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    //! See [`Job`] and [`Work`].

    use std::fmt;
    use std::mem;

    use super::JobState;
    use crate::core::compiler::fingerprint::DirtyReason;
    use crate::util::CargoResult;

    /// Represents a unit of [`Work`] with a [`Freshness`] for caller
    /// to determine whether to re-execute or not.
    pub struct Job {
        work: Work,
        fresh: Freshness,
    }

    /// The basic unit of work.
    ///
    /// Each proc should send its description before starting.
    /// It should send either once or close immediately.
    pub struct Work {
        inner: Box<dyn FnOnce(&JobState<'_, '_>) -> CargoResult<()> + Send>,
    }

    impl Work {
        /// Creates a unit of work.
        pub fn new<F>(f: F) -> Work
        where
            F: FnOnce(&JobState<'_, '_>) -> CargoResult<()> + Send + 'static,
        {
            Work { inner: Box::new(f) }
        }

        /// Creates a unit of work that does nothing.
        pub fn noop() -> Work {
            Work::new(|_| Ok(()))
        }

        /// Consumes this work by running it.
        pub fn call(self, tx: &JobState<'_, '_>) -> CargoResult<()> {
            (self.inner)(tx)
        }

        /// Creates a new unit of work that chains `next` after ourself.
        pub fn then(self, next: Work) -> Work {
            Work::new(move |state| {
                self.call(state)?;
                next.call(state)
            })
        }
    }

    impl Job {
        /// Creates a new job that does nothing.
        pub fn new_fresh() -> Job {
            Job {
                work: Work::noop(),
                fresh: Freshness::Fresh,
            }
        }

        /// Creates a new job representing a unit of work.
        pub fn new_dirty(work: Work, dirty_reason: Option<DirtyReason>) -> Job {
            Job {
                work,
                fresh: Freshness::Dirty(dirty_reason),
            }
        }

        /// Consumes this job by running it, returning the result of the
        /// computation.
        pub fn run(self, state: &JobState<'_, '_>) -> CargoResult<()> {
            self.work.call(state)
        }

        /// Returns whether this job was fresh/dirty, where "fresh" means we're
        /// likely to perform just some small bookkeeping where "dirty" means we'll
        /// probably do something slow like invoke rustc.
        pub fn freshness(&self) -> &Freshness {
            &self.fresh
        }

        /// Chains the given work by putting it in front of our own unit of work.
        pub fn before(&mut self, next: Work) {
            let prev = mem::replace(&mut self.work, Work::noop());
            self.work = next.then(prev);
        }
    }

    impl fmt::Debug for Job {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            write!(f, "Job {{ ... }}")
        }
    }

    /// Indication of the freshness of a package.
    ///
    /// A fresh package does not necessarily need to be rebuilt (unless a dependency
    /// was also rebuilt), and a dirty package must always be rebuilt.
    #[derive(Debug, Clone)]
    pub enum Freshness {
        Fresh,
        Dirty(Option<DirtyReason>),
    }

    impl Freshness {
        pub fn is_dirty(&self) -> bool {
            matches!(self, Freshness::Dirty(_))
        }

        pub fn is_fresh(&self) -> bool {
            matches!(self, Freshness::Fresh)
        }
    }