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    // Copyright 2018 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.

    #![unstable(feature = "futures_api",
                reason = "futures in libcore are unstable",
                issue = "50547")]

    use fmt;
    use future::{FutureObj, LocalFutureObj};

    /// A task executor.
    ///
    /// Futures are polled until completion by tasks, a kind of lightweight
    /// "thread". A *task executor* is responsible for the creation of these tasks
    /// and the coordination of their execution on real operating system threads. In
    /// particular, whenever a task signals that it can make further progress via a
    /// wake-up notification, it is the responsibility of the task executor to put
    /// the task into a queue to continue executing it, i.e. polling the future in
    /// it, later.
    pub trait Executor {
        /// Spawns a new task with the given future. The future will be polled until
        /// completion.
        ///
        /// # Errors
        ///
        /// The executor may be unable to spawn tasks, either because it has
        /// been shut down or is resource-constrained.
        fn spawn_obj(
            &mut self,
            future: FutureObj<'static, ()>,
        ) -> Result<(), SpawnObjError>;

        /// Determines whether the executor is able to spawn new tasks.
        ///
        /// # Returns
        ///
        /// An `Ok` return means the executor is *likely* (but not guaranteed)
        /// to accept a subsequent spawn attempt. Likewise, an `Err` return
        /// means that `spawn` is likely, but not guaranteed, to yield an error.
        #[inline]
        fn status(&self) -> Result<(), SpawnErrorKind> {
            Ok(())
        }
    }

    /// Provides the reason that an executor was unable to spawn.
    pub struct SpawnErrorKind {
        _hidden: (),
    }

    impl fmt::Debug for SpawnErrorKind {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            f.debug_tuple("SpawnErrorKind")
                .field(&"shutdown")
                .finish()
        }
    }

    impl SpawnErrorKind {
        /// Spawning is failing because the executor has been shut down.
        pub fn shutdown() -> SpawnErrorKind {
            SpawnErrorKind { _hidden: () }
        }

        /// Check whether this error is the `shutdown` error.
        pub fn is_shutdown(&self) -> bool {
            true
        }
    }

    /// The result of a failed spawn
    #[derive(Debug)]
    pub struct SpawnObjError {
        /// The kind of error
        pub kind: SpawnErrorKind,

        /// The future for which spawning inside a task was attempted
        pub future: FutureObj<'static, ()>,
    }

    /// The result of a failed spawn
    #[derive(Debug)]
    pub struct SpawnLocalObjError {
        /// The kind of error
        pub kind: SpawnErrorKind,

        /// The future for which spawning inside a task was attempted
        pub future: LocalFutureObj<'static, ()>,
    }

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