library/core/src/result.rs

   1 //! Error handling with the `Result` type.
   2 //!
   3 //! [`Result<T, E>`][`Result`] is the type used for returning and propagating
   4 //! errors. It is an enum with the variants, [`Ok(T)`], representing
   5 //! success and containing a value, and [`Err(E)`], representing error
   6 //! and containing an error value.
   7 //!
   8 //! ```
   9 //! # #[allow(dead_code)]
  10 //! enum Result<T, E> {
  11 //!    Ok(T),
  12 //!    Err(E),
  13 //! }
  14 //! ```
  15 //!
  16 //! Functions return [`Result`] whenever errors are expected and
  17 //! recoverable. In the `std` crate, [`Result`] is most prominently used
  18 //! for [I/O](../../std/io/index.html).
  19 //!
  20 //! A simple function returning [`Result`] might be
  21 //! defined and used like so:
  22 //!
  23 //! ```
  24 //! #[derive(Debug)]
  25 //! enum Version { Version1, Version2 }
  26 //!
  27 //! fn parse_version(header: &[u8]) -> Result<Version, &'static str> {
  28 //!     match header.get(0) {
  29 //!         None => Err("invalid header length"),
  30 //!         Some(&1) => Ok(Version::Version1),
  31 //!         Some(&2) => Ok(Version::Version2),
  32 //!         Some(_) => Err("invalid version"),
  33 //!     }
  34 //! }
  35 //!
  36 //! let version = parse_version(&[1, 2, 3, 4]);
  37 //! match version {
  38 //!     Ok(v) => println!("working with version: {:?}", v),
  39 //!     Err(e) => println!("error parsing header: {:?}", e),
  40 //! }
  41 //! ```
  42 //!
  43 //! Pattern matching on [`Result`]s is clear and straightforward for
  44 //! simple cases, but [`Result`] comes with some convenience methods
  45 //! that make working with it more succinct.
  46 //!
  47 //! ```
  48 //! 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 [`Result`]`<T, `[`io::Error`]`>`.*
  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
 221 //! [`write_all`]: ../../std/io/trait.Write.html#method.write_all
 222 //! [`io::Result`]: ../../std/io/type.Result.html
 223 //! [`?`]: crate::ops::Try
 224 //! [`Ok(T)`]: Ok
 225 //! [`Err(E)`]: Err
 226 //! [`io::Error`]: ../../std/io/struct.Error.html
 227
 228 #![stable(feature = "rust1", since = "1.0.0")]
 229
 230 use crate::iter::{self, FromIterator, FusedIterator, TrustedLen};
 231 use crate::ops::{self, Deref, DerefMut};
 232 use crate::{convert, fmt, hint};
 233
 234 /// `Result` is a type that represents either success ([`Ok`]) or failure ([`Err`]).
 235 ///
 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
 243     #[lang = "Ok"]
 244     #[stable(feature = "rust1", since = "1.0.0")]
 245     Ok(#[stable(feature = "rust1", since = "1.0.0")] T),
 246
 247     /// Contains the error value
 248     #[lang = "Err"]
 249     #[stable(feature = "rust1", since = "1.0.0")]
 250     Err(#[stable(feature = "rust1", since = "1.0.0")] E),
 251 }
 252
 253 /////////////////////////////////////////////////////////////////////////////
 254 // Type implementation
 255 /////////////////////////////////////////////////////////////////////////////
 256
 257 impl<T, E> Result<T, E> {
 258     /////////////////////////////////////////////////////////////////////////
 259     // Querying the contained values
 260     /////////////////////////////////////////////////////////////////////////
 261
 262     /// Returns `true` if the result is [`Ok`].
 263     ///
 264     /// # Examples
 265     ///
 266     /// Basic usage:
 267     ///
 268     /// ```
 269     /// let x: Result<i32, &str> = Ok(-3);
 270     /// assert_eq!(x.is_ok(), true);
 271     ///
 272     /// let x: Result<i32, &str> = Err("Some error message");
 273     /// assert_eq!(x.is_ok(), false);
 274     /// ```
 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")]
 277     #[inline]
 278     #[stable(feature = "rust1", since = "1.0.0")]
 279     pub const fn is_ok(&self) -> bool {
 280         matches!(*self, Ok(_))
 281     }
 282
 283     /// Returns `true` if the result is [`Err`].
 284     ///
 285     /// # Examples
 286     ///
 287     /// Basic usage:
 288     ///
 289     /// ```
 290     /// let x: Result<i32, &str> = Ok(-3);
 291     /// assert_eq!(x.is_err(), false);
 292     ///
 293     /// let x: Result<i32, &str> = Err("Some error message");
 294     /// assert_eq!(x.is_err(), true);
 295     /// ```
 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")]
 298     #[inline]
 299     #[stable(feature = "rust1", since = "1.0.0")]
 300     pub const fn is_err(&self) -> bool {
 301         !self.is_ok()
 302     }
 303
 304     /// Returns `true` if the result is an [`Ok`] value containing the given value.
 305     ///
 306     /// # Examples
 307     ///
 308     /// ```
 309     /// #![feature(option_result_contains)]
 310     ///
 311     /// let x: Result<u32, &str> = Ok(2);
 312     /// assert_eq!(x.contains(&2), true);
 313     ///
 314     /// let x: Result<u32, &str> = Ok(3);
 315     /// assert_eq!(x.contains(&2), false);
 316     ///
 317     /// let x: Result<u32, &str> = Err("Some error message");
 318     /// assert_eq!(x.contains(&2), false);
 319     /// ```
 320     #[must_use]
 321     #[inline]
 322     #[unstable(feature = "option_result_contains", issue = "62358")]
 323     pub fn contains<U>(&self, x: &U) -> bool
 324     where
 325         U: PartialEq<T>,
 326     {
 327         match self {
 328             Ok(y) => x == y,
 329             Err(_) => false,
 330         }
 331     }
 332
 333     /// Returns `true` if the result is an [`Err`] value containing the given value.
 334     ///
 335     /// # Examples
 336     ///
 337     /// ```
 338     /// #![feature(result_contains_err)]
 339     ///
 340     /// let x: Result<u32, &str> = Ok(2);
 341     /// assert_eq!(x.contains_err(&"Some error message"), false);
 342     ///
 343     /// let x: Result<u32, &str> = Err("Some error message");
 344     /// assert_eq!(x.contains_err(&"Some error message"), true);
 345     ///
 346     /// let x: Result<u32, &str> = Err("Some other error message");
 347     /// assert_eq!(x.contains_err(&"Some error message"), false);
 348     /// ```
 349     #[must_use]
 350     #[inline]
 351     #[unstable(feature = "result_contains_err", issue = "62358")]
 352     pub fn contains_err<F>(&self, f: &F) -> bool
 353     where
 354         F: PartialEq<E>,
 355     {
 356         match self {
 357             Ok(_) => false,
 358             Err(e) => f == e,
 359         }
 360     }
 361
 362     /////////////////////////////////////////////////////////////////////////
 363     // Adapter for each variant
 364     /////////////////////////////////////////////////////////////////////////
 365
 366     /// Converts from `Result<T, E>` to [`Option<T>`].
 367     ///
 368     /// Converts `self` into an [`Option<T>`], consuming `self`,
 369     /// and discarding the error, if any.
 370     ///
 371     /// # Examples
 372     ///
 373     /// Basic usage:
 374     ///
 375     /// ```
 376     /// let x: Result<u32, &str> = Ok(2);
 377     /// assert_eq!(x.ok(), Some(2));
 378     ///
 379     /// let x: Result<u32, &str> = Err("Nothing here");
 380     /// assert_eq!(x.ok(), None);
 381     /// ```
 382     #[inline]
 383     #[stable(feature = "rust1", since = "1.0.0")]
 384     pub fn ok(self) -> Option<T> {
 385         match self {
 386             Ok(x) => Some(x),
 387             Err(_) => None,
 388         }
 389     }
 390
 391     /// Converts from `Result<T, E>` to [`Option<E>`].
 392     ///
 393     /// Converts `self` into an [`Option<E>`], consuming `self`,
 394     /// and discarding the success value, if any.
 395     ///
 396     /// # Examples
 397     ///
 398     /// Basic usage:
 399     ///
 400     /// ```
 401     /// let x: Result<u32, &str> = Ok(2);
 402     /// assert_eq!(x.err(), None);
 403     ///
 404     /// let x: Result<u32, &str> = Err("Nothing here");
 405     /// assert_eq!(x.err(), Some("Nothing here"));
 406     /// ```
 407     #[inline]
 408     #[stable(feature = "rust1", since = "1.0.0")]
 409     pub fn err(self) -> Option<E> {
 410         match self {
 411             Ok(_) => None,
 412             Err(x) => Some(x),
 413         }
 414     }
 415
 416     /////////////////////////////////////////////////////////////////////////
 417     // Adapter for working with references
 418     /////////////////////////////////////////////////////////////////////////
 419
 420     /// Converts from `&Result<T, E>` to `Result<&T, &E>`.
 421     ///
 422     /// Produces a new `Result`, containing a reference
 423     /// into the original, leaving the original in place.
 424     ///
 425     /// # Examples
 426     ///
 427     /// Basic usage:
 428     ///
 429     /// ```
 430     /// let x: Result<u32, &str> = Ok(2);
 431     /// assert_eq!(x.as_ref(), Ok(&2));
 432     ///
 433     /// let x: Result<u32, &str> = Err("Error");
 434     /// assert_eq!(x.as_ref(), Err(&"Error"));
 435     /// ```
 436     #[inline]
 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> {
 440         match *self {
 441             Ok(ref x) => Ok(x),
 442             Err(ref x) => Err(x),
 443         }
 444     }
 445
 446     /// Converts from `&mut Result<T, E>` to `Result<&mut T, &mut E>`.
 447     ///
 448     /// # Examples
 449     ///
 450     /// Basic usage:
 451     ///
 452     /// ```
 453     /// fn mutate(r: &mut Result<i32, i32>) {
 454     ///     match r.as_mut() {
 455     ///         Ok(v) => *v = 42,
 456     ///         Err(e) => *e = 0,
 457     ///     }
 458     /// }
 459     ///
 460     /// let mut x: Result<i32, i32> = Ok(2);
 461     /// mutate(&mut x);
 462     /// assert_eq!(x.unwrap(), 42);
 463     ///
 464     /// let mut x: Result<i32, i32> = Err(13);
 465     /// mutate(&mut x);
 466     /// assert_eq!(x.unwrap_err(), 0);
 467     /// ```
 468     #[inline]
 469     #[stable(feature = "rust1", since = "1.0.0")]
 470     pub fn as_mut(&mut self) -> Result<&mut T, &mut E> {
 471         match *self {
 472             Ok(ref mut x) => Ok(x),
 473             Err(ref mut x) => Err(x),
 474         }
 475     }
 476
 477     /////////////////////////////////////////////////////////////////////////
 478     // Transforming contained values
 479     /////////////////////////////////////////////////////////////////////////
 480
 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.
 483     ///
 484     /// This function can be used to compose the results of two functions.
 485     ///
 486     /// # Examples
 487     ///
 488     /// Print the numbers on each line of a string multiplied by two.
 489     ///
 490     /// ```
 491     /// let line = "1\n2\n3\n4\n";
 492     ///
 493     /// for num in line.lines() {
 494     ///     match num.parse::<i32>().map(|i| i * 2) {
 495     ///         Ok(n) => println!("{}", n),
 496     ///         Err(..) => {}
 497     ///     }
 498     /// }
 499     /// ```
 500     #[inline]
 501     #[stable(feature = "rust1", since = "1.0.0")]
 502     pub fn map<U, F: FnOnce(T) -> U>(self, op: F) -> Result<U, E> {
 503         match self {
 504             Ok(t) => Ok(op(t)),
 505             Err(e) => Err(e),
 506         }
 507     }
 508
 509     /// Applies a function to the contained value (if [`Ok`]),
 510     /// or returns the provided default (if [`Err`]).
 511     ///
 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.
 515     ///
 516     /// [`map_or_else`]: Result::map_or_else
 517     ///
 518     /// # Examples
 519     ///
 520     /// ```
 521     /// let x: Result<_, &str> = Ok("foo");
 522     /// assert_eq!(x.map_or(42, |v| v.len()), 3);
 523     ///
 524     /// let x: Result<&str, _> = Err("bar");
 525     /// assert_eq!(x.map_or(42, |v| v.len()), 42);
 526     /// ```
 527     #[inline]
 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 {
 530         match self {
 531             Ok(t) => f(t),
 532             Err(_) => default,
 533         }
 534     }
 535
 536     /// Maps a `Result<T, E>` to `U` by applying a function to a
 537     /// contained [`Ok`] value, or a fallback function to a
 538     /// contained [`Err`] value.
 539     ///
 540     /// This function can be used to unpack a successful result
 541     /// while handling an error.
 542     ///
 543     ///
 544     /// # Examples
 545     ///
 546     /// Basic usage:
 547     ///
 548     /// ```
 549     /// let k = 21;
 550     ///
 551     /// let x : Result<_, &str> = Ok("foo");
 552     /// assert_eq!(x.map_or_else(|e| k * 2, |v| v.len()), 3);
 553     ///
 554     /// let x : Result<&str, _> = Err("bar");
 555     /// assert_eq!(x.map_or_else(|e| k * 2, |v| v.len()), 42);
 556     /// ```
 557     #[inline]
 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 {
 560         match self {
 561             Ok(t) => f(t),
 562             Err(e) => default(e),
 563         }
 564     }
 565
 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.
 568     ///
 569     /// This function can be used to pass through a successful result while handling
 570     /// an error.
 571     ///
 572     ///
 573     /// # Examples
 574     ///
 575     /// Basic usage:
 576     ///
 577     /// ```
 578     /// fn stringify(x: u32) -> String { format!("error code: {}", x) }
 579     ///
 580     /// let x: Result<u32, u32> = Ok(2);
 581     /// assert_eq!(x.map_err(stringify), Ok(2));
 582     ///
 583     /// let x: Result<u32, u32> = Err(13);
 584     /// assert_eq!(x.map_err(stringify), Err("error code: 13".to_string()));
 585     /// ```
 586     #[inline]
 587     #[stable(feature = "rust1", since = "1.0.0")]
 588     pub fn map_err<F, O: FnOnce(E) -> F>(self, op: O) -> Result<T, F> {
 589         match self {
 590             Ok(t) => Ok(t),
 591             Err(e) => Err(op(e)),
 592         }
 593     }
 594
 595     /////////////////////////////////////////////////////////////////////////
 596     // Iterator constructors
 597     /////////////////////////////////////////////////////////////////////////
 598
 599     /// Returns an iterator over the possibly contained value.
 600     ///
 601     /// The iterator yields one value if the result is [`Result::Ok`], otherwise none.
 602     ///
 603     /// # Examples
 604     ///
 605     /// Basic usage:
 606     ///
 607     /// ```
 608     /// let x: Result<u32, &str> = Ok(7);
 609     /// assert_eq!(x.iter().next(), Some(&7));
 610     ///
 611     /// let x: Result<u32, &str> = Err("nothing!");
 612     /// assert_eq!(x.iter().next(), None);
 613     /// ```
 614     #[inline]
 615     #[stable(feature = "rust1", since = "1.0.0")]
 616     pub fn iter(&self) -> Iter<'_, T> {
 617         Iter { inner: self.as_ref().ok() }
 618     }
 619
 620     /// Returns a mutable iterator over the possibly contained value.
 621     ///
 622     /// The iterator yields one value if the result is [`Result::Ok`], otherwise none.
 623     ///
 624     /// # Examples
 625     ///
 626     /// Basic usage:
 627     ///
 628     /// ```
 629     /// let mut x: Result<u32, &str> = Ok(7);
 630     /// match x.iter_mut().next() {
 631     ///     Some(v) => *v = 40,
 632     ///     None => {},
 633     /// }
 634     /// assert_eq!(x, Ok(40));
 635     ///
 636     /// let mut x: Result<u32, &str> = Err("nothing!");
 637     /// assert_eq!(x.iter_mut().next(), None);
 638     /// ```
 639     #[inline]
 640     #[stable(feature = "rust1", since = "1.0.0")]
 641     pub fn iter_mut(&mut self) -> IterMut<'_, T> {
 642         IterMut { inner: self.as_mut().ok() }
 643     }
 644
 645     ////////////////////////////////////////////////////////////////////////
 646     // Boolean operations on the values, eager and lazy
 647     /////////////////////////////////////////////////////////////////////////
 648
 649     /// Returns `res` if the result is [`Ok`], otherwise returns the [`Err`] value of `self`.
 650     ///
 651     ///
 652     /// # Examples
 653     ///
 654     /// Basic usage:
 655     ///
 656     /// ```
 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"));
 660     ///
 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"));
 664     ///
 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"));
 668     ///
 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"));
 672     /// ```
 673     #[inline]
 674     #[stable(feature = "rust1", since = "1.0.0")]
 675     pub fn and<U>(self, res: Result<U, E>) -> Result<U, E> {
 676         match self {
 677             Ok(_) => res,
 678             Err(e) => Err(e),
 679         }
 680     }
 681
 682     /// Calls `op` if the result is [`Ok`], otherwise returns the [`Err`] value of `self`.
 683     ///
 684     ///
 685     /// This function can be used for control flow based on `Result` values.
 686     ///
 687     /// # Examples
 688     ///
 689     /// Basic usage:
 690     ///
 691     /// ```
 692     /// fn sq(x: u32) -> Result<u32, u32> { Ok(x * x) }
 693     /// fn err(x: u32) -> Result<u32, u32> { Err(x) }
 694     ///
 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));
 699     /// ```
 700     #[inline]
 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> {
 703         match self {
 704             Ok(t) => op(t),
 705             Err(e) => Err(e),
 706         }
 707     }
 708
 709     /// Returns `res` if the result is [`Err`], otherwise returns the [`Ok`] value of `self`.
 710     ///
 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.
 714     ///
 715     /// [`or_else`]: Result::or_else
 716     ///
 717     /// # Examples
 718     ///
 719     /// Basic usage:
 720     ///
 721     /// ```
 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));
 725     ///
 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));
 729     ///
 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"));
 733     ///
 734     /// let x: Result<u32, &str> = Ok(2);
 735     /// let y: Result<u32, &str> = Ok(100);
 736     /// assert_eq!(x.or(y), Ok(2));
 737     /// ```
 738     #[inline]
 739     #[stable(feature = "rust1", since = "1.0.0")]
 740     pub fn or<F>(self, res: Result<T, F>) -> Result<T, F> {
 741         match self {
 742             Ok(v) => Ok(v),
 743             Err(_) => res,
 744         }
 745     }
 746
 747     /// Calls `op` if the result is [`Err`], otherwise returns the [`Ok`] value of `self`.
 748     ///
 749     /// This function can be used for control flow based on result values.
 750     ///
 751     ///
 752     /// # Examples
 753     ///
 754     /// Basic usage:
 755     ///
 756     /// ```
 757     /// fn sq(x: u32) -> Result<u32, u32> { Ok(x * x) }
 758     /// fn err(x: u32) -> Result<u32, u32> { Err(x) }
 759     ///
 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));
 764     /// ```
 765     #[inline]
 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> {
 768         match self {
 769             Ok(t) => Ok(t),
 770             Err(e) => op(e),
 771         }
 772     }
 773
 774     /// Returns the contained [`Ok`] value or a provided default.
 775     ///
 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.
 779     ///
 780     /// [`unwrap_or_else`]: Result::unwrap_or_else
 781     ///
 782     /// # Examples
 783     ///
 784     /// Basic usage:
 785     ///
 786     /// ```
 787     /// let default = 2;
 788     /// let x: Result<u32, &str> = Ok(9);
 789     /// assert_eq!(x.unwrap_or(default), 9);
 790     ///
 791     /// let x: Result<u32, &str> = Err("error");
 792     /// assert_eq!(x.unwrap_or(default), default);
 793     /// ```
 794     #[inline]
 795     #[stable(feature = "rust1", since = "1.0.0")]
 796     pub fn unwrap_or(self, default: T) -> T {
 797         match self {
 798             Ok(t) => t,
 799             Err(_) => default,
 800         }
 801     }
 802
 803     /// Returns the contained [`Ok`] value or computes it from a closure.
 804     ///
 805     ///
 806     /// # Examples
 807     ///
 808     /// Basic usage:
 809     ///
 810     /// ```
 811     /// fn count(x: &str) -> usize { x.len() }
 812     ///
 813     /// assert_eq!(Ok(2).unwrap_or_else(count), 2);
 814     /// assert_eq!(Err("foo").unwrap_or_else(count), 3);
 815     /// ```
 816     #[inline]
 817     #[stable(feature = "rust1", since = "1.0.0")]
 818     pub fn unwrap_or_else<F: FnOnce(E) -> T>(self, op: F) -> T {
 819         match self {
 820             Ok(t) => t,
 821             Err(e) => op(e),
 822         }
 823     }
 824
 825     /// Returns the contained [`Ok`] value, consuming the `self` value,
 826     /// without checking that the value is not an [`Err`].
 827     ///
 828     /// # Safety
 829     ///
 830     /// Calling this method on an [`Err`] is *[undefined behavior]*.
 831     ///
 832     /// [undefined behavior]: https://doc.rust-lang.org/reference/behavior-considered-undefined.html
 833     ///
 834     /// # Examples
 835     ///
 836     /// ```
 837     /// #![feature(option_result_unwrap_unchecked)]
 838     /// let x: Result<u32, &str> = Ok(2);
 839     /// assert_eq!(unsafe { x.unwrap_unchecked() }, 2);
 840     /// ```
 841     ///
 842     /// ```no_run
 843     /// #![feature(option_result_unwrap_unchecked)]
 844     /// let x: Result<u32, &str> = Err("emergency failure");
 845     /// unsafe { x.unwrap_unchecked(); } // Undefined behavior!
 846     /// ```
 847     #[inline]
 848     #[track_caller]
 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());
 852         match self {
 853             Ok(t) => t,
 854             // SAFETY: the safety contract must be upheld by the caller.
 855             Err(_) => unsafe { hint::unreachable_unchecked() },
 856         }
 857     }
 858
 859     /// Returns the contained [`Err`] value, consuming the `self` value,
 860     /// without checking that the value is not an [`Ok`].
 861     ///
 862     /// # Safety
 863     ///
 864     /// Calling this method on an [`Ok`] is *[undefined behavior]*.
 865     ///
 866     /// [undefined behavior]: https://doc.rust-lang.org/reference/behavior-considered-undefined.html
 867     ///
 868     /// # Examples
 869     ///
 870     /// ```no_run
 871     /// #![feature(option_result_unwrap_unchecked)]
 872     /// let x: Result<u32, &str> = Ok(2);
 873     /// unsafe { x.unwrap_err_unchecked() }; // Undefined behavior!
 874     /// ```
 875     ///
 876     /// ```
 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");
 880     /// ```
 881     #[inline]
 882     #[track_caller]
 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());
 886         match self {
 887             // SAFETY: the safety contract must be upheld by the caller.
 888             Ok(_) => unsafe { hint::unreachable_unchecked() },
 889             Err(e) => e,
 890         }
 891     }
 892 }
 893
 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
 896     /// `Ok` part.
 897     ///
 898     /// # Examples
 899     ///
 900     /// ```
 901     /// #![feature(result_copied)]
 902     /// let val = 12;
 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));
 907     /// ```
 908     #[unstable(feature = "result_copied", reason = "newly added", issue = "63168")]
 909     pub fn copied(self) -> Result<T, E> {
 910         self.map(|&t| t)
 911     }
 912 }
 913
 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
 916     /// `Ok` part.
 917     ///
 918     /// # Examples
 919     ///
 920     /// ```
 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));
 927     /// ```
 928     #[unstable(feature = "result_copied", reason = "newly added", issue = "63168")]
 929     pub fn copied(self) -> Result<T, E> {
 930         self.map(|&mut t| t)
 931     }
 932 }
 933
 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
 936     /// `Ok` part.
 937     ///
 938     /// # Examples
 939     ///
 940     /// ```
 941     /// #![feature(result_cloned)]
 942     /// let val = 12;
 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));
 947     /// ```
 948     #[unstable(feature = "result_cloned", reason = "newly added", issue = "63168")]
 949     pub fn cloned(self) -> Result<T, E> {
 950         self.map(|t| t.clone())
 951     }
 952 }
 953
 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
 956     /// `Ok` part.
 957     ///
 958     /// # Examples
 959     ///
 960     /// ```
 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));
 967     /// ```
 968     #[unstable(feature = "result_cloned", reason = "newly added", issue = "63168")]
 969     pub fn cloned(self) -> Result<T, E> {
 970         self.map(|t| t.clone())
 971     }
 972 }
 973
 974 impl<T, E: fmt::Debug> Result<T, E> {
 975     /// Returns the contained [`Ok`] value, consuming the `self` value.
 976     ///
 977     /// # Panics
 978     ///
 979     /// Panics if the value is an [`Err`], with a panic message including the
 980     /// passed message, and the content of the [`Err`].
 981     ///
 982     ///
 983     /// # Examples
 984     ///
 985     /// Basic usage:
 986     ///
 987     /// ```{.should_panic}
 988     /// let x: Result<u32, &str> = Err("emergency failure");
 989     /// x.expect("Testing expect"); // panics with `Testing expect: emergency failure`
 990     /// ```
 991     #[inline]
 992     #[track_caller]
 993     #[stable(feature = "result_expect", since = "1.4.0")]
 994     pub fn expect(self, msg: &str) -> T {
 995         match self {
 996             Ok(t) => t,
 997             Err(e) => unwrap_failed(msg, &e),
 998         }
 999     }
1000
1001     /// Returns the contained [`Ok`] value, consuming the `self` value.
1002     ///
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`].
1007     ///
1008     /// [`unwrap_or`]: Result::unwrap_or
1009     /// [`unwrap_or_else`]: Result::unwrap_or_else
1010     /// [`unwrap_or_default`]: Result::unwrap_or_default
1011     ///
1012     /// # Panics
1013     ///
1014     /// Panics if the value is an [`Err`], with a panic message provided by the
1015     /// [`Err`]'s value.
1016     ///
1017     ///
1018     /// # Examples
1019     ///
1020     /// Basic usage:
1021     ///
1022     /// ```
1023     /// let x: Result<u32, &str> = Ok(2);
1024     /// assert_eq!(x.unwrap(), 2);
1025     /// ```
1026     ///
1027     /// ```{.should_panic}
1028     /// let x: Result<u32, &str> = Err("emergency failure");
1029     /// x.unwrap(); // panics with `emergency failure`
1030     /// ```
1031     #[inline]
1032     #[track_caller]
1033     #[stable(feature = "rust1", since = "1.0.0")]
1034     pub fn unwrap(self) -> T {
1035         match self {
1036             Ok(t) => t,
1037             Err(e) => unwrap_failed("called `Result::unwrap()` on an `Err` value", &e),
1038         }
1039     }
1040 }
1041
1042 impl<T: fmt::Debug, E> Result<T, E> {
1043     /// Returns the contained [`Err`] value, consuming the `self` value.
1044     ///
1045     /// # Panics
1046     ///
1047     /// Panics if the value is an [`Ok`], with a panic message including the
1048     /// passed message, and the content of the [`Ok`].
1049     ///
1050     ///
1051     /// # Examples
1052     ///
1053     /// Basic usage:
1054     ///
1055     /// ```{.should_panic}
1056     /// let x: Result<u32, &str> = Ok(10);
1057     /// x.expect_err("Testing expect_err"); // panics with `Testing expect_err: 10`
1058     /// ```
1059     #[inline]
1060     #[track_caller]
1061     #[stable(feature = "result_expect_err", since = "1.17.0")]
1062     pub fn expect_err(self, msg: &str) -> E {
1063         match self {
1064             Ok(t) => unwrap_failed(msg, &t),
1065             Err(e) => e,
1066         }
1067     }
1068
1069     /// Returns the contained [`Err`] value, consuming the `self` value.
1070     ///
1071     /// # Panics
1072     ///
1073     /// Panics if the value is an [`Ok`], with a custom panic message provided
1074     /// by the [`Ok`]'s value.
1075     ///
1076     /// # Examples
1077     ///
1078     /// ```{.should_panic}
1079     /// let x: Result<u32, &str> = Ok(2);
1080     /// x.unwrap_err(); // panics with `2`
1081     /// ```
1082     ///
1083     /// ```
1084     /// let x: Result<u32, &str> = Err("emergency failure");
1085     /// assert_eq!(x.unwrap_err(), "emergency failure");
1086     /// ```
1087     #[inline]
1088     #[track_caller]
1089     #[stable(feature = "rust1", since = "1.0.0")]
1090     pub fn unwrap_err(self) -> E {
1091         match self {
1092             Ok(t) => unwrap_failed("called `Result::unwrap_err()` on an `Ok` value", &t),
1093             Err(e) => e,
1094         }
1095     }
1096 }
1097
1098 impl<T: Default, E> Result<T, E> {
1099     /// Returns the contained [`Ok`] value or a default
1100     ///
1101     /// Consumes the `self` argument then, if [`Ok`], returns the contained
1102     /// value, otherwise if [`Err`], returns the default value for that
1103     /// type.
1104     ///
1105     /// # Examples
1106     ///
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.
1111     ///
1112     /// ```
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();
1117     ///
1118     /// assert_eq!(1909, good_year);
1119     /// assert_eq!(0, bad_year);
1120     /// ```
1121     ///
1122     /// [`parse`]: str::parse
1123     /// [`FromStr`]: crate::str::FromStr
1124     #[inline]
1125     #[stable(feature = "result_unwrap_or_default", since = "1.16.0")]
1126     pub fn unwrap_or_default(self) -> T {
1127         match self {
1128             Ok(x) => x,
1129             Err(_) => Default::default(),
1130         }
1131     }
1132 }
1133
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.
1137     ///
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.
1143     ///
1144     /// [`unwrap`]: Result::unwrap
1145     ///
1146     /// # Examples
1147     ///
1148     /// Basic usage:
1149     ///
1150     /// ```
1151     /// # #![feature(never_type)]
1152     /// # #![feature(unwrap_infallible)]
1153     ///
1154     /// fn only_good_news() -> Result<String, !> {
1155     ///     Ok("this is fine".into())
1156     /// }
1157     ///
1158     /// let s: String = only_good_news().into_ok();
1159     /// println!("{}", s);
1160     /// ```
1161     #[inline]
1162     pub fn into_ok(self) -> T {
1163         match self {
1164             Ok(x) => x,
1165             Err(e) => e.into(),
1166         }
1167     }
1168 }
1169
1170 impl<T: Deref, E> Result<T, E> {
1171     /// Converts from `Result<T, E>` (or `&Result<T, E>`) to `Result<&<T as Deref>::Target, &E>`.
1172     ///
1173     /// Coerces the [`Ok`] variant of the original [`Result`] via [`Deref`](crate::ops::Deref)
1174     /// and returns the new [`Result`].
1175     ///
1176     /// # Examples
1177     ///
1178     /// ```
1179     /// let x: Result<String, u32> = Ok("hello".to_string());
1180     /// let y: Result<&str, &u32> = Ok("hello");
1181     /// assert_eq!(x.as_deref(), y);
1182     ///
1183     /// let x: Result<String, u32> = Err(42);
1184     /// let y: Result<&str, &u32> = Err(&42);
1185     /// assert_eq!(x.as_deref(), y);
1186     /// ```
1187     #[stable(feature = "inner_deref", since = "1.47.0")]
1188     pub fn as_deref(&self) -> Result<&T::Target, &E> {
1189         self.as_ref().map(|t| t.deref())
1190     }
1191 }
1192
1193 impl<T: DerefMut, E> Result<T, E> {
1194     /// Converts from `Result<T, E>` (or `&mut Result<T, E>`) to `Result<&mut <T as DerefMut>::Target, &mut E>`.
1195     ///
1196     /// Coerces the [`Ok`] variant of the original [`Result`] via [`DerefMut`](crate::ops::DerefMut)
1197     /// and returns the new [`Result`].
1198     ///
1199     /// # Examples
1200     ///
1201     /// ```
1202     /// let mut s = "HELLO".to_string();
1203     /// let mut x: Result<String, u32> = Ok("hello".to_string());
1204     /// let y: Result<&mut str, &mut u32> = Ok(&mut s);
1205     /// assert_eq!(x.as_deref_mut().map(|x| { x.make_ascii_uppercase(); x }), y);
1206     ///
1207     /// let mut i = 42;
1208     /// let mut x: Result<String, u32> = Err(42);
1209     /// let y: Result<&mut str, &mut u32> = Err(&mut i);
1210     /// assert_eq!(x.as_deref_mut().map(|x| { x.make_ascii_uppercase(); x }), y);
1211     /// ```
1212     #[stable(feature = "inner_deref", since = "1.47.0")]
1213     pub fn as_deref_mut(&mut self) -> Result<&mut T::Target, &mut E> {
1214         self.as_mut().map(|t| t.deref_mut())
1215     }
1216 }
1217
1218 impl<T, E> Result<Option<T>, E> {
1219     /// Transposes a `Result` of an `Option` into an `Option` of a `Result`.
1220     ///
1221     /// `Ok(None)` will be mapped to `None`.
1222     /// `Ok(Some(_))` and `Err(_)` will be mapped to `Some(Ok(_))` and `Some(Err(_))`.
1223     ///
1224     /// # Examples
1225     ///
1226     /// ```
1227     /// #[derive(Debug, Eq, PartialEq)]
1228     /// struct SomeErr;
1229     ///
1230     /// let x: Result<Option<i32>, SomeErr> = Ok(Some(5));
1231     /// let y: Option<Result<i32, SomeErr>> = Some(Ok(5));
1232     /// assert_eq!(x.transpose(), y);
1233     /// ```
1234     #[inline]
1235     #[stable(feature = "transpose_result", since = "1.33.0")]
1236     pub fn transpose(self) -> Option<Result<T, E>> {
1237         match self {
1238             Ok(Some(x)) => Some(Ok(x)),
1239             Ok(None) => None,
1240             Err(e) => Some(Err(e)),
1241         }
1242     }
1243 }
1244
1245 impl<T, E> Result<Result<T, E>, E> {
1246     /// Converts from `Result<Result<T, E>, E>` to `Result<T, E>`
1247     ///
1248     /// # Examples
1249     ///
1250     /// Basic usage:
1251     ///
1252     /// ```
1253     /// #![feature(result_flattening)]
1254     /// let x: Result<Result<&'static str, u32>, u32> = Ok(Ok("hello"));
1255     /// assert_eq!(Ok("hello"), x.flatten());
1256     ///
1257     /// let x: Result<Result<&'static str, u32>, u32> = Ok(Err(6));
1258     /// assert_eq!(Err(6), x.flatten());
1259     ///
1260     /// let x: Result<Result<&'static str, u32>, u32> = Err(6);
1261     /// assert_eq!(Err(6), x.flatten());
1262     /// ```
1263     ///
1264     /// Flattening only removes one level of nesting at a time:
1265     ///
1266     /// ```
1267     /// #![feature(result_flattening)]
1268     /// let x: Result<Result<Result<&'static str, u32>, u32>, u32> = Ok(Ok(Ok("hello")));
1269     /// assert_eq!(Ok(Ok("hello")), x.flatten());
1270     /// assert_eq!(Ok("hello"), x.flatten().flatten());
1271     /// ```
1272     #[inline]
1273     #[unstable(feature = "result_flattening", issue = "70142")]
1274     pub fn flatten(self) -> Result<T, E> {
1275         self.and_then(convert::identity)
1276     }
1277 }
1278
1279 // This is a separate function to reduce the code size of the methods
1280 #[inline(never)]
1281 #[cold]
1282 #[track_caller]
1283 fn unwrap_failed(msg: &str, error: &dyn fmt::Debug) -> ! {
1284     panic!("{}: {:?}", msg, error)
1285 }
1286
1287 /////////////////////////////////////////////////////////////////////////////
1288 // Trait implementations
1289 /////////////////////////////////////////////////////////////////////////////
1290
1291 #[stable(feature = "rust1", since = "1.0.0")]
1292 impl<T: Clone, E: Clone> Clone for Result<T, E> {
1293     #[inline]
1294     fn clone(&self) -> Self {
1295         match self {
1296             Ok(x) => Ok(x.clone()),
1297             Err(x) => Err(x.clone()),
1298         }
1299     }
1300
1301     #[inline]
1302     fn clone_from(&mut self, source: &Self) {
1303         match (self, source) {
1304             (Ok(to), Ok(from)) => to.clone_from(from),
1305             (Err(to), Err(from)) => to.clone_from(from),
1306             (to, from) => *to = from.clone(),
1307         }
1308     }
1309 }
1310
1311 #[stable(feature = "rust1", since = "1.0.0")]
1312 impl<T, E> IntoIterator for Result<T, E> {
1313     type Item = T;
1314     type IntoIter = IntoIter<T>;
1315
1316     /// Returns a consuming iterator over the possibly contained value.
1317     ///
1318     /// The iterator yields one value if the result is [`Result::Ok`], otherwise none.
1319     ///
1320     /// # Examples
1321     ///
1322     /// Basic usage:
1323     ///
1324     /// ```
1325     /// let x: Result<u32, &str> = Ok(5);
1326     /// let v: Vec<u32> = x.into_iter().collect();
1327     /// assert_eq!(v, [5]);
1328     ///
1329     /// let x: Result<u32, &str> = Err("nothing!");
1330     /// let v: Vec<u32> = x.into_iter().collect();
1331     /// assert_eq!(v, []);
1332     /// ```
1333     #[inline]
1334     fn into_iter(self) -> IntoIter<T> {
1335         IntoIter { inner: self.ok() }
1336     }
1337 }
1338
1339 #[stable(since = "1.4.0", feature = "result_iter")]
1340 impl<'a, T, E> IntoIterator for &'a Result<T, E> {
1341     type Item = &'a T;
1342     type IntoIter = Iter<'a, T>;
1343
1344     fn into_iter(self) -> Iter<'a, T> {
1345         self.iter()
1346     }
1347 }
1348
1349 #[stable(since = "1.4.0", feature = "result_iter")]
1350 impl<'a, T, E> IntoIterator for &'a mut Result<T, E> {
1351     type Item = &'a mut T;
1352     type IntoIter = IterMut<'a, T>;
1353
1354     fn into_iter(self) -> IterMut<'a, T> {
1355         self.iter_mut()
1356     }
1357 }
1358
1359 /////////////////////////////////////////////////////////////////////////////
1360 // The Result Iterators
1361 /////////////////////////////////////////////////////////////////////////////
1362
1363 /// An iterator over a reference to the [`Ok`] variant of a [`Result`].
1364 ///
1365 /// The iterator yields one value if the result is [`Ok`], otherwise none.
1366 ///
1367 /// Created by [`Result::iter`].
1368 #[derive(Debug)]
1369 #[stable(feature = "rust1", since = "1.0.0")]
1370 pub struct Iter<'a, T: 'a> {
1371     inner: Option<&'a T>,
1372 }
1373
1374 #[stable(feature = "rust1", since = "1.0.0")]
1375 impl<'a, T> Iterator for Iter<'a, T> {
1376     type Item = &'a T;
1377
1378     #[inline]
1379     fn next(&mut self) -> Option<&'a T> {
1380         self.inner.take()
1381     }
1382     #[inline]
1383     fn size_hint(&self) -> (usize, Option<usize>) {
1384         let n = if self.inner.is_some() { 1 } else { 0 };
1385         (n, Some(n))
1386     }
1387 }
1388
1389 #[stable(feature = "rust1", since = "1.0.0")]
1390 impl<'a, T> DoubleEndedIterator for Iter<'a, T> {
1391     #[inline]
1392     fn next_back(&mut self) -> Option<&'a T> {
1393         self.inner.take()
1394     }
1395 }
1396
1397 #[stable(feature = "rust1", since = "1.0.0")]
1398 impl<T> ExactSizeIterator for Iter<'_, T> {}
1399
1400 #[stable(feature = "fused", since = "1.26.0")]
1401 impl<T> FusedIterator for Iter<'_, T> {}
1402
1403 #[unstable(feature = "trusted_len", issue = "37572")]
1404 unsafe impl<A> TrustedLen for Iter<'_, A> {}
1405
1406 #[stable(feature = "rust1", since = "1.0.0")]
1407 impl<T> Clone for Iter<'_, T> {
1408     #[inline]
1409     fn clone(&self) -> Self {
1410         Iter { inner: self.inner }
1411     }
1412 }
1413
1414 /// An iterator over a mutable reference to the [`Ok`] variant of a [`Result`].
1415 ///
1416 /// Created by [`Result::iter_mut`].
1417 #[derive(Debug)]
1418 #[stable(feature = "rust1", since = "1.0.0")]
1419 pub struct IterMut<'a, T: 'a> {
1420     inner: Option<&'a mut T>,
1421 }
1422
1423 #[stable(feature = "rust1", since = "1.0.0")]
1424 impl<'a, T> Iterator for IterMut<'a, T> {
1425     type Item = &'a mut T;
1426
1427     #[inline]
1428     fn next(&mut self) -> Option<&'a mut T> {
1429         self.inner.take()
1430     }
1431     #[inline]
1432     fn size_hint(&self) -> (usize, Option<usize>) {
1433         let n = if self.inner.is_some() { 1 } else { 0 };
1434         (n, Some(n))
1435     }
1436 }
1437
1438 #[stable(feature = "rust1", since = "1.0.0")]
1439 impl<'a, T> DoubleEndedIterator for IterMut<'a, T> {
1440     #[inline]
1441     fn next_back(&mut self) -> Option<&'a mut T> {
1442         self.inner.take()
1443     }
1444 }
1445
1446 #[stable(feature = "rust1", since = "1.0.0")]
1447 impl<T> ExactSizeIterator for IterMut<'_, T> {}
1448
1449 #[stable(feature = "fused", since = "1.26.0")]
1450 impl<T> FusedIterator for IterMut<'_, T> {}
1451
1452 #[unstable(feature = "trusted_len", issue = "37572")]
1453 unsafe impl<A> TrustedLen for IterMut<'_, A> {}
1454
1455 /// An iterator over the value in a [`Ok`] variant of a [`Result`].
1456 ///
1457 /// The iterator yields one value if the result is [`Ok`], otherwise none.
1458 ///
1459 /// This struct is created by the [`into_iter`] method on
1460 /// [`Result`] (provided by the [`IntoIterator`] trait).
1461 ///
1462 /// [`into_iter`]: IntoIterator::into_iter
1463 #[derive(Clone, Debug)]
1464 #[stable(feature = "rust1", since = "1.0.0")]
1465 pub struct IntoIter<T> {
1466     inner: Option<T>,
1467 }
1468
1469 #[stable(feature = "rust1", since = "1.0.0")]
1470 impl<T> Iterator for IntoIter<T> {
1471     type Item = T;
1472
1473     #[inline]
1474     fn next(&mut self) -> Option<T> {
1475         self.inner.take()
1476     }
1477     #[inline]
1478     fn size_hint(&self) -> (usize, Option<usize>) {
1479         let n = if self.inner.is_some() { 1 } else { 0 };
1480         (n, Some(n))
1481     }
1482 }
1483
1484 #[stable(feature = "rust1", since = "1.0.0")]
1485 impl<T> DoubleEndedIterator for IntoIter<T> {
1486     #[inline]
1487     fn next_back(&mut self) -> Option<T> {
1488         self.inner.take()
1489     }
1490 }
1491
1492 #[stable(feature = "rust1", since = "1.0.0")]
1493 impl<T> ExactSizeIterator for IntoIter<T> {}
1494
1495 #[stable(feature = "fused", since = "1.26.0")]
1496 impl<T> FusedIterator for IntoIter<T> {}
1497
1498 #[unstable(feature = "trusted_len", issue = "37572")]
1499 unsafe impl<A> TrustedLen for IntoIter<A> {}
1500
1501 /////////////////////////////////////////////////////////////////////////////
1502 // FromIterator
1503 /////////////////////////////////////////////////////////////////////////////
1504
1505 #[stable(feature = "rust1", since = "1.0.0")]
1506 impl<A, E, V: FromIterator<A>> FromIterator<Result<A, E>> for Result<V, E> {
1507     /// Takes each element in the `Iterator`: if it is an `Err`, no further
1508     /// elements are taken, and the `Err` is returned. Should no `Err` occur, a
1509     /// container with the values of each `Result` is returned.
1510     ///
1511     /// Here is an example which increments every integer in a vector,
1512     /// checking for overflow:
1513     ///
1514     /// ```
1515     /// let v = vec![1, 2];
1516     /// let res: Result<Vec<u32>, &'static str> = v.iter().map(|x: &u32|
1517     ///     x.checked_add(1).ok_or("Overflow!")
1518     /// ).collect();
1519     /// assert_eq!(res, Ok(vec![2, 3]));
1520     /// ```
1521     ///
1522     /// Here is another example that tries to subtract one from another list
1523     /// of integers, this time checking for underflow:
1524     ///
1525     /// ```
1526     /// let v = vec![1, 2, 0];
1527     /// let res: Result<Vec<u32>, &'static str> = v.iter().map(|x: &u32|
1528     ///     x.checked_sub(1).ok_or("Underflow!")
1529     /// ).collect();
1530     /// assert_eq!(res, Err("Underflow!"));
1531     /// ```
1532     ///
1533     /// Here is a variation on the previous example, showing that no
1534     /// further elements are taken from `iter` after the first `Err`.
1535     ///
1536     /// ```
1537     /// let v = vec![3, 2, 1, 10];
1538     /// let mut shared = 0;
1539     /// let res: Result<Vec<u32>, &'static str> = v.iter().map(|x: &u32| {
1540     ///     shared += x;
1541     ///     x.checked_sub(2).ok_or("Underflow!")
1542     /// }).collect();
1543     /// assert_eq!(res, Err("Underflow!"));
1544     /// assert_eq!(shared, 6);
1545     /// ```
1546     ///
1547     /// Since the third element caused an underflow, no further elements were taken,
1548     /// so the final value of `shared` is 6 (= `3 + 2 + 1`), not 16.
1549     #[inline]
1550     fn from_iter<I: IntoIterator<Item = Result<A, E>>>(iter: I) -> Result<V, E> {
1551         // FIXME(#11084): This could be replaced with Iterator::scan when this
1552         // performance bug is closed.
1553
1554         iter::process_results(iter.into_iter(), |i| i.collect())
1555     }
1556 }
1557
1558 #[unstable(feature = "try_trait", issue = "42327")]
1559 impl<T, E> ops::Try for Result<T, E> {
1560     type Ok = T;
1561     type Error = E;
1562
1563     #[inline]
1564     fn into_result(self) -> Self {
1565         self
1566     }
1567
1568     #[inline]
1569     fn from_ok(v: T) -> Self {
1570         Ok(v)
1571     }
1572
1573     #[inline]
1574     fn from_error(v: E) -> Self {
1575         Err(v)
1576     }
1577 }