src/libcore/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`]: enum.Result.html#method.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 //! [`?`]: ../../std/macro.try.html
 224 //! [`Result`]: enum.Result.html
 225 //! [`Ok(T)`]: enum.Result.html#variant.Ok
 226 //! [`Err(E)`]: enum.Result.html#variant.Err
 227 //! [`io::Error`]: ../../std/io/struct.Error.html
 228 //! [`Ok`]: enum.Result.html#variant.Ok
 229 //! [`Err`]: enum.Result.html#variant.Err
 230
 231 #![stable(feature = "rust1", since = "1.0.0")]
 232
 233 use crate::fmt;
 234 use crate::iter::{self, FromIterator, FusedIterator, TrustedLen};
 235 use crate::ops::{self, Deref, DerefMut};
 236
 237 /// `Result` is a type that represents either success ([`Ok`]) or failure ([`Err`]).
 238 ///
 239 /// See the [`std::result`](index.html) module documentation for details.
 240 ///
 241 /// [`Ok`]: enum.Result.html#variant.Ok
 242 /// [`Err`]: enum.Result.html#variant.Err
 243 #[derive(Copy, PartialEq, PartialOrd, Eq, Ord, Debug, Hash)]
 244 #[must_use = "this `Result` may be an `Err` variant, which should be handled"]
 245 #[rustc_diagnostic_item = "result_type"]
 246 #[stable(feature = "rust1", since = "1.0.0")]
 247 pub enum Result<T, E> {
 248     /// Contains the success value
 249     #[stable(feature = "rust1", since = "1.0.0")]
 250     Ok(#[stable(feature = "rust1", since = "1.0.0")] T),
 251
 252     /// Contains the error value
 253     #[stable(feature = "rust1", since = "1.0.0")]
 254     Err(#[stable(feature = "rust1", since = "1.0.0")] E),
 255 }
 256
 257 /////////////////////////////////////////////////////////////////////////////
 258 // Type implementation
 259 /////////////////////////////////////////////////////////////////////////////
 260
 261 impl<T, E> Result<T, E> {
 262     /////////////////////////////////////////////////////////////////////////
 263     // Querying the contained values
 264     /////////////////////////////////////////////////////////////////////////
 265
 266     /// Returns `true` if the result is [`Ok`].
 267     ///
 268     /// [`Ok`]: enum.Result.html#variant.Ok
 269     ///
 270     /// # Examples
 271     ///
 272     /// Basic usage:
 273     ///
 274     /// ```
 275     /// let x: Result<i32, &str> = Ok(-3);
 276     /// assert_eq!(x.is_ok(), true);
 277     ///
 278     /// let x: Result<i32, &str> = Err("Some error message");
 279     /// assert_eq!(x.is_ok(), false);
 280     /// ```
 281     #[must_use = "if you intended to assert that this is ok, consider `.unwrap()` instead"]
 282     #[rustc_const_unstable(feature = "const_result", issue = "67520")]
 283     #[inline]
 284     #[stable(feature = "rust1", since = "1.0.0")]
 285     pub const fn is_ok(&self) -> bool {
 286         matches!(*self, Ok(_))
 287     }
 288
 289     /// Returns `true` if the result is [`Err`].
 290     ///
 291     /// [`Err`]: enum.Result.html#variant.Err
 292     ///
 293     /// # Examples
 294     ///
 295     /// Basic usage:
 296     ///
 297     /// ```
 298     /// let x: Result<i32, &str> = Ok(-3);
 299     /// assert_eq!(x.is_err(), false);
 300     ///
 301     /// let x: Result<i32, &str> = Err("Some error message");
 302     /// assert_eq!(x.is_err(), true);
 303     /// ```
 304     #[must_use = "if you intended to assert that this is err, consider `.unwrap_err()` instead"]
 305     #[rustc_const_unstable(feature = "const_result", issue = "67520")]
 306     #[inline]
 307     #[stable(feature = "rust1", since = "1.0.0")]
 308     pub const fn is_err(&self) -> bool {
 309         !self.is_ok()
 310     }
 311
 312     /// Returns `true` if the result is an [`Ok`] value containing the given value.
 313     ///
 314     /// # Examples
 315     ///
 316     /// ```
 317     /// #![feature(option_result_contains)]
 318     ///
 319     /// let x: Result<u32, &str> = Ok(2);
 320     /// assert_eq!(x.contains(&2), true);
 321     ///
 322     /// let x: Result<u32, &str> = Ok(3);
 323     /// assert_eq!(x.contains(&2), false);
 324     ///
 325     /// let x: Result<u32, &str> = Err("Some error message");
 326     /// assert_eq!(x.contains(&2), false);
 327     /// ```
 328     #[must_use]
 329     #[inline]
 330     #[unstable(feature = "option_result_contains", issue = "62358")]
 331     pub fn contains<U>(&self, x: &U) -> bool
 332     where
 333         U: PartialEq<T>,
 334     {
 335         match self {
 336             Ok(y) => x == y,
 337             Err(_) => false,
 338         }
 339     }
 340
 341     /// Returns `true` if the result is an [`Err`] value containing the given value.
 342     ///
 343     /// # Examples
 344     ///
 345     /// ```
 346     /// #![feature(result_contains_err)]
 347     ///
 348     /// let x: Result<u32, &str> = Ok(2);
 349     /// assert_eq!(x.contains_err(&"Some error message"), false);
 350     ///
 351     /// let x: Result<u32, &str> = Err("Some error message");
 352     /// assert_eq!(x.contains_err(&"Some error message"), true);
 353     ///
 354     /// let x: Result<u32, &str> = Err("Some other error message");
 355     /// assert_eq!(x.contains_err(&"Some error message"), false);
 356     /// ```
 357     #[must_use]
 358     #[inline]
 359     #[unstable(feature = "result_contains_err", issue = "62358")]
 360     pub fn contains_err<F>(&self, f: &F) -> bool
 361     where
 362         F: PartialEq<E>,
 363     {
 364         match self {
 365             Ok(_) => false,
 366             Err(e) => f == e,
 367         }
 368     }
 369
 370     /////////////////////////////////////////////////////////////////////////
 371     // Adapter for each variant
 372     /////////////////////////////////////////////////////////////////////////
 373
 374     /// Converts from `Result<T, E>` to [`Option<T>`].
 375     ///
 376     /// Converts `self` into an [`Option<T>`], consuming `self`,
 377     /// and discarding the error, if any.
 378     ///
 379     /// [`Option<T>`]: ../../std/option/enum.Option.html
 380     ///
 381     /// # Examples
 382     ///
 383     /// Basic usage:
 384     ///
 385     /// ```
 386     /// let x: Result<u32, &str> = Ok(2);
 387     /// assert_eq!(x.ok(), Some(2));
 388     ///
 389     /// let x: Result<u32, &str> = Err("Nothing here");
 390     /// assert_eq!(x.ok(), None);
 391     /// ```
 392     #[inline]
 393     #[stable(feature = "rust1", since = "1.0.0")]
 394     pub fn ok(self) -> Option<T> {
 395         match self {
 396             Ok(x) => Some(x),
 397             Err(_) => None,
 398         }
 399     }
 400
 401     /// Converts from `Result<T, E>` to [`Option<E>`].
 402     ///
 403     /// Converts `self` into an [`Option<E>`], consuming `self`,
 404     /// and discarding the success value, if any.
 405     ///
 406     /// [`Option<E>`]: ../../std/option/enum.Option.html
 407     ///
 408     /// # Examples
 409     ///
 410     /// Basic usage:
 411     ///
 412     /// ```
 413     /// let x: Result<u32, &str> = Ok(2);
 414     /// assert_eq!(x.err(), None);
 415     ///
 416     /// let x: Result<u32, &str> = Err("Nothing here");
 417     /// assert_eq!(x.err(), Some("Nothing here"));
 418     /// ```
 419     #[inline]
 420     #[stable(feature = "rust1", since = "1.0.0")]
 421     pub fn err(self) -> Option<E> {
 422         match self {
 423             Ok(_) => None,
 424             Err(x) => Some(x),
 425         }
 426     }
 427
 428     /////////////////////////////////////////////////////////////////////////
 429     // Adapter for working with references
 430     /////////////////////////////////////////////////////////////////////////
 431
 432     /// Converts from `&Result<T, E>` to `Result<&T, &E>`.
 433     ///
 434     /// Produces a new `Result`, containing a reference
 435     /// into the original, leaving the original in place.
 436     ///
 437     /// # Examples
 438     ///
 439     /// Basic usage:
 440     ///
 441     /// ```
 442     /// let x: Result<u32, &str> = Ok(2);
 443     /// assert_eq!(x.as_ref(), Ok(&2));
 444     ///
 445     /// let x: Result<u32, &str> = Err("Error");
 446     /// assert_eq!(x.as_ref(), Err(&"Error"));
 447     /// ```
 448     #[inline]
 449     #[rustc_const_unstable(feature = "const_result", issue = "67520")]
 450     #[stable(feature = "rust1", since = "1.0.0")]
 451     pub const fn as_ref(&self) -> Result<&T, &E> {
 452         match *self {
 453             Ok(ref x) => Ok(x),
 454             Err(ref x) => Err(x),
 455         }
 456     }
 457
 458     /// Converts from `&mut Result<T, E>` to `Result<&mut T, &mut E>`.
 459     ///
 460     /// # Examples
 461     ///
 462     /// Basic usage:
 463     ///
 464     /// ```
 465     /// fn mutate(r: &mut Result<i32, i32>) {
 466     ///     match r.as_mut() {
 467     ///         Ok(v) => *v = 42,
 468     ///         Err(e) => *e = 0,
 469     ///     }
 470     /// }
 471     ///
 472     /// let mut x: Result<i32, i32> = Ok(2);
 473     /// mutate(&mut x);
 474     /// assert_eq!(x.unwrap(), 42);
 475     ///
 476     /// let mut x: Result<i32, i32> = Err(13);
 477     /// mutate(&mut x);
 478     /// assert_eq!(x.unwrap_err(), 0);
 479     /// ```
 480     #[inline]
 481     #[stable(feature = "rust1", since = "1.0.0")]
 482     pub fn as_mut(&mut self) -> Result<&mut T, &mut E> {
 483         match *self {
 484             Ok(ref mut x) => Ok(x),
 485             Err(ref mut x) => Err(x),
 486         }
 487     }
 488
 489     /////////////////////////////////////////////////////////////////////////
 490     // Transforming contained values
 491     /////////////////////////////////////////////////////////////////////////
 492
 493     /// Maps a `Result<T, E>` to `Result<U, E>` by applying a function to a
 494     /// contained [`Ok`] value, leaving an [`Err`] value untouched.
 495     ///
 496     /// This function can be used to compose the results of two functions.
 497     ///
 498     /// [`Ok`]: enum.Result.html#variant.Ok
 499     /// [`Err`]: enum.Result.html#variant.Err
 500     ///
 501     /// # Examples
 502     ///
 503     /// Print the numbers on each line of a string multiplied by two.
 504     ///
 505     /// ```
 506     /// let line = "1\n2\n3\n4\n";
 507     ///
 508     /// for num in line.lines() {
 509     ///     match num.parse::<i32>().map(|i| i * 2) {
 510     ///         Ok(n) => println!("{}", n),
 511     ///         Err(..) => {}
 512     ///     }
 513     /// }
 514     /// ```
 515     #[inline]
 516     #[stable(feature = "rust1", since = "1.0.0")]
 517     pub fn map<U, F: FnOnce(T) -> U>(self, op: F) -> Result<U, E> {
 518         match self {
 519             Ok(t) => Ok(op(t)),
 520             Err(e) => Err(e),
 521         }
 522     }
 523
 524     /// Applies a function to the contained value (if any),
 525     /// or returns the provided default (if not).
 526     ///
 527     /// Arguments passed to `map_or` are eagerly evaluated; if you are passing
 528     /// the result of a function call, it is recommended to use [`map_or_else`],
 529     /// which is lazily evaluated.
 530     ///
 531     /// [`map_or_else`]: #method.map_or_else
 532     ///
 533     /// # Examples
 534     ///
 535     /// ```
 536     /// let x: Result<_, &str> = Ok("foo");
 537     /// assert_eq!(x.map_or(42, |v| v.len()), 3);
 538     ///
 539     /// let x: Result<&str, _> = Err("bar");
 540     /// assert_eq!(x.map_or(42, |v| v.len()), 42);
 541     /// ```
 542     #[inline]
 543     #[stable(feature = "result_map_or", since = "1.41.0")]
 544     pub fn map_or<U, F: FnOnce(T) -> U>(self, default: U, f: F) -> U {
 545         match self {
 546             Ok(t) => f(t),
 547             Err(_) => default,
 548         }
 549     }
 550
 551     /// Maps a `Result<T, E>` to `U` by applying a function to a
 552     /// contained [`Ok`] value, or a fallback function to a
 553     /// contained [`Err`] value.
 554     ///
 555     /// This function can be used to unpack a successful result
 556     /// while handling an error.
 557     ///
 558     /// [`Ok`]: enum.Result.html#variant.Ok
 559     /// [`Err`]: enum.Result.html#variant.Err
 560     ///
 561     /// # Examples
 562     ///
 563     /// Basic usage:
 564     ///
 565     /// ```
 566     /// let k = 21;
 567     ///
 568     /// let x : Result<_, &str> = Ok("foo");
 569     /// assert_eq!(x.map_or_else(|e| k * 2, |v| v.len()), 3);
 570     ///
 571     /// let x : Result<&str, _> = Err("bar");
 572     /// assert_eq!(x.map_or_else(|e| k * 2, |v| v.len()), 42);
 573     /// ```
 574     #[inline]
 575     #[stable(feature = "result_map_or_else", since = "1.41.0")]
 576     pub fn map_or_else<U, D: FnOnce(E) -> U, F: FnOnce(T) -> U>(self, default: D, f: F) -> U {
 577         match self {
 578             Ok(t) => f(t),
 579             Err(e) => default(e),
 580         }
 581     }
 582
 583     /// Maps a `Result<T, E>` to `Result<T, F>` by applying a function to a
 584     /// contained [`Err`] value, leaving an [`Ok`] value untouched.
 585     ///
 586     /// This function can be used to pass through a successful result while handling
 587     /// an error.
 588     ///
 589     /// [`Ok`]: enum.Result.html#variant.Ok
 590     /// [`Err`]: enum.Result.html#variant.Err
 591     ///
 592     /// # Examples
 593     ///
 594     /// Basic usage:
 595     ///
 596     /// ```
 597     /// fn stringify(x: u32) -> String { format!("error code: {}", x) }
 598     ///
 599     /// let x: Result<u32, u32> = Ok(2);
 600     /// assert_eq!(x.map_err(stringify), Ok(2));
 601     ///
 602     /// let x: Result<u32, u32> = Err(13);
 603     /// assert_eq!(x.map_err(stringify), Err("error code: 13".to_string()));
 604     /// ```
 605     #[inline]
 606     #[stable(feature = "rust1", since = "1.0.0")]
 607     pub fn map_err<F, O: FnOnce(E) -> F>(self, op: O) -> Result<T, F> {
 608         match self {
 609             Ok(t) => Ok(t),
 610             Err(e) => Err(op(e)),
 611         }
 612     }
 613
 614     /////////////////////////////////////////////////////////////////////////
 615     // Iterator constructors
 616     /////////////////////////////////////////////////////////////////////////
 617
 618     /// Returns an iterator over the possibly contained value.
 619     ///
 620     /// The iterator yields one value if the result is [`Result::Ok`], otherwise none.
 621     ///
 622     /// # Examples
 623     ///
 624     /// Basic usage:
 625     ///
 626     /// ```
 627     /// let x: Result<u32, &str> = Ok(7);
 628     /// assert_eq!(x.iter().next(), Some(&7));
 629     ///
 630     /// let x: Result<u32, &str> = Err("nothing!");
 631     /// assert_eq!(x.iter().next(), None);
 632     /// ```
 633     #[inline]
 634     #[stable(feature = "rust1", since = "1.0.0")]
 635     pub fn iter(&self) -> Iter<'_, T> {
 636         Iter { inner: self.as_ref().ok() }
 637     }
 638
 639     /// Returns a mutable iterator over the possibly contained value.
 640     ///
 641     /// The iterator yields one value if the result is [`Result::Ok`], otherwise none.
 642     ///
 643     /// # Examples
 644     ///
 645     /// Basic usage:
 646     ///
 647     /// ```
 648     /// let mut x: Result<u32, &str> = Ok(7);
 649     /// match x.iter_mut().next() {
 650     ///     Some(v) => *v = 40,
 651     ///     None => {},
 652     /// }
 653     /// assert_eq!(x, Ok(40));
 654     ///
 655     /// let mut x: Result<u32, &str> = Err("nothing!");
 656     /// assert_eq!(x.iter_mut().next(), None);
 657     /// ```
 658     #[inline]
 659     #[stable(feature = "rust1", since = "1.0.0")]
 660     pub fn iter_mut(&mut self) -> IterMut<'_, T> {
 661         IterMut { inner: self.as_mut().ok() }
 662     }
 663
 664     ////////////////////////////////////////////////////////////////////////
 665     // Boolean operations on the values, eager and lazy
 666     /////////////////////////////////////////////////////////////////////////
 667
 668     /// Returns `res` if the result is [`Ok`], otherwise returns the [`Err`] value of `self`.
 669     ///
 670     /// [`Ok`]: enum.Result.html#variant.Ok
 671     /// [`Err`]: enum.Result.html#variant.Err
 672     ///
 673     /// # Examples
 674     ///
 675     /// Basic usage:
 676     ///
 677     /// ```
 678     /// let x: Result<u32, &str> = Ok(2);
 679     /// let y: Result<&str, &str> = Err("late error");
 680     /// assert_eq!(x.and(y), Err("late error"));
 681     ///
 682     /// let x: Result<u32, &str> = Err("early error");
 683     /// let y: Result<&str, &str> = Ok("foo");
 684     /// assert_eq!(x.and(y), Err("early error"));
 685     ///
 686     /// let x: Result<u32, &str> = Err("not a 2");
 687     /// let y: Result<&str, &str> = Err("late error");
 688     /// assert_eq!(x.and(y), Err("not a 2"));
 689     ///
 690     /// let x: Result<u32, &str> = Ok(2);
 691     /// let y: Result<&str, &str> = Ok("different result type");
 692     /// assert_eq!(x.and(y), Ok("different result type"));
 693     /// ```
 694     #[inline]
 695     #[stable(feature = "rust1", since = "1.0.0")]
 696     pub fn and<U>(self, res: Result<U, E>) -> Result<U, E> {
 697         match self {
 698             Ok(_) => res,
 699             Err(e) => Err(e),
 700         }
 701     }
 702
 703     /// Calls `op` if the result is [`Ok`], otherwise returns the [`Err`] value of `self`.
 704     ///
 705     /// [`Ok`]: enum.Result.html#variant.Ok
 706     /// [`Err`]: enum.Result.html#variant.Err
 707     ///
 708     /// This function can be used for control flow based on `Result` values.
 709     ///
 710     /// # Examples
 711     ///
 712     /// Basic usage:
 713     ///
 714     /// ```
 715     /// fn sq(x: u32) -> Result<u32, u32> { Ok(x * x) }
 716     /// fn err(x: u32) -> Result<u32, u32> { Err(x) }
 717     ///
 718     /// assert_eq!(Ok(2).and_then(sq).and_then(sq), Ok(16));
 719     /// assert_eq!(Ok(2).and_then(sq).and_then(err), Err(4));
 720     /// assert_eq!(Ok(2).and_then(err).and_then(sq), Err(2));
 721     /// assert_eq!(Err(3).and_then(sq).and_then(sq), Err(3));
 722     /// ```
 723     #[inline]
 724     #[stable(feature = "rust1", since = "1.0.0")]
 725     pub fn and_then<U, F: FnOnce(T) -> Result<U, E>>(self, op: F) -> Result<U, E> {
 726         match self {
 727             Ok(t) => op(t),
 728             Err(e) => Err(e),
 729         }
 730     }
 731
 732     /// Returns `res` if the result is [`Err`], otherwise returns the [`Ok`] value of `self`.
 733     ///
 734     /// Arguments passed to `or` are eagerly evaluated; if you are passing the
 735     /// result of a function call, it is recommended to use [`or_else`], which is
 736     /// lazily evaluated.
 737     ///
 738     /// [`Ok`]: enum.Result.html#variant.Ok
 739     /// [`Err`]: enum.Result.html#variant.Err
 740     /// [`or_else`]: #method.or_else
 741     ///
 742     /// # Examples
 743     ///
 744     /// Basic usage:
 745     ///
 746     /// ```
 747     /// let x: Result<u32, &str> = Ok(2);
 748     /// let y: Result<u32, &str> = Err("late error");
 749     /// assert_eq!(x.or(y), Ok(2));
 750     ///
 751     /// let x: Result<u32, &str> = Err("early error");
 752     /// let y: Result<u32, &str> = Ok(2);
 753     /// assert_eq!(x.or(y), Ok(2));
 754     ///
 755     /// let x: Result<u32, &str> = Err("not a 2");
 756     /// let y: Result<u32, &str> = Err("late error");
 757     /// assert_eq!(x.or(y), Err("late error"));
 758     ///
 759     /// let x: Result<u32, &str> = Ok(2);
 760     /// let y: Result<u32, &str> = Ok(100);
 761     /// assert_eq!(x.or(y), Ok(2));
 762     /// ```
 763     #[inline]
 764     #[stable(feature = "rust1", since = "1.0.0")]
 765     pub fn or<F>(self, res: Result<T, F>) -> Result<T, F> {
 766         match self {
 767             Ok(v) => Ok(v),
 768             Err(_) => res,
 769         }
 770     }
 771
 772     /// Calls `op` if the result is [`Err`], otherwise returns the [`Ok`] value of `self`.
 773     ///
 774     /// This function can be used for control flow based on result values.
 775     ///
 776     /// [`Ok`]: enum.Result.html#variant.Ok
 777     /// [`Err`]: enum.Result.html#variant.Err
 778     ///
 779     /// # Examples
 780     ///
 781     /// Basic usage:
 782     ///
 783     /// ```
 784     /// fn sq(x: u32) -> Result<u32, u32> { Ok(x * x) }
 785     /// fn err(x: u32) -> Result<u32, u32> { Err(x) }
 786     ///
 787     /// assert_eq!(Ok(2).or_else(sq).or_else(sq), Ok(2));
 788     /// assert_eq!(Ok(2).or_else(err).or_else(sq), Ok(2));
 789     /// assert_eq!(Err(3).or_else(sq).or_else(err), Ok(9));
 790     /// assert_eq!(Err(3).or_else(err).or_else(err), Err(3));
 791     /// ```
 792     #[inline]
 793     #[stable(feature = "rust1", since = "1.0.0")]
 794     pub fn or_else<F, O: FnOnce(E) -> Result<T, F>>(self, op: O) -> Result<T, F> {
 795         match self {
 796             Ok(t) => Ok(t),
 797             Err(e) => op(e),
 798         }
 799     }
 800
 801     /// Returns the contained [`Ok`] value or a provided default.
 802     ///
 803     /// Arguments passed to `unwrap_or` are eagerly evaluated; if you are passing
 804     /// the result of a function call, it is recommended to use [`unwrap_or_else`],
 805     /// which is lazily evaluated.
 806     ///
 807     /// [`Ok`]: enum.Result.html#variant.Ok
 808     /// [`Err`]: enum.Result.html#variant.Err
 809     /// [`unwrap_or_else`]: #method.unwrap_or_else
 810     ///
 811     /// # Examples
 812     ///
 813     /// Basic usage:
 814     ///
 815     /// ```
 816     /// let default = 2;
 817     /// let x: Result<u32, &str> = Ok(9);
 818     /// assert_eq!(x.unwrap_or(default), 9);
 819     ///
 820     /// let x: Result<u32, &str> = Err("error");
 821     /// assert_eq!(x.unwrap_or(default), default);
 822     /// ```
 823     #[inline]
 824     #[stable(feature = "rust1", since = "1.0.0")]
 825     pub fn unwrap_or(self, default: T) -> T {
 826         match self {
 827             Ok(t) => t,
 828             Err(_) => default,
 829         }
 830     }
 831
 832     /// Returns the contained [`Ok`] value or computes it from a closure.
 833     ///
 834     /// [`Ok`]: enum.Result.html#variant.Ok
 835     ///
 836     /// # Examples
 837     ///
 838     /// Basic usage:
 839     ///
 840     /// ```
 841     /// fn count(x: &str) -> usize { x.len() }
 842     ///
 843     /// assert_eq!(Ok(2).unwrap_or_else(count), 2);
 844     /// assert_eq!(Err("foo").unwrap_or_else(count), 3);
 845     /// ```
 846     #[inline]
 847     #[stable(feature = "rust1", since = "1.0.0")]
 848     pub fn unwrap_or_else<F: FnOnce(E) -> T>(self, op: F) -> T {
 849         match self {
 850             Ok(t) => t,
 851             Err(e) => op(e),
 852         }
 853     }
 854 }
 855
 856 impl<T: Copy, E> Result<&T, E> {
 857     /// Maps a `Result<&T, E>` to a `Result<T, E>` by copying the contents of the
 858     /// `Ok` part.
 859     ///
 860     /// # Examples
 861     ///
 862     /// ```
 863     /// #![feature(result_copied)]
 864     /// let val = 12;
 865     /// let x: Result<&i32, i32> = Ok(&val);
 866     /// assert_eq!(x, Ok(&12));
 867     /// let copied = x.copied();
 868     /// assert_eq!(copied, Ok(12));
 869     /// ```
 870     #[unstable(feature = "result_copied", reason = "newly added", issue = "63168")]
 871     pub fn copied(self) -> Result<T, E> {
 872         self.map(|&t| t)
 873     }
 874 }
 875
 876 impl<T: Copy, E> Result<&mut T, E> {
 877     /// Maps a `Result<&mut T, E>` to a `Result<T, E>` by copying the contents of the
 878     /// `Ok` part.
 879     ///
 880     /// # Examples
 881     ///
 882     /// ```
 883     /// #![feature(result_copied)]
 884     /// let mut val = 12;
 885     /// let x: Result<&mut i32, i32> = Ok(&mut val);
 886     /// assert_eq!(x, Ok(&mut 12));
 887     /// let copied = x.copied();
 888     /// assert_eq!(copied, Ok(12));
 889     /// ```
 890     #[unstable(feature = "result_copied", reason = "newly added", issue = "63168")]
 891     pub fn copied(self) -> Result<T, E> {
 892         self.map(|&mut t| t)
 893     }
 894 }
 895
 896 impl<T: Clone, E> Result<&T, E> {
 897     /// Maps a `Result<&T, E>` to a `Result<T, E>` by cloning the contents of the
 898     /// `Ok` part.
 899     ///
 900     /// # Examples
 901     ///
 902     /// ```
 903     /// #![feature(result_cloned)]
 904     /// let val = 12;
 905     /// let x: Result<&i32, i32> = Ok(&val);
 906     /// assert_eq!(x, Ok(&12));
 907     /// let cloned = x.cloned();
 908     /// assert_eq!(cloned, Ok(12));
 909     /// ```
 910     #[unstable(feature = "result_cloned", reason = "newly added", issue = "63168")]
 911     pub fn cloned(self) -> Result<T, E> {
 912         self.map(|t| t.clone())
 913     }
 914 }
 915
 916 impl<T: Clone, E> Result<&mut T, E> {
 917     /// Maps a `Result<&mut T, E>` to a `Result<T, E>` by cloning the contents of the
 918     /// `Ok` part.
 919     ///
 920     /// # Examples
 921     ///
 922     /// ```
 923     /// #![feature(result_cloned)]
 924     /// let mut val = 12;
 925     /// let x: Result<&mut i32, i32> = Ok(&mut val);
 926     /// assert_eq!(x, Ok(&mut 12));
 927     /// let cloned = x.cloned();
 928     /// assert_eq!(cloned, Ok(12));
 929     /// ```
 930     #[unstable(feature = "result_cloned", reason = "newly added", issue = "63168")]
 931     pub fn cloned(self) -> Result<T, E> {
 932         self.map(|t| t.clone())
 933     }
 934 }
 935
 936 impl<T, E: fmt::Debug> Result<T, E> {
 937     /// Returns the contained [`Ok`] value, consuming the `self` value.
 938     ///
 939     /// # Panics
 940     ///
 941     /// Panics if the value is an [`Err`], with a panic message including the
 942     /// passed message, and the content of the [`Err`].
 943     ///
 944     /// [`Ok`]: enum.Result.html#variant.Ok
 945     /// [`Err`]: enum.Result.html#variant.Err
 946     ///
 947     /// # Examples
 948     ///
 949     /// Basic usage:
 950     ///
 951     /// ```{.should_panic}
 952     /// let x: Result<u32, &str> = Err("emergency failure");
 953     /// x.expect("Testing expect"); // panics with `Testing expect: emergency failure`
 954     /// ```
 955     #[inline]
 956     #[track_caller]
 957     #[stable(feature = "result_expect", since = "1.4.0")]
 958     pub fn expect(self, msg: &str) -> T {
 959         match self {
 960             Ok(t) => t,
 961             Err(e) => unwrap_failed(msg, &e),
 962         }
 963     }
 964
 965     /// Returns the contained [`Ok`] value, consuming the `self` value.
 966     ///
 967     /// Because this function may panic, its use is generally discouraged.
 968     /// Instead, prefer to use pattern matching and handle the [`Err`]
 969     /// case explicitly, or call [`unwrap_or`], [`unwrap_or_else`], or
 970     /// [`unwrap_or_default`].
 971     ///
 972     /// [`unwrap_or`]: #method.unwrap_or
 973     /// [`unwrap_or_else`]: #method.unwrap_or_else
 974     /// [`unwrap_or_default`]: #method.unwrap_or_default
 975     ///
 976     /// # Panics
 977     ///
 978     /// Panics if the value is an [`Err`], with a panic message provided by the
 979     /// [`Err`]'s value.
 980     ///
 981     /// [`Ok`]: enum.Result.html#variant.Ok
 982     /// [`Err`]: enum.Result.html#variant.Err
 983     ///
 984     /// # Examples
 985     ///
 986     /// Basic usage:
 987     ///
 988     /// ```
 989     /// let x: Result<u32, &str> = Ok(2);
 990     /// assert_eq!(x.unwrap(), 2);
 991     /// ```
 992     ///
 993     /// ```{.should_panic}
 994     /// let x: Result<u32, &str> = Err("emergency failure");
 995     /// x.unwrap(); // panics with `emergency failure`
 996     /// ```
 997     #[inline]
 998     #[track_caller]
 999     #[stable(feature = "rust1", since = "1.0.0")]
1000     pub fn unwrap(self) -> T {
1001         match self {
1002             Ok(t) => t,
1003             Err(e) => unwrap_failed("called `Result::unwrap()` on an `Err` value", &e),
1004         }
1005     }
1006 }
1007
1008 impl<T: fmt::Debug, E> Result<T, E> {
1009     /// Returns the contained [`Err`] value, consuming the `self` value.
1010     ///
1011     /// # Panics
1012     ///
1013     /// Panics if the value is an [`Ok`], with a panic message including the
1014     /// passed message, and the content of the [`Ok`].
1015     ///
1016     /// [`Ok`]: enum.Result.html#variant.Ok
1017     /// [`Err`]: enum.Result.html#variant.Err
1018     ///
1019     /// # Examples
1020     ///
1021     /// Basic usage:
1022     ///
1023     /// ```{.should_panic}
1024     /// let x: Result<u32, &str> = Ok(10);
1025     /// x.expect_err("Testing expect_err"); // panics with `Testing expect_err: 10`
1026     /// ```
1027     #[inline]
1028     #[track_caller]
1029     #[stable(feature = "result_expect_err", since = "1.17.0")]
1030     pub fn expect_err(self, msg: &str) -> E {
1031         match self {
1032             Ok(t) => unwrap_failed(msg, &t),
1033             Err(e) => e,
1034         }
1035     }
1036
1037     /// Returns the contained [`Err`] value, consuming the `self` value.
1038     ///
1039     /// # Panics
1040     ///
1041     /// Panics if the value is an [`Ok`], with a custom panic message provided
1042     /// by the [`Ok`]'s value.
1043     ///
1044     /// [`Ok`]: enum.Result.html#variant.Ok
1045     /// [`Err`]: enum.Result.html#variant.Err
1046     ///
1047     ///
1048     /// # Examples
1049     ///
1050     /// ```{.should_panic}
1051     /// let x: Result<u32, &str> = Ok(2);
1052     /// x.unwrap_err(); // panics with `2`
1053     /// ```
1054     ///
1055     /// ```
1056     /// let x: Result<u32, &str> = Err("emergency failure");
1057     /// assert_eq!(x.unwrap_err(), "emergency failure");
1058     /// ```
1059     #[inline]
1060     #[track_caller]
1061     #[stable(feature = "rust1", since = "1.0.0")]
1062     pub fn unwrap_err(self) -> E {
1063         match self {
1064             Ok(t) => unwrap_failed("called `Result::unwrap_err()` on an `Ok` value", &t),
1065             Err(e) => e,
1066         }
1067     }
1068 }
1069
1070 impl<T: Default, E> Result<T, E> {
1071     /// Returns the contained [`Ok`] value or a default
1072     ///
1073     /// Consumes the `self` argument then, if [`Ok`], returns the contained
1074     /// value, otherwise if [`Err`], returns the default value for that
1075     /// type.
1076     ///
1077     /// # Examples
1078     ///
1079     /// Converts a string to an integer, turning poorly-formed strings
1080     /// into 0 (the default value for integers). [`parse`] converts
1081     /// a string to any other type that implements [`FromStr`], returning an
1082     /// [`Err`] on error.
1083     ///
1084     /// ```
1085     /// let good_year_from_input = "1909";
1086     /// let bad_year_from_input = "190blarg";
1087     /// let good_year = good_year_from_input.parse().unwrap_or_default();
1088     /// let bad_year = bad_year_from_input.parse().unwrap_or_default();
1089     ///
1090     /// assert_eq!(1909, good_year);
1091     /// assert_eq!(0, bad_year);
1092     /// ```
1093     ///
1094     /// [`parse`]: ../../std/primitive.str.html#method.parse
1095     /// [`FromStr`]: ../../std/str/trait.FromStr.html
1096     /// [`Ok`]: enum.Result.html#variant.Ok
1097     /// [`Err`]: enum.Result.html#variant.Err
1098     #[inline]
1099     #[stable(feature = "result_unwrap_or_default", since = "1.16.0")]
1100     pub fn unwrap_or_default(self) -> T {
1101         match self {
1102             Ok(x) => x,
1103             Err(_) => Default::default(),
1104         }
1105     }
1106 }
1107
1108 #[unstable(feature = "unwrap_infallible", reason = "newly added", issue = "61695")]
1109 impl<T, E: Into<!>> Result<T, E> {
1110     /// Returns the contained [`Ok`] value, but never panics.
1111     ///
1112     /// Unlike [`unwrap`], this method is known to never panic on the
1113     /// result types it is implemented for. Therefore, it can be used
1114     /// instead of `unwrap` as a maintainability safeguard that will fail
1115     /// to compile if the error type of the `Result` is later changed
1116     /// to an error that can actually occur.
1117     ///
1118     /// [`Ok`]: enum.Result.html#variant.Ok
1119     /// [`Err`]: enum.Result.html#variant.Err
1120     /// [`unwrap`]: enum.Result.html#method.unwrap
1121     ///
1122     /// # Examples
1123     ///
1124     /// Basic usage:
1125     ///
1126     /// ```
1127     /// # #![feature(never_type)]
1128     /// # #![feature(unwrap_infallible)]
1129     ///
1130     /// fn only_good_news() -> Result<String, !> {
1131     ///     Ok("this is fine".into())
1132     /// }
1133     ///
1134     /// let s: String = only_good_news().into_ok();
1135     /// println!("{}", s);
1136     /// ```
1137     #[inline]
1138     pub fn into_ok(self) -> T {
1139         match self {
1140             Ok(x) => x,
1141             Err(e) => e.into(),
1142         }
1143     }
1144 }
1145
1146 #[unstable(feature = "inner_deref", reason = "newly added", issue = "50264")]
1147 impl<T: Deref, E> Result<T, E> {
1148     /// Converts from `Result<T, E>` (or `&Result<T, E>`) to `Result<&T::Target, &E>`.
1149     ///
1150     /// Leaves the original `Result` in-place, creating a new one containing a reference to the
1151     /// `Ok` type's `Deref::Target` type.
1152     pub fn as_deref(&self) -> Result<&T::Target, &E> {
1153         self.as_ref().map(|t| t.deref())
1154     }
1155 }
1156
1157 #[unstable(feature = "inner_deref", reason = "newly added", issue = "50264")]
1158 impl<T, E: Deref> Result<T, E> {
1159     /// Converts from `Result<T, E>` (or `&Result<T, E>`) to `Result<&T, &E::Target>`.
1160     ///
1161     /// Leaves the original `Result` in-place, creating a new one containing a reference to the
1162     /// `Err` type's `Deref::Target` type.
1163     pub fn as_deref_err(&self) -> Result<&T, &E::Target> {
1164         self.as_ref().map_err(|e| e.deref())
1165     }
1166 }
1167
1168 #[unstable(feature = "inner_deref", reason = "newly added", issue = "50264")]
1169 impl<T: DerefMut, E> Result<T, E> {
1170     /// Converts from `Result<T, E>` (or `&mut Result<T, E>`) to `Result<&mut T::Target, &mut E>`.
1171     ///
1172     /// Leaves the original `Result` in-place, creating a new one containing a mutable reference to
1173     /// the `Ok` type's `Deref::Target` type.
1174     pub fn as_deref_mut(&mut self) -> Result<&mut T::Target, &mut E> {
1175         self.as_mut().map(|t| t.deref_mut())
1176     }
1177 }
1178
1179 #[unstable(feature = "inner_deref", reason = "newly added", issue = "50264")]
1180 impl<T, E: DerefMut> Result<T, E> {
1181     /// Converts from `Result<T, E>` (or `&mut Result<T, E>`) to `Result<&mut T, &mut E::Target>`.
1182     ///
1183     /// Leaves the original `Result` in-place, creating a new one containing a mutable reference to
1184     /// the `Err` type's `Deref::Target` type.
1185     pub fn as_deref_mut_err(&mut self) -> Result<&mut T, &mut E::Target> {
1186         self.as_mut().map_err(|e| e.deref_mut())
1187     }
1188 }
1189
1190 impl<T, E> Result<Option<T>, E> {
1191     /// Transposes a `Result` of an `Option` into an `Option` of a `Result`.
1192     ///
1193     /// `Ok(None)` will be mapped to `None`.
1194     /// `Ok(Some(_))` and `Err(_)` will be mapped to `Some(Ok(_))` and `Some(Err(_))`.
1195     ///
1196     /// # Examples
1197     ///
1198     /// ```
1199     /// #[derive(Debug, Eq, PartialEq)]
1200     /// struct SomeErr;
1201     ///
1202     /// let x: Result<Option<i32>, SomeErr> = Ok(Some(5));
1203     /// let y: Option<Result<i32, SomeErr>> = Some(Ok(5));
1204     /// assert_eq!(x.transpose(), y);
1205     /// ```
1206     #[inline]
1207     #[stable(feature = "transpose_result", since = "1.33.0")]
1208     pub fn transpose(self) -> Option<Result<T, E>> {
1209         match self {
1210             Ok(Some(x)) => Some(Ok(x)),
1211             Ok(None) => None,
1212             Err(e) => Some(Err(e)),
1213         }
1214     }
1215 }
1216
1217 // This is a separate function to reduce the code size of the methods
1218 #[inline(never)]
1219 #[cold]
1220 #[track_caller]
1221 fn unwrap_failed(msg: &str, error: &dyn fmt::Debug) -> ! {
1222     panic!("{}: {:?}", msg, error)
1223 }
1224
1225 /////////////////////////////////////////////////////////////////////////////
1226 // Trait implementations
1227 /////////////////////////////////////////////////////////////////////////////
1228
1229 #[stable(feature = "rust1", since = "1.0.0")]
1230 impl<T: Clone, E: Clone> Clone for Result<T, E> {
1231     #[inline]
1232     fn clone(&self) -> Self {
1233         match self {
1234             Ok(x) => Ok(x.clone()),
1235             Err(x) => Err(x.clone()),
1236         }
1237     }
1238
1239     #[inline]
1240     fn clone_from(&mut self, source: &Self) {
1241         match (self, source) {
1242             (Ok(to), Ok(from)) => to.clone_from(from),
1243             (Err(to), Err(from)) => to.clone_from(from),
1244             (to, from) => *to = from.clone(),
1245         }
1246     }
1247 }
1248
1249 #[stable(feature = "rust1", since = "1.0.0")]
1250 impl<T, E> IntoIterator for Result<T, E> {
1251     type Item = T;
1252     type IntoIter = IntoIter<T>;
1253
1254     /// Returns a consuming iterator over the possibly contained value.
1255     ///
1256     /// The iterator yields one value if the result is [`Result::Ok`], otherwise none.
1257     ///
1258     /// # Examples
1259     ///
1260     /// Basic usage:
1261     ///
1262     /// ```
1263     /// let x: Result<u32, &str> = Ok(5);
1264     /// let v: Vec<u32> = x.into_iter().collect();
1265     /// assert_eq!(v, [5]);
1266     ///
1267     /// let x: Result<u32, &str> = Err("nothing!");
1268     /// let v: Vec<u32> = x.into_iter().collect();
1269     /// assert_eq!(v, []);
1270     /// ```
1271     #[inline]
1272     fn into_iter(self) -> IntoIter<T> {
1273         IntoIter { inner: self.ok() }
1274     }
1275 }
1276
1277 #[stable(since = "1.4.0", feature = "result_iter")]
1278 impl<'a, T, E> IntoIterator for &'a Result<T, E> {
1279     type Item = &'a T;
1280     type IntoIter = Iter<'a, T>;
1281
1282     fn into_iter(self) -> Iter<'a, T> {
1283         self.iter()
1284     }
1285 }
1286
1287 #[stable(since = "1.4.0", feature = "result_iter")]
1288 impl<'a, T, E> IntoIterator for &'a mut Result<T, E> {
1289     type Item = &'a mut T;
1290     type IntoIter = IterMut<'a, T>;
1291
1292     fn into_iter(self) -> IterMut<'a, T> {
1293         self.iter_mut()
1294     }
1295 }
1296
1297 /////////////////////////////////////////////////////////////////////////////
1298 // The Result Iterators
1299 /////////////////////////////////////////////////////////////////////////////
1300
1301 /// An iterator over a reference to the [`Ok`] variant of a [`Result`].
1302 ///
1303 /// The iterator yields one value if the result is [`Ok`], otherwise none.
1304 ///
1305 /// Created by [`Result::iter`].
1306 ///
1307 /// [`Ok`]: enum.Result.html#variant.Ok
1308 /// [`Result`]: enum.Result.html
1309 /// [`Result::iter`]: enum.Result.html#method.iter
1310 #[derive(Debug)]
1311 #[stable(feature = "rust1", since = "1.0.0")]
1312 pub struct Iter<'a, T: 'a> {
1313     inner: Option<&'a T>,
1314 }
1315
1316 #[stable(feature = "rust1", since = "1.0.0")]
1317 impl<'a, T> Iterator for Iter<'a, T> {
1318     type Item = &'a T;
1319
1320     #[inline]
1321     fn next(&mut self) -> Option<&'a T> {
1322         self.inner.take()
1323     }
1324     #[inline]
1325     fn size_hint(&self) -> (usize, Option<usize>) {
1326         let n = if self.inner.is_some() { 1 } else { 0 };
1327         (n, Some(n))
1328     }
1329 }
1330
1331 #[stable(feature = "rust1", since = "1.0.0")]
1332 impl<'a, T> DoubleEndedIterator for Iter<'a, T> {
1333     #[inline]
1334     fn next_back(&mut self) -> Option<&'a T> {
1335         self.inner.take()
1336     }
1337 }
1338
1339 #[stable(feature = "rust1", since = "1.0.0")]
1340 impl<T> ExactSizeIterator for Iter<'_, T> {}
1341
1342 #[stable(feature = "fused", since = "1.26.0")]
1343 impl<T> FusedIterator for Iter<'_, T> {}
1344
1345 #[unstable(feature = "trusted_len", issue = "37572")]
1346 unsafe impl<A> TrustedLen for Iter<'_, A> {}
1347
1348 #[stable(feature = "rust1", since = "1.0.0")]
1349 impl<T> Clone for Iter<'_, T> {
1350     #[inline]
1351     fn clone(&self) -> Self {
1352         Iter { inner: self.inner }
1353     }
1354 }
1355
1356 /// An iterator over a mutable reference to the [`Ok`] variant of a [`Result`].
1357 ///
1358 /// Created by [`Result::iter_mut`].
1359 ///
1360 /// [`Ok`]: enum.Result.html#variant.Ok
1361 /// [`Result`]: enum.Result.html
1362 /// [`Result::iter_mut`]: enum.Result.html#method.iter_mut
1363 #[derive(Debug)]
1364 #[stable(feature = "rust1", since = "1.0.0")]
1365 pub struct IterMut<'a, T: 'a> {
1366     inner: Option<&'a mut T>,
1367 }
1368
1369 #[stable(feature = "rust1", since = "1.0.0")]
1370 impl<'a, T> Iterator for IterMut<'a, T> {
1371     type Item = &'a mut T;
1372
1373     #[inline]
1374     fn next(&mut self) -> Option<&'a mut T> {
1375         self.inner.take()
1376     }
1377     #[inline]
1378     fn size_hint(&self) -> (usize, Option<usize>) {
1379         let n = if self.inner.is_some() { 1 } else { 0 };
1380         (n, Some(n))
1381     }
1382 }
1383
1384 #[stable(feature = "rust1", since = "1.0.0")]
1385 impl<'a, T> DoubleEndedIterator for IterMut<'a, T> {
1386     #[inline]
1387     fn next_back(&mut self) -> Option<&'a mut T> {
1388         self.inner.take()
1389     }
1390 }
1391
1392 #[stable(feature = "rust1", since = "1.0.0")]
1393 impl<T> ExactSizeIterator for IterMut<'_, T> {}
1394
1395 #[stable(feature = "fused", since = "1.26.0")]
1396 impl<T> FusedIterator for IterMut<'_, T> {}
1397
1398 #[unstable(feature = "trusted_len", issue = "37572")]
1399 unsafe impl<A> TrustedLen for IterMut<'_, A> {}
1400
1401 /// An iterator over the value in a [`Ok`] variant of a [`Result`].
1402 ///
1403 /// The iterator yields one value if the result is [`Ok`], otherwise none.
1404 ///
1405 /// This struct is created by the [`into_iter`] method on
1406 /// [`Result`] (provided by the [`IntoIterator`] trait).
1407 ///
1408 /// [`Ok`]: enum.Result.html#variant.Ok
1409 /// [`Result`]: enum.Result.html
1410 /// [`into_iter`]: ../iter/trait.IntoIterator.html#tymethod.into_iter
1411 /// [`IntoIterator`]: ../iter/trait.IntoIterator.html
1412 #[derive(Clone, Debug)]
1413 #[stable(feature = "rust1", since = "1.0.0")]
1414 pub struct IntoIter<T> {
1415     inner: Option<T>,
1416 }
1417
1418 #[stable(feature = "rust1", since = "1.0.0")]
1419 impl<T> Iterator for IntoIter<T> {
1420     type Item = T;
1421
1422     #[inline]
1423     fn next(&mut self) -> Option<T> {
1424         self.inner.take()
1425     }
1426     #[inline]
1427     fn size_hint(&self) -> (usize, Option<usize>) {
1428         let n = if self.inner.is_some() { 1 } else { 0 };
1429         (n, Some(n))
1430     }
1431 }
1432
1433 #[stable(feature = "rust1", since = "1.0.0")]
1434 impl<T> DoubleEndedIterator for IntoIter<T> {
1435     #[inline]
1436     fn next_back(&mut self) -> Option<T> {
1437         self.inner.take()
1438     }
1439 }
1440
1441 #[stable(feature = "rust1", since = "1.0.0")]
1442 impl<T> ExactSizeIterator for IntoIter<T> {}
1443
1444 #[stable(feature = "fused", since = "1.26.0")]
1445 impl<T> FusedIterator for IntoIter<T> {}
1446
1447 #[unstable(feature = "trusted_len", issue = "37572")]
1448 unsafe impl<A> TrustedLen for IntoIter<A> {}
1449
1450 /////////////////////////////////////////////////////////////////////////////
1451 // FromIterator
1452 /////////////////////////////////////////////////////////////////////////////
1453
1454 #[stable(feature = "rust1", since = "1.0.0")]
1455 impl<A, E, V: FromIterator<A>> FromIterator<Result<A, E>> for Result<V, E> {
1456     /// Takes each element in the `Iterator`: if it is an `Err`, no further
1457     /// elements are taken, and the `Err` is returned. Should no `Err` occur, a
1458     /// container with the values of each `Result` is returned.
1459     ///
1460     /// Here is an example which increments every integer in a vector,
1461     /// checking for overflow:
1462     ///
1463     /// ```
1464     /// let v = vec![1, 2];
1465     /// let res: Result<Vec<u32>, &'static str> = v.iter().map(|x: &u32|
1466     ///     x.checked_add(1).ok_or("Overflow!")
1467     /// ).collect();
1468     /// assert_eq!(res, Ok(vec![2, 3]));
1469     /// ```
1470     ///
1471     /// Here is another example that tries to subtract one from another list
1472     /// of integers, this time checking for underflow:
1473     ///
1474     /// ```
1475     /// let v = vec![1, 2, 0];
1476     /// let res: Result<Vec<u32>, &'static str> = v.iter().map(|x: &u32|
1477     ///     x.checked_sub(1).ok_or("Underflow!")
1478     /// ).collect();
1479     /// assert_eq!(res, Err("Underflow!"));
1480     /// ```
1481     ///
1482     /// Here is a variation on the previous example, showing that no
1483     /// further elements are taken from `iter` after the first `Err`.
1484     ///
1485     /// ```
1486     /// let v = vec![3, 2, 1, 10];
1487     /// let mut shared = 0;
1488     /// let res: Result<Vec<u32>, &'static str> = v.iter().map(|x: &u32| {
1489     ///     shared += x;
1490     ///     x.checked_sub(2).ok_or("Underflow!")
1491     /// }).collect();
1492     /// assert_eq!(res, Err("Underflow!"));
1493     /// assert_eq!(shared, 6);
1494     /// ```
1495     ///
1496     /// Since the third element caused an underflow, no further elements were taken,
1497     /// so the final value of `shared` is 6 (= `3 + 2 + 1`), not 16.
1498     #[inline]
1499     fn from_iter<I: IntoIterator<Item = Result<A, E>>>(iter: I) -> Result<V, E> {
1500         // FIXME(#11084): This could be replaced with Iterator::scan when this
1501         // performance bug is closed.
1502
1503         iter::process_results(iter.into_iter(), |i| i.collect())
1504     }
1505 }
1506
1507 #[unstable(feature = "try_trait", issue = "42327")]
1508 impl<T, E> ops::Try for Result<T, E> {
1509     type Ok = T;
1510     type Error = E;
1511
1512     #[inline]
1513     fn into_result(self) -> Self {
1514         self
1515     }
1516
1517     #[inline]
1518     fn from_ok(v: T) -> Self {
1519         Ok(v)
1520     }
1521
1522     #[inline]
1523     fn from_error(v: E) -> Self {
1524         Err(v)
1525     }
1526 }