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    // Copyright 2012-2014 The Rust Project Developers. See the COPYRIGHT
    // file at the top-level directory of this distribution and at
    // http://rust-lang.org/COPYRIGHT.
    //
    // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
    // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
    // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
    // option. This file may not be copied, modified, or distributed
    // except according to those terms.

    //! Error handling with the `Result` type
    //!
    //! `Result<T, E>` is the type used for returning and propagating
    //! errors. It is an enum with the variants, `Ok(T)`, representing
    //! success and containing a value, and `Err(E)`, representing error
    //! and containing an error value.
    //!
    //! ```
    //! # #[allow(dead_code)]
    //! enum Result<T, E> {
    //!    Ok(T),
    //!    Err(E)
    //! }
    //! ```
    //!
    //! Functions return `Result` whenever errors are expected and
    //! recoverable. In the `std` crate `Result` is most prominently used
    //! for [I/O](../../std/io/index.html).
    //!
    //! A simple function returning `Result` might be
    //! defined and used like so:
    //!
    //! ```
    //! #[derive(Debug)]
    //! enum Version { Version1, Version2 }
    //!
    //! fn parse_version(header: &[u8]) -> Result<Version, &'static str> {
    //!     match header.get(0) {
    //!         None => Err("invalid header length"),
    //!         Some(&1) => Ok(Version::Version1),
    //!         Some(&2) => Ok(Version::Version2),
    //!         Some(_) => Err("invalid version")
    //!     }
    //! }
    //!
    //! let version = parse_version(&[1, 2, 3, 4]);
    //! match version {
    //!     Ok(v) => println!("working with version: {:?}", v),
    //!     Err(e) => println!("error parsing header: {:?}", e),
    //! }
    //! ```
    //!
    //! Pattern matching on `Result`s is clear and straightforward for
    //! simple cases, but `Result` comes with some convenience methods
    //! that make working with it more succinct.
    //!
    //! ```
    //! let good_result: Result<i32, i32> = Ok(10);
    //! let bad_result: Result<i32, i32> = Err(10);
    //!
    //! // The `is_ok` and `is_err` methods do what they say.
    //! assert!(good_result.is_ok() && !good_result.is_err());
    //! assert!(bad_result.is_err() && !bad_result.is_ok());
    //!
    //! // `map` consumes the `Result` and produces another.
    //! let good_result: Result<i32, i32> = good_result.map(|i| i + 1);
    //! let bad_result: Result<i32, i32> = bad_result.map(|i| i - 1);
    //!
    //! // Use `and_then` to continue the computation.
    //! let good_result: Result<bool, i32> = good_result.and_then(|i| Ok(i == 11));
    //!
    //! // Use `or_else` to handle the error.
    //! let bad_result: Result<i32, i32> = bad_result.or_else(|i| Ok(i + 20));
    //!
    //! // Consume the result and return the contents with `unwrap`.
    //! let final_awesome_result = good_result.unwrap();
    //! ```
    //!
    //! # Results must be used
    //!
    //! A common problem with using return values to indicate errors is
    //! that it is easy to ignore the return value, thus failing to handle
    //! the error. Result is annotated with the #[must_use] attribute,
    //! which will cause the compiler to issue a warning when a Result
    //! value is ignored. This makes `Result` especially useful with
    //! functions that may encounter errors but don't otherwise return a
    //! useful value.
    //!
    //! Consider the `write_all` method defined for I/O types
    //! by the [`Write`](../../std/io/trait.Write.html) trait:
    //!
    //! ```
    //! use std::io;
    //!
    //! trait Write {
    //!     fn write_all(&mut self, bytes: &[u8]) -> Result<(), io::Error>;
    //! }
    //! ```
    //!
    //! *Note: The actual definition of `Write` uses `io::Result`, which
    //! is just a synonym for `Result<T, io::Error>`.*
    //!
    //! This method doesn't produce a value, but the write may
    //! fail. It's crucial to handle the error case, and *not* write
    //! something like this:
    //!
    //! ```no_run
    //! # #![allow(unused_must_use)] // \o/
    //! use std::fs::File;
    //! use std::io::prelude::*;
    //!
    //! let mut file = File::create("valuable_data.txt").unwrap();
    //! // If `write_all` errors, then we'll never know, because the return
    //! // value is ignored.
    //! file.write_all(b"important message");
    //! ```
    //!
    //! If you *do* write that in Rust, the compiler will give you a
    //! warning (by default, controlled by the `unused_must_use` lint).
    //!
    //! You might instead, if you don't want to handle the error, simply
    //! assert success with `expect`. This will panic if the
    //! write fails, providing a marginally useful message indicating why:
    //!
    //! ```{.no_run}
    //! use std::fs::File;
    //! use std::io::prelude::*;
    //!
    //! let mut file = File::create("valuable_data.txt").unwrap();
    //! file.write_all(b"important message").expect("failed to write message");
    //! ```
    //!
    //! You might also simply assert success:
    //!
    //! ```{.no_run}
    //! # use std::fs::File;
    //! # use std::io::prelude::*;
    //! # let mut file = File::create("valuable_data.txt").unwrap();
    //! assert!(file.write_all(b"important message").is_ok());
    //! ```
    //!
    //! Or propagate the error up the call stack with `try!`:
    //!
    //! ```
    //! # use std::fs::File;
    //! # use std::io::prelude::*;
    //! # use std::io;
    //! # #[allow(dead_code)]
    //! fn write_message() -> io::Result<()> {
    //!     let mut file = try!(File::create("valuable_data.txt"));
    //!     try!(file.write_all(b"important message"));
    //!     Ok(())
    //! }
    //! ```
    //!
    //! # The `try!` macro
    //!
    //! When writing code that calls many functions that return the
    //! `Result` type, the error handling can be tedious.  The `try!`
    //! macro hides some of the boilerplate of propagating errors up the
    //! call stack.
    //!
    //! It replaces this:
    //!
    //! ```
    //! # #![allow(dead_code)]
    //! use std::fs::File;
    //! use std::io::prelude::*;
    //! use std::io;
    //!
    //! struct Info {
    //!     name: String,
    //!     age: i32,
    //!     rating: i32,
    //! }
    //!
    //! fn write_info(info: &Info) -> io::Result<()> {
    //!     let mut file = try!(File::create("my_best_friends.txt"));
    //!     // Early return on error
    //!     if let Err(e) = file.write_all(format!("name: {}\n", info.name).as_bytes()) {
    //!         return Err(e)
    //!     }
    //!     if let Err(e) = file.write_all(format!("age: {}\n", info.age).as_bytes()) {
    //!         return Err(e)
    //!     }
    //!     if let Err(e) = file.write_all(format!("rating: {}\n", info.rating).as_bytes()) {
    //!         return Err(e)
    //!     }
    //!     Ok(())
    //! }
    //! ```
    //!
    //! With this:
    //!
    //! ```
    //! # #![allow(dead_code)]
    //! use std::fs::File;
    //! use std::io::prelude::*;
    //! use std::io;
    //!
    //! struct Info {
    //!     name: String,
    //!     age: i32,
    //!     rating: i32,
    //! }
    //!
    //! fn write_info(info: &Info) -> io::Result<()> {
    //!     let mut file = try!(File::create("my_best_friends.txt"));
    //!     // Early return on error
    //!     try!(file.write_all(format!("name: {}\n", info.name).as_bytes()));
    //!     try!(file.write_all(format!("age: {}\n", info.age).as_bytes()));
    //!     try!(file.write_all(format!("rating: {}\n", info.rating).as_bytes()));
    //!     Ok(())
    //! }
    //! ```
    //!
    //! *It's much nicer!*
    //!
    //! Wrapping an expression in `try!` will result in the unwrapped
    //! success (`Ok`) value, unless the result is `Err`, in which case
    //! `Err` is returned early from the enclosing function. Its simple definition
    //! makes it clear:
    //!
    //! ```
    //! macro_rules! try {
    //!     ($e:expr) => (match $e { Ok(e) => e, Err(e) => return Err(e) })
    //! }
    //! ```
    //!
    //! `try!` is imported by the prelude and is available everywhere, but it can only
    //! be used in functions that return `Result` because of the early return of
    //! `Err` that it provides.

    #![stable(feature = "rust1", since = "1.0.0")]

    use self::Result::{Ok, Err};

    use clone::Clone;
    use fmt;
    use iter::{Iterator, DoubleEndedIterator, FromIterator, ExactSizeIterator, IntoIterator};
    use ops::FnOnce;
    use option::Option::{self, None, Some};

    /// `Result` is a type that represents either success (`Ok`) or failure (`Err`).
    ///
    /// See the [`std::result`](index.html) module documentation for details.
    #[derive(Clone, Copy, PartialEq, PartialOrd, Eq, Ord, Debug, Hash)]
    #[must_use]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub enum Result<T, E> {
        /// Contains the success value
        #[stable(feature = "rust1", since = "1.0.0")]
        Ok(#[cfg_attr(not(stage0), stable(feature = "rust1", since = "1.0.0"))] T),

        /// Contains the error value
        #[stable(feature = "rust1", since = "1.0.0")]
        Err(#[cfg_attr(not(stage0), stable(feature = "rust1", since = "1.0.0"))] E)
    }

    /////////////////////////////////////////////////////////////////////////////
    // Type implementation
    /////////////////////////////////////////////////////////////////////////////

    impl<T, E> Result<T, E> {
        /////////////////////////////////////////////////////////////////////////
        // Querying the contained values
        /////////////////////////////////////////////////////////////////////////

        /// Returns true if the result is `Ok`
        ///
        /// # Examples
        ///
        /// ```
        /// let x: Result<i32, &str> = Ok(-3);
        /// assert_eq!(x.is_ok(), true);
        ///
        /// let x: Result<i32, &str> = Err("Some error message");
        /// assert_eq!(x.is_ok(), false);
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn is_ok(&self) -> bool {
            match *self {
                Ok(_) => true,
                Err(_) => false
            }
        }

        /// Returns true if the result is `Err`
        ///
        /// # Examples
        ///
        /// ```
        /// let x: Result<i32, &str> = Ok(-3);
        /// assert_eq!(x.is_err(), false);
        ///
        /// let x: Result<i32, &str> = Err("Some error message");
        /// assert_eq!(x.is_err(), true);
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn is_err(&self) -> bool {
            !self.is_ok()
        }

        /////////////////////////////////////////////////////////////////////////
        // Adapter for each variant
        /////////////////////////////////////////////////////////////////////////

        /// Converts from `Result<T, E>` to `Option<T>`
        ///
        /// Converts `self` into an `Option<T>`, consuming `self`,
        /// and discarding the error, if any.
        ///
        /// # Examples
        ///
        /// ```
        /// let x: Result<u32, &str> = Ok(2);
        /// assert_eq!(x.ok(), Some(2));
        ///
        /// let x: Result<u32, &str> = Err("Nothing here");
        /// assert_eq!(x.ok(), None);
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn ok(self) -> Option<T> {
            match self {
                Ok(x)  => Some(x),
                Err(_) => None,
            }
        }

        /// Converts from `Result<T, E>` to `Option<E>`
        ///
        /// Converts `self` into an `Option<E>`, consuming `self`,
        /// and discarding the success value, if any.
        ///
        /// # Examples
        ///
        /// ```
        /// let x: Result<u32, &str> = Ok(2);
        /// assert_eq!(x.err(), None);
        ///
        /// let x: Result<u32, &str> = Err("Nothing here");
        /// assert_eq!(x.err(), Some("Nothing here"));
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn err(self) -> Option<E> {
            match self {
                Ok(_)  => None,
                Err(x) => Some(x),
            }
        }

        /////////////////////////////////////////////////////////////////////////
        // Adapter for working with references
        /////////////////////////////////////////////////////////////////////////

        /// Converts from `Result<T, E>` to `Result<&T, &E>`
        ///
        /// Produces a new `Result`, containing a reference
        /// into the original, leaving the original in place.
        ///
        /// ```
        /// let x: Result<u32, &str> = Ok(2);
        /// assert_eq!(x.as_ref(), Ok(&2));
        ///
        /// let x: Result<u32, &str> = Err("Error");
        /// assert_eq!(x.as_ref(), Err(&"Error"));
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn as_ref(&self) -> Result<&T, &E> {
            match *self {
                Ok(ref x) => Ok(x),
                Err(ref x) => Err(x),
            }
        }

        /// Converts from `Result<T, E>` to `Result<&mut T, &mut E>`
        ///
        /// ```
        /// fn mutate(r: &mut Result<i32, i32>) {
        ///     match r.as_mut() {
        ///         Ok(&mut ref mut v) => *v = 42,
        ///         Err(&mut ref mut e) => *e = 0,
        ///     }
        /// }
        ///
        /// let mut x: Result<i32, i32> = Ok(2);
        /// mutate(&mut x);
        /// assert_eq!(x.unwrap(), 42);
        ///
        /// let mut x: Result<i32, i32> = Err(13);
        /// mutate(&mut x);
        /// assert_eq!(x.unwrap_err(), 0);
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn as_mut(&mut self) -> Result<&mut T, &mut E> {
            match *self {
                Ok(ref mut x) => Ok(x),
                Err(ref mut x) => Err(x),
            }
        }

        /////////////////////////////////////////////////////////////////////////
        // Transforming contained values
        /////////////////////////////////////////////////////////////////////////

        /// Maps a `Result<T, E>` to `Result<U, E>` by applying a function to a
        /// contained `Ok` value, leaving an `Err` value untouched.
        ///
        /// This function can be used to compose the results of two functions.
        ///
        /// # Examples
        ///
        /// Print the numbers on each line of a string multiplied by two.
        ///
        /// ```
        /// let line = "1\n2\n3\n4\n";
        ///
        /// for num in line.lines() {
        ///     match num.parse::<i32>().map(|i| i * 2) {
        ///         Ok(n) => println!("{}", n),
        ///         Err(..) => {}
        ///     }
        /// }
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn map<U, F: FnOnce(T) -> U>(self, op: F) -> Result<U,E> {
            match self {
                Ok(t) => Ok(op(t)),
                Err(e) => Err(e)
            }
        }

        /// Maps a `Result<T, E>` to `Result<T, F>` by applying a function to a
        /// contained `Err` value, leaving an `Ok` value untouched.
        ///
        /// This function can be used to pass through a successful result while handling
        /// an error.
        ///
        /// # Examples
        ///
        /// ```
        /// fn stringify(x: u32) -> String { format!("error code: {}", x) }
        ///
        /// let x: Result<u32, u32> = Ok(2);
        /// assert_eq!(x.map_err(stringify), Ok(2));
        ///
        /// let x: Result<u32, u32> = Err(13);
        /// assert_eq!(x.map_err(stringify), Err("error code: 13".to_string()));
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn map_err<F, O: FnOnce(E) -> F>(self, op: O) -> Result<T,F> {
            match self {
                Ok(t) => Ok(t),
                Err(e) => Err(op(e))
            }
        }

        /////////////////////////////////////////////////////////////////////////
        // Iterator constructors
        /////////////////////////////////////////////////////////////////////////

        /// Returns an iterator over the possibly contained value.
        ///
        /// # Examples
        ///
        /// ```
        /// let x: Result<u32, &str> = Ok(7);
        /// assert_eq!(x.iter().next(), Some(&7));
        ///
        /// let x: Result<u32, &str> = Err("nothing!");
        /// assert_eq!(x.iter().next(), None);
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn iter(&self) -> Iter<T> {
            Iter { inner: self.as_ref().ok() }
        }

        /// Returns a mutable iterator over the possibly contained value.
        ///
        /// # Examples
        ///
        /// ```
        /// let mut x: Result<u32, &str> = Ok(7);
        /// match x.iter_mut().next() {
        ///     Some(v) => *v = 40,
        ///     None => {},
        /// }
        /// assert_eq!(x, Ok(40));
        ///
        /// let mut x: Result<u32, &str> = Err("nothing!");
        /// assert_eq!(x.iter_mut().next(), None);
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn iter_mut(&mut self) -> IterMut<T> {
            IterMut { inner: self.as_mut().ok() }
        }

        ////////////////////////////////////////////////////////////////////////
        // Boolean operations on the values, eager and lazy
        /////////////////////////////////////////////////////////////////////////

        /// Returns `res` if the result is `Ok`, otherwise returns the `Err` value of `self`.
        ///
        /// # Examples
        ///
        /// ```
        /// let x: Result<u32, &str> = Ok(2);
        /// let y: Result<&str, &str> = Err("late error");
        /// assert_eq!(x.and(y), Err("late error"));
        ///
        /// let x: Result<u32, &str> = Err("early error");
        /// let y: Result<&str, &str> = Ok("foo");
        /// assert_eq!(x.and(y), Err("early error"));
        ///
        /// let x: Result<u32, &str> = Err("not a 2");
        /// let y: Result<&str, &str> = Err("late error");
        /// assert_eq!(x.and(y), Err("not a 2"));
        ///
        /// let x: Result<u32, &str> = Ok(2);
        /// let y: Result<&str, &str> = Ok("different result type");
        /// assert_eq!(x.and(y), Ok("different result type"));
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn and<U>(self, res: Result<U, E>) -> Result<U, E> {
            match self {
                Ok(_) => res,
                Err(e) => Err(e),
            }
        }

        /// Calls `op` if the result is `Ok`, otherwise returns the `Err` value of `self`.
        ///
        /// This function can be used for control flow based on result values.
        ///
        /// # Examples
        ///
        /// ```
        /// fn sq(x: u32) -> Result<u32, u32> { Ok(x * x) }
        /// fn err(x: u32) -> Result<u32, u32> { Err(x) }
        ///
        /// assert_eq!(Ok(2).and_then(sq).and_then(sq), Ok(16));
        /// assert_eq!(Ok(2).and_then(sq).and_then(err), Err(4));
        /// assert_eq!(Ok(2).and_then(err).and_then(sq), Err(2));
        /// assert_eq!(Err(3).and_then(sq).and_then(sq), Err(3));
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn and_then<U, F: FnOnce(T) -> Result<U, E>>(self, op: F) -> Result<U, E> {
            match self {
                Ok(t) => op(t),
                Err(e) => Err(e),
            }
        }

        /// Returns `res` if the result is `Err`, otherwise returns the `Ok` value of `self`.
        ///
        /// # Examples
        ///
        /// ```
        /// let x: Result<u32, &str> = Ok(2);
        /// let y: Result<u32, &str> = Err("late error");
        /// assert_eq!(x.or(y), Ok(2));
        ///
        /// let x: Result<u32, &str> = Err("early error");
        /// let y: Result<u32, &str> = Ok(2);
        /// assert_eq!(x.or(y), Ok(2));
        ///
        /// let x: Result<u32, &str> = Err("not a 2");
        /// let y: Result<u32, &str> = Err("late error");
        /// assert_eq!(x.or(y), Err("late error"));
        ///
        /// let x: Result<u32, &str> = Ok(2);
        /// let y: Result<u32, &str> = Ok(100);
        /// assert_eq!(x.or(y), Ok(2));
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn or<F>(self, res: Result<T, F>) -> Result<T, F> {
            match self {
                Ok(v) => Ok(v),
                Err(_) => res,
            }
        }

        /// Calls `op` if the result is `Err`, otherwise returns the `Ok` value of `self`.
        ///
        /// This function can be used for control flow based on result values.
        ///
        /// # Examples
        ///
        /// ```
        /// fn sq(x: u32) -> Result<u32, u32> { Ok(x * x) }
        /// fn err(x: u32) -> Result<u32, u32> { Err(x) }
        ///
        /// assert_eq!(Ok(2).or_else(sq).or_else(sq), Ok(2));
        /// assert_eq!(Ok(2).or_else(err).or_else(sq), Ok(2));
        /// assert_eq!(Err(3).or_else(sq).or_else(err), Ok(9));
        /// assert_eq!(Err(3).or_else(err).or_else(err), Err(3));
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn or_else<F, O: FnOnce(E) -> Result<T, F>>(self, op: O) -> Result<T, F> {
            match self {
                Ok(t) => Ok(t),
                Err(e) => op(e),
            }
        }

        /// Unwraps a result, yielding the content of an `Ok`.
        /// Else it returns `optb`.
        ///
        /// # Examples
        ///
        /// ```
        /// let optb = 2;
        /// let x: Result<u32, &str> = Ok(9);
        /// assert_eq!(x.unwrap_or(optb), 9);
        ///
        /// let x: Result<u32, &str> = Err("error");
        /// assert_eq!(x.unwrap_or(optb), optb);
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn unwrap_or(self, optb: T) -> T {
            match self {
                Ok(t) => t,
                Err(_) => optb
            }
        }

        /// Unwraps a result, yielding the content of an `Ok`.
        /// If the value is an `Err` then it calls `op` with its value.
        ///
        /// # Examples
        ///
        /// ```
        /// fn count(x: &str) -> usize { x.len() }
        ///
        /// assert_eq!(Ok(2).unwrap_or_else(count), 2);
        /// assert_eq!(Err("foo").unwrap_or_else(count), 3);
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn unwrap_or_else<F: FnOnce(E) -> T>(self, op: F) -> T {
            match self {
                Ok(t) => t,
                Err(e) => op(e)
            }
        }
    }

    impl<T, E: fmt::Debug> Result<T, E> {
        /// Unwraps a result, yielding the content of an `Ok`.
        ///
        /// # Panics
        ///
        /// Panics if the value is an `Err`, with a panic message provided by the
        /// `Err`'s value.
        ///
        /// # Examples
        ///
        /// ```
        /// let x: Result<u32, &str> = Ok(2);
        /// assert_eq!(x.unwrap(), 2);
        /// ```
        ///
        /// ```{.should_panic}
        /// let x: Result<u32, &str> = Err("emergency failure");
        /// x.unwrap(); // panics with `emergency failure`
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn unwrap(self) -> T {
            match self {
                Ok(t) => t,
                Err(e) =>
                    panic!("called `Result::unwrap()` on an `Err` value: {:?}", e)
            }
        }

        /// Unwraps a result, yielding the content of an `Ok`.
        ///
        /// # Panics
        ///
        /// Panics if the value is an `Err`, with a panic message including the
        /// passed message, and the content of the `Err`.
        ///
        /// # Examples
        /// ```{.should_panic}
        /// let x: Result<u32, &str> = Err("emergency failure");
        /// x.expect("Testing expect"); // panics with `Testing expect: emergency failure`
        /// ```
        #[inline]
        #[stable(feature = "result_expect", since = "1.4.0")]
        pub fn expect(self, msg: &str) -> T {
            match self {
                Ok(t) => t,
                Err(e) => panic!("{}: {:?}", msg, e),
            }
        }
    }

    impl<T: fmt::Debug, E> Result<T, E> {
        /// Unwraps a result, yielding the content of an `Err`.
        ///
        /// # Panics
        ///
        /// Panics if the value is an `Ok`, with a custom panic message provided
        /// by the `Ok`'s value.
        ///
        /// # Examples
        ///
        /// ```{.should_panic}
        /// let x: Result<u32, &str> = Ok(2);
        /// x.unwrap_err(); // panics with `2`
        /// ```
        ///
        /// ```
        /// let x: Result<u32, &str> = Err("emergency failure");
        /// assert_eq!(x.unwrap_err(), "emergency failure");
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn unwrap_err(self) -> E {
            match self {
                Ok(t) =>
                    panic!("called `Result::unwrap_err()` on an `Ok` value: {:?}", t),
                Err(e) => e
            }
        }
    }

    /////////////////////////////////////////////////////////////////////////////
    // Trait implementations
    /////////////////////////////////////////////////////////////////////////////

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T, E> IntoIterator for Result<T, E> {
        type Item = T;
        type IntoIter = IntoIter<T>;

        /// Returns a consuming iterator over the possibly contained value.
        ///
        /// # Examples
        ///
        /// ```
        /// let x: Result<u32, &str> = Ok(5);
        /// let v: Vec<u32> = x.into_iter().collect();
        /// assert_eq!(v, [5]);
        ///
        /// let x: Result<u32, &str> = Err("nothing!");
        /// let v: Vec<u32> = x.into_iter().collect();
        /// assert_eq!(v, []);
        /// ```
        #[inline]
        fn into_iter(self) -> IntoIter<T> {
            IntoIter { inner: self.ok() }
        }
    }

    #[stable(since = "1.4.0", feature = "result_iter")]
    impl<'a, T, E> IntoIterator for &'a Result<T, E> {
        type Item = &'a T;
        type IntoIter = Iter<'a, T>;

        fn into_iter(self) -> Iter<'a, T> {
            self.iter()
        }
    }

    #[stable(since = "1.4.0", feature = "result_iter")]
    impl<'a, T, E> IntoIterator for &'a mut Result<T, E> {
        type Item = &'a mut T;
        type IntoIter = IterMut<'a, T>;

        fn into_iter(mut self) -> IterMut<'a, T> {
            self.iter_mut()
        }
    }

    /////////////////////////////////////////////////////////////////////////////
    // The Result Iterators
    /////////////////////////////////////////////////////////////////////////////

    /// An iterator over a reference to the `Ok` variant of a `Result`.
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct Iter<'a, T: 'a> { inner: Option<&'a T> }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, T> Iterator for Iter<'a, T> {
        type Item = &'a T;

        #[inline]
        fn next(&mut self) -> Option<&'a T> { self.inner.take() }
        #[inline]
        fn size_hint(&self) -> (usize, Option<usize>) {
            let n = if self.inner.is_some() {1} else {0};
            (n, Some(n))
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, T> DoubleEndedIterator for Iter<'a, T> {
        #[inline]
        fn next_back(&mut self) -> Option<&'a T> { self.inner.take() }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, T> ExactSizeIterator for Iter<'a, T> {}

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, T> Clone for Iter<'a, T> {
        fn clone(&self) -> Iter<'a, T> { Iter { inner: self.inner } }
    }

    /// An iterator over a mutable reference to the `Ok` variant of a `Result`.
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct IterMut<'a, T: 'a> { inner: Option<&'a mut T> }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, T> Iterator for IterMut<'a, T> {
        type Item = &'a mut T;

        #[inline]
        fn next(&mut self) -> Option<&'a mut T> { self.inner.take() }
        #[inline]
        fn size_hint(&self) -> (usize, Option<usize>) {
            let n = if self.inner.is_some() {1} else {0};
            (n, Some(n))
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, T> DoubleEndedIterator for IterMut<'a, T> {
        #[inline]
        fn next_back(&mut self) -> Option<&'a mut T> { self.inner.take() }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, T> ExactSizeIterator for IterMut<'a, T> {}

    /// An iterator over the value in a `Ok` variant of a `Result`.
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct IntoIter<T> { inner: Option<T> }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T> Iterator for IntoIter<T> {
        type Item = T;

        #[inline]
        fn next(&mut self) -> Option<T> { self.inner.take() }
        #[inline]
        fn size_hint(&self) -> (usize, Option<usize>) {
            let n = if self.inner.is_some() {1} else {0};
            (n, Some(n))
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T> DoubleEndedIterator for IntoIter<T> {
        #[inline]
        fn next_back(&mut self) -> Option<T> { self.inner.take() }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T> ExactSizeIterator for IntoIter<T> {}

    /////////////////////////////////////////////////////////////////////////////
    // FromIterator
    /////////////////////////////////////////////////////////////////////////////

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<A, E, V: FromIterator<A>> FromIterator<Result<A, E>> for Result<V, E> {
        /// Takes each element in the `Iterator`: if it is an `Err`, no further
        /// elements are taken, and the `Err` is returned. Should no `Err` occur, a
        /// container with the values of each `Result` is returned.
        ///
        /// Here is an example which increments every integer in a vector,
        /// checking for overflow:
        ///
        /// ```
        /// use std::u32;
        ///
        /// let v = vec!(1, 2);
        /// let res: Result<Vec<u32>, &'static str> = v.iter().map(|&x: &u32|
        ///     if x == u32::MAX { Err("Overflow!") }
        ///     else { Ok(x + 1) }
        /// ).collect();
        /// assert!(res == Ok(vec!(2, 3)));
        /// ```
        #[inline]
        fn from_iter<I: IntoIterator<Item=Result<A, E>>>(iter: I) -> Result<V, E> {
            // FIXME(#11084): This could be replaced with Iterator::scan when this
            // performance bug is closed.

            struct Adapter<Iter, E> {
                iter: Iter,
                err: Option<E>,
            }

            impl<T, E, Iter: Iterator<Item=Result<T, E>>> Iterator for Adapter<Iter, E> {
                type Item = T;

                #[inline]
                fn next(&mut self) -> Option<T> {
                    match self.iter.next() {
                        Some(Ok(value)) => Some(value),
                        Some(Err(err)) => {
                            self.err = Some(err);
                            None
                        }
                        None => None,
                    }
                }
            }

            let mut adapter = Adapter { iter: iter.into_iter(), err: None };
            let v: V = FromIterator::from_iter(adapter.by_ref());

            match adapter.err {
                Some(err) => Err(err),
                None => Ok(v),
            }
        }
    }

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