "768"]
440 //! .into_iter()
441 //! .map(u8::from_str)
442 //! // Save clones of the raw `Result` values to inspect
443 //! .inspect(|x| results.push(x.clone()))
444 //! // Challenge: explain how this captures only the `Err` values
445 //! .inspect(|x| errs.extend(x.clone().err()))
446 //! .flatten()
447 //! .collect();
448 //! assert_eq!(errs.len(), 3);
449 //! assert_eq!(nums, [17, 99]);
450 //! println!("results {:?}", results);
451 //! println!("errs {:?}", errs);
452 //! println!("nums {:?}", nums);
453 //! ```
454 //!
455 //! ## Collecting into `Result`
456 //!
457 //! [`Result`] implements the [`FromIterator`][impl-FromIterator] trait,
458 //! which allows an iterator over [`Result`] values to be collected into a
459 //! [`Result`] of a collection of each contained value of the original
460 //! [`Result`] values, or [`Err`] if any of the elements was [`Err`].
461 //!
462 //! [impl-FromIterator]: Result#impl-FromIterator%3CResult%3CA%2C%20E%3E%3E
463 //!
464 //! ```
465 //! let v = vec![Ok(2), Ok(4), Err("err!"), Ok(8)];
466 //! let res: Result<Vec<_>, &str> = v.into_iter().collect();
467 //! assert_eq!(res, Err("err!"));
468 //! let v = vec![Ok(2), Ok(4), Ok(8)];
469 //! let res: Result<Vec<_>, &str> = v.into_iter().collect();
470 //! assert_eq!(res, Ok(vec![2, 4, 8]));
471 //! ```
472 //!
473 //! [`Result`] also implements the [`Product`][impl-Product] and
474 //! [`Sum`][impl-Sum] traits, allowing an iterator over [`Result`] values
475 //! to provide the [`product`][Iterator::product] and
476 //! [`sum`][Iterator::sum] methods.
477 //!
478 //! [impl-Product]: Result#impl-Product%3CResult%3CU%2C%20E%3E%3E
479 //! [impl-Sum]: Result#impl-Sum%3CResult%3CU%2C%20E%3E%3E
480 //!
481 //! ```
482 //! let v = vec![Err("error!"), Ok(1), Ok(2), Ok(3), Err("foo")];
483 //! let res: Result<i32, &str> = v.into_iter().sum();
484 //! assert_eq!(res, Err("error!"));
485 //! let v: Vec<Result<i32, &str>> = vec![Ok(1), Ok(2), Ok(21)];
486 //! let res: Result<i32, &str> = v.into_iter().product();
487 //! assert_eq!(res, Ok(42));
488 //! ```
489
490 #![stable(feature = "rust1", since = "1.0.0")]
491
492 use crate::iter::{self, FromIterator, FusedIterator, TrustedLen};
493 use crate::ops::{self, ControlFlow, Deref, DerefMut};
494 use crate::{convert, fmt, hint};
495
496 /// `Result` is a type that represents either success ([`Ok`]) or failure ([`Err`]).
497 ///
498 /// See the [module documentation](self) for details.
499 #[derive(Copy, PartialEq, PartialOrd, Eq, Ord, Debug, Hash)]
500 #[must_use = "this `Result` may be an `Err` variant, which should be handled"]
501 #[rustc_diagnostic_item = "Result"]
502 #[stable(feature = "rust1", since = "1.0.0")]
503 pub enum Result<T, E> {
504 /// Contains the success value
505 #[lang = "Ok"]
506 #[stable(feature = "rust1", since = "1.0.0")]
507 Ok(#[stable(feature = "rust1", since = "1.0.0")] T),
508
509 /// Contains the error value
510 #[lang = "Err"]
511 #[stable(feature = "rust1", since = "1.0.0")]
512 Err(#[stable(feature = "rust1", since = "1.0.0")] E),
513 }
514
515 /////////////////////////////////////////////////////////////////////////////
516 // Type implementation
517 /////////////////////////////////////////////////////////////////////////////
518
519 impl<T, E> Result<T, E> {
520 /////////////////////////////////////////////////////////////////////////
521 // Querying the contained values
522 /////////////////////////////////////////////////////////////////////////
523
524 /// Returns `true` if the result is [`Ok`].
525 ///
526 /// # Examples
527 ///
528 /// Basic usage:
529 ///
530 /// ```
531 /// let x: Result<i32, &str> = Ok(-3);
532 /// assert_eq!(x.is_ok(), true);
533 ///
534 /// let x: Result<i32, &str> = Err("Some error message");
535 /// assert_eq!(x.is_ok(), false);
536 /// ```
537 #[must_use = "if you intended to assert that this is ok, consider `.unwrap()` instead"]
538 #[rustc_const_stable(feature = "const_result", since = "1.48.0")]
539 #[inline]
540 #[stable(feature = "rust1", since = "1.0.0")]
is_ok(&self) -> bool541 pub const fn is_ok(&self) -> bool {
542 matches!(*self, Ok(_))
543 }
544
545 /// Returns `true` if the result is [`Err`].
546 ///
547 /// # Examples
548 ///
549 /// Basic usage:
550 ///
551 /// ```
552 /// let x: Result<i32, &str> = Ok(-3);
553 /// assert_eq!(x.is_err(), false);
554 ///
555 /// let x: Result<i32, &str> = Err("Some error message");
556 /// assert_eq!(x.is_err(), true);
557 /// ```
558 #[must_use = "if you intended to assert that this is err, consider `.unwrap_err()` instead"]
559 #[rustc_const_stable(feature = "const_result", since = "1.48.0")]
560 #[inline]
561 #[stable(feature = "rust1", since = "1.0.0")]
is_err(&self) -> bool562 pub const fn is_err(&self) -> bool {
563 !self.is_ok()
564 }
565
566 /// Returns `true` if the result is an [`Ok`] value containing the given value.
567 ///
568 /// # Examples
569 ///
570 /// ```
571 /// #![feature(option_result_contains)]
572 ///
573 /// let x: Result<u32, &str> = Ok(2);
574 /// assert_eq!(x.contains(&2), true);
575 ///
576 /// let x: Result<u32, &str> = Ok(3);
577 /// assert_eq!(x.contains(&2), false);
578 ///
579 /// let x: Result<u32, &str> = Err("Some error message");
580 /// assert_eq!(x.contains(&2), false);
581 /// ```
582 #[must_use]
583 #[inline]
584 #[unstable(feature = "option_result_contains", issue = "62358")]
contains<U>(&self, x: &U) -> bool where U: PartialEq<T>,585 pub fn contains<U>(&self, x: &U) -> bool
586 where
587 U: PartialEq<T>,
588 {
589 match self {
590 Ok(y) => x == y,
591 Err(_) => false,
592 }
593 }
594
595 /// Returns `true` if the result is an [`Err`] value containing the given value.
596 ///
597 /// # Examples
598 ///
599 /// ```
600 /// #![feature(result_contains_err)]
601 ///
602 /// let x: Result<u32, &str> = Ok(2);
603 /// assert_eq!(x.contains_err(&"Some error message"), false);
604 ///
605 /// let x: Result<u32, &str> = Err("Some error message");
606 /// assert_eq!(x.contains_err(&"Some error message"), true);
607 ///
608 /// let x: Result<u32, &str> = Err("Some other error message");
609 /// assert_eq!(x.contains_err(&"Some error message"), false);
610 /// ```
611 #[must_use]
612 #[inline]
613 #[unstable(feature = "result_contains_err", issue = "62358")]
contains_err<F>(&self, f: &F) -> bool where F: PartialEq<E>,614 pub fn contains_err<F>(&self, f: &F) -> bool
615 where
616 F: PartialEq<E>,
617 {
618 match self {
619 Ok(_) => false,
620 Err(e) => f == e,
621 }
622 }
623
624 /////////////////////////////////////////////////////////////////////////
625 // Adapter for each variant
626 /////////////////////////////////////////////////////////////////////////
627
628 /// Converts from `Result<T, E>` to [`Option<T>`].
629 ///
630 /// Converts `self` into an [`Option<T>`], consuming `self`,
631 /// and discarding the error, if any.
632 ///
633 /// # Examples
634 ///
635 /// Basic usage:
636 ///
637 /// ```
638 /// let x: Result<u32, &str> = Ok(2);
639 /// assert_eq!(x.ok(), Some(2));
640 ///
641 /// let x: Result<u32, &str> = Err("Nothing here");
642 /// assert_eq!(x.ok(), None);
643 /// ```
644 #[inline]
645 #[stable(feature = "rust1", since = "1.0.0")]
ok(self) -> Option<T>646 pub fn ok(self) -> Option<T> {
647 match self {
648 Ok(x) => Some(x),
649 Err(_) => None,
650 }
651 }
652
653 /// Converts from `Result<T, E>` to [`Option<E>`].
654 ///
655 /// Converts `self` into an [`Option<E>`], consuming `self`,
656 /// and discarding the success value, if any.
657 ///
658 /// # Examples
659 ///
660 /// Basic usage:
661 ///
662 /// ```
663 /// let x: Result<u32, &str> = Ok(2);
664 /// assert_eq!(x.err(), None);
665 ///
666 /// let x: Result<u32, &str> = Err("Nothing here");
667 /// assert_eq!(x.err(), Some("Nothing here"));
668 /// ```
669 #[inline]
670 #[stable(feature = "rust1", since = "1.0.0")]
err(self) -> Option<E>671 pub fn err(self) -> Option<E> {
672 match self {
673 Ok(_) => None,
674 Err(x) => Some(x),
675 }
676 }
677
678 /////////////////////////////////////////////////////////////////////////
679 // Adapter for working with references
680 /////////////////////////////////////////////////////////////////////////
681
682 /// Converts from `&Result<T, E>` to `Result<&T, &E>`.
683 ///
684 /// Produces a new `Result`, containing a reference
685 /// into the original, leaving the original in place.
686 ///
687 /// # Examples
688 ///
689 /// Basic usage:
690 ///
691 /// ```
692 /// let x: Result<u32, &str> = Ok(2);
693 /// assert_eq!(x.as_ref(), Ok(&2));
694 ///
695 /// let x: Result<u32, &str> = Err("Error");
696 /// assert_eq!(x.as_ref(), Err(&"Error"));
697 /// ```
698 #[inline]
699 #[rustc_const_stable(feature = "const_result", since = "1.48.0")]
700 #[stable(feature = "rust1", since = "1.0.0")]
as_ref(&self) -> Result<&T, &E>701 pub const fn as_ref(&self) -> Result<&T, &E> {
702 match *self {
703 Ok(ref x) => Ok(x),
704 Err(ref x) => Err(x),
705 }
706 }
707
708 /// Converts from `&mut Result<T, E>` to `Result<&mut T, &mut E>`.
709 ///
710 /// # Examples
711 ///
712 /// Basic usage:
713 ///
714 /// ```
715 /// fn mutate(r: &mut Result<i32, i32>) {
716 /// match r.as_mut() {
717 /// Ok(v) => *v = 42,
718 /// Err(e) => *e = 0,
719 /// }
720 /// }
721 ///
722 /// let mut x: Result<i32, i32> = Ok(2);
723 /// mutate(&mut x);
724 /// assert_eq!(x.unwrap(), 42);
725 ///
726 /// let mut x: Result<i32, i32> = Err(13);
727 /// mutate(&mut x);
728 /// assert_eq!(x.unwrap_err(), 0);
729 /// ```
730 #[inline]
731 #[stable(feature = "rust1", since = "1.0.0")]
732 #[rustc_const_unstable(feature = "const_result", issue = "82814")]
as_mut(&mut self) -> Result<&mut T, &mut E>733 pub const fn as_mut(&mut self) -> Result<&mut T, &mut E> {
734 match *self {
735 Ok(ref mut x) => Ok(x),
736 Err(ref mut x) => Err(x),
737 }
738 }
739
740 /////////////////////////////////////////////////////////////////////////
741 // Transforming contained values
742 /////////////////////////////////////////////////////////////////////////
743
744 /// Maps a `Result<T, E>` to `Result<U, E>` by applying a function to a
745 /// contained [`Ok`] value, leaving an [`Err`] value untouched.
746 ///
747 /// This function can be used to compose the results of two functions.
748 ///
749 /// # Examples
750 ///
751 /// Print the numbers on each line of a string multiplied by two.
752 ///
753 /// ```
754 /// let line = "1\n2\n3\n4\n";
755 ///
756 /// for num in line.lines() {
757 /// match num.parse::<i32>().map(|i| i * 2) {
758 /// Ok(n) => println!("{}", n),
759 /// Err(..) => {}
760 /// }
761 /// }
762 /// ```
763 #[inline]
764 #[stable(feature = "rust1", since = "1.0.0")]
map<U, F: FnOnce(T) -> U>(self, op: F) -> Result<U, E>765 pub fn map<U, F: FnOnce(T) -> U>(self, op: F) -> Result<U, E> {
766 match self {
767 Ok(t) => Ok(op(t)),
768 Err(e) => Err(e),
769 }
770 }
771
772 /// Returns the provided default (if [`Err`]), or
773 /// applies a function to the contained value (if [`Ok`]),
774 ///
775 /// Arguments passed to `map_or` are eagerly evaluated; if you are passing
776 /// the result of a function call, it is recommended to use [`map_or_else`],
777 /// which is lazily evaluated.
778 ///
779 /// [`map_or_else`]: Result::map_or_else
780 ///
781 /// # Examples
782 ///
783 /// ```
784 /// let x: Result<_, &str> = Ok("foo");
785 /// assert_eq!(x.map_or(42, |v| v.len()), 3);
786 ///
787 /// let x: Result<&str, _> = Err("bar");
788 /// assert_eq!(x.map_or(42, |v| v.len()), 42);
789 /// ```
790 #[inline]
791 #[stable(feature = "result_map_or", since = "1.41.0")]
map_or<U, F: FnOnce(T) -> U>(self, default: U, f: F) -> U792 pub fn map_or<U, F: FnOnce(T) -> U>(self, default: U, f: F) -> U {
793 match self {
794 Ok(t) => f(t),
795 Err(_) => default,
796 }
797 }
798
799 /// Maps a `Result<T, E>` to `U` by applying fallback function `default` to
800 /// a contained [`Err`] value, or function `f` to a contained [`Ok`] value.
801 ///
802 /// This function can be used to unpack a successful result
803 /// while handling an error.
804 ///
805 ///
806 /// # Examples
807 ///
808 /// Basic usage:
809 ///
810 /// ```
811 /// let k = 21;
812 ///
813 /// let x : Result<_, &str> = Ok("foo");
814 /// assert_eq!(x.map_or_else(|e| k * 2, |v| v.len()), 3);
815 ///
816 /// let x : Result<&str, _> = Err("bar");
817 /// assert_eq!(x.map_or_else(|e| k * 2, |v| v.len()), 42);
818 /// ```
819 #[inline]
820 #[stable(feature = "result_map_or_else", since = "1.41.0")]
map_or_else<U, D: FnOnce(E) -> U, F: FnOnce(T) -> U>(self, default: D, f: F) -> U821 pub fn map_or_else<U, D: FnOnce(E) -> U, F: FnOnce(T) -> U>(self, default: D, f: F) -> U {
822 match self {
823 Ok(t) => f(t),
824 Err(e) => default(e),
825 }
826 }
827
828 /// Maps a `Result<T, E>` to `Result<T, F>` by applying a function to a
829 /// contained [`Err`] value, leaving an [`Ok`] value untouched.
830 ///
831 /// This function can be used to pass through a successful result while handling
832 /// an error.
833 ///
834 ///
835 /// # Examples
836 ///
837 /// Basic usage:
838 ///
839 /// ```
840 /// fn stringify(x: u32) -> String { format!("error code: {}", x) }
841 ///
842 /// let x: Result<u32, u32> = Ok(2);
843 /// assert_eq!(x.map_err(stringify), Ok(2));
844 ///
845 /// let x: Result<u32, u32> = Err(13);
846 /// assert_eq!(x.map_err(stringify), Err("error code: 13".to_string()));
847 /// ```
848 #[inline]
849 #[stable(feature = "rust1", since = "1.0.0")]
map_err<F, O: FnOnce(E) -> F>(self, op: O) -> Result<T, F>850 pub fn map_err<F, O: FnOnce(E) -> F>(self, op: O) -> Result<T, F> {
851 match self {
852 Ok(t) => Ok(t),
853 Err(e) => Err(op(e)),
854 }
855 }
856
857 /////////////////////////////////////////////////////////////////////////
858 // Iterator constructors
859 /////////////////////////////////////////////////////////////////////////
860
861 /// Returns an iterator over the possibly contained value.
862 ///
863 /// The iterator yields one value if the result is [`Result::Ok`], otherwise none.
864 ///
865 /// # Examples
866 ///
867 /// Basic usage:
868 ///
869 /// ```
870 /// let x: Result<u32, &str> = Ok(7);
871 /// assert_eq!(x.iter().next(), Some(&7));
872 ///
873 /// let x: Result<u32, &str> = Err("nothing!");
874 /// assert_eq!(x.iter().next(), None);
875 /// ```
876 #[inline]
877 #[stable(feature = "rust1", since = "1.0.0")]
iter(&self) -> Iter<'_, T>878 pub fn iter(&self) -> Iter<'_, T> {
879 Iter { inner: self.as_ref().ok() }
880 }
881
882 /// Returns a mutable iterator over the possibly contained value.
883 ///
884 /// The iterator yields one value if the result is [`Result::Ok`], otherwise none.
885 ///
886 /// # Examples
887 ///
888 /// Basic usage:
889 ///
890 /// ```
891 /// let mut x: Result<u32, &str> = Ok(7);
892 /// match x.iter_mut().next() {
893 /// Some(v) => *v = 40,
894 /// None => {},
895 /// }
896 /// assert_eq!(x, Ok(40));
897 ///
898 /// let mut x: Result<u32, &str> = Err("nothing!");
899 /// assert_eq!(x.iter_mut().next(), None);
900 /// ```
901 #[inline]
902 #[stable(feature = "rust1", since = "1.0.0")]
iter_mut(&mut self) -> IterMut<'_, T>903 pub fn iter_mut(&mut self) -> IterMut<'_, T> {
904 IterMut { inner: self.as_mut().ok() }
905 }
906
907 ////////////////////////////////////////////////////////////////////////
908 // Boolean operations on the values, eager and lazy
909 /////////////////////////////////////////////////////////////////////////
910
911 /// Returns `res` if the result is [`Ok`], otherwise returns the [`Err`] value of `self`.
912 ///
913 ///
914 /// # Examples
915 ///
916 /// Basic usage:
917 ///
918 /// ```
919 /// let x: Result<u32, &str> = Ok(2);
920 /// let y: Result<&str, &str> = Err("late error");
921 /// assert_eq!(x.and(y), Err("late error"));
922 ///
923 /// let x: Result<u32, &str> = Err("early error");
924 /// let y: Result<&str, &str> = Ok("foo");
925 /// assert_eq!(x.and(y), Err("early error"));
926 ///
927 /// let x: Result<u32, &str> = Err("not a 2");
928 /// let y: Result<&str, &str> = Err("late error");
929 /// assert_eq!(x.and(y), Err("not a 2"));
930 ///
931 /// let x: Result<u32, &str> = Ok(2);
932 /// let y: Result<&str, &str> = Ok("different result type");
933 /// assert_eq!(x.and(y), Ok("different result type"));
934 /// ```
935 #[inline]
936 #[stable(feature = "rust1", since = "1.0.0")]
and<U>(self, res: Result<U, E>) -> Result<U, E>937 pub fn and<U>(self, res: Result<U, E>) -> Result<U, E> {
938 match self {
939 Ok(_) => res,
940 Err(e) => Err(e),
941 }
942 }
943
944 /// Calls `op` if the result is [`Ok`], otherwise returns the [`Err`] value of `self`.
945 ///
946 ///
947 /// This function can be used for control flow based on `Result` values.
948 ///
949 /// # Examples
950 ///
951 /// Basic usage:
952 ///
953 /// ```
954 /// fn sq(x: u32) -> Result<u32, u32> { Ok(x * x) }
955 /// fn err(x: u32) -> Result<u32, u32> { Err(x) }
956 ///
957 /// assert_eq!(Ok(2).and_then(sq).and_then(sq), Ok(16));
958 /// assert_eq!(Ok(2).and_then(sq).and_then(err), Err(4));
959 /// assert_eq!(Ok(2).and_then(err).and_then(sq), Err(2));
960 /// assert_eq!(Err(3).and_then(sq).and_then(sq), Err(3));
961 /// ```
962 #[inline]
963 #[stable(feature = "rust1", since = "1.0.0")]
and_then<U, F: FnOnce(T) -> Result<U, E>>(self, op: F) -> Result<U, E>964 pub fn and_then<U, F: FnOnce(T) -> Result<U, E>>(self, op: F) -> Result<U, E> {
965 match self {
966 Ok(t) => op(t),
967 Err(e) => Err(e),
968 }
969 }
970
971 /// Returns `res` if the result is [`Err`], otherwise returns the [`Ok`] value of `self`.
972 ///
973 /// Arguments passed to `or` are eagerly evaluated; if you are passing the
974 /// result of a function call, it is recommended to use [`or_else`], which is
975 /// lazily evaluated.
976 ///
977 /// [`or_else`]: Result::or_else
978 ///
979 /// # Examples
980 ///
981 /// Basic usage:
982 ///
983 /// ```
984 /// let x: Result<u32, &str> = Ok(2);
985 /// let y: Result<u32, &str> = Err("late error");
986 /// assert_eq!(x.or(y), Ok(2));
987 ///
988 /// let x: Result<u32, &str> = Err("early error");
989 /// let y: Result<u32, &str> = Ok(2);
990 /// assert_eq!(x.or(y), Ok(2));
991 ///
992 /// let x: Result<u32, &str> = Err("not a 2");
993 /// let y: Result<u32, &str> = Err("late error");
994 /// assert_eq!(x.or(y), Err("late error"));
995 ///
996 /// let x: Result<u32, &str> = Ok(2);
997 /// let y: Result<u32, &str> = Ok(100);
998 /// assert_eq!(x.or(y), Ok(2));
999 /// ```
1000 #[inline]
1001 #[stable(feature = "rust1", since = "1.0.0")]
or<F>(self, res: Result<T, F>) -> Result<T, F>1002 pub fn or<F>(self, res: Result<T, F>) -> Result<T, F> {
1003 match self {
1004 Ok(v) => Ok(v),
1005 Err(_) => res,
1006 }
1007 }
1008
1009 /// Calls `op` if the result is [`Err`], otherwise returns the [`Ok`] value of `self`.
1010 ///
1011 /// This function can be used for control flow based on result values.
1012 ///
1013 ///
1014 /// # Examples
1015 ///
1016 /// Basic usage:
1017 ///
1018 /// ```
1019 /// fn sq(x: u32) -> Result<u32, u32> { Ok(x * x) }
1020 /// fn err(x: u32) -> Result<u32, u32> { Err(x) }
1021 ///
1022 /// assert_eq!(Ok(2).or_else(sq).or_else(sq), Ok(2));
1023 /// assert_eq!(Ok(2).or_else(err).or_else(sq), Ok(2));
1024 /// assert_eq!(Err(3).or_else(sq).or_else(err), Ok(9));
1025 /// assert_eq!(Err(3).or_else(err).or_else(err), Err(3));
1026 /// ```
1027 #[inline]
1028 #[stable(feature = "rust1", since = "1.0.0")]
or_else<F, O: FnOnce(E) -> Result<T, F>>(self, op: O) -> Result<T, F>1029 pub fn or_else<F, O: FnOnce(E) -> Result<T, F>>(self, op: O) -> Result<T, F> {
1030 match self {
1031 Ok(t) => Ok(t),
1032 Err(e) => op(e),
1033 }
1034 }
1035
1036 /// Returns the contained [`Ok`] value or a provided default.
1037 ///
1038 /// Arguments passed to `unwrap_or` are eagerly evaluated; if you are passing
1039 /// the result of a function call, it is recommended to use [`unwrap_or_else`],
1040 /// which is lazily evaluated.
1041 ///
1042 /// [`unwrap_or_else`]: Result::unwrap_or_else
1043 ///
1044 /// # Examples
1045 ///
1046 /// Basic usage:
1047 ///
1048 /// ```
1049 /// let default = 2;
1050 /// let x: Result<u32, &str> = Ok(9);
1051 /// assert_eq!(x.unwrap_or(default), 9);
1052 ///
1053 /// let x: Result<u32, &str> = Err("error");
1054 /// assert_eq!(x.unwrap_or(default), default);
1055 /// ```
1056 #[inline]
1057 #[stable(feature = "rust1", since = "1.0.0")]
unwrap_or(self, default: T) -> T1058 pub fn unwrap_or(self, default: T) -> T {
1059 match self {
1060 Ok(t) => t,
1061 Err(_) => default,
1062 }
1063 }
1064
1065 /// Returns the contained [`Ok`] value or computes it from a closure.
1066 ///
1067 ///
1068 /// # Examples
1069 ///
1070 /// Basic usage:
1071 ///
1072 /// ```
1073 /// fn count(x: &str) -> usize { x.len() }
1074 ///
1075 /// assert_eq!(Ok(2).unwrap_or_else(count), 2);
1076 /// assert_eq!(Err("foo").unwrap_or_else(count), 3);
1077 /// ```
1078 #[inline]
1079 #[stable(feature = "rust1", since = "1.0.0")]
unwrap_or_else<F: FnOnce(E) -> T>(self, op: F) -> T1080 pub fn unwrap_or_else<F: FnOnce(E) -> T>(self, op: F) -> T {
1081 match self {
1082 Ok(t) => t,
1083 Err(e) => op(e),
1084 }
1085 }
1086
1087 /// Returns the contained [`Ok`] value, consuming the `self` value,
1088 /// without checking that the value is not an [`Err`].
1089 ///
1090 /// # Safety
1091 ///
1092 /// Calling this method on an [`Err`] is *[undefined behavior]*.
1093 ///
1094 /// [undefined behavior]: https://doc.rust-lang.org/reference/behavior-considered-undefined.html
1095 ///
1096 /// # Examples
1097 ///
1098 /// ```
1099 /// let x: Result<u32, &str> = Ok(2);
1100 /// assert_eq!(unsafe { x.unwrap_unchecked() }, 2);
1101 /// ```
1102 ///
1103 /// ```no_run
1104 /// let x: Result<u32, &str> = Err("emergency failure");
1105 /// unsafe { x.unwrap_unchecked(); } // Undefined behavior!
1106 /// ```
1107 #[inline]
1108 #[track_caller]
1109 #[stable(feature = "option_result_unwrap_unchecked", since = "1.58.0")]
unwrap_unchecked(self) -> T1110 pub unsafe fn unwrap_unchecked(self) -> T {
1111 debug_assert!(self.is_ok());
1112 match self {
1113 Ok(t) => t,
1114 // SAFETY: the safety contract must be upheld by the caller.
1115 Err(_) => unsafe { hint::unreachable_unchecked() },
1116 }
1117 }
1118
1119 /// Returns the contained [`Err`] value, consuming the `self` value,
1120 /// without checking that the value is not an [`Ok`].
1121 ///
1122 /// # Safety
1123 ///
1124 /// Calling this method on an [`Ok`] is *[undefined behavior]*.
1125 ///
1126 /// [undefined behavior]: https://doc.rust-lang.org/reference/behavior-considered-undefined.html
1127 ///
1128 /// # Examples
1129 ///
1130 /// ```no_run
1131 /// let x: Result<u32, &str> = Ok(2);
1132 /// unsafe { x.unwrap_err_unchecked() }; // Undefined behavior!
1133 /// ```
1134 ///
1135 /// ```
1136 /// let x: Result<u32, &str> = Err("emergency failure");
1137 /// assert_eq!(unsafe { x.unwrap_err_unchecked() }, "emergency failure");
1138 /// ```
1139 #[inline]
1140 #[track_caller]
1141 #[stable(feature = "option_result_unwrap_unchecked", since = "1.58.0")]
unwrap_err_unchecked(self) -> E1142 pub unsafe fn unwrap_err_unchecked(self) -> E {
1143 debug_assert!(self.is_err());
1144 match self {
1145 // SAFETY: the safety contract must be upheld by the caller.
1146 Ok(_) => unsafe { hint::unreachable_unchecked() },
1147 Err(e) => e,
1148 }
1149 }
1150 }
1151
1152 impl<T: Copy, E> Result<&T, E> {
1153 /// Maps a `Result<&T, E>` to a `Result<T, E>` by copying the contents of the
1154 /// `Ok` part.
1155 ///
1156 /// # Examples
1157 ///
1158 /// ```
1159 /// #![feature(result_copied)]
1160 /// let val = 12;
1161 /// let x: Result<&i32, i32> = Ok(&val);
1162 /// assert_eq!(x, Ok(&12));
1163 /// let copied = x.copied();
1164 /// assert_eq!(copied, Ok(12));
1165 /// ```
1166 #[unstable(feature = "result_copied", reason = "newly added", issue = "63168")]
copied(self) -> Result<T, E>1167 pub fn copied(self) -> Result<T, E> {
1168 self.map(|&t| t)
1169 }
1170 }
1171
1172 impl<T: Copy, E> Result<&mut T, E> {
1173 /// Maps a `Result<&mut T, E>` to a `Result<T, E>` by copying the contents of the
1174 /// `Ok` part.
1175 ///
1176 /// # Examples
1177 ///
1178 /// ```
1179 /// #![feature(result_copied)]
1180 /// let mut val = 12;
1181 /// let x: Result<&mut i32, i32> = Ok(&mut val);
1182 /// assert_eq!(x, Ok(&mut 12));
1183 /// let copied = x.copied();
1184 /// assert_eq!(copied, Ok(12));
1185 /// ```
1186 #[unstable(feature = "result_copied", reason = "newly added", issue = "63168")]
copied(self) -> Result<T, E>1187 pub fn copied(self) -> Result<T, E> {
1188 self.map(|&mut t| t)
1189 }
1190 }
1191
1192 impl<T: Clone, E> Result<&T, E> {
1193 /// Maps a `Result<&T, E>` to a `Result<T, E>` by cloning the contents of the
1194 /// `Ok` part.
1195 ///
1196 /// # Examples
1197 ///
1198 /// ```
1199 /// #![feature(result_cloned)]
1200 /// let val = 12;
1201 /// let x: Result<&i32, i32> = Ok(&val);
1202 /// assert_eq!(x, Ok(&12));
1203 /// let cloned = x.cloned();
1204 /// assert_eq!(cloned, Ok(12));
1205 /// ```
1206 #[unstable(feature = "result_cloned", reason = "newly added", issue = "63168")]
cloned(self) -> Result<T, E>1207 pub fn cloned(self) -> Result<T, E> {
1208 self.map(|t| t.clone())
1209 }
1210 }
1211
1212 impl<T: Clone, E> Result<&mut T, E> {
1213 /// Maps a `Result<&mut T, E>` to a `Result<T, E>` by cloning the contents of the
1214 /// `Ok` part.
1215 ///
1216 /// # Examples
1217 ///
1218 /// ```
1219 /// #![feature(result_cloned)]
1220 /// let mut val = 12;
1221 /// let x: Result<&mut i32, i32> = Ok(&mut val);
1222 /// assert_eq!(x, Ok(&mut 12));
1223 /// let cloned = x.cloned();
1224 /// assert_eq!(cloned, Ok(12));
1225 /// ```
1226 #[unstable(feature = "result_cloned", reason = "newly added", issue = "63168")]
cloned(self) -> Result<T, E>1227 pub fn cloned(self) -> Result<T, E> {
1228 self.map(|t| t.clone())
1229 }
1230 }
1231
1232 impl<T, E: fmt::Debug> Result<T, E> {
1233 /// Returns the contained [`Ok`] value, consuming the `self` value.
1234 ///
1235 /// # Panics
1236 ///
1237 /// Panics if the value is an [`Err`], with a panic message including the
1238 /// passed message, and the content of the [`Err`].
1239 ///
1240 ///
1241 /// # Examples
1242 ///
1243 /// Basic usage:
1244 ///
1245 /// ```should_panic
1246 /// let x: Result<u32, &str> = Err("emergency failure");
1247 /// x.expect("Testing expect"); // panics with `Testing expect: emergency failure`
1248 /// ```
1249 #[inline]
1250 #[track_caller]
1251 #[stable(feature = "result_expect", since = "1.4.0")]
expect(self, msg: &str) -> T1252 pub fn expect(self, msg: &str) -> T {
1253 match self {
1254 Ok(t) => t,
1255 Err(e) => unwrap_failed(msg, &e),
1256 }
1257 }
1258
1259 /// Returns the contained [`Ok`] value, consuming the `self` value.
1260 ///
1261 /// Because this function may panic, its use is generally discouraged.
1262 /// Instead, prefer to use pattern matching and handle the [`Err`]
1263 /// case explicitly, or call [`unwrap_or`], [`unwrap_or_else`], or
1264 /// [`unwrap_or_default`].
1265 ///
1266 /// [`unwrap_or`]: Result::unwrap_or
1267 /// [`unwrap_or_else`]: Result::unwrap_or_else
1268 /// [`unwrap_or_default`]: Result::unwrap_or_default
1269 ///
1270 /// # Panics
1271 ///
1272 /// Panics if the value is an [`Err`], with a panic message provided by the
1273 /// [`Err`]'s value.
1274 ///
1275 ///
1276 /// # Examples
1277 ///
1278 /// Basic usage:
1279 ///
1280 /// ```
1281 /// let x: Result<u32, &str> = Ok(2);
1282 /// assert_eq!(x.unwrap(), 2);
1283 /// ```
1284 ///
1285 /// ```should_panic
1286 /// let x: Result<u32, &str> = Err("emergency failure");
1287 /// x.unwrap(); // panics with `emergency failure`
1288 /// ```
1289 #[inline]
1290 #[track_caller]
1291 #[stable(feature = "rust1", since = "1.0.0")]
unwrap(self) -> T1292 pub fn unwrap(self) -> T {
1293 match self {
1294 Ok(t) => t,
1295 Err(e) => unwrap_failed("called `Result::unwrap()` on an `Err` value", &e),
1296 }
1297 }
1298 }
1299
1300 impl<T: fmt::Debug, E> Result<T, E> {
1301 /// Returns the contained [`Err`] value, consuming the `self` value.
1302 ///
1303 /// # Panics
1304 ///
1305 /// Panics if the value is an [`Ok`], with a panic message including the
1306 /// passed message, and the content of the [`Ok`].
1307 ///
1308 ///
1309 /// # Examples
1310 ///
1311 /// Basic usage:
1312 ///
1313 /// ```should_panic
1314 /// let x: Result<u32, &str> = Ok(10);
1315 /// x.expect_err("Testing expect_err"); // panics with `Testing expect_err: 10`
1316 /// ```
1317 #[inline]
1318 #[track_caller]
1319 #[stable(feature = "result_expect_err", since = "1.17.0")]
expect_err(self, msg: &str) -> E1320 pub fn expect_err(self, msg: &str) -> E {
1321 match self {
1322 Ok(t) => unwrap_failed(msg, &t),
1323 Err(e) => e,
1324 }
1325 }
1326
1327 /// Returns the contained [`Err`] value, consuming the `self` value.
1328 ///
1329 /// # Panics
1330 ///
1331 /// Panics if the value is an [`Ok`], with a custom panic message provided
1332 /// by the [`Ok`]'s value.
1333 ///
1334 /// # Examples
1335 ///
1336 /// ```should_panic
1337 /// let x: Result<u32, &str> = Ok(2);
1338 /// x.unwrap_err(); // panics with `2`
1339 /// ```
1340 ///
1341 /// ```
1342 /// let x: Result<u32, &str> = Err("emergency failure");
1343 /// assert_eq!(x.unwrap_err(), "emergency failure");
1344 /// ```
1345 #[inline]
1346 #[track_caller]
1347 #[stable(feature = "rust1", since = "1.0.0")]
unwrap_err(self) -> E1348 pub fn unwrap_err(self) -> E {
1349 match self {
1350 Ok(t) => unwrap_failed("called `Result::unwrap_err()` on an `Ok` value", &t),
1351 Err(e) => e,
1352 }
1353 }
1354 }
1355
1356 impl<T: Default, E> Result<T, E> {
1357 /// Returns the contained [`Ok`] value or a default
1358 ///
1359 /// Consumes the `self` argument then, if [`Ok`], returns the contained
1360 /// value, otherwise if [`Err`], returns the default value for that
1361 /// type.
1362 ///
1363 /// # Examples
1364 ///
1365 /// Converts a string to an integer, turning poorly-formed strings
1366 /// into 0 (the default value for integers). [`parse`] converts
1367 /// a string to any other type that implements [`FromStr`], returning an
1368 /// [`Err`] on error.
1369 ///
1370 /// ```
1371 /// let good_year_from_input = "1909";
1372 /// let bad_year_from_input = "190blarg";
1373 /// let good_year = good_year_from_input.parse().unwrap_or_default();
1374 /// let bad_year = bad_year_from_input.parse().unwrap_or_default();
1375 ///
1376 /// assert_eq!(1909, good_year);
1377 /// assert_eq!(0, bad_year);
1378 /// ```
1379 ///
1380 /// [`parse`]: str::parse
1381 /// [`FromStr`]: crate::str::FromStr
1382 #[inline]
1383 #[stable(feature = "result_unwrap_or_default", since = "1.16.0")]
unwrap_or_default(self) -> T1384 pub fn unwrap_or_default(self) -> T {
1385 match self {
1386 Ok(x) => x,
1387 Err(_) => Default::default(),
1388 }
1389 }
1390 }
1391
1392 #[unstable(feature = "unwrap_infallible", reason = "newly added", issue = "61695")]
1393 impl<T, E: Into<!>> Result<T, E> {
1394 /// Returns the contained [`Ok`] value, but never panics.
1395 ///
1396 /// Unlike [`unwrap`], this method is known to never panic on the
1397 /// result types it is implemented for. Therefore, it can be used
1398 /// instead of `unwrap` as a maintainability safeguard that will fail
1399 /// to compile if the error type of the `Result` is later changed
1400 /// to an error that can actually occur.
1401 ///
1402 /// [`unwrap`]: Result::unwrap
1403 ///
1404 /// # Examples
1405 ///
1406 /// Basic usage:
1407 ///
1408 /// ```
1409 /// # #![feature(never_type)]
1410 /// # #![feature(unwrap_infallible)]
1411 ///
1412 /// fn only_good_news() -> Result<String, !> {
1413 /// Ok("this is fine".into())
1414 /// }
1415 ///
1416 /// let s: String = only_good_news().into_ok();
1417 /// println!("{}", s);
1418 /// ```
1419 #[inline]
into_ok(self) -> T1420 pub fn into_ok(self) -> T {
1421 match self {
1422 Ok(x) => x,
1423 Err(e) => e.into(),
1424 }
1425 }
1426 }
1427
1428 #[unstable(feature = "unwrap_infallible", reason = "newly added", issue = "61695")]
1429 impl<T: Into<!>, E> Result<T, E> {
1430 /// Returns the contained [`Err`] value, but never panics.
1431 ///
1432 /// Unlike [`unwrap_err`], this method is known to never panic on the
1433 /// result types it is implemented for. Therefore, it can be used
1434 /// instead of `unwrap_err` as a maintainability safeguard that will fail
1435 /// to compile if the ok type of the `Result` is later changed
1436 /// to a type that can actually occur.
1437 ///
1438 /// [`unwrap_err`]: Result::unwrap_err
1439 ///
1440 /// # Examples
1441 ///
1442 /// Basic usage:
1443 ///
1444 /// ```
1445 /// # #![feature(never_type)]
1446 /// # #![feature(unwrap_infallible)]
1447 ///
1448 /// fn only_bad_news() -> Result<!, String> {
1449 /// Err("Oops, it failed".into())
1450 /// }
1451 ///
1452 /// let error: String = only_bad_news().into_err();
1453 /// println!("{}", error);
1454 /// ```
1455 #[inline]
into_err(self) -> E1456 pub fn into_err(self) -> E {
1457 match self {
1458 Ok(x) => x.into(),
1459 Err(e) => e,
1460 }
1461 }
1462 }
1463
1464 impl<T: Deref, E> Result<T, E> {
1465 /// Converts from `Result<T, E>` (or `&Result<T, E>`) to `Result<&<T as Deref>::Target, &E>`.
1466 ///
1467 /// Coerces the [`Ok`] variant of the original [`Result`] via [`Deref`](crate::ops::Deref)
1468 /// and returns the new [`Result`].
1469 ///
1470 /// # Examples
1471 ///
1472 /// ```
1473 /// let x: Result<String, u32> = Ok("hello".to_string());
1474 /// let y: Result<&str, &u32> = Ok("hello");
1475 /// assert_eq!(x.as_deref(), y);
1476 ///
1477 /// let x: Result<String, u32> = Err(42);
1478 /// let y: Result<&str, &u32> = Err(&42);
1479 /// assert_eq!(x.as_deref(), y);
1480 /// ```
1481 #[stable(feature = "inner_deref", since = "1.47.0")]
as_deref(&self) -> Result<&T::Target, &E>1482 pub fn as_deref(&self) -> Result<&T::Target, &E> {
1483 self.as_ref().map(|t| t.deref())
1484 }
1485 }
1486
1487 impl<T: DerefMut, E> Result<T, E> {
1488 /// Converts from `Result<T, E>` (or `&mut Result<T, E>`) to `Result<&mut <T as DerefMut>::Target, &mut E>`.
1489 ///
1490 /// Coerces the [`Ok`] variant of the original [`Result`] via [`DerefMut`](crate::ops::DerefMut)
1491 /// and returns the new [`Result`].
1492 ///
1493 /// # Examples
1494 ///
1495 /// ```
1496 /// let mut s = "HELLO".to_string();
1497 /// let mut x: Result<String, u32> = Ok("hello".to_string());
1498 /// let y: Result<&mut str, &mut u32> = Ok(&mut s);
1499 /// assert_eq!(x.as_deref_mut().map(|x| { x.make_ascii_uppercase(); x }), y);
1500 ///
1501 /// let mut i = 42;
1502 /// let mut x: Result<String, u32> = Err(42);
1503 /// let y: Result<&mut str, &mut u32> = Err(&mut i);
1504 /// assert_eq!(x.as_deref_mut().map(|x| { x.make_ascii_uppercase(); x }), y);
1505 /// ```
1506 #[stable(feature = "inner_deref", since = "1.47.0")]
as_deref_mut(&mut self) -> Result<&mut T::Target, &mut E>1507 pub fn as_deref_mut(&mut self) -> Result<&mut T::Target, &mut E> {
1508 self.as_mut().map(|t| t.deref_mut())
1509 }
1510 }
1511
1512 impl<T, E> Result<Option<T>, E> {
1513 /// Transposes a `Result` of an `Option` into an `Option` of a `Result`.
1514 ///
1515 /// `Ok(None)` will be mapped to `None`.
1516 /// `Ok(Some(_))` and `Err(_)` will be mapped to `Some(Ok(_))` and `Some(Err(_))`.
1517 ///
1518 /// # Examples
1519 ///
1520 /// ```
1521 /// #[derive(Debug, Eq, PartialEq)]
1522 /// struct SomeErr;
1523 ///
1524 /// let x: Result<Option<i32>, SomeErr> = Ok(Some(5));
1525 /// let y: Option<Result<i32, SomeErr>> = Some(Ok(5));
1526 /// assert_eq!(x.transpose(), y);
1527 /// ```
1528 #[inline]
1529 #[stable(feature = "transpose_result", since = "1.33.0")]
1530 #[rustc_const_unstable(feature = "const_result", issue = "82814")]
transpose(self) -> Option<Result<T, E>>1531 pub const fn transpose(self) -> Option<Result<T, E>> {
1532 match self {
1533 Ok(Some(x)) => Some(Ok(x)),
1534 Ok(None) => None,
1535 Err(e) => Some(Err(e)),
1536 }
1537 }
1538 }
1539
1540 impl<T, E> Result<Result<T, E>, E> {
1541 /// Converts from `Result<Result<T, E>, E>` to `Result<T, E>`
1542 ///
1543 /// # Examples
1544 ///
1545 /// Basic usage:
1546 ///
1547 /// ```
1548 /// #![feature(result_flattening)]
1549 /// let x: Result<Result<&'static str, u32>, u32> = Ok(Ok("hello"));
1550 /// assert_eq!(Ok("hello"), x.flatten());
1551 ///
1552 /// let x: Result<Result<&'static str, u32>, u32> = Ok(Err(6));
1553 /// assert_eq!(Err(6), x.flatten());
1554 ///
1555 /// let x: Result<Result<&'static str, u32>, u32> = Err(6);
1556 /// assert_eq!(Err(6), x.flatten());
1557 /// ```
1558 ///
1559 /// Flattening only removes one level of nesting at a time:
1560 ///
1561 /// ```
1562 /// #![feature(result_flattening)]
1563 /// let x: Result<Result<Result<&'static str, u32>, u32>, u32> = Ok(Ok(Ok("hello")));
1564 /// assert_eq!(Ok(Ok("hello")), x.flatten());
1565 /// assert_eq!(Ok("hello"), x.flatten().flatten());
1566 /// ```
1567 #[inline]
1568 #[unstable(feature = "result_flattening", issue = "70142")]
flatten(self) -> Result<T, E>1569 pub fn flatten(self) -> Result<T, E> {
1570 self.and_then(convert::identity)
1571 }
1572 }
1573
1574 impl<T> Result<T, T> {
1575 /// Returns the [`Ok`] value if `self` is `Ok`, and the [`Err`] value if
1576 /// `self` is `Err`.
1577 ///
1578 /// In other words, this function returns the value (the `T`) of a
1579 /// `Result<T, T>`, regardless of whether or not that result is `Ok` or
1580 /// `Err`.
1581 ///
1582 /// This can be useful in conjunction with APIs such as
1583 /// [`Atomic*::compare_exchange`], or [`slice::binary_search`], but only in
1584 /// cases where you don't care if the result was `Ok` or not.
1585 ///
1586 /// [`Atomic*::compare_exchange`]: crate::sync::atomic::AtomicBool::compare_exchange
1587 ///
1588 /// # Examples
1589 ///
1590 /// ```
1591 /// #![feature(result_into_ok_or_err)]
1592 /// let ok: Result<u32, u32> = Ok(3);
1593 /// let err: Result<u32, u32> = Err(4);
1594 ///
1595 /// assert_eq!(ok.into_ok_or_err(), 3);
1596 /// assert_eq!(err.into_ok_or_err(), 4);
1597 /// ```
1598 #[inline]
1599 #[unstable(feature = "result_into_ok_or_err", reason = "newly added", issue = "82223")]
into_ok_or_err(self) -> T1600 pub const fn into_ok_or_err(self) -> T {
1601 match self {
1602 Ok(v) => v,
1603 Err(v) => v,
1604 }
1605 }
1606 }
1607
1608 // This is a separate function to reduce the code size of the methods
1609 #[inline(never)]
1610 #[cold]
1611 #[track_caller]
unwrap_failed(msg: &str, error: &dyn fmt::Debug) -> !1612 fn unwrap_failed(msg: &str, error: &dyn fmt::Debug) -> ! {
1613 panic!("{}: {:?}", msg, error)
1614 }
1615
1616 /////////////////////////////////////////////////////////////////////////////
1617 // Trait implementations
1618 /////////////////////////////////////////////////////////////////////////////
1619
1620 #[stable(feature = "rust1", since = "1.0.0")]
1621 impl<T: Clone, E: Clone> Clone for Result<T, E> {
1622 #[inline]
clone(&self) -> Self1623 fn clone(&self) -> Self {
1624 match self {
1625 Ok(x) => Ok(x.clone()),
1626 Err(x) => Err(x.clone()),
1627 }
1628 }
1629
1630 #[inline]
clone_from(&mut self, source: &Self)1631 fn clone_from(&mut self, source: &Self) {
1632 match (self, source) {
1633 (Ok(to), Ok(from)) => to.clone_from(from),
1634 (Err(to), Err(from)) => to.clone_from(from),
1635 (to, from) => *to = from.clone(),
1636 }
1637 }
1638 }
1639
1640 #[stable(feature = "rust1", since = "1.0.0")]
1641 impl<T, E> IntoIterator for Result<T, E> {
1642 type Item = T;
1643 type IntoIter = IntoIter<T>;
1644
1645 /// Returns a consuming iterator over the possibly contained value.
1646 ///
1647 /// The iterator yields one value if the result is [`Result::Ok`], otherwise none.
1648 ///
1649 /// # Examples
1650 ///
1651 /// Basic usage:
1652 ///
1653 /// ```
1654 /// let x: Result<u32, &str> = Ok(5);
1655 /// let v: Vec<u32> = x.into_iter().collect();
1656 /// assert_eq!(v, [5]);
1657 ///
1658 /// let x: Result<u32, &str> = Err("nothing!");
1659 /// let v: Vec<u32> = x.into_iter().collect();
1660 /// assert_eq!(v, []);
1661 /// ```
1662 #[inline]
into_iter(self) -> IntoIter<T>1663 fn into_iter(self) -> IntoIter<T> {
1664 IntoIter { inner: self.ok() }
1665 }
1666 }
1667
1668 #[stable(since = "1.4.0", feature = "result_iter")]
1669 impl<'a, T, E> IntoIterator for &'a Result<T, E> {
1670 type Item = &'a T;
1671 type IntoIter = Iter<'a, T>;
1672
into_iter(self) -> Iter<'a, T>1673 fn into_iter(self) -> Iter<'a, T> {
1674 self.iter()
1675 }
1676 }
1677
1678 #[stable(since = "1.4.0", feature = "result_iter")]
1679 impl<'a, T, E> IntoIterator for &'a mut Result<T, E> {
1680 type Item = &'a mut T;
1681 type IntoIter = IterMut<'a, T>;
1682
into_iter(self) -> IterMut<'a, T>1683 fn into_iter(self) -> IterMut<'a, T> {
1684 self.iter_mut()
1685 }
1686 }
1687
1688 /////////////////////////////////////////////////////////////////////////////
1689 // The Result Iterators
1690 /////////////////////////////////////////////////////////////////////////////
1691
1692 /// An iterator over a reference to the [`Ok`] variant of a [`Result`].
1693 ///
1694 /// The iterator yields one value if the result is [`Ok`], otherwise none.
1695 ///
1696 /// Created by [`Result::iter`].
1697 #[derive(Debug)]
1698 #[stable(feature = "rust1", since = "1.0.0")]
1699 pub struct Iter<'a, T: 'a> {
1700 inner: Option<&'a T>,
1701 }
1702
1703 #[stable(feature = "rust1", since = "1.0.0")]
1704 impl<'a, T> Iterator for Iter<'a, T> {
1705 type Item = &'a T;
1706
1707 #[inline]
next(&mut self) -> Option<&'a T>1708 fn next(&mut self) -> Option<&'a T> {
1709 self.inner.take()
1710 }
1711 #[inline]
size_hint(&self) -> (usize, Option<usize>)1712 fn size_hint(&self) -> (usize, Option<usize>) {
1713 let n = if self.inner.is_some() { 1 } else { 0 };
1714 (n, Some(n))
1715 }
1716 }
1717
1718 #[stable(feature = "rust1", since = "1.0.0")]
1719 impl<'a, T> DoubleEndedIterator for Iter<'a, T> {
1720 #[inline]
next_back(&mut self) -> Option<&'a T>1721 fn next_back(&mut self) -> Option<&'a T> {
1722 self.inner.take()
1723 }
1724 }
1725
1726 #[stable(feature = "rust1", since = "1.0.0")]
1727 impl<T> ExactSizeIterator for Iter<'_, T> {}
1728
1729 #[stable(feature = "fused", since = "1.26.0")]
1730 impl<T> FusedIterator for Iter<'_, T> {}
1731
1732 #[unstable(feature = "trusted_len", issue = "37572")]
1733 unsafe impl<A> TrustedLen for Iter<'_, A> {}
1734
1735 #[stable(feature = "rust1", since = "1.0.0")]
1736 impl<T> Clone for Iter<'_, T> {
1737 #[inline]
clone(&self) -> Self1738 fn clone(&self) -> Self {
1739 Iter { inner: self.inner }
1740 }
1741 }
1742
1743 /// An iterator over a mutable reference to the [`Ok`] variant of a [`Result`].
1744 ///
1745 /// Created by [`Result::iter_mut`].
1746 #[derive(Debug)]
1747 #[stable(feature = "rust1", since = "1.0.0")]
1748 pub struct IterMut<'a, T: 'a> {
1749 inner: Option<&'a mut T>,
1750 }
1751
1752 #[stable(feature = "rust1", since = "1.0.0")]
1753 impl<'a, T> Iterator for IterMut<'a, T> {
1754 type Item = &'a mut T;
1755
1756 #[inline]
next(&mut self) -> Option<&'a mut T>1757 fn next(&mut self) -> Option<&'a mut T> {
1758 self.inner.take()
1759 }
1760 #[inline]
size_hint(&self) -> (usize, Option<usize>)1761 fn size_hint(&self) -> (usize, Option<usize>) {
1762 let n = if self.inner.is_some() { 1 } else { 0 };
1763 (n, Some(n))
1764 }
1765 }
1766
1767 #[stable(feature = "rust1", since = "1.0.0")]
1768 impl<'a, T> DoubleEndedIterator for IterMut<'a, T> {
1769 #[inline]
next_back(&mut self) -> Option<&'a mut T>1770 fn next_back(&mut self) -> Option<&'a mut T> {
1771 self.inner.take()
1772 }
1773 }
1774
1775 #[stable(feature = "rust1", since = "1.0.0")]
1776 impl<T> ExactSizeIterator for IterMut<'_, T> {}
1777
1778 #[stable(feature = "fused", since = "1.26.0")]
1779 impl<T> FusedIterator for IterMut<'_, T> {}
1780
1781 #[unstable(feature = "trusted_len", issue = "37572")]
1782 unsafe impl<A> TrustedLen for IterMut<'_, A> {}
1783
1784 /// An iterator over the value in a [`Ok`] variant of a [`Result`].
1785 ///
1786 /// The iterator yields one value if the result is [`Ok`], otherwise none.
1787 ///
1788 /// This struct is created by the [`into_iter`] method on
1789 /// [`Result`] (provided by the [`IntoIterator`] trait).
1790 ///
1791 /// [`into_iter`]: IntoIterator::into_iter
1792 #[derive(Clone, Debug)]
1793 #[stable(feature = "rust1", since = "1.0.0")]
1794 pub struct IntoIter<T> {
1795 inner: Option<T>,
1796 }
1797
1798 #[stable(feature = "rust1", since = "1.0.0")]
1799 impl<T> Iterator for IntoIter<T> {
1800 type Item = T;
1801
1802 #[inline]
next(&mut self) -> Option<T>1803 fn next(&mut self) -> Option<T> {
1804 self.inner.take()
1805 }
1806 #[inline]
size_hint(&self) -> (usize, Option<usize>)1807 fn size_hint(&self) -> (usize, Option<usize>) {
1808 let n = if self.inner.is_some() { 1 } else { 0 };
1809 (n, Some(n))
1810 }
1811 }
1812
1813 #[stable(feature = "rust1", since = "1.0.0")]
1814 impl<T> DoubleEndedIterator for IntoIter<T> {
1815 #[inline]
next_back(&mut self) -> Option<T>1816 fn next_back(&mut self) -> Option<T> {
1817 self.inner.take()
1818 }
1819 }
1820
1821 #[stable(feature = "rust1", since = "1.0.0")]
1822 impl<T> ExactSizeIterator for IntoIter<T> {}
1823
1824 #[stable(feature = "fused", since = "1.26.0")]
1825 impl<T> FusedIterator for IntoIter<T> {}
1826
1827 #[unstable(feature = "trusted_len", issue = "37572")]
1828 unsafe impl<A> TrustedLen for IntoIter<A> {}
1829
1830 /////////////////////////////////////////////////////////////////////////////
1831 // FromIterator
1832 /////////////////////////////////////////////////////////////////////////////
1833
1834 #[stable(feature = "rust1", since = "1.0.0")]
1835 impl<A, E, V: FromIterator<A>> FromIterator<Result<A, E>> for Result<V, E> {
1836 /// Takes each element in the `Iterator`: if it is an `Err`, no further
1837 /// elements are taken, and the `Err` is returned. Should no `Err` occur, a
1838 /// container with the values of each `Result` is returned.
1839 ///
1840 /// Here is an example which increments every integer in a vector,
1841 /// checking for overflow:
1842 ///
1843 /// ```
1844 /// let v = vec![1, 2];
1845 /// let res: Result<Vec<u32>, &'static str> = v.iter().map(|x: &u32|
1846 /// x.checked_add(1).ok_or("Overflow!")
1847 /// ).collect();
1848 /// assert_eq!(res, Ok(vec![2, 3]));
1849 /// ```
1850 ///
1851 /// Here is another example that tries to subtract one from another list
1852 /// of integers, this time checking for underflow:
1853 ///
1854 /// ```
1855 /// let v = vec![1, 2, 0];
1856 /// let res: Result<Vec<u32>, &'static str> = v.iter().map(|x: &u32|
1857 /// x.checked_sub(1).ok_or("Underflow!")
1858 /// ).collect();
1859 /// assert_eq!(res, Err("Underflow!"));
1860 /// ```
1861 ///
1862 /// Here is a variation on the previous example, showing that no
1863 /// further elements are taken from `iter` after the first `Err`.
1864 ///
1865 /// ```
1866 /// let v = vec![3, 2, 1, 10];
1867 /// let mut shared = 0;
1868 /// let res: Result<Vec<u32>, &'static str> = v.iter().map(|x: &u32| {
1869 /// shared += x;
1870 /// x.checked_sub(2).ok_or("Underflow!")
1871 /// }).collect();
1872 /// assert_eq!(res, Err("Underflow!"));
1873 /// assert_eq!(shared, 6);
1874 /// ```
1875 ///
1876 /// Since the third element caused an underflow, no further elements were taken,
1877 /// so the final value of `shared` is 6 (= `3 + 2 + 1`), not 16.
1878 #[inline]
from_iter<I: IntoIterator<Item = Result<A, E>>>(iter: I) -> Result<V, E>1879 fn from_iter<I: IntoIterator<Item = Result<A, E>>>(iter: I) -> Result<V, E> {
1880 // FIXME(#11084): This could be replaced with Iterator::scan when this
1881 // performance bug is closed.
1882
1883 iter::process_results(iter.into_iter(), |i| i.collect())
1884 }
1885 }
1886
1887 #[unstable(feature = "try_trait_v2", issue = "84277")]
1888 impl<T, E> ops::Try for Result<T, E> {
1889 type Output = T;
1890 type Residual = Result<convert::Infallible, E>;
1891
1892 #[inline]
from_output(output: Self::Output) -> Self1893 fn from_output(output: Self::Output) -> Self {
1894 Ok(output)
1895 }
1896
1897 #[inline]
branch(self) -> ControlFlow<Self::Residual, Self::Output>1898 fn branch(self) -> ControlFlow<Self::Residual, Self::Output> {
1899 match self {
1900 Ok(v) => ControlFlow::Continue(v),
1901 Err(e) => ControlFlow::Break(Err(e)),
1902 }
1903 }
1904 }
1905
1906 #[unstable(feature = "try_trait_v2", issue = "84277")]
1907 impl<T, E, F: From<E>> ops::FromResidual<Result<convert::Infallible, E>> for Result<T, F> {
1908 #[inline]
from_residual(residual: Result<convert::Infallible, E>) -> Self1909 fn from_residual(residual: Result<convert::Infallible, E>) -> Self {
1910 match residual {
1911 Err(e) => Err(From::from(e)),
1912 }
1913 }
1914 }
1915