src/libcore/result.rs

   1 // Copyright 2012-2014 The Rust Project Developers. See the COPYRIGHT
   2 // file at the top-level directory of this distribution and at
   3 // http://rust-lang.org/COPYRIGHT.
   4 //
   5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
   6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
   7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
   8 // option. This file may not be copied, modified, or distributed
   9 // except according to those terms.
  10
  11 //! Error handling with the `Result` type.
  12 //!
  13 //! [`Result<T, E>`][`Result`] is the type used for returning and propagating
  14 //! errors. It is an enum with the variants, [`Ok(T)`], representing
  15 //! success and containing a value, and [`Err(E)`], representing error
  16 //! and containing an error value.
  17 //!
  18 //! ```
  19 //! # #[allow(dead_code)]
  20 //! enum Result<T, E> {
  21 //!    Ok(T),
  22 //!    Err(E),
  23 //! }
  24 //! ```
  25 //!
  26 //! Functions return [`Result`] whenever errors are expected and
  27 //! recoverable. In the `std` crate, [`Result`] is most prominently used
  28 //! for [I/O](../../std/io/index.html).
  29 //!
  30 //! A simple function returning [`Result`] might be
  31 //! defined and used like so:
  32 //!
  33 //! ```
  34 //! #[derive(Debug)]
  35 //! enum Version { Version1, Version2 }
  36 //!
  37 //! fn parse_version(header: &[u8]) -> Result<Version, &'static str> {
  38 //!     match header.get(0) {
  39 //!         None => Err("invalid header length"),
  40 //!         Some(&1) => Ok(Version::Version1),
  41 //!         Some(&2) => Ok(Version::Version2),
  42 //!         Some(_) => Err("invalid version"),
  43 //!     }
  44 //! }
  45 //!
  46 //! let version = parse_version(&[1, 2, 3, 4]);
  47 //! match version {
  48 //!     Ok(v) => println!("working with version: {:?}", v),
  49 //!     Err(e) => println!("error parsing header: {:?}", e),
  50 //! }
  51 //! ```
  52 //!
  53 //! Pattern matching on [`Result`]s is clear and straightforward for
  54 //! simple cases, but [`Result`] comes with some convenience methods
  55 //! that make working with it more succinct.
  56 //!
  57 //! ```
  58 //! let good_result: Result<i32, i32> = Ok(10);
  59 //! let bad_result: Result<i32, i32> = Err(10);
  60 //!
  61 //! // The `is_ok` and `is_err` methods do what they say.
  62 //! assert!(good_result.is_ok() && !good_result.is_err());
  63 //! assert!(bad_result.is_err() && !bad_result.is_ok());
  64 //!
  65 //! // `map` consumes the `Result` and produces another.
  66 //! let good_result: Result<i32, i32> = good_result.map(|i| i + 1);
  67 //! let bad_result: Result<i32, i32> = bad_result.map(|i| i - 1);
  68 //!
  69 //! // Use `and_then` to continue the computation.
  70 //! let good_result: Result<bool, i32> = good_result.and_then(|i| Ok(i == 11));
  71 //!
  72 //! // Use `or_else` to handle the error.
  73 //! let bad_result: Result<i32, i32> = bad_result.or_else(|i| Ok(i + 20));
  74 //!
  75 //! // Consume the result and return the contents with `unwrap`.
  76 //! let final_awesome_result = good_result.unwrap();
  77 //! ```
  78 //!
  79 //! # Results must be used
  80 //!
  81 //! A common problem with using return values to indicate errors is
  82 //! that it is easy to ignore the return value, thus failing to handle
  83 //! the error. [`Result`] is annotated with the `#[must_use]` attribute,
  84 //! which will cause the compiler to issue a warning when a Result
  85 //! value is ignored. This makes [`Result`] especially useful with
  86 //! functions that may encounter errors but don't otherwise return a
  87 //! useful value.
  88 //!
  89 //! Consider the [`write_all`] method defined for I/O types
  90 //! by the [`Write`] trait:
  91 //!
  92 //! ```
  93 //! use std::io;
  94 //!
  95 //! trait Write {
  96 //!     fn write_all(&mut self, bytes: &[u8]) -> Result<(), io::Error>;
  97 //! }
  98 //! ```
  99 //!
 100 //! *Note: The actual definition of [`Write`] uses [`io::Result`], which
 101 //! is just a synonym for [`Result`]`<T, `[`io::Error`]`>`.*
 102 //!
 103 //! This method doesn't produce a value, but the write may
 104 //! fail. It's crucial to handle the error case, and *not* write
 105 //! something like this:
 106 //!
 107 //! ```no_run
 108 //! # #![allow(unused_must_use)] // \o/
 109 //! use std::fs::File;
 110 //! use std::io::prelude::*;
 111 //!
 112 //! let mut file = File::create("valuable_data.txt").unwrap();
 113 //! // If `write_all` errors, then we'll never know, because the return
 114 //! // value is ignored.
 115 //! file.write_all(b"important message");
 116 //! ```
 117 //!
 118 //! If you *do* write that in Rust, the compiler will give you a
 119 //! warning (by default, controlled by the `unused_must_use` lint).
 120 //!
 121 //! You might instead, if you don't want to handle the error, simply
 122 //! assert success with [`expect`]. This will panic if the
 123 //! write fails, providing a marginally useful message indicating why:
 124 //!
 125 //! ```{.no_run}
 126 //! use std::fs::File;
 127 //! use std::io::prelude::*;
 128 //!
 129 //! let mut file = File::create("valuable_data.txt").unwrap();
 130 //! file.write_all(b"important message").expect("failed to write message");
 131 //! ```
 132 //!
 133 //! You might also simply assert success:
 134 //!
 135 //! ```{.no_run}
 136 //! # use std::fs::File;
 137 //! # use std::io::prelude::*;
 138 //! # let mut file = File::create("valuable_data.txt").unwrap();
 139 //! assert!(file.write_all(b"important message").is_ok());
 140 //! ```
 141 //!
 142 //! Or propagate the error up the call stack with [`?`]:
 143 //!
 144 //! ```
 145 //! # use std::fs::File;
 146 //! # use std::io::prelude::*;
 147 //! # use std::io;
 148 //! # #[allow(dead_code)]
 149 //! fn write_message() -> io::Result<()> {
 150 //!     let mut file = File::create("valuable_data.txt")?;
 151 //!     file.write_all(b"important message")?;
 152 //!     Ok(())
 153 //! }
 154 //! ```
 155 //!
 156 //! # The question mark operator, `?`
 157 //!
 158 //! When writing code that calls many functions that return the
 159 //! [`Result`] type, the error handling can be tedious. The question mark
 160 //! operator, [`?`], hides some of the boilerplate of propagating errors
 161 //! up the call stack.
 162 //!
 163 //! It replaces this:
 164 //!
 165 //! ```
 166 //! # #![allow(dead_code)]
 167 //! use std::fs::File;
 168 //! use std::io::prelude::*;
 169 //! use std::io;
 170 //!
 171 //! struct Info {
 172 //!     name: String,
 173 //!     age: i32,
 174 //!     rating: i32,
 175 //! }
 176 //!
 177 //! fn write_info(info: &Info) -> io::Result<()> {
 178 //!     // Early return on error
 179 //!     let mut file = match File::create("my_best_friends.txt") {
 180 //!            Err(e) => return Err(e),
 181 //!            Ok(f) => f,
 182 //!     };
 183 //!     if let Err(e) = file.write_all(format!("name: {}\n", info.name).as_bytes()) {
 184 //!         return Err(e)
 185 //!     }
 186 //!     if let Err(e) = file.write_all(format!("age: {}\n", info.age).as_bytes()) {
 187 //!         return Err(e)
 188 //!     }
 189 //!     if let Err(e) = file.write_all(format!("rating: {}\n", info.rating).as_bytes()) {
 190 //!         return Err(e)
 191 //!     }
 192 //!     Ok(())
 193 //! }
 194 //! ```
 195 //!
 196 //! With this:
 197 //!
 198 //! ```
 199 //! # #![allow(dead_code)]
 200 //! use std::fs::File;
 201 //! use std::io::prelude::*;
 202 //! use std::io;
 203 //!
 204 //! struct Info {
 205 //!     name: String,
 206 //!     age: i32,
 207 //!     rating: i32,
 208 //! }
 209 //!
 210 //! fn write_info(info: &Info) -> io::Result<()> {
 211 //!     let mut file = File::create("my_best_friends.txt")?;
 212 //!     // Early return on error
 213 //!     file.write_all(format!("name: {}\n", info.name).as_bytes())?;
 214 //!     file.write_all(format!("age: {}\n", info.age).as_bytes())?;
 215 //!     file.write_all(format!("rating: {}\n", info.rating).as_bytes())?;
 216 //!     Ok(())
 217 //! }
 218 //! ```
 219 //!
 220 //! *It's much nicer!*
 221 //!
 222 //! Ending the expression with [`?`] will result in the unwrapped
 223 //! success ([`Ok`]) value, unless the result is [`Err`], in which case
 224 //! [`Err`] is returned early from the enclosing function.
 225 //!
 226 //! [`?`] can only be used in functions that return [`Result`] because of the
 227 //! early return of [`Err`] that it provides.
 228 //!
 229 //! [`expect`]: enum.Result.html#method.expect
 230 //! [`Write`]: ../../std/io/trait.Write.html
 231 //! [`write_all`]: ../../std/io/trait.Write.html#method.write_all
 232 //! [`io::Result`]: ../../std/io/type.Result.html
 233 //! [`?`]: ../../std/macro.try.html
 234 //! [`Result`]: enum.Result.html
 235 //! [`Ok(T)`]: enum.Result.html#variant.Ok
 236 //! [`Err(E)`]: enum.Result.html#variant.Err
 237 //! [`io::Error`]: ../../std/io/struct.Error.html
 238 //! [`Ok`]: enum.Result.html#variant.Ok
 239 //! [`Err`]: enum.Result.html#variant.Err
 240
 241 #![stable(feature = "rust1", since = "1.0.0")]
 242
 243 use fmt;
 244 use iter::{FromIterator, FusedIterator, TrustedLen};
 245 use ops::{self, Deref};
 246
 247 /// `Result` is a type that represents either success ([`Ok`]) or failure ([`Err`]).
 248 ///
 249 /// See the [`std::result`](index.html) module documentation for details.
 250 ///
 251 /// [`Ok`]: enum.Result.html#variant.Ok
 252 /// [`Err`]: enum.Result.html#variant.Err
 253 #[derive(Clone, Copy, PartialEq, PartialOrd, Eq, Ord, Debug, Hash)]
 254 #[must_use = "this `Result` may be an `Err` variant, which should be handled"]
 255 #[stable(feature = "rust1", since = "1.0.0")]
 256 pub enum Result<T, E> {
 257     /// Contains the success value
 258     #[stable(feature = "rust1", since = "1.0.0")]
 259     Ok(#[stable(feature = "rust1", since = "1.0.0")] T),
 260
 261     /// Contains the error value
 262     #[stable(feature = "rust1", since = "1.0.0")]
 263     Err(#[stable(feature = "rust1", since = "1.0.0")] E),
 264 }
 265
 266 /////////////////////////////////////////////////////////////////////////////
 267 // Type implementation
 268 /////////////////////////////////////////////////////////////////////////////
 269
 270 impl<T, E> Result<T, E> {
 271     /////////////////////////////////////////////////////////////////////////
 272     // Querying the contained values
 273     /////////////////////////////////////////////////////////////////////////
 274
 275     /// Returns `true` if the result is [`Ok`].
 276     ///
 277     /// [`Ok`]: enum.Result.html#variant.Ok
 278     ///
 279     /// # Examples
 280     ///
 281     /// Basic usage:
 282     ///
 283     /// ```
 284     /// let x: Result<i32, &str> = Ok(-3);
 285     /// assert_eq!(x.is_ok(), true);
 286     ///
 287     /// let x: Result<i32, &str> = Err("Some error message");
 288     /// assert_eq!(x.is_ok(), false);
 289     /// ```
 290     #[inline]
 291     #[stable(feature = "rust1", since = "1.0.0")]
 292     pub fn is_ok(&self) -> bool {
 293         match *self {
 294             Ok(_) => true,
 295             Err(_) => false
 296         }
 297     }
 298
 299     /// Returns `true` if the result is [`Err`].
 300     ///
 301     /// [`Err`]: enum.Result.html#variant.Err
 302     ///
 303     /// # Examples
 304     ///
 305     /// Basic usage:
 306     ///
 307     /// ```
 308     /// let x: Result<i32, &str> = Ok(-3);
 309     /// assert_eq!(x.is_err(), false);
 310     ///
 311     /// let x: Result<i32, &str> = Err("Some error message");
 312     /// assert_eq!(x.is_err(), true);
 313     /// ```
 314     #[inline]
 315     #[stable(feature = "rust1", since = "1.0.0")]
 316     pub fn is_err(&self) -> bool {
 317         !self.is_ok()
 318     }
 319
 320     /////////////////////////////////////////////////////////////////////////
 321     // Adapter for each variant
 322     /////////////////////////////////////////////////////////////////////////
 323
 324     /// Converts from `Result<T, E>` to [`Option<T>`].
 325     ///
 326     /// Converts `self` into an [`Option<T>`], consuming `self`,
 327     /// and discarding the error, if any.
 328     ///
 329     /// [`Option<T>`]: ../../std/option/enum.Option.html
 330     ///
 331     /// # Examples
 332     ///
 333     /// Basic usage:
 334     ///
 335     /// ```
 336     /// let x: Result<u32, &str> = Ok(2);
 337     /// assert_eq!(x.ok(), Some(2));
 338     ///
 339     /// let x: Result<u32, &str> = Err("Nothing here");
 340     /// assert_eq!(x.ok(), None);
 341     /// ```
 342     #[inline]
 343     #[stable(feature = "rust1", since = "1.0.0")]
 344     pub fn ok(self) -> Option<T> {
 345         match self {
 346             Ok(x)  => Some(x),
 347             Err(_) => None,
 348         }
 349     }
 350
 351     /// Converts from `Result<T, E>` to [`Option<E>`].
 352     ///
 353     /// Converts `self` into an [`Option<E>`], consuming `self`,
 354     /// and discarding the success value, if any.
 355     ///
 356     /// [`Option<E>`]: ../../std/option/enum.Option.html
 357     ///
 358     /// # Examples
 359     ///
 360     /// Basic usage:
 361     ///
 362     /// ```
 363     /// let x: Result<u32, &str> = Ok(2);
 364     /// assert_eq!(x.err(), None);
 365     ///
 366     /// let x: Result<u32, &str> = Err("Nothing here");
 367     /// assert_eq!(x.err(), Some("Nothing here"));
 368     /// ```
 369     #[inline]
 370     #[stable(feature = "rust1", since = "1.0.0")]
 371     pub fn err(self) -> Option<E> {
 372         match self {
 373             Ok(_)  => None,
 374             Err(x) => Some(x),
 375         }
 376     }
 377
 378     /////////////////////////////////////////////////////////////////////////
 379     // Adapter for working with references
 380     /////////////////////////////////////////////////////////////////////////
 381
 382     /// Converts from `Result<T, E>` to `Result<&T, &E>`.
 383     ///
 384     /// Produces a new `Result`, containing a reference
 385     /// into the original, leaving the original in place.
 386     ///
 387     /// # Examples
 388     ///
 389     /// Basic usage:
 390     ///
 391     /// ```
 392     /// let x: Result<u32, &str> = Ok(2);
 393     /// assert_eq!(x.as_ref(), Ok(&2));
 394     ///
 395     /// let x: Result<u32, &str> = Err("Error");
 396     /// assert_eq!(x.as_ref(), Err(&"Error"));
 397     /// ```
 398     #[inline]
 399     #[stable(feature = "rust1", since = "1.0.0")]
 400     pub fn as_ref(&self) -> Result<&T, &E> {
 401         match *self {
 402             Ok(ref x) => Ok(x),
 403             Err(ref x) => Err(x),
 404         }
 405     }
 406
 407     /// Converts from `Result<T, E>` to `Result<&mut T, &mut E>`.
 408     ///
 409     /// # Examples
 410     ///
 411     /// Basic usage:
 412     ///
 413     /// ```
 414     /// fn mutate(r: &mut Result<i32, i32>) {
 415     ///     match r.as_mut() {
 416     ///         Ok(v) => *v = 42,
 417     ///         Err(e) => *e = 0,
 418     ///     }
 419     /// }
 420     ///
 421     /// let mut x: Result<i32, i32> = Ok(2);
 422     /// mutate(&mut x);
 423     /// assert_eq!(x.unwrap(), 42);
 424     ///
 425     /// let mut x: Result<i32, i32> = Err(13);
 426     /// mutate(&mut x);
 427     /// assert_eq!(x.unwrap_err(), 0);
 428     /// ```
 429     #[inline]
 430     #[stable(feature = "rust1", since = "1.0.0")]
 431     pub fn as_mut(&mut self) -> Result<&mut T, &mut E> {
 432         match *self {
 433             Ok(ref mut x) => Ok(x),
 434             Err(ref mut x) => Err(x),
 435         }
 436     }
 437
 438     /////////////////////////////////////////////////////////////////////////
 439     // Transforming contained values
 440     /////////////////////////////////////////////////////////////////////////
 441
 442     /// Maps a `Result<T, E>` to `Result<U, E>` by applying a function to a
 443     /// contained [`Ok`] value, leaving an [`Err`] value untouched.
 444     ///
 445     /// This function can be used to compose the results of two functions.
 446     ///
 447     /// [`Ok`]: enum.Result.html#variant.Ok
 448     /// [`Err`]: enum.Result.html#variant.Err
 449     ///
 450     /// # Examples
 451     ///
 452     /// Print the numbers on each line of a string multiplied by two.
 453     ///
 454     /// ```
 455     /// let line = "1\n2\n3\n4\n";
 456     ///
 457     /// for num in line.lines() {
 458     ///     match num.parse::<i32>().map(|i| i * 2) {
 459     ///         Ok(n) => println!("{}", n),
 460     ///         Err(..) => {}
 461     ///     }
 462     /// }
 463     /// ```
 464     #[inline]
 465     #[stable(feature = "rust1", since = "1.0.0")]
 466     pub fn map<U, F: FnOnce(T) -> U>(self, op: F) -> Result<U,E> {
 467         match self {
 468             Ok(t) => Ok(op(t)),
 469             Err(e) => Err(e)
 470         }
 471     }
 472
 473     /// Maps a `Result<T, E>` to `U` by applying a function to a
 474     /// contained [`Ok`] value, or a fallback function to a
 475     /// contained [`Err`] value.
 476     ///
 477     /// This function can be used to unpack a successful result
 478     /// while handling an error.
 479     ///
 480     /// [`Ok`]: enum.Result.html#variant.Ok
 481     /// [`Err`]: enum.Result.html#variant.Err
 482     ///
 483     /// # Examples
 484     ///
 485     /// Basic usage:
 486     ///
 487     /// ```
 488     /// #![feature(result_map_or_else)]
 489     /// let k = 21;
 490     ///
 491     /// let x : Result<_, &str> = Ok("foo");
 492     /// assert_eq!(x.map_or_else(|e| k * 2, |v| v.len()), 3);
 493     ///
 494     /// let x : Result<&str, _> = Err("bar");
 495     /// assert_eq!(x.map_or_else(|e| k * 2, |v| v.len()), 42);
 496     /// ```
 497     #[inline]
 498     #[unstable(feature = "result_map_or_else", issue = "53268")]
 499     pub fn map_or_else<U, M: FnOnce(T) -> U, F: FnOnce(E) -> U>(self, fallback: F, map: M) -> U {
 500         self.map(map).unwrap_or_else(fallback)
 501     }
 502
 503     /// Maps a `Result<T, E>` to `Result<T, F>` by applying a function to a
 504     /// contained [`Err`] value, leaving an [`Ok`] value untouched.
 505     ///
 506     /// This function can be used to pass through a successful result while handling
 507     /// an error.
 508     ///
 509     /// [`Ok`]: enum.Result.html#variant.Ok
 510     /// [`Err`]: enum.Result.html#variant.Err
 511     ///
 512     /// # Examples
 513     ///
 514     /// Basic usage:
 515     ///
 516     /// ```
 517     /// fn stringify(x: u32) -> String { format!("error code: {}", x) }
 518     ///
 519     /// let x: Result<u32, u32> = Ok(2);
 520     /// assert_eq!(x.map_err(stringify), Ok(2));
 521     ///
 522     /// let x: Result<u32, u32> = Err(13);
 523     /// assert_eq!(x.map_err(stringify), Err("error code: 13".to_string()));
 524     /// ```
 525     #[inline]
 526     #[stable(feature = "rust1", since = "1.0.0")]
 527     pub fn map_err<F, O: FnOnce(E) -> F>(self, op: O) -> Result<T,F> {
 528         match self {
 529             Ok(t) => Ok(t),
 530             Err(e) => Err(op(e))
 531         }
 532     }
 533
 534     /////////////////////////////////////////////////////////////////////////
 535     // Iterator constructors
 536     /////////////////////////////////////////////////////////////////////////
 537
 538     /// Returns an iterator over the possibly contained value.
 539     ///
 540     /// The iterator yields one value if the result is [`Result::Ok`], otherwise none.
 541     ///
 542     /// # Examples
 543     ///
 544     /// Basic usage:
 545     ///
 546     /// ```
 547     /// let x: Result<u32, &str> = Ok(7);
 548     /// assert_eq!(x.iter().next(), Some(&7));
 549     ///
 550     /// let x: Result<u32, &str> = Err("nothing!");
 551     /// assert_eq!(x.iter().next(), None);
 552     /// ```
 553     #[inline]
 554     #[stable(feature = "rust1", since = "1.0.0")]
 555     pub fn iter(&self) -> Iter<T> {
 556         Iter { inner: self.as_ref().ok() }
 557     }
 558
 559     /// Returns a mutable iterator over the possibly contained value.
 560     ///
 561     /// The iterator yields one value if the result is [`Result::Ok`], otherwise none.
 562     ///
 563     /// # Examples
 564     ///
 565     /// Basic usage:
 566     ///
 567     /// ```
 568     /// let mut x: Result<u32, &str> = Ok(7);
 569     /// match x.iter_mut().next() {
 570     ///     Some(v) => *v = 40,
 571     ///     None => {},
 572     /// }
 573     /// assert_eq!(x, Ok(40));
 574     ///
 575     /// let mut x: Result<u32, &str> = Err("nothing!");
 576     /// assert_eq!(x.iter_mut().next(), None);
 577     /// ```
 578     #[inline]
 579     #[stable(feature = "rust1", since = "1.0.0")]
 580     pub fn iter_mut(&mut self) -> IterMut<T> {
 581         IterMut { inner: self.as_mut().ok() }
 582     }
 583
 584     ////////////////////////////////////////////////////////////////////////
 585     // Boolean operations on the values, eager and lazy
 586     /////////////////////////////////////////////////////////////////////////
 587
 588     /// Returns `res` if the result is [`Ok`], otherwise returns the [`Err`] value of `self`.
 589     ///
 590     /// [`Ok`]: enum.Result.html#variant.Ok
 591     /// [`Err`]: enum.Result.html#variant.Err
 592     ///
 593     /// # Examples
 594     ///
 595     /// Basic usage:
 596     ///
 597     /// ```
 598     /// let x: Result<u32, &str> = Ok(2);
 599     /// let y: Result<&str, &str> = Err("late error");
 600     /// assert_eq!(x.and(y), Err("late error"));
 601     ///
 602     /// let x: Result<u32, &str> = Err("early error");
 603     /// let y: Result<&str, &str> = Ok("foo");
 604     /// assert_eq!(x.and(y), Err("early error"));
 605     ///
 606     /// let x: Result<u32, &str> = Err("not a 2");
 607     /// let y: Result<&str, &str> = Err("late error");
 608     /// assert_eq!(x.and(y), Err("not a 2"));
 609     ///
 610     /// let x: Result<u32, &str> = Ok(2);
 611     /// let y: Result<&str, &str> = Ok("different result type");
 612     /// assert_eq!(x.and(y), Ok("different result type"));
 613     /// ```
 614     #[inline]
 615     #[stable(feature = "rust1", since = "1.0.0")]
 616     pub fn and<U>(self, res: Result<U, E>) -> Result<U, E> {
 617         match self {
 618             Ok(_) => res,
 619             Err(e) => Err(e),
 620         }
 621     }
 622
 623     /// Calls `op` if the result is [`Ok`], otherwise returns the [`Err`] value of `self`.
 624     ///
 625     /// [`Ok`]: enum.Result.html#variant.Ok
 626     /// [`Err`]: enum.Result.html#variant.Err
 627     ///
 628     /// This function can be used for control flow based on `Result` values.
 629     ///
 630     /// # Examples
 631     ///
 632     /// Basic usage:
 633     ///
 634     /// ```
 635     /// fn sq(x: u32) -> Result<u32, u32> { Ok(x * x) }
 636     /// fn err(x: u32) -> Result<u32, u32> { Err(x) }
 637     ///
 638     /// assert_eq!(Ok(2).and_then(sq).and_then(sq), Ok(16));
 639     /// assert_eq!(Ok(2).and_then(sq).and_then(err), Err(4));
 640     /// assert_eq!(Ok(2).and_then(err).and_then(sq), Err(2));
 641     /// assert_eq!(Err(3).and_then(sq).and_then(sq), Err(3));
 642     /// ```
 643     #[inline]
 644     #[stable(feature = "rust1", since = "1.0.0")]
 645     pub fn and_then<U, F: FnOnce(T) -> Result<U, E>>(self, op: F) -> Result<U, E> {
 646         match self {
 647             Ok(t) => op(t),
 648             Err(e) => Err(e),
 649         }
 650     }
 651
 652     /// Returns `res` if the result is [`Err`], otherwise returns the [`Ok`] value of `self`.
 653     ///
 654     /// Arguments passed to `or` are eagerly evaluated; if you are passing the
 655     /// result of a function call, it is recommended to use [`or_else`], which is
 656     /// lazily evaluated.
 657     ///
 658     /// [`Ok`]: enum.Result.html#variant.Ok
 659     /// [`Err`]: enum.Result.html#variant.Err
 660     /// [`or_else`]: #method.or_else
 661     ///
 662     /// # Examples
 663     ///
 664     /// Basic usage:
 665     ///
 666     /// ```
 667     /// let x: Result<u32, &str> = Ok(2);
 668     /// let y: Result<u32, &str> = Err("late error");
 669     /// assert_eq!(x.or(y), Ok(2));
 670     ///
 671     /// let x: Result<u32, &str> = Err("early error");
 672     /// let y: Result<u32, &str> = Ok(2);
 673     /// assert_eq!(x.or(y), Ok(2));
 674     ///
 675     /// let x: Result<u32, &str> = Err("not a 2");
 676     /// let y: Result<u32, &str> = Err("late error");
 677     /// assert_eq!(x.or(y), Err("late error"));
 678     ///
 679     /// let x: Result<u32, &str> = Ok(2);
 680     /// let y: Result<u32, &str> = Ok(100);
 681     /// assert_eq!(x.or(y), Ok(2));
 682     /// ```
 683     #[inline]
 684     #[stable(feature = "rust1", since = "1.0.0")]
 685     pub fn or<F>(self, res: Result<T, F>) -> Result<T, F> {
 686         match self {
 687             Ok(v) => Ok(v),
 688             Err(_) => res,
 689         }
 690     }
 691
 692     /// Calls `op` if the result is [`Err`], otherwise returns the [`Ok`] value of `self`.
 693     ///
 694     /// This function can be used for control flow based on result values.
 695     ///
 696     /// [`Ok`]: enum.Result.html#variant.Ok
 697     /// [`Err`]: enum.Result.html#variant.Err
 698     ///
 699     /// # Examples
 700     ///
 701     /// Basic usage:
 702     ///
 703     /// ```
 704     /// fn sq(x: u32) -> Result<u32, u32> { Ok(x * x) }
 705     /// fn err(x: u32) -> Result<u32, u32> { Err(x) }
 706     ///
 707     /// assert_eq!(Ok(2).or_else(sq).or_else(sq), Ok(2));
 708     /// assert_eq!(Ok(2).or_else(err).or_else(sq), Ok(2));
 709     /// assert_eq!(Err(3).or_else(sq).or_else(err), Ok(9));
 710     /// assert_eq!(Err(3).or_else(err).or_else(err), Err(3));
 711     /// ```
 712     #[inline]
 713     #[stable(feature = "rust1", since = "1.0.0")]
 714     pub fn or_else<F, O: FnOnce(E) -> Result<T, F>>(self, op: O) -> Result<T, F> {
 715         match self {
 716             Ok(t) => Ok(t),
 717             Err(e) => op(e),
 718         }
 719     }
 720
 721     /// Unwraps a result, yielding the content of an [`Ok`].
 722     /// Else, it returns `optb`.
 723     ///
 724     /// Arguments passed to `unwrap_or` are eagerly evaluated; if you are passing
 725     /// the result of a function call, it is recommended to use [`unwrap_or_else`],
 726     /// which is lazily evaluated.
 727     ///
 728     /// [`Ok`]: enum.Result.html#variant.Ok
 729     /// [`Err`]: enum.Result.html#variant.Err
 730     /// [`unwrap_or_else`]: #method.unwrap_or_else
 731     ///
 732     /// # Examples
 733     ///
 734     /// Basic usage:
 735     ///
 736     /// ```
 737     /// let optb = 2;
 738     /// let x: Result<u32, &str> = Ok(9);
 739     /// assert_eq!(x.unwrap_or(optb), 9);
 740     ///
 741     /// let x: Result<u32, &str> = Err("error");
 742     /// assert_eq!(x.unwrap_or(optb), optb);
 743     /// ```
 744     #[inline]
 745     #[stable(feature = "rust1", since = "1.0.0")]
 746     pub fn unwrap_or(self, optb: T) -> T {
 747         match self {
 748             Ok(t) => t,
 749             Err(_) => optb
 750         }
 751     }
 752
 753     /// Unwraps a result, yielding the content of an [`Ok`].
 754     /// If the value is an [`Err`] then it calls `op` with its value.
 755     ///
 756     /// [`Ok`]: enum.Result.html#variant.Ok
 757     /// [`Err`]: enum.Result.html#variant.Err
 758     ///
 759     /// # Examples
 760     ///
 761     /// Basic usage:
 762     ///
 763     /// ```
 764     /// fn count(x: &str) -> usize { x.len() }
 765     ///
 766     /// assert_eq!(Ok(2).unwrap_or_else(count), 2);
 767     /// assert_eq!(Err("foo").unwrap_or_else(count), 3);
 768     /// ```
 769     #[inline]
 770     #[stable(feature = "rust1", since = "1.0.0")]
 771     pub fn unwrap_or_else<F: FnOnce(E) -> T>(self, op: F) -> T {
 772         match self {
 773             Ok(t) => t,
 774             Err(e) => op(e)
 775         }
 776     }
 777 }
 778
 779 impl<T, E: fmt::Debug> Result<T, E> {
 780     /// Unwraps a result, yielding the content of an [`Ok`].
 781     ///
 782     /// # Panics
 783     ///
 784     /// Panics if the value is an [`Err`], with a panic message provided by the
 785     /// [`Err`]'s value.
 786     ///
 787     /// [`Ok`]: enum.Result.html#variant.Ok
 788     /// [`Err`]: enum.Result.html#variant.Err
 789     ///
 790     /// # Examples
 791     ///
 792     /// Basic usage:
 793     ///
 794     /// ```
 795     /// let x: Result<u32, &str> = Ok(2);
 796     /// assert_eq!(x.unwrap(), 2);
 797     /// ```
 798     ///
 799     /// ```{.should_panic}
 800     /// let x: Result<u32, &str> = Err("emergency failure");
 801     /// x.unwrap(); // panics with `emergency failure`
 802     /// ```
 803     #[inline]
 804     #[stable(feature = "rust1", since = "1.0.0")]
 805     pub fn unwrap(self) -> T {
 806         match self {
 807             Ok(t) => t,
 808             Err(e) => unwrap_failed("called `Result::unwrap()` on an `Err` value", e),
 809         }
 810     }
 811
 812     /// Unwraps a result, yielding the content of an [`Ok`].
 813     ///
 814     /// # Panics
 815     ///
 816     /// Panics if the value is an [`Err`], with a panic message including the
 817     /// passed message, and the content of the [`Err`].
 818     ///
 819     /// [`Ok`]: enum.Result.html#variant.Ok
 820     /// [`Err`]: enum.Result.html#variant.Err
 821     ///
 822     /// # Examples
 823     ///
 824     /// Basic usage:
 825     ///
 826     /// ```{.should_panic}
 827     /// let x: Result<u32, &str> = Err("emergency failure");
 828     /// x.expect("Testing expect"); // panics with `Testing expect: emergency failure`
 829     /// ```
 830     #[inline]
 831     #[stable(feature = "result_expect", since = "1.4.0")]
 832     pub fn expect(self, msg: &str) -> T {
 833         match self {
 834             Ok(t) => t,
 835             Err(e) => unwrap_failed(msg, e),
 836         }
 837     }
 838 }
 839
 840 impl<T: fmt::Debug, E> Result<T, E> {
 841     /// Unwraps a result, yielding the content of an [`Err`].
 842     ///
 843     /// # Panics
 844     ///
 845     /// Panics if the value is an [`Ok`], with a custom panic message provided
 846     /// by the [`Ok`]'s value.
 847     ///
 848     /// [`Ok`]: enum.Result.html#variant.Ok
 849     /// [`Err`]: enum.Result.html#variant.Err
 850     ///
 851     ///
 852     /// # Examples
 853     ///
 854     /// ```{.should_panic}
 855     /// let x: Result<u32, &str> = Ok(2);
 856     /// x.unwrap_err(); // panics with `2`
 857     /// ```
 858     ///
 859     /// ```
 860     /// let x: Result<u32, &str> = Err("emergency failure");
 861     /// assert_eq!(x.unwrap_err(), "emergency failure");
 862     /// ```
 863     #[inline]
 864     #[stable(feature = "rust1", since = "1.0.0")]
 865     pub fn unwrap_err(self) -> E {
 866         match self {
 867             Ok(t) => unwrap_failed("called `Result::unwrap_err()` on an `Ok` value", t),
 868             Err(e) => e,
 869         }
 870     }
 871
 872     /// Unwraps a result, yielding the content of an [`Err`].
 873     ///
 874     /// # Panics
 875     ///
 876     /// Panics if the value is an [`Ok`], with a panic message including the
 877     /// passed message, and the content of the [`Ok`].
 878     ///
 879     /// [`Ok`]: enum.Result.html#variant.Ok
 880     /// [`Err`]: enum.Result.html#variant.Err
 881     ///
 882     /// # Examples
 883     ///
 884     /// Basic usage:
 885     ///
 886     /// ```{.should_panic}
 887     /// let x: Result<u32, &str> = Ok(10);
 888     /// x.expect_err("Testing expect_err"); // panics with `Testing expect_err: 10`
 889     /// ```
 890     #[inline]
 891     #[stable(feature = "result_expect_err", since = "1.17.0")]
 892     pub fn expect_err(self, msg: &str) -> E {
 893         match self {
 894             Ok(t) => unwrap_failed(msg, t),
 895             Err(e) => e,
 896         }
 897     }
 898 }
 899
 900 impl<T: Default, E> Result<T, E> {
 901     /// Returns the contained value or a default
 902     ///
 903     /// Consumes the `self` argument then, if [`Ok`], returns the contained
 904     /// value, otherwise if [`Err`], returns the default value for that
 905     /// type.
 906     ///
 907     /// # Examples
 908     ///
 909     /// Convert a string to an integer, turning poorly-formed strings
 910     /// into 0 (the default value for integers). [`parse`] converts
 911     /// a string to any other type that implements [`FromStr`], returning an
 912     /// [`Err`] on error.
 913     ///
 914     /// ```
 915     /// let good_year_from_input = "1909";
 916     /// let bad_year_from_input = "190blarg";
 917     /// let good_year = good_year_from_input.parse().unwrap_or_default();
 918     /// let bad_year = bad_year_from_input.parse().unwrap_or_default();
 919     ///
 920     /// assert_eq!(1909, good_year);
 921     /// assert_eq!(0, bad_year);
 922     /// ```
 923     ///
 924     /// [`parse`]: ../../std/primitive.str.html#method.parse
 925     /// [`FromStr`]: ../../std/str/trait.FromStr.html
 926     /// [`Ok`]: enum.Result.html#variant.Ok
 927     /// [`Err`]: enum.Result.html#variant.Err
 928     #[inline]
 929     #[stable(feature = "result_unwrap_or_default", since = "1.16.0")]
 930     pub fn unwrap_or_default(self) -> T {
 931         match self {
 932             Ok(x) => x,
 933             Err(_) => Default::default(),
 934         }
 935     }
 936 }
 937
 938 #[unstable(feature = "inner_deref", reason = "newly added", issue = "50264")]
 939 impl<T: Deref, E> Result<T, E> {
 940     /// Converts from `&Result<T, E>` to `Result<&T::Target, &E>`.
 941     ///
 942     /// Leaves the original Result in-place, creating a new one with a reference
 943     /// to the original one, additionally coercing the `Ok` arm of the Result via
 944     /// `Deref`.
 945     pub fn deref_ok(&self) -> Result<&T::Target, &E> {
 946         self.as_ref().map(|t| t.deref())
 947     }
 948 }
 949
 950 #[unstable(feature = "inner_deref", reason = "newly added", issue = "50264")]
 951 impl<T, E: Deref> Result<T, E> {
 952     /// Converts from `&Result<T, E>` to `Result<&T, &E::Target>`.
 953     ///
 954     /// Leaves the original Result in-place, creating a new one with a reference
 955     /// to the original one, additionally coercing the `Err` arm of the Result via
 956     /// `Deref`.
 957     pub fn deref_err(&self) -> Result<&T, &E::Target>
 958     {
 959         self.as_ref().map_err(|e| e.deref())
 960     }
 961 }
 962
 963 #[unstable(feature = "inner_deref", reason = "newly added", issue = "50264")]
 964 impl<T: Deref, E: Deref> Result<T, E> {
 965     /// Converts from `&Result<T, E>` to `Result<&T::Target, &E::Target>`.
 966     ///
 967     /// Leaves the original Result in-place, creating a new one with a reference
 968     /// to the original one, additionally coercing both the `Ok` and `Err` arms
 969     /// of the Result via `Deref`.
 970     pub fn deref(&self) -> Result<&T::Target, &E::Target>
 971     {
 972         self.as_ref().map(|t| t.deref()).map_err(|e| e.deref())
 973     }
 974 }
 975
 976 impl<T, E> Result<Option<T>, E> {
 977     /// Transposes a `Result` of an `Option` into an `Option` of a `Result`.
 978     ///
 979     /// `Ok(None)` will be mapped to `None`.
 980     /// `Ok(Some(_))` and `Err(_)` will be mapped to `Some(Ok(_))` and `Some(Err(_))`.
 981     ///
 982     /// # Examples
 983     ///
 984     /// ```
 985     /// #![feature(transpose_result)]
 986     ///
 987     /// #[derive(Debug, Eq, PartialEq)]
 988     /// struct SomeErr;
 989     ///
 990     /// let x: Result<Option<i32>, SomeErr> = Ok(Some(5));
 991     /// let y: Option<Result<i32, SomeErr>> = Some(Ok(5));
 992     /// assert_eq!(x.transpose(), y);
 993     /// ```
 994     #[inline]
 995     #[unstable(feature = "transpose_result", issue = "47338")]
 996     pub fn transpose(self) -> Option<Result<T, E>> {
 997         match self {
 998             Ok(Some(x)) => Some(Ok(x)),
 999             Ok(None) => None,
1000             Err(e) => Some(Err(e)),
1001         }
1002     }
1003 }
1004
1005 // This is a separate function to reduce the code size of the methods
1006 #[inline(never)]
1007 #[cold]
1008 fn unwrap_failed<E: fmt::Debug>(msg: &str, error: E) -> ! {
1009     panic!("{}: {:?}", msg, error)
1010 }
1011
1012 /////////////////////////////////////////////////////////////////////////////
1013 // Trait implementations
1014 /////////////////////////////////////////////////////////////////////////////
1015
1016 #[stable(feature = "rust1", since = "1.0.0")]
1017 impl<T, E> IntoIterator for Result<T, E> {
1018     type Item = T;
1019     type IntoIter = IntoIter<T>;
1020
1021     /// Returns a consuming iterator over the possibly contained value.
1022     ///
1023     /// The iterator yields one value if the result is [`Result::Ok`], otherwise none.
1024     ///
1025     /// # Examples
1026     ///
1027     /// Basic usage:
1028     ///
1029     /// ```
1030     /// let x: Result<u32, &str> = Ok(5);
1031     /// let v: Vec<u32> = x.into_iter().collect();
1032     /// assert_eq!(v, [5]);
1033     ///
1034     /// let x: Result<u32, &str> = Err("nothing!");
1035     /// let v: Vec<u32> = x.into_iter().collect();
1036     /// assert_eq!(v, []);
1037     /// ```
1038     #[inline]
1039     fn into_iter(self) -> IntoIter<T> {
1040         IntoIter { inner: self.ok() }
1041     }
1042 }
1043
1044 #[stable(since = "1.4.0", feature = "result_iter")]
1045 impl<'a, T, E> IntoIterator for &'a Result<T, E> {
1046     type Item = &'a T;
1047     type IntoIter = Iter<'a, T>;
1048
1049     fn into_iter(self) -> Iter<'a, T> {
1050         self.iter()
1051     }
1052 }
1053
1054 #[stable(since = "1.4.0", feature = "result_iter")]
1055 impl<'a, T, E> IntoIterator for &'a mut Result<T, E> {
1056     type Item = &'a mut T;
1057     type IntoIter = IterMut<'a, T>;
1058
1059     fn into_iter(self) -> IterMut<'a, T> {
1060         self.iter_mut()
1061     }
1062 }
1063
1064 /////////////////////////////////////////////////////////////////////////////
1065 // The Result Iterators
1066 /////////////////////////////////////////////////////////////////////////////
1067
1068 /// An iterator over a reference to the [`Ok`] variant of a [`Result`].
1069 ///
1070 /// The iterator yields one value if the result is [`Ok`], otherwise none.
1071 ///
1072 /// Created by [`Result::iter`].
1073 ///
1074 /// [`Ok`]: enum.Result.html#variant.Ok
1075 /// [`Result`]: enum.Result.html
1076 /// [`Result::iter`]: enum.Result.html#method.iter
1077 #[derive(Debug)]
1078 #[stable(feature = "rust1", since = "1.0.0")]
1079 pub struct Iter<'a, T: 'a> { inner: Option<&'a T> }
1080
1081 #[stable(feature = "rust1", since = "1.0.0")]
1082 impl<'a, T> Iterator for Iter<'a, T> {
1083     type Item = &'a T;
1084
1085     #[inline]
1086     fn next(&mut self) -> Option<&'a T> { self.inner.take() }
1087     #[inline]
1088     fn size_hint(&self) -> (usize, Option<usize>) {
1089         let n = if self.inner.is_some() {1} else {0};
1090         (n, Some(n))
1091     }
1092 }
1093
1094 #[stable(feature = "rust1", since = "1.0.0")]
1095 impl<'a, T> DoubleEndedIterator for Iter<'a, T> {
1096     #[inline]
1097     fn next_back(&mut self) -> Option<&'a T> { self.inner.take() }
1098 }
1099
1100 #[stable(feature = "rust1", since = "1.0.0")]
1101 impl<T> ExactSizeIterator for Iter<'_, T> {}
1102
1103 #[stable(feature = "fused", since = "1.26.0")]
1104 impl<T> FusedIterator for Iter<'_, T> {}
1105
1106 #[unstable(feature = "trusted_len", issue = "37572")]
1107 unsafe impl<A> TrustedLen for Iter<'_, A> {}
1108
1109 #[stable(feature = "rust1", since = "1.0.0")]
1110 impl<T> Clone for Iter<'_, T> {
1111     #[inline]
1112     fn clone(&self) -> Self { Iter { inner: self.inner } }
1113 }
1114
1115 /// An iterator over a mutable reference to the [`Ok`] variant of a [`Result`].
1116 ///
1117 /// Created by [`Result::iter_mut`].
1118 ///
1119 /// [`Ok`]: enum.Result.html#variant.Ok
1120 /// [`Result`]: enum.Result.html
1121 /// [`Result::iter_mut`]: enum.Result.html#method.iter_mut
1122 #[derive(Debug)]
1123 #[stable(feature = "rust1", since = "1.0.0")]
1124 pub struct IterMut<'a, T: 'a> { inner: Option<&'a mut T> }
1125
1126 #[stable(feature = "rust1", since = "1.0.0")]
1127 impl<'a, T> Iterator for IterMut<'a, T> {
1128     type Item = &'a mut T;
1129
1130     #[inline]
1131     fn next(&mut self) -> Option<&'a mut T> { self.inner.take() }
1132     #[inline]
1133     fn size_hint(&self) -> (usize, Option<usize>) {
1134         let n = if self.inner.is_some() {1} else {0};
1135         (n, Some(n))
1136     }
1137 }
1138
1139 #[stable(feature = "rust1", since = "1.0.0")]
1140 impl<'a, T> DoubleEndedIterator for IterMut<'a, T> {
1141     #[inline]
1142     fn next_back(&mut self) -> Option<&'a mut T> { self.inner.take() }
1143 }
1144
1145 #[stable(feature = "rust1", since = "1.0.0")]
1146 impl<T> ExactSizeIterator for IterMut<'_, T> {}
1147
1148 #[stable(feature = "fused", since = "1.26.0")]
1149 impl<T> FusedIterator for IterMut<'_, T> {}
1150
1151 #[unstable(feature = "trusted_len", issue = "37572")]
1152 unsafe impl<A> TrustedLen for IterMut<'_, A> {}
1153
1154 /// An iterator over the value in a [`Ok`] variant of a [`Result`].
1155 ///
1156 /// The iterator yields one value if the result is [`Ok`], otherwise none.
1157 ///
1158 /// This struct is created by the [`into_iter`] method on
1159 /// [`Result`][`Result`] (provided by the [`IntoIterator`] trait).
1160 ///
1161 /// [`Ok`]: enum.Result.html#variant.Ok
1162 /// [`Result`]: enum.Result.html
1163 /// [`into_iter`]: ../iter/trait.IntoIterator.html#tymethod.into_iter
1164 /// [`IntoIterator`]: ../iter/trait.IntoIterator.html
1165 #[derive(Clone, Debug)]
1166 #[stable(feature = "rust1", since = "1.0.0")]
1167 pub struct IntoIter<T> { inner: Option<T> }
1168
1169 #[stable(feature = "rust1", since = "1.0.0")]
1170 impl<T> Iterator for IntoIter<T> {
1171     type Item = T;
1172
1173     #[inline]
1174     fn next(&mut self) -> Option<T> { self.inner.take() }
1175     #[inline]
1176     fn size_hint(&self) -> (usize, Option<usize>) {
1177         let n = if self.inner.is_some() {1} else {0};
1178         (n, Some(n))
1179     }
1180 }
1181
1182 #[stable(feature = "rust1", since = "1.0.0")]
1183 impl<T> DoubleEndedIterator for IntoIter<T> {
1184     #[inline]
1185     fn next_back(&mut self) -> Option<T> { self.inner.take() }
1186 }
1187
1188 #[stable(feature = "rust1", since = "1.0.0")]
1189 impl<T> ExactSizeIterator for IntoIter<T> {}
1190
1191 #[stable(feature = "fused", since = "1.26.0")]
1192 impl<T> FusedIterator for IntoIter<T> {}
1193
1194 #[unstable(feature = "trusted_len", issue = "37572")]
1195 unsafe impl<A> TrustedLen for IntoIter<A> {}
1196
1197 /////////////////////////////////////////////////////////////////////////////
1198 // FromIterator
1199 /////////////////////////////////////////////////////////////////////////////
1200
1201 #[stable(feature = "rust1", since = "1.0.0")]
1202 impl<A, E, V: FromIterator<A>> FromIterator<Result<A, E>> for Result<V, E> {
1203     /// Takes each element in the `Iterator`: if it is an `Err`, no further
1204     /// elements are taken, and the `Err` is returned. Should no `Err` occur, a
1205     /// container with the values of each `Result` is returned.
1206     ///
1207     /// Here is an example which increments every integer in a vector,
1208     /// checking for overflow:
1209     ///
1210     /// ```
1211     /// let v = vec![1, 2];
1212     /// let res: Result<Vec<u32>, &'static str> = v.iter().map(|x: &u32|
1213     ///     x.checked_add(1).ok_or("Overflow!")
1214     /// ).collect();
1215     /// assert!(res == Ok(vec![2, 3]));
1216     /// ```
1217     #[inline]
1218     fn from_iter<I: IntoIterator<Item=Result<A, E>>>(iter: I) -> Result<V, E> {
1219         // FIXME(#11084): This could be replaced with Iterator::scan when this
1220         // performance bug is closed.
1221
1222         struct Adapter<Iter, E> {
1223             iter: Iter,
1224             err: Option<E>,
1225         }
1226
1227         impl<T, E, Iter: Iterator<Item=Result<T, E>>> Iterator for Adapter<Iter, E> {
1228             type Item = T;
1229
1230             #[inline]
1231             fn next(&mut self) -> Option<T> {
1232                 match self.iter.next() {
1233                     Some(Ok(value)) => Some(value),
1234                     Some(Err(err)) => {
1235                         self.err = Some(err);
1236                         None
1237                     }
1238                     None => None,
1239                 }
1240             }
1241
1242             fn size_hint(&self) -> (usize, Option<usize>) {
1243                 let (_min, max) = self.iter.size_hint();
1244                 (0, max)
1245             }
1246         }
1247
1248         let mut adapter = Adapter { iter: iter.into_iter(), err: None };
1249         let v: V = FromIterator::from_iter(adapter.by_ref());
1250
1251         match adapter.err {
1252             Some(err) => Err(err),
1253             None => Ok(v),
1254         }
1255     }
1256 }
1257
1258 #[unstable(feature = "try_trait", issue = "42327")]
1259 impl<T,E> ops::Try for Result<T, E> {
1260     type Ok = T;
1261     type Error = E;
1262
1263     #[inline]
1264     fn into_result(self) -> Self {
1265         self
1266     }
1267
1268     #[inline]
1269     fn from_ok(v: T) -> Self {
1270         Ok(v)
1271     }
1272
1273     #[inline]
1274     fn from_error(v: E) -> Self {
1275         Err(v)
1276     }
1277 }