result.rs.html -- source
Description: Source to the Rust file `src/libcore/result.rs`.
Keywords: rust, rustlang, rust-lang

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    //! Error handling with the `Result` type.
    //!
    //! [`Result<T, E>`][`Result`] 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`] 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 [`?`]:
    //!
    //! ```
    //! # use std::fs::File;
    //! # use std::io::prelude::*;
    //! # use std::io;
    //! # #[allow(dead_code)]
    //! fn write_message() -> io::Result<()> {
    //!     let mut file = File::create("valuable_data.txt")?;
    //!     file.write_all(b"important message")?;
    //!     Ok(())
    //! }
    //! ```
    //!
    //! # The question mark operator, `?`
    //!
    //! When writing code that calls many functions that return the
    //! [`Result`] type, the error handling can be tedious. The question mark
    //! operator, [`?`], 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<()> {
    //!     // Early return on error
    //!     let mut file = match File::create("my_best_friends.txt") {
    //!            Err(e) => return Err(e),
    //!            Ok(f) => f,
    //!     };
    //!     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 = File::create("my_best_friends.txt")?;
    //!     // Early return on error
    //!     file.write_all(format!("name: {}\n", info.name).as_bytes())?;
    //!     file.write_all(format!("age: {}\n", info.age).as_bytes())?;
    //!     file.write_all(format!("rating: {}\n", info.rating).as_bytes())?;
    //!     Ok(())
    //! }
    //! ```
    //!
    //! *It's much nicer!*
    //!
    //! Ending the expression with [`?`] will result in the unwrapped
    //! success ([`Ok`]) value, unless the result is [`Err`], in which case
    //! [`Err`] is returned early from the enclosing function.
    //!
    //! [`?`] can only be used in functions that return [`Result`] because of the
    //! early return of [`Err`] that it provides.
    //!
    //! [`expect`]: enum.Result.html#method.expect
    //! [`Write`]: ../../std/io/trait.Write.html
    //! [`write_all`]: ../../std/io/trait.Write.html#method.write_all
    //! [`io::Result`]: ../../std/io/type.Result.html
    //! [`?`]: ../../std/macro.try.html
    //! [`Result`]: enum.Result.html
    //! [`Ok(T)`]: enum.Result.html#variant.Ok
    //! [`Err(E)`]: enum.Result.html#variant.Err
    //! [`io::Error`]: ../../std/io/struct.Error.html
    //! [`Ok`]: enum.Result.html#variant.Ok
    //! [`Err`]: enum.Result.html#variant.Err

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

    use crate::fmt;
    use crate::iter::{self, FromIterator, FusedIterator, TrustedLen};
    use crate::ops::{self, Deref, DerefMut};

    /// `Result` is a type that represents either success ([`Ok`]) or failure ([`Err`]).
    ///
    /// See the [`std::result`](index.html) module documentation for details.
    ///
    /// [`Ok`]: enum.Result.html#variant.Ok
    /// [`Err`]: enum.Result.html#variant.Err
    #[derive(Copy, PartialEq, PartialOrd, Eq, Ord, Debug, Hash)]
    #[must_use = "this `Result` may be an `Err` variant, which should be handled"]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub enum Result<T, E> {
        /// Contains the success value
        #[stable(feature = "rust1", since = "1.0.0")]
        Ok(#[stable(feature = "rust1", since = "1.0.0")] T),

        /// Contains the error value
        #[stable(feature = "rust1", since = "1.0.0")]
        Err(#[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`].
        ///
        /// [`Ok`]: enum.Result.html#variant.Ok
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// 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);
        /// ```
        #[must_use = "if you intended to assert that this is ok, consider `.unwrap()` instead"]
        #[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`].
        ///
        /// [`Err`]: enum.Result.html#variant.Err
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// 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);
        /// ```
        #[must_use = "if you intended to assert that this is err, consider `.unwrap_err()` instead"]
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn is_err(&self) -> bool {
            !self.is_ok()
        }

        /// Returns `true` if the result is an [`Ok`] value containing the given value.
        ///
        /// # Examples
        ///
        /// ```
        /// #![feature(option_result_contains)]
        ///
        /// let x: Result<u32, &str> = Ok(2);
        /// assert_eq!(x.contains(&2), true);
        ///
        /// let x: Result<u32, &str> = Ok(3);
        /// assert_eq!(x.contains(&2), false);
        ///
        /// let x: Result<u32, &str> = Err("Some error message");
        /// assert_eq!(x.contains(&2), false);
        /// ```
        #[must_use]
        #[inline]
        #[unstable(feature = "option_result_contains", issue = "62358")]
        pub fn contains<U>(&self, x: &U) -> bool where U: PartialEq<T> {
            match self {
                Ok(y) => x == y,
                Err(_) => false
            }
        }

        /// Returns `true` if the result is an [`Err`] value containing the given value.
        ///
        /// # Examples
        ///
        /// ```
        /// #![feature(result_contains_err)]
        ///
        /// let x: Result<u32, &str> = Ok(2);
        /// assert_eq!(x.contains_err(&"Some error message"), false);
        ///
        /// let x: Result<u32, &str> = Err("Some error message");
        /// assert_eq!(x.contains_err(&"Some error message"), true);
        ///
        /// let x: Result<u32, &str> = Err("Some other error message");
        /// assert_eq!(x.contains_err(&"Some error message"), false);
        /// ```
        #[must_use]
        #[inline]
        #[unstable(feature = "result_contains_err", issue = "62358")]
        pub fn contains_err<F>(&self, f: &F) -> bool where F: PartialEq<E> {
            match self {
                Ok(_) => false,
                Err(e) => f == e
            }
        }

        /////////////////////////////////////////////////////////////////////////
        // 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.
        ///
        /// [`Option<T>`]: ../../std/option/enum.Option.html
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// 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.
        ///
        /// [`Option<E>`]: ../../std/option/enum.Option.html
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// 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.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// 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 `&mut Result<T, E>` to `Result<&mut T, &mut E>`.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// fn mutate(r: &mut Result<i32, i32>) {
        ///     match r.as_mut() {
        ///         Ok(v) => *v = 42,
        ///         Err(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.
        ///
        /// [`Ok`]: enum.Result.html#variant.Ok
        /// [`Err`]: enum.Result.html#variant.Err
        ///
        /// # 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 `U` by applying a function to a
        /// contained [`Ok`] value, or a fallback function to a
        /// contained [`Err`] value.
        ///
        /// This function can be used to unpack a successful result
        /// while handling an error.
        ///
        /// [`Ok`]: enum.Result.html#variant.Ok
        /// [`Err`]: enum.Result.html#variant.Err
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// #![feature(result_map_or_else)]
        /// let k = 21;
        ///
        /// let x : Result<_, &str> = Ok("foo");
        /// assert_eq!(x.map_or_else(|e| k * 2, |v| v.len()), 3);
        ///
        /// let x : Result<&str, _> = Err("bar");
        /// assert_eq!(x.map_or_else(|e| k * 2, |v| v.len()), 42);
        /// ```
        #[inline]
        #[unstable(feature = "result_map_or_else", issue = "53268")]
        pub fn map_or_else<U, M: FnOnce(T) -> U, F: FnOnce(E) -> U>(self, fallback: F, map: M) -> U {
            self.map(map).unwrap_or_else(fallback)
        }

        /// 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.
        ///
        /// [`Ok`]: enum.Result.html#variant.Ok
        /// [`Err`]: enum.Result.html#variant.Err
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// 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.
        ///
        /// The iterator yields one value if the result is [`Result::Ok`], otherwise none.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// 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.
        ///
        /// The iterator yields one value if the result is [`Result::Ok`], otherwise none.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// 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`.
        ///
        /// [`Ok`]: enum.Result.html#variant.Ok
        /// [`Err`]: enum.Result.html#variant.Err
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// 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`.
        ///
        /// [`Ok`]: enum.Result.html#variant.Ok
        /// [`Err`]: enum.Result.html#variant.Err
        ///
        /// This function can be used for control flow based on `Result` values.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// 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`.
        ///
        /// Arguments passed to `or` are eagerly evaluated; if you are passing the
        /// result of a function call, it is recommended to use [`or_else`], which is
        /// lazily evaluated.
        ///
        /// [`Ok`]: enum.Result.html#variant.Ok
        /// [`Err`]: enum.Result.html#variant.Err
        /// [`or_else`]: #method.or_else
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// 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.
        ///
        /// [`Ok`]: enum.Result.html#variant.Ok
        /// [`Err`]: enum.Result.html#variant.Err
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// 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`.
        ///
        /// Arguments passed to `unwrap_or` are eagerly evaluated; if you are passing
        /// the result of a function call, it is recommended to use [`unwrap_or_else`],
        /// which is lazily evaluated.
        ///
        /// [`Ok`]: enum.Result.html#variant.Ok
        /// [`Err`]: enum.Result.html#variant.Err
        /// [`unwrap_or_else`]: #method.unwrap_or_else
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// 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.
        ///
        /// [`Ok`]: enum.Result.html#variant.Ok
        /// [`Err`]: enum.Result.html#variant.Err
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// 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.
        ///
        /// [`Ok`]: enum.Result.html#variant.Ok
        /// [`Err`]: enum.Result.html#variant.Err
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// 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) => unwrap_failed("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`].
        ///
        /// [`Ok`]: enum.Result.html#variant.Ok
        /// [`Err`]: enum.Result.html#variant.Err
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```{.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) => unwrap_failed(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.
        ///
        /// [`Ok`]: enum.Result.html#variant.Ok
        /// [`Err`]: enum.Result.html#variant.Err
        ///
        ///
        /// # 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) => unwrap_failed("called `Result::unwrap_err()` on an `Ok` value", &t),
                Err(e) => e,
            }
        }

        /// Unwraps a result, yielding the content of an [`Err`].
        ///
        /// # Panics
        ///
        /// Panics if the value is an [`Ok`], with a panic message including the
        /// passed message, and the content of the [`Ok`].
        ///
        /// [`Ok`]: enum.Result.html#variant.Ok
        /// [`Err`]: enum.Result.html#variant.Err
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```{.should_panic}
        /// let x: Result<u32, &str> = Ok(10);
        /// x.expect_err("Testing expect_err"); // panics with `Testing expect_err: 10`
        /// ```
        #[inline]
        #[stable(feature = "result_expect_err", since = "1.17.0")]
        pub fn expect_err(self, msg: &str) -> E {
            match self {
                Ok(t) => unwrap_failed(msg, &t),
                Err(e) => e,
            }
        }
    }

    impl<T: Default, E> Result<T, E> {
        /// Returns the contained value or a default
        ///
        /// Consumes the `self` argument then, if [`Ok`], returns the contained
        /// value, otherwise if [`Err`], returns the default value for that
        /// type.
        ///
        /// # Examples
        ///
        /// Converts a string to an integer, turning poorly-formed strings
        /// into 0 (the default value for integers). [`parse`] converts
        /// a string to any other type that implements [`FromStr`], returning an
        /// [`Err`] on error.
        ///
        /// ```
        /// let good_year_from_input = "1909";
        /// let bad_year_from_input = "190blarg";
        /// let good_year = good_year_from_input.parse().unwrap_or_default();
        /// let bad_year = bad_year_from_input.parse().unwrap_or_default();
        ///
        /// assert_eq!(1909, good_year);
        /// assert_eq!(0, bad_year);
        /// ```
        ///
        /// [`parse`]: ../../std/primitive.str.html#method.parse
        /// [`FromStr`]: ../../std/str/trait.FromStr.html
        /// [`Ok`]: enum.Result.html#variant.Ok
        /// [`Err`]: enum.Result.html#variant.Err
        #[inline]
        #[stable(feature = "result_unwrap_or_default", since = "1.16.0")]
        pub fn unwrap_or_default(self) -> T {
            match self {
                Ok(x) => x,
                Err(_) => Default::default(),
            }
        }
    }

    #[unstable(feature = "inner_deref", reason = "newly added", issue = "50264")]
    impl<T: Deref, E> Result<T, E> {
        /// Converts from `Result<T, E>` (or `&Result<T, E>`) to `Result<&T::Target, &E>`.
        ///
        /// Leaves the original `Result` in-place, creating a new one containing a reference to the
        /// `Ok` type's `Deref::Target` type.
        pub fn as_deref_ok(&self) -> Result<&T::Target, &E> {
            self.as_ref().map(|t| t.deref())
        }
    }

    #[unstable(feature = "inner_deref", reason = "newly added", issue = "50264")]
    impl<T, E: Deref> Result<T, E> {
        /// Converts from `Result<T, E>` (or `&Result<T, E>`) to `Result<&T, &E::Target>`.
        ///
        /// Leaves the original `Result` in-place, creating a new one containing a reference to the
        /// `Err` type's `Deref::Target` type.
        pub fn as_deref_err(&self) -> Result<&T, &E::Target>
        {
            self.as_ref().map_err(|e| e.deref())
        }
    }

    #[unstable(feature = "inner_deref", reason = "newly added", issue = "50264")]
    impl<T: Deref, E: Deref> Result<T, E> {
        /// Converts from `Result<T, E>` (or `&Result<T, E>`) to `Result<&T::Target, &E::Target>`.
        ///
        /// Leaves the original `Result` in-place, creating a new one containing a reference to both
        /// the `Ok` and `Err` types' `Deref::Target` types.
        pub fn as_deref(&self) -> Result<&T::Target, &E::Target>
        {
            self.as_ref().map(|t| t.deref()).map_err(|e| e.deref())
        }
    }

    #[unstable(feature = "inner_deref", reason = "newly added", issue = "50264")]
    impl<T: DerefMut, E> Result<T, E> {
        /// Converts from `Result<T, E>` (or `&mut Result<T, E>`) to `Result<&mut T::Target, &mut E>`.
        ///
        /// Leaves the original `Result` in-place, creating a new one containing a mutable reference to
        /// the `Ok` type's `Deref::Target` type.
        pub fn as_deref_mut_ok(&mut self) -> Result<&mut T::Target, &mut E> {
            self.as_mut().map(|t| t.deref_mut())
        }
    }

    #[unstable(feature = "inner_deref", reason = "newly added", issue = "50264")]
    impl<T, E: DerefMut> Result<T, E> {
        /// Converts from `Result<T, E>` (or `&mut Result<T, E>`) to `Result<&mut T, &mut E::Target>`.
        ///
        /// Leaves the original `Result` in-place, creating a new one containing a mutable reference to
        /// the `Err` type's `Deref::Target` type.
        pub fn as_deref_mut_err(&mut self) -> Result<&mut T, &mut E::Target>
        {
            self.as_mut().map_err(|e| e.deref_mut())
        }
    }

    #[unstable(feature = "inner_deref", reason = "newly added", issue = "50264")]
    impl<T: DerefMut, E: DerefMut> Result<T, E> {
        /// Converts from `Result<T, E>` (or `&mut Result<T, E>`) to
        /// `Result<&mut T::Target, &mut E::Target>`.
        ///
        /// Leaves the original `Result` in-place, creating a new one containing a mutable reference to
        /// both the `Ok` and `Err` types' `Deref::Target` types.
        pub fn as_deref_mut(&mut self) -> Result<&mut T::Target, &mut E::Target>
        {
            self.as_mut().map(|t| t.deref_mut()).map_err(|e| e.deref_mut())
        }
    }

    impl<T, E> Result<Option<T>, E> {
        /// Transposes a `Result` of an `Option` into an `Option` of a `Result`.
        ///
        /// `Ok(None)` will be mapped to `None`.
        /// `Ok(Some(_))` and `Err(_)` will be mapped to `Some(Ok(_))` and `Some(Err(_))`.
        ///
        /// # Examples
        ///
        /// ```
        /// #[derive(Debug, Eq, PartialEq)]
        /// struct SomeErr;
        ///
        /// let x: Result<Option<i32>, SomeErr> = Ok(Some(5));
        /// let y: Option<Result<i32, SomeErr>> = Some(Ok(5));
        /// assert_eq!(x.transpose(), y);
        /// ```
        #[inline]
        #[stable(feature = "transpose_result", since = "1.33.0")]
        pub fn transpose(self) -> Option<Result<T, E>> {
            match self {
                Ok(Some(x)) => Some(Ok(x)),
                Ok(None) => None,
                Err(e) => Some(Err(e)),
            }
        }
    }

    // This is a separate function to reduce the code size of the methods
    #[inline(never)]
    #[cold]
    fn unwrap_failed(msg: &str, error: &dyn fmt::Debug) -> ! {
        panic!("{}: {:?}", msg, error)
    }

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

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: Clone, E: Clone> Clone for Result<T, E> {
        #[inline]
        fn clone(&self) -> Self {
            match self {
                Ok(x) => Ok(x.clone()),
                Err(x) => Err(x.clone()),
            }
        }

        #[inline]
        fn clone_from(&mut self, source: &Self) {
            match (self, source) {
                (Ok(to), Ok(from)) => to.clone_from(from),
                (Err(to), Err(from)) => to.clone_from(from),
                (to, from) => *to = from.clone(),
            }
        }
    }

    #[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.
        ///
        /// The iterator yields one value if the result is [`Result::Ok`], otherwise none.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// 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(self) -> IterMut<'a, T> {
            self.iter_mut()
        }
    }

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

    /// An iterator over a reference to the [`Ok`] variant of a [`Result`].
    ///
    /// The iterator yields one value if the result is [`Ok`], otherwise none.
    ///
    /// Created by [`Result::iter`].
    ///
    /// [`Ok`]: enum.Result.html#variant.Ok
    /// [`Result`]: enum.Result.html
    /// [`Result::iter`]: enum.Result.html#method.iter
    #[derive(Debug)]
    #[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<T> ExactSizeIterator for Iter<'_, T> {}

    #[stable(feature = "fused", since = "1.26.0")]
    impl<T> FusedIterator for Iter<'_, T> {}

    #[unstable(feature = "trusted_len", issue = "37572")]
    unsafe impl<A> TrustedLen for Iter<'_, A> {}

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

    /// An iterator over a mutable reference to the [`Ok`] variant of a [`Result`].
    ///
    /// Created by [`Result::iter_mut`].
    ///
    /// [`Ok`]: enum.Result.html#variant.Ok
    /// [`Result`]: enum.Result.html
    /// [`Result::iter_mut`]: enum.Result.html#method.iter_mut
    #[derive(Debug)]
    #[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<T> ExactSizeIterator for IterMut<'_, T> {}

    #[stable(feature = "fused", since = "1.26.0")]
    impl<T> FusedIterator for IterMut<'_, T> {}

    #[unstable(feature = "trusted_len", issue = "37572")]
    unsafe impl<A> TrustedLen for IterMut<'_, A> {}

    /// An iterator over the value in a [`Ok`] variant of a [`Result`].
    ///
    /// The iterator yields one value if the result is [`Ok`], otherwise none.
    ///
    /// This struct is created by the [`into_iter`] method on
    /// [`Result`][`Result`] (provided by the [`IntoIterator`] trait).
    ///
    /// [`Ok`]: enum.Result.html#variant.Ok
    /// [`Result`]: enum.Result.html
    /// [`into_iter`]: ../iter/trait.IntoIterator.html#tymethod.into_iter
    /// [`IntoIterator`]: ../iter/trait.IntoIterator.html
    #[derive(Clone, Debug)]
    #[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> {}

    #[stable(feature = "fused", since = "1.26.0")]
    impl<T> FusedIterator for IntoIter<T> {}

    #[unstable(feature = "trusted_len", issue = "37572")]
    unsafe impl<A> TrustedLen for IntoIter<A> {}

    /////////////////////////////////////////////////////////////////////////////
    // 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:
        ///
        /// ```
        /// let v = vec![1, 2];
        /// let res: Result<Vec<u32>, &'static str> = v.iter().map(|x: &u32|
        ///     x.checked_add(1).ok_or("Overflow!")
        /// ).collect();
        /// assert_eq!(res, Ok(vec![2, 3]));
        /// ```
        ///
        /// Here is another example that tries to subtract one from another list
        /// of integers, this time checking for underflow:
        ///
        /// ```
        /// let v = vec![1, 2, 0];
        /// let res: Result<Vec<u32>, &'static str> = v.iter().map(|x: &u32|
        ///     x.checked_sub(1).ok_or("Underflow!")
        /// ).collect();
        /// assert_eq!(res, Err("Underflow!"));
        /// ```
        ///
        /// Here is a variation on the previous example, showing that no
        /// further elements are taken from `iter` after the first `Err`.
        ///
        /// ```
        /// let v = vec![3, 2, 1, 10];
        /// let mut shared = 0;
        /// let res: Result<Vec<u32>, &'static str> = v.iter().map(|x: &u32| {
        ///     shared += x;
        ///     x.checked_sub(2).ok_or("Underflow!")
        /// }).collect();
        /// assert_eq!(res, Err("Underflow!"));
        /// assert_eq!(shared, 6);
        /// ```
        ///
        /// Since the third element caused an underflow, no further elements were taken,
        /// so the final value of `shared` is 6 (= `3 + 2 + 1`), not 16.
        #[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.

            iter::process_results(iter.into_iter(), |i| i.collect())
        }
    }

    #[unstable(feature = "try_trait", issue = "42327")]
    impl<T,E> ops::Try for Result<T, E> {
        type Ok = T;
        type Error = E;

        #[inline]
        fn into_result(self) -> Self {
            self
        }

        #[inline]
        fn from_ok(v: T) -> Self {
            Ok(v)
        }

        #[inline]
        fn from_error(v: E) -> Self {
            Err(v)
        }
    }

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