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    // Copyright 2012 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.

    /// A trait for customizing the behavior of the `?` operator.
    ///
    /// A type implementing `Try` is one that has a canonical way to view it
    /// in terms of a success/failure dichotomy.  This trait allows both
    /// extracting those success or failure values from an existing instance and
    /// creating a new instance from a success or failure value.
    #[unstable(feature = "try_trait", issue = "42327")]
    #[rustc_on_unimplemented(
       on(all(
           any(from_method="from_error", from_method="from_ok"),
           from_desugaring="?"),
          message="the `?` operator can only be used in a \
                   function that returns `Result` or `Option` \
                   (or another type that implements `{Try}`)",
          label="cannot use the `?` operator in a function that returns `{Self}`"),
       on(all(from_method="into_result", from_desugaring="?"),
          message="the `?` operator can only be applied to values \
                   that implement `{Try}`",
          label="the `?` operator cannot be applied to type `{Self}`")
    )]
    #[doc(alias = "?")]
    pub trait Try {
        /// The type of this value when viewed as successful.
        #[unstable(feature = "try_trait", issue = "42327")]
        type Ok;
        /// The type of this value when viewed as failed.
        #[unstable(feature = "try_trait", issue = "42327")]
        type Error;

        /// Applies the "?" operator. A return of `Ok(t)` means that the
        /// execution should continue normally, and the result of `?` is the
        /// value `t`. A return of `Err(e)` means that execution should branch
        /// to the innermost enclosing `catch`, or return from the function.
        ///
        /// If an `Err(e)` result is returned, the value `e` will be "wrapped"
        /// in the return type of the enclosing scope (which must itself implement
        /// `Try`). Specifically, the value `X::from_error(From::from(e))`
        /// is returned, where `X` is the return type of the enclosing function.
        #[unstable(feature = "try_trait", issue = "42327")]
        fn into_result(self) -> Result<Self::Ok, Self::Error>;

        /// Wrap an error value to construct the composite result. For example,
        /// `Result::Err(x)` and `Result::from_error(x)` are equivalent.
        #[unstable(feature = "try_trait", issue = "42327")]
        fn from_error(v: Self::Error) -> Self;

        /// Wrap an OK value to construct the composite result. For example,
        /// `Result::Ok(x)` and `Result::from_ok(x)` are equivalent.
        #[unstable(feature = "try_trait", issue = "42327")]
        fn from_ok(v: Self::Ok) -> Self;
    }

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