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    // Copyright 2017 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.

    use error::Error;
    #[cfg(target_arch = "wasm32")]
    mod exit {
        pub const SUCCESS: i32 = 0;
        pub const FAILURE: i32 = 1;
    }
    #[cfg(not(target_arch = "wasm32"))]
    mod exit {
        use libc;
        pub const SUCCESS: i32 = libc::EXIT_SUCCESS;
        pub const FAILURE: i32 = libc::EXIT_FAILURE;
    }

    /// A trait for implementing arbitrary return types in the `main` function.
    ///
    /// The c-main function only supports to return integers as return type.
    /// So, every type implementing the `Termination` trait has to be converted
    /// to an integer.
    ///
    /// The default implementations are returning `libc::EXIT_SUCCESS` to indicate
    /// a successful execution. In case of a failure, `libc::EXIT_FAILURE` is returned.
    #[cfg_attr(not(test), lang = "termination")]
    #[unstable(feature = "termination_trait", issue = "43301")]
    #[rustc_on_unimplemented =
      "`main` can only return types that implement {Termination}, not `{Self}`"]
    pub trait Termination {
        /// Is called to get the representation of the value as status code.
        /// This status code is returned to the operating system.
        fn report(self) -> i32;
    }

    #[unstable(feature = "termination_trait", issue = "43301")]
    impl Termination for () {
        fn report(self) -> i32 { exit::SUCCESS }
    }

    #[unstable(feature = "termination_trait", issue = "43301")]
    impl<T: Termination, E: Error> Termination for Result<T, E> {
        fn report(self) -> i32 {
            match self {
                Ok(val) => val.report(),
                Err(err) => {
                    print_error(err);
                    exit::FAILURE
                }
            }
        }
    }

    #[unstable(feature = "termination_trait", issue = "43301")]
    fn print_error<E: Error>(err: E) {
        eprintln!("Error: {}", err.description());

        if let Some(ref err) = err.cause() {
            eprintln!("Caused by: {}", err.description());
        }
    }

    #[unstable(feature = "termination_trait", issue = "43301")]
    impl Termination for ! {
        fn report(self) -> i32 { unreachable!(); }
    }

    #[unstable(feature = "termination_trait", issue = "43301")]
    impl Termination for bool {
        fn report(self) -> i32 {
            if self { exit::SUCCESS } else { exit::FAILURE }
        }
    }

    #[unstable(feature = "termination_trait", issue = "43301")]
    impl Termination for i32 {
        fn report(self) -> i32 {
            self
        }
    }

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