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    use std::marker;

    use super::{local, Shared};

    /// An RAII-style guard for pinning the current epoch.
    ///
    /// A guard must be acquired before most operations on an `Atomic` pointer. On
    /// destruction, it unpins the epoch.
    #[must_use]
    #[derive(Debug)]
    pub struct Guard {
        _marker: marker::PhantomData<*mut ()>, // !Send and !Sync
    }

    /// Pin the current epoch.
    ///
    /// Threads generally pin before interacting with a lock-free data
    /// structure. Pinning requires a full memory barrier, so is somewhat
    /// expensive. It is rentrant -- you can safely acquire nested guards, and only
    /// the first guard requires a barrier. Thus, in cases where you expect to
    /// perform several lock-free operations in quick succession, you may consider
    /// pinning around the entire set of operations.
    pub fn pin() -> Guard {
        local::with_participant(|p| {
            let entered = p.enter();

            let g = Guard {
                _marker: marker::PhantomData,
            };

            if entered && p.should_gc() {
                p.try_collect(&g);
            }

            g
        })
    }

    impl Guard {
        /// Assert that the value is no longer reachable from a lock-free data
        /// structure and should be collected when sufficient epochs have passed.
        pub unsafe fn unlinked<T>(&self, val: Shared<T>) {
            local::with_participant(|p| p.reclaim(val.as_raw()))
        }

        /// Move the thread-local garbage into the global set of garbage.
        pub fn migrate_garbage(&self) {
            local::with_participant(|p| p.migrate_garbage())
        }
    }

    impl Drop for Guard {
        fn drop(&mut self) {
            local::with_participant(|p| p.exit());
        }
    }

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