1 //! Error handling with the `Result` type.
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.
9 //! # #[allow(dead_code)]
10 //! enum Result<T, E> {
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).
20 //! A simple function returning [`Result`] might be
21 //! defined and used like so:
25 //! enum Version { Version1, Version2 }
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"),
36 //! let version = parse_version(&[1, 2, 3, 4]);
38 //! Ok(v) => println!("working with version: {:?}", v),
39 //! Err(e) => println!("error parsing header: {:?}", e),
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.
48 //! let good_result: Result<i32, i32> = Ok(10);
49 //! let bad_result: Result<i32, i32> = Err(10);
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());
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);
59 //! // Use `and_then` to continue the computation.
60 //! let good_result: Result<bool, i32> = good_result.and_then(|i| Ok(i == 11));
62 //! // Use `or_else` to handle the error.
63 //! let bad_result: Result<i32, i32> = bad_result.or_else(|i| Ok(i + 20));
65 //! // Consume the result and return the contents with `unwrap`.
66 //! let final_awesome_result = good_result.unwrap();
69 //! # Results must be used
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
79 //! Consider the [`write_all`] method defined for I/O types
80 //! by the [`Write`] trait:
86 //! fn write_all(&mut self, bytes: &[u8]) -> Result<(), io::Error>;
90 //! *Note: The actual definition of [`Write`] uses [`io::Result`], which
91 //! is just a synonym for [`Result`]`<T, `[`io::Error`]`>`.*
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:
98 //! # #![allow(unused_must_use)] // \o/
99 //! use std::fs::File;
100 //! use std::io::prelude::*;
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");
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).
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:
116 //! use std::fs::File;
117 //! use std::io::prelude::*;
119 //! let mut file = File::create("valuable_data.txt").unwrap();
120 //! file.write_all(b"important message").expect("failed to write message");
123 //! You might also simply assert success:
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());
132 //! Or propagate the error up the call stack with [`?`]:
135 //! # use std::fs::File;
136 //! # use std::io::prelude::*;
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")?;
146 //! # The question mark operator, `?`
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.
153 //! It replaces this:
156 //! # #![allow(dead_code)]
157 //! use std::fs::File;
158 //! use std::io::prelude::*;
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),
173 //! if let Err(e) = file.write_all(format!("name: {}\n", info.name).as_bytes()) {
176 //! if let Err(e) = file.write_all(format!("age: {}\n", info.age).as_bytes()) {
179 //! if let Err(e) = file.write_all(format!("rating: {}\n", info.rating).as_bytes()) {
189 //! # #![allow(dead_code)]
190 //! use std::fs::File;
191 //! use std::io::prelude::*;
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())?;
210 //! *It's much nicer!*
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.
216 //! [`?`] can only be used in functions that return [`Result`] because of the
217 //! early return of [`Err`] that it provides.
219 //! [`expect`]: Result::expect
220 //! [`Write`]: ../../std/io/trait.Write.html
221 //! [`write_all`]: ../../std/io/trait.Write.html#method.write_all
222 //! [`io::Result`]: ../../std/io/type.Result.html
223 //! [`?`]: crate::ops::Try
226 //! [`io::Error`]: ../../std/io/struct.Error.html
228 #![stable(feature = "rust1", since = "1.0.0")]
230 use crate::iter
::{self, FromIterator, FusedIterator, TrustedLen}
;
231 use crate::ops
::{self, ControlFlow, Deref, DerefMut}
;
232 use crate::{convert, fmt, hint}
;
234 /// `Result` is a type that represents either success ([`Ok`]) or failure ([`Err`]).
236 /// See the [module documentation](self) for details.
237 #[derive(Copy, PartialEq, PartialOrd, Eq, Ord, Debug, Hash)]
238 #[must_use = "this `Result` may be an `Err` variant, which should be handled"]
239 #[rustc_diagnostic_item = "result_type"]
240 #[stable(feature = "rust1", since = "1.0.0")]
241 pub enum Result
<T
, E
> {
242 /// Contains the success value
244 #[stable(feature = "rust1", since = "1.0.0")]
245 Ok(#[stable(feature = "rust1", since = "1.0.0")] T),
247 /// Contains the error value
249 #[stable(feature = "rust1", since = "1.0.0")]
250 Err(#[stable(feature = "rust1", since = "1.0.0")] E),
253 /////////////////////////////////////////////////////////////////////////////
254 // Type implementation
255 /////////////////////////////////////////////////////////////////////////////
257 impl<T
, E
> Result
<T
, E
> {
258 /////////////////////////////////////////////////////////////////////////
259 // Querying the contained values
260 /////////////////////////////////////////////////////////////////////////
262 /// Returns `true` if the result is [`Ok`].
269 /// let x: Result<i32, &str> = Ok(-3);
270 /// assert_eq!(x.is_ok(), true);
272 /// let x: Result<i32, &str> = Err("Some error message");
273 /// assert_eq!(x.is_ok(), false);
275 #[must_use = "if you intended to assert that this is ok, consider `.unwrap()` instead"]
276 #[rustc_const_stable(feature = "const_result", since = "1.48.0")]
278 #[stable(feature = "rust1", since = "1.0.0")]
279 pub const fn is_ok(&self) -> bool
{
280 matches
!(*self, Ok(_
))
283 /// Returns `true` if the result is [`Err`].
290 /// let x: Result<i32, &str> = Ok(-3);
291 /// assert_eq!(x.is_err(), false);
293 /// let x: Result<i32, &str> = Err("Some error message");
294 /// assert_eq!(x.is_err(), true);
296 #[must_use = "if you intended to assert that this is err, consider `.unwrap_err()` instead"]
297 #[rustc_const_stable(feature = "const_result", since = "1.48.0")]
299 #[stable(feature = "rust1", since = "1.0.0")]
300 pub const fn is_err(&self) -> bool
{
304 /// Returns `true` if the result is an [`Ok`] value containing the given value.
309 /// #![feature(option_result_contains)]
311 /// let x: Result<u32, &str> = Ok(2);
312 /// assert_eq!(x.contains(&2), true);
314 /// let x: Result<u32, &str> = Ok(3);
315 /// assert_eq!(x.contains(&2), false);
317 /// let x: Result<u32, &str> = Err("Some error message");
318 /// assert_eq!(x.contains(&2), false);
322 #[unstable(feature = "option_result_contains", issue = "62358")]
323 pub fn contains
<U
>(&self, x
: &U
) -> bool
333 /// Returns `true` if the result is an [`Err`] value containing the given value.
338 /// #![feature(result_contains_err)]
340 /// let x: Result<u32, &str> = Ok(2);
341 /// assert_eq!(x.contains_err(&"Some error message"), false);
343 /// let x: Result<u32, &str> = Err("Some error message");
344 /// assert_eq!(x.contains_err(&"Some error message"), true);
346 /// let x: Result<u32, &str> = Err("Some other error message");
347 /// assert_eq!(x.contains_err(&"Some error message"), false);
351 #[unstable(feature = "result_contains_err", issue = "62358")]
352 pub fn contains_err
<F
>(&self, f
: &F
) -> bool
362 /////////////////////////////////////////////////////////////////////////
363 // Adapter for each variant
364 /////////////////////////////////////////////////////////////////////////
366 /// Converts from `Result<T, E>` to [`Option<T>`].
368 /// Converts `self` into an [`Option<T>`], consuming `self`,
369 /// and discarding the error, if any.
376 /// let x: Result<u32, &str> = Ok(2);
377 /// assert_eq!(x.ok(), Some(2));
379 /// let x: Result<u32, &str> = Err("Nothing here");
380 /// assert_eq!(x.ok(), None);
383 #[stable(feature = "rust1", since = "1.0.0")]
384 pub fn ok(self) -> Option
<T
> {
391 /// Converts from `Result<T, E>` to [`Option<E>`].
393 /// Converts `self` into an [`Option<E>`], consuming `self`,
394 /// and discarding the success value, if any.
401 /// let x: Result<u32, &str> = Ok(2);
402 /// assert_eq!(x.err(), None);
404 /// let x: Result<u32, &str> = Err("Nothing here");
405 /// assert_eq!(x.err(), Some("Nothing here"));
408 #[stable(feature = "rust1", since = "1.0.0")]
409 pub fn err(self) -> Option
<E
> {
416 /////////////////////////////////////////////////////////////////////////
417 // Adapter for working with references
418 /////////////////////////////////////////////////////////////////////////
420 /// Converts from `&Result<T, E>` to `Result<&T, &E>`.
422 /// Produces a new `Result`, containing a reference
423 /// into the original, leaving the original in place.
430 /// let x: Result<u32, &str> = Ok(2);
431 /// assert_eq!(x.as_ref(), Ok(&2));
433 /// let x: Result<u32, &str> = Err("Error");
434 /// assert_eq!(x.as_ref(), Err(&"Error"));
437 #[rustc_const_stable(feature = "const_result", since = "1.48.0")]
438 #[stable(feature = "rust1", since = "1.0.0")]
439 pub const fn as_ref(&self) -> Result
<&T
, &E
> {
442 Err(ref x
) => Err(x
),
446 /// Converts from `&mut Result<T, E>` to `Result<&mut T, &mut E>`.
453 /// fn mutate(r: &mut Result<i32, i32>) {
454 /// match r.as_mut() {
455 /// Ok(v) => *v = 42,
456 /// Err(e) => *e = 0,
460 /// let mut x: Result<i32, i32> = Ok(2);
462 /// assert_eq!(x.unwrap(), 42);
464 /// let mut x: Result<i32, i32> = Err(13);
466 /// assert_eq!(x.unwrap_err(), 0);
469 #[stable(feature = "rust1", since = "1.0.0")]
470 pub fn as_mut(&mut self) -> Result
<&mut T
, &mut E
> {
472 Ok(ref mut x
) => Ok(x
),
473 Err(ref mut x
) => Err(x
),
477 /////////////////////////////////////////////////////////////////////////
478 // Transforming contained values
479 /////////////////////////////////////////////////////////////////////////
481 /// Maps a `Result<T, E>` to `Result<U, E>` by applying a function to a
482 /// contained [`Ok`] value, leaving an [`Err`] value untouched.
484 /// This function can be used to compose the results of two functions.
488 /// Print the numbers on each line of a string multiplied by two.
491 /// let line = "1\n2\n3\n4\n";
493 /// for num in line.lines() {
494 /// match num.parse::<i32>().map(|i| i * 2) {
495 /// Ok(n) => println!("{}", n),
501 #[stable(feature = "rust1", since = "1.0.0")]
502 pub fn map
<U
, F
: FnOnce(T
) -> U
>(self, op
: F
) -> Result
<U
, E
> {
509 /// Returns the provided default (if [`Err`]), or
510 /// applies a function to the contained value (if [`Ok`]),
512 /// Arguments passed to `map_or` are eagerly evaluated; if you are passing
513 /// the result of a function call, it is recommended to use [`map_or_else`],
514 /// which is lazily evaluated.
516 /// [`map_or_else`]: Result::map_or_else
521 /// let x: Result<_, &str> = Ok("foo");
522 /// assert_eq!(x.map_or(42, |v| v.len()), 3);
524 /// let x: Result<&str, _> = Err("bar");
525 /// assert_eq!(x.map_or(42, |v| v.len()), 42);
528 #[stable(feature = "result_map_or", since = "1.41.0")]
529 pub fn map_or
<U
, F
: FnOnce(T
) -> U
>(self, default: U
, f
: F
) -> U
{
536 /// Maps a `Result<T, E>` to `U` by applying a fallback function to a
537 /// contained [`Err`] value, or a default function to a
538 /// contained [`Ok`] value.
540 /// This function can be used to unpack a successful result
541 /// while handling an error.
551 /// let x : Result<_, &str> = Ok("foo");
552 /// assert_eq!(x.map_or_else(|e| k * 2, |v| v.len()), 3);
554 /// let x : Result<&str, _> = Err("bar");
555 /// assert_eq!(x.map_or_else(|e| k * 2, |v| v.len()), 42);
558 #[stable(feature = "result_map_or_else", since = "1.41.0")]
559 pub fn map_or_else
<U
, D
: FnOnce(E
) -> U
, F
: FnOnce(T
) -> U
>(self, default: D
, f
: F
) -> U
{
562 Err(e
) => default(e
),
566 /// Maps a `Result<T, E>` to `Result<T, F>` by applying a function to a
567 /// contained [`Err`] value, leaving an [`Ok`] value untouched.
569 /// This function can be used to pass through a successful result while handling
578 /// fn stringify(x: u32) -> String { format!("error code: {}", x) }
580 /// let x: Result<u32, u32> = Ok(2);
581 /// assert_eq!(x.map_err(stringify), Ok(2));
583 /// let x: Result<u32, u32> = Err(13);
584 /// assert_eq!(x.map_err(stringify), Err("error code: 13".to_string()));
587 #[stable(feature = "rust1", since = "1.0.0")]
588 pub fn map_err
<F
, O
: FnOnce(E
) -> F
>(self, op
: O
) -> Result
<T
, F
> {
591 Err(e
) => Err(op(e
)),
595 /////////////////////////////////////////////////////////////////////////
596 // Iterator constructors
597 /////////////////////////////////////////////////////////////////////////
599 /// Returns an iterator over the possibly contained value.
601 /// The iterator yields one value if the result is [`Result::Ok`], otherwise none.
608 /// let x: Result<u32, &str> = Ok(7);
609 /// assert_eq!(x.iter().next(), Some(&7));
611 /// let x: Result<u32, &str> = Err("nothing!");
612 /// assert_eq!(x.iter().next(), None);
615 #[stable(feature = "rust1", since = "1.0.0")]
616 pub fn iter(&self) -> Iter
<'_
, T
> {
617 Iter { inner: self.as_ref().ok() }
620 /// Returns a mutable iterator over the possibly contained value.
622 /// The iterator yields one value if the result is [`Result::Ok`], otherwise none.
629 /// let mut x: Result<u32, &str> = Ok(7);
630 /// match x.iter_mut().next() {
631 /// Some(v) => *v = 40,
634 /// assert_eq!(x, Ok(40));
636 /// let mut x: Result<u32, &str> = Err("nothing!");
637 /// assert_eq!(x.iter_mut().next(), None);
640 #[stable(feature = "rust1", since = "1.0.0")]
641 pub fn iter_mut(&mut self) -> IterMut
<'_
, T
> {
642 IterMut { inner: self.as_mut().ok() }
645 ////////////////////////////////////////////////////////////////////////
646 // Boolean operations on the values, eager and lazy
647 /////////////////////////////////////////////////////////////////////////
649 /// Returns `res` if the result is [`Ok`], otherwise returns the [`Err`] value of `self`.
657 /// let x: Result<u32, &str> = Ok(2);
658 /// let y: Result<&str, &str> = Err("late error");
659 /// assert_eq!(x.and(y), Err("late error"));
661 /// let x: Result<u32, &str> = Err("early error");
662 /// let y: Result<&str, &str> = Ok("foo");
663 /// assert_eq!(x.and(y), Err("early error"));
665 /// let x: Result<u32, &str> = Err("not a 2");
666 /// let y: Result<&str, &str> = Err("late error");
667 /// assert_eq!(x.and(y), Err("not a 2"));
669 /// let x: Result<u32, &str> = Ok(2);
670 /// let y: Result<&str, &str> = Ok("different result type");
671 /// assert_eq!(x.and(y), Ok("different result type"));
674 #[stable(feature = "rust1", since = "1.0.0")]
675 pub fn and
<U
>(self, res
: Result
<U
, E
>) -> Result
<U
, E
> {
682 /// Calls `op` if the result is [`Ok`], otherwise returns the [`Err`] value of `self`.
685 /// This function can be used for control flow based on `Result` values.
692 /// fn sq(x: u32) -> Result<u32, u32> { Ok(x * x) }
693 /// fn err(x: u32) -> Result<u32, u32> { Err(x) }
695 /// assert_eq!(Ok(2).and_then(sq).and_then(sq), Ok(16));
696 /// assert_eq!(Ok(2).and_then(sq).and_then(err), Err(4));
697 /// assert_eq!(Ok(2).and_then(err).and_then(sq), Err(2));
698 /// assert_eq!(Err(3).and_then(sq).and_then(sq), Err(3));
701 #[stable(feature = "rust1", since = "1.0.0")]
702 pub fn and_then
<U
, F
: FnOnce(T
) -> Result
<U
, E
>>(self, op
: F
) -> Result
<U
, E
> {
709 /// Returns `res` if the result is [`Err`], otherwise returns the [`Ok`] value of `self`.
711 /// Arguments passed to `or` are eagerly evaluated; if you are passing the
712 /// result of a function call, it is recommended to use [`or_else`], which is
713 /// lazily evaluated.
715 /// [`or_else`]: Result::or_else
722 /// let x: Result<u32, &str> = Ok(2);
723 /// let y: Result<u32, &str> = Err("late error");
724 /// assert_eq!(x.or(y), Ok(2));
726 /// let x: Result<u32, &str> = Err("early error");
727 /// let y: Result<u32, &str> = Ok(2);
728 /// assert_eq!(x.or(y), Ok(2));
730 /// let x: Result<u32, &str> = Err("not a 2");
731 /// let y: Result<u32, &str> = Err("late error");
732 /// assert_eq!(x.or(y), Err("late error"));
734 /// let x: Result<u32, &str> = Ok(2);
735 /// let y: Result<u32, &str> = Ok(100);
736 /// assert_eq!(x.or(y), Ok(2));
739 #[stable(feature = "rust1", since = "1.0.0")]
740 pub fn or
<F
>(self, res
: Result
<T
, F
>) -> Result
<T
, F
> {
747 /// Calls `op` if the result is [`Err`], otherwise returns the [`Ok`] value of `self`.
749 /// This function can be used for control flow based on result values.
757 /// fn sq(x: u32) -> Result<u32, u32> { Ok(x * x) }
758 /// fn err(x: u32) -> Result<u32, u32> { Err(x) }
760 /// assert_eq!(Ok(2).or_else(sq).or_else(sq), Ok(2));
761 /// assert_eq!(Ok(2).or_else(err).or_else(sq), Ok(2));
762 /// assert_eq!(Err(3).or_else(sq).or_else(err), Ok(9));
763 /// assert_eq!(Err(3).or_else(err).or_else(err), Err(3));
766 #[stable(feature = "rust1", since = "1.0.0")]
767 pub fn or_else
<F
, O
: FnOnce(E
) -> Result
<T
, F
>>(self, op
: O
) -> Result
<T
, F
> {
774 /// Returns the contained [`Ok`] value or a provided default.
776 /// Arguments passed to `unwrap_or` are eagerly evaluated; if you are passing
777 /// the result of a function call, it is recommended to use [`unwrap_or_else`],
778 /// which is lazily evaluated.
780 /// [`unwrap_or_else`]: Result::unwrap_or_else
788 /// let x: Result<u32, &str> = Ok(9);
789 /// assert_eq!(x.unwrap_or(default), 9);
791 /// let x: Result<u32, &str> = Err("error");
792 /// assert_eq!(x.unwrap_or(default), default);
795 #[stable(feature = "rust1", since = "1.0.0")]
796 pub fn unwrap_or(self, default: T
) -> T
{
803 /// Returns the contained [`Ok`] value or computes it from a closure.
811 /// fn count(x: &str) -> usize { x.len() }
813 /// assert_eq!(Ok(2).unwrap_or_else(count), 2);
814 /// assert_eq!(Err("foo").unwrap_or_else(count), 3);
817 #[stable(feature = "rust1", since = "1.0.0")]
818 pub fn unwrap_or_else
<F
: FnOnce(E
) -> T
>(self, op
: F
) -> T
{
825 /// Returns the contained [`Ok`] value, consuming the `self` value,
826 /// without checking that the value is not an [`Err`].
830 /// Calling this method on an [`Err`] is *[undefined behavior]*.
832 /// [undefined behavior]: https://doc.rust-lang.org/reference/behavior-considered-undefined.html
837 /// #![feature(option_result_unwrap_unchecked)]
838 /// let x: Result<u32, &str> = Ok(2);
839 /// assert_eq!(unsafe { x.unwrap_unchecked() }, 2);
843 /// #![feature(option_result_unwrap_unchecked)]
844 /// let x: Result<u32, &str> = Err("emergency failure");
845 /// unsafe { x.unwrap_unchecked(); } // Undefined behavior!
849 #[unstable(feature = "option_result_unwrap_unchecked", reason = "newly added", issue = "81383")]
850 pub unsafe fn unwrap_unchecked(self) -> T
{
851 debug_assert
!(self.is_ok());
854 // SAFETY: the safety contract must be upheld by the caller.
855 Err(_
) => unsafe { hint::unreachable_unchecked() }
,
859 /// Returns the contained [`Err`] value, consuming the `self` value,
860 /// without checking that the value is not an [`Ok`].
864 /// Calling this method on an [`Ok`] is *[undefined behavior]*.
866 /// [undefined behavior]: https://doc.rust-lang.org/reference/behavior-considered-undefined.html
871 /// #![feature(option_result_unwrap_unchecked)]
872 /// let x: Result<u32, &str> = Ok(2);
873 /// unsafe { x.unwrap_err_unchecked() }; // Undefined behavior!
877 /// #![feature(option_result_unwrap_unchecked)]
878 /// let x: Result<u32, &str> = Err("emergency failure");
879 /// assert_eq!(unsafe { x.unwrap_err_unchecked() }, "emergency failure");
883 #[unstable(feature = "option_result_unwrap_unchecked", reason = "newly added", issue = "81383")]
884 pub unsafe fn unwrap_err_unchecked(self) -> E
{
885 debug_assert
!(self.is_err());
887 // SAFETY: the safety contract must be upheld by the caller.
888 Ok(_
) => unsafe { hint::unreachable_unchecked() }
,
894 impl<T
: Copy
, E
> Result
<&T
, E
> {
895 /// Maps a `Result<&T, E>` to a `Result<T, E>` by copying the contents of the
901 /// #![feature(result_copied)]
903 /// let x: Result<&i32, i32> = Ok(&val);
904 /// assert_eq!(x, Ok(&12));
905 /// let copied = x.copied();
906 /// assert_eq!(copied, Ok(12));
908 #[unstable(feature = "result_copied", reason = "newly added", issue = "63168")]
909 pub fn copied(self) -> Result
<T
, E
> {
914 impl<T
: Copy
, E
> Result
<&mut T
, E
> {
915 /// Maps a `Result<&mut T, E>` to a `Result<T, E>` by copying the contents of the
921 /// #![feature(result_copied)]
922 /// let mut val = 12;
923 /// let x: Result<&mut i32, i32> = Ok(&mut val);
924 /// assert_eq!(x, Ok(&mut 12));
925 /// let copied = x.copied();
926 /// assert_eq!(copied, Ok(12));
928 #[unstable(feature = "result_copied", reason = "newly added", issue = "63168")]
929 pub fn copied(self) -> Result
<T
, E
> {
934 impl<T
: Clone
, E
> Result
<&T
, E
> {
935 /// Maps a `Result<&T, E>` to a `Result<T, E>` by cloning the contents of the
941 /// #![feature(result_cloned)]
943 /// let x: Result<&i32, i32> = Ok(&val);
944 /// assert_eq!(x, Ok(&12));
945 /// let cloned = x.cloned();
946 /// assert_eq!(cloned, Ok(12));
948 #[unstable(feature = "result_cloned", reason = "newly added", issue = "63168")]
949 pub fn cloned(self) -> Result
<T
, E
> {
950 self.map(|t
| t
.clone())
954 impl<T
: Clone
, E
> Result
<&mut T
, E
> {
955 /// Maps a `Result<&mut T, E>` to a `Result<T, E>` by cloning the contents of the
961 /// #![feature(result_cloned)]
962 /// let mut val = 12;
963 /// let x: Result<&mut i32, i32> = Ok(&mut val);
964 /// assert_eq!(x, Ok(&mut 12));
965 /// let cloned = x.cloned();
966 /// assert_eq!(cloned, Ok(12));
968 #[unstable(feature = "result_cloned", reason = "newly added", issue = "63168")]
969 pub fn cloned(self) -> Result
<T
, E
> {
970 self.map(|t
| t
.clone())
974 impl<T
, E
: fmt
::Debug
> Result
<T
, E
> {
975 /// Returns the contained [`Ok`] value, consuming the `self` value.
979 /// Panics if the value is an [`Err`], with a panic message including the
980 /// passed message, and the content of the [`Err`].
988 /// let x: Result<u32, &str> = Err("emergency failure");
989 /// x.expect("Testing expect"); // panics with `Testing expect: emergency failure`
993 #[stable(feature = "result_expect", since = "1.4.0")]
994 pub fn expect(self, msg
: &str) -> T
{
997 Err(e
) => unwrap_failed(msg
, &e
),
1001 /// Returns the contained [`Ok`] value, consuming the `self` value.
1003 /// Because this function may panic, its use is generally discouraged.
1004 /// Instead, prefer to use pattern matching and handle the [`Err`]
1005 /// case explicitly, or call [`unwrap_or`], [`unwrap_or_else`], or
1006 /// [`unwrap_or_default`].
1008 /// [`unwrap_or`]: Result::unwrap_or
1009 /// [`unwrap_or_else`]: Result::unwrap_or_else
1010 /// [`unwrap_or_default`]: Result::unwrap_or_default
1014 /// Panics if the value is an [`Err`], with a panic message provided by the
1015 /// [`Err`]'s value.
1023 /// let x: Result<u32, &str> = Ok(2);
1024 /// assert_eq!(x.unwrap(), 2);
1028 /// let x: Result<u32, &str> = Err("emergency failure");
1029 /// x.unwrap(); // panics with `emergency failure`
1033 #[stable(feature = "rust1", since = "1.0.0")]
1034 pub fn unwrap(self) -> T
{
1037 Err(e
) => unwrap_failed("called `Result::unwrap()` on an `Err` value", &e
),
1042 impl<T
: fmt
::Debug
, E
> Result
<T
, E
> {
1043 /// Returns the contained [`Err`] value, consuming the `self` value.
1047 /// Panics if the value is an [`Ok`], with a panic message including the
1048 /// passed message, and the content of the [`Ok`].
1056 /// let x: Result<u32, &str> = Ok(10);
1057 /// x.expect_err("Testing expect_err"); // panics with `Testing expect_err: 10`
1061 #[stable(feature = "result_expect_err", since = "1.17.0")]
1062 pub fn expect_err(self, msg
: &str) -> E
{
1064 Ok(t
) => unwrap_failed(msg
, &t
),
1069 /// Returns the contained [`Err`] value, consuming the `self` value.
1073 /// Panics if the value is an [`Ok`], with a custom panic message provided
1074 /// by the [`Ok`]'s value.
1079 /// let x: Result<u32, &str> = Ok(2);
1080 /// x.unwrap_err(); // panics with `2`
1084 /// let x: Result<u32, &str> = Err("emergency failure");
1085 /// assert_eq!(x.unwrap_err(), "emergency failure");
1089 #[stable(feature = "rust1", since = "1.0.0")]
1090 pub fn unwrap_err(self) -> E
{
1092 Ok(t
) => unwrap_failed("called `Result::unwrap_err()` on an `Ok` value", &t
),
1098 impl<T
: Default
, E
> Result
<T
, E
> {
1099 /// Returns the contained [`Ok`] value or a default
1101 /// Consumes the `self` argument then, if [`Ok`], returns the contained
1102 /// value, otherwise if [`Err`], returns the default value for that
1107 /// Converts a string to an integer, turning poorly-formed strings
1108 /// into 0 (the default value for integers). [`parse`] converts
1109 /// a string to any other type that implements [`FromStr`], returning an
1110 /// [`Err`] on error.
1113 /// let good_year_from_input = "1909";
1114 /// let bad_year_from_input = "190blarg";
1115 /// let good_year = good_year_from_input.parse().unwrap_or_default();
1116 /// let bad_year = bad_year_from_input.parse().unwrap_or_default();
1118 /// assert_eq!(1909, good_year);
1119 /// assert_eq!(0, bad_year);
1122 /// [`parse`]: str::parse
1123 /// [`FromStr`]: crate::str::FromStr
1125 #[stable(feature = "result_unwrap_or_default", since = "1.16.0")]
1126 pub fn unwrap_or_default(self) -> T
{
1129 Err(_
) => Default
::default(),
1134 #[unstable(feature = "unwrap_infallible", reason = "newly added", issue = "61695")]
1135 impl<T
, E
: Into
<!>> Result
<T
, E
> {
1136 /// Returns the contained [`Ok`] value, but never panics.
1138 /// Unlike [`unwrap`], this method is known to never panic on the
1139 /// result types it is implemented for. Therefore, it can be used
1140 /// instead of `unwrap` as a maintainability safeguard that will fail
1141 /// to compile if the error type of the `Result` is later changed
1142 /// to an error that can actually occur.
1144 /// [`unwrap`]: Result::unwrap
1151 /// # #![feature(never_type)]
1152 /// # #![feature(unwrap_infallible)]
1154 /// fn only_good_news() -> Result<String, !> {
1155 /// Ok("this is fine".into())
1158 /// let s: String = only_good_news().into_ok();
1159 /// println!("{}", s);
1162 pub fn into_ok(self) -> T
{
1170 #[unstable(feature = "unwrap_infallible", reason = "newly added", issue = "61695")]
1171 impl<T
: Into
<!>, E
> Result
<T
, E
> {
1172 /// Returns the contained [`Err`] value, but never panics.
1174 /// Unlike [`unwrap_err`], this method is known to never panic on the
1175 /// result types it is implemented for. Therefore, it can be used
1176 /// instead of `unwrap_err` as a maintainability safeguard that will fail
1177 /// to compile if the ok type of the `Result` is later changed
1178 /// to a type that can actually occur.
1180 /// [`unwrap_err`]: Result::unwrap_err
1187 /// # #![feature(never_type)]
1188 /// # #![feature(unwrap_infallible)]
1190 /// fn only_bad_news() -> Result<!, String> {
1191 /// Err("Oops, it failed".into())
1194 /// let error: String = only_bad_news().into_err();
1195 /// println!("{}", error);
1198 pub fn into_err(self) -> E
{
1206 impl<T
: Deref
, E
> Result
<T
, E
> {
1207 /// Converts from `Result<T, E>` (or `&Result<T, E>`) to `Result<&<T as Deref>::Target, &E>`.
1209 /// Coerces the [`Ok`] variant of the original [`Result`] via [`Deref`](crate::ops::Deref)
1210 /// and returns the new [`Result`].
1215 /// let x: Result<String, u32> = Ok("hello".to_string());
1216 /// let y: Result<&str, &u32> = Ok("hello");
1217 /// assert_eq!(x.as_deref(), y);
1219 /// let x: Result<String, u32> = Err(42);
1220 /// let y: Result<&str, &u32> = Err(&42);
1221 /// assert_eq!(x.as_deref(), y);
1223 #[stable(feature = "inner_deref", since = "1.47.0")]
1224 pub fn as_deref(&self) -> Result
<&T
::Target
, &E
> {
1225 self.as_ref().map(|t
| t
.deref())
1229 impl<T
: DerefMut
, E
> Result
<T
, E
> {
1230 /// Converts from `Result<T, E>` (or `&mut Result<T, E>`) to `Result<&mut <T as DerefMut>::Target, &mut E>`.
1232 /// Coerces the [`Ok`] variant of the original [`Result`] via [`DerefMut`](crate::ops::DerefMut)
1233 /// and returns the new [`Result`].
1238 /// let mut s = "HELLO".to_string();
1239 /// let mut x: Result<String, u32> = Ok("hello".to_string());
1240 /// let y: Result<&mut str, &mut u32> = Ok(&mut s);
1241 /// assert_eq!(x.as_deref_mut().map(|x| { x.make_ascii_uppercase(); x }), y);
1244 /// let mut x: Result<String, u32> = Err(42);
1245 /// let y: Result<&mut str, &mut u32> = Err(&mut i);
1246 /// assert_eq!(x.as_deref_mut().map(|x| { x.make_ascii_uppercase(); x }), y);
1248 #[stable(feature = "inner_deref", since = "1.47.0")]
1249 pub fn as_deref_mut(&mut self) -> Result
<&mut T
::Target
, &mut E
> {
1250 self.as_mut().map(|t
| t
.deref_mut())
1254 impl<T
, E
> Result
<Option
<T
>, E
> {
1255 /// Transposes a `Result` of an `Option` into an `Option` of a `Result`.
1257 /// `Ok(None)` will be mapped to `None`.
1258 /// `Ok(Some(_))` and `Err(_)` will be mapped to `Some(Ok(_))` and `Some(Err(_))`.
1263 /// #[derive(Debug, Eq, PartialEq)]
1266 /// let x: Result<Option<i32>, SomeErr> = Ok(Some(5));
1267 /// let y: Option<Result<i32, SomeErr>> = Some(Ok(5));
1268 /// assert_eq!(x.transpose(), y);
1271 #[stable(feature = "transpose_result", since = "1.33.0")]
1272 #[rustc_const_unstable(feature = "const_result", issue = "82814")]
1273 pub const fn transpose(self) -> Option
<Result
<T
, E
>> {
1275 Ok(Some(x
)) => Some(Ok(x
)),
1277 Err(e
) => Some(Err(e
)),
1282 impl<T
, E
> Result
<Result
<T
, E
>, E
> {
1283 /// Converts from `Result<Result<T, E>, E>` to `Result<T, E>`
1290 /// #![feature(result_flattening)]
1291 /// let x: Result<Result<&'static str, u32>, u32> = Ok(Ok("hello"));
1292 /// assert_eq!(Ok("hello"), x.flatten());
1294 /// let x: Result<Result<&'static str, u32>, u32> = Ok(Err(6));
1295 /// assert_eq!(Err(6), x.flatten());
1297 /// let x: Result<Result<&'static str, u32>, u32> = Err(6);
1298 /// assert_eq!(Err(6), x.flatten());
1301 /// Flattening only removes one level of nesting at a time:
1304 /// #![feature(result_flattening)]
1305 /// let x: Result<Result<Result<&'static str, u32>, u32>, u32> = Ok(Ok(Ok("hello")));
1306 /// assert_eq!(Ok(Ok("hello")), x.flatten());
1307 /// assert_eq!(Ok("hello"), x.flatten().flatten());
1310 #[unstable(feature = "result_flattening", issue = "70142")]
1311 pub fn flatten(self) -> Result
<T
, E
> {
1312 self.and_then(convert
::identity
)
1316 impl<T
> Result
<T
, T
> {
1317 /// Returns the [`Ok`] value if `self` is `Ok`, and the [`Err`] value if
1318 /// `self` is `Err`.
1320 /// In other words, this function returns the value (the `T`) of a
1321 /// `Result<T, T>`, regardless of whether or not that result is `Ok` or
1324 /// This can be useful in conjunction with APIs such as
1325 /// [`Atomic*::compare_exchange`], or [`slice::binary_search`], but only in
1326 /// cases where you don't care if the result was `Ok` or not.
1328 /// [`Atomic*::compare_exchange`]: crate::sync::atomic::AtomicBool::compare_exchange
1333 /// #![feature(result_into_ok_or_err)]
1334 /// let ok: Result<u32, u32> = Ok(3);
1335 /// let err: Result<u32, u32> = Err(4);
1337 /// assert_eq!(ok.into_ok_or_err(), 3);
1338 /// assert_eq!(err.into_ok_or_err(), 4);
1341 #[unstable(feature = "result_into_ok_or_err", reason = "newly added", issue = "82223")]
1342 pub const fn into_ok_or_err(self) -> T
{
1350 // This is a separate function to reduce the code size of the methods
1354 fn unwrap_failed(msg
: &str, error
: &dyn fmt
::Debug
) -> ! {
1355 panic
!("{}: {:?}", msg
, error
)
1358 /////////////////////////////////////////////////////////////////////////////
1359 // Trait implementations
1360 /////////////////////////////////////////////////////////////////////////////
1362 #[stable(feature = "rust1", since = "1.0.0")]
1363 impl<T
: Clone
, E
: Clone
> Clone
for Result
<T
, E
> {
1365 fn clone(&self) -> Self {
1367 Ok(x
) => Ok(x
.clone()),
1368 Err(x
) => Err(x
.clone()),
1373 fn clone_from(&mut self, source
: &Self) {
1374 match (self, source
) {
1375 (Ok(to
), Ok(from
)) => to
.clone_from(from
),
1376 (Err(to
), Err(from
)) => to
.clone_from(from
),
1377 (to
, from
) => *to
= from
.clone(),
1382 #[stable(feature = "rust1", since = "1.0.0")]
1383 impl<T
, E
> IntoIterator
for Result
<T
, E
> {
1385 type IntoIter
= IntoIter
<T
>;
1387 /// Returns a consuming iterator over the possibly contained value.
1389 /// The iterator yields one value if the result is [`Result::Ok`], otherwise none.
1396 /// let x: Result<u32, &str> = Ok(5);
1397 /// let v: Vec<u32> = x.into_iter().collect();
1398 /// assert_eq!(v, [5]);
1400 /// let x: Result<u32, &str> = Err("nothing!");
1401 /// let v: Vec<u32> = x.into_iter().collect();
1402 /// assert_eq!(v, []);
1405 fn into_iter(self) -> IntoIter
<T
> {
1406 IntoIter { inner: self.ok() }
1410 #[stable(since = "1.4.0", feature = "result_iter")]
1411 impl<'a
, T
, E
> IntoIterator
for &'a Result
<T
, E
> {
1413 type IntoIter
= Iter
<'a
, T
>;
1415 fn into_iter(self) -> Iter
<'a
, T
> {
1420 #[stable(since = "1.4.0", feature = "result_iter")]
1421 impl<'a
, T
, E
> IntoIterator
for &'a
mut Result
<T
, E
> {
1422 type Item
= &'a
mut T
;
1423 type IntoIter
= IterMut
<'a
, T
>;
1425 fn into_iter(self) -> IterMut
<'a
, T
> {
1430 /////////////////////////////////////////////////////////////////////////////
1431 // The Result Iterators
1432 /////////////////////////////////////////////////////////////////////////////
1434 /// An iterator over a reference to the [`Ok`] variant of a [`Result`].
1436 /// The iterator yields one value if the result is [`Ok`], otherwise none.
1438 /// Created by [`Result::iter`].
1440 #[stable(feature = "rust1", since = "1.0.0")]
1441 pub struct Iter
<'a
, T
: 'a
> {
1442 inner
: Option
<&'a T
>,
1445 #[stable(feature = "rust1", since = "1.0.0")]
1446 impl<'a
, T
> Iterator
for Iter
<'a
, T
> {
1450 fn next(&mut self) -> Option
<&'a T
> {
1454 fn size_hint(&self) -> (usize, Option
<usize>) {
1455 let n
= if self.inner
.is_some() { 1 }
else { 0 }
;
1460 #[stable(feature = "rust1", since = "1.0.0")]
1461 impl<'a
, T
> DoubleEndedIterator
for Iter
<'a
, T
> {
1463 fn next_back(&mut self) -> Option
<&'a T
> {
1468 #[stable(feature = "rust1", since = "1.0.0")]
1469 impl<T
> ExactSizeIterator
for Iter
<'_
, T
> {}
1471 #[stable(feature = "fused", since = "1.26.0")]
1472 impl<T
> FusedIterator
for Iter
<'_
, T
> {}
1474 #[unstable(feature = "trusted_len", issue = "37572")]
1475 unsafe impl<A
> TrustedLen
for Iter
<'_
, A
> {}
1477 #[stable(feature = "rust1", since = "1.0.0")]
1478 impl<T
> Clone
for Iter
<'_
, T
> {
1480 fn clone(&self) -> Self {
1481 Iter { inner: self.inner }
1485 /// An iterator over a mutable reference to the [`Ok`] variant of a [`Result`].
1487 /// Created by [`Result::iter_mut`].
1489 #[stable(feature = "rust1", since = "1.0.0")]
1490 pub struct IterMut
<'a
, T
: 'a
> {
1491 inner
: Option
<&'a
mut T
>,
1494 #[stable(feature = "rust1", since = "1.0.0")]
1495 impl<'a
, T
> Iterator
for IterMut
<'a
, T
> {
1496 type Item
= &'a
mut T
;
1499 fn next(&mut self) -> Option
<&'a
mut T
> {
1503 fn size_hint(&self) -> (usize, Option
<usize>) {
1504 let n
= if self.inner
.is_some() { 1 }
else { 0 }
;
1509 #[stable(feature = "rust1", since = "1.0.0")]
1510 impl<'a
, T
> DoubleEndedIterator
for IterMut
<'a
, T
> {
1512 fn next_back(&mut self) -> Option
<&'a
mut T
> {
1517 #[stable(feature = "rust1", since = "1.0.0")]
1518 impl<T
> ExactSizeIterator
for IterMut
<'_
, T
> {}
1520 #[stable(feature = "fused", since = "1.26.0")]
1521 impl<T
> FusedIterator
for IterMut
<'_
, T
> {}
1523 #[unstable(feature = "trusted_len", issue = "37572")]
1524 unsafe impl<A
> TrustedLen
for IterMut
<'_
, A
> {}
1526 /// An iterator over the value in a [`Ok`] variant of a [`Result`].
1528 /// The iterator yields one value if the result is [`Ok`], otherwise none.
1530 /// This struct is created by the [`into_iter`] method on
1531 /// [`Result`] (provided by the [`IntoIterator`] trait).
1533 /// [`into_iter`]: IntoIterator::into_iter
1534 #[derive(Clone, Debug)]
1535 #[stable(feature = "rust1", since = "1.0.0")]
1536 pub struct IntoIter
<T
> {
1540 #[stable(feature = "rust1", since = "1.0.0")]
1541 impl<T
> Iterator
for IntoIter
<T
> {
1545 fn next(&mut self) -> Option
<T
> {
1549 fn size_hint(&self) -> (usize, Option
<usize>) {
1550 let n
= if self.inner
.is_some() { 1 }
else { 0 }
;
1555 #[stable(feature = "rust1", since = "1.0.0")]
1556 impl<T
> DoubleEndedIterator
for IntoIter
<T
> {
1558 fn next_back(&mut self) -> Option
<T
> {
1563 #[stable(feature = "rust1", since = "1.0.0")]
1564 impl<T
> ExactSizeIterator
for IntoIter
<T
> {}
1566 #[stable(feature = "fused", since = "1.26.0")]
1567 impl<T
> FusedIterator
for IntoIter
<T
> {}
1569 #[unstable(feature = "trusted_len", issue = "37572")]
1570 unsafe impl<A
> TrustedLen
for IntoIter
<A
> {}
1572 /////////////////////////////////////////////////////////////////////////////
1574 /////////////////////////////////////////////////////////////////////////////
1576 #[stable(feature = "rust1", since = "1.0.0")]
1577 impl<A
, E
, V
: FromIterator
<A
>> FromIterator
<Result
<A
, E
>> for Result
<V
, E
> {
1578 /// Takes each element in the `Iterator`: if it is an `Err`, no further
1579 /// elements are taken, and the `Err` is returned. Should no `Err` occur, a
1580 /// container with the values of each `Result` is returned.
1582 /// Here is an example which increments every integer in a vector,
1583 /// checking for overflow:
1586 /// let v = vec![1, 2];
1587 /// let res: Result<Vec<u32>, &'static str> = v.iter().map(|x: &u32|
1588 /// x.checked_add(1).ok_or("Overflow!")
1590 /// assert_eq!(res, Ok(vec![2, 3]));
1593 /// Here is another example that tries to subtract one from another list
1594 /// of integers, this time checking for underflow:
1597 /// let v = vec![1, 2, 0];
1598 /// let res: Result<Vec<u32>, &'static str> = v.iter().map(|x: &u32|
1599 /// x.checked_sub(1).ok_or("Underflow!")
1601 /// assert_eq!(res, Err("Underflow!"));
1604 /// Here is a variation on the previous example, showing that no
1605 /// further elements are taken from `iter` after the first `Err`.
1608 /// let v = vec![3, 2, 1, 10];
1609 /// let mut shared = 0;
1610 /// let res: Result<Vec<u32>, &'static str> = v.iter().map(|x: &u32| {
1612 /// x.checked_sub(2).ok_or("Underflow!")
1614 /// assert_eq!(res, Err("Underflow!"));
1615 /// assert_eq!(shared, 6);
1618 /// Since the third element caused an underflow, no further elements were taken,
1619 /// so the final value of `shared` is 6 (= `3 + 2 + 1`), not 16.
1621 fn from_iter
<I
: IntoIterator
<Item
= Result
<A
, E
>>>(iter
: I
) -> Result
<V
, E
> {
1622 // FIXME(#11084): This could be replaced with Iterator::scan when this
1623 // performance bug is closed.
1625 iter
::process_results(iter
.into_iter(), |i
| i
.collect())
1629 #[unstable(feature = "try_trait", issue = "42327")]
1630 impl<T
, E
> ops
::Try
for Result
<T
, E
> {
1635 fn into_result(self) -> Self {
1640 fn from_ok(v
: T
) -> Self {
1645 fn from_error(v
: E
) -> Self {
1650 #[unstable(feature = "try_trait_v2", issue = "84277")]
1651 impl<T
, E
> ops
::TryV2
for Result
<T
, E
> {
1653 type Residual
= Result
<convert
::Infallible
, E
>;
1656 fn from_output(output
: Self::Output
) -> Self {
1661 fn branch(self) -> ControlFlow
<Self::Residual
, Self::Output
> {
1663 Ok(v
) => ControlFlow
::Continue(v
),
1664 Err(e
) => ControlFlow
::Break(Err(e
)),
1669 #[unstable(feature = "try_trait_v2", issue = "84277")]
1670 impl<T
, E
, F
: From
<E
>> ops
::FromResidual
<Result
<convert
::Infallible
, E
>> for Result
<T
, F
> {
1672 fn from_residual(residual
: Result
<convert
::Infallible
, E
>) -> Self {
1674 Err(e
) => Err(From
::from(e
)),