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    //! Dispatches trace events to [`Subscriber`]s.
    //!
    //! The _dispatcher_ is the component of the tracing system which is responsible
    //! for forwarding trace data from the instrumentation points that generate it
    //! to the subscriber that collects it.
    //!
    //! # Using the Trace Dispatcher
    //!
    //! Every thread in a program using `tracing` has a _default subscriber_. When
    //! events occur, or spans are created, they are dispatched to the thread's
    //! current subscriber.
    //!
    //! ## Setting the Default Subscriber
    //!
    //! By default, the current subscriber is an empty implementation that does
    //! nothing. To use a subscriber implementation, it must be set as the default.
    //! There are two methods for doing so: [`with_default`] and
    //! [`set_global_default`]. `with_default` sets the default subscriber for the
    //! duration of a scope, while `set_global_default` sets a default subscriber
    //! for the entire process.
    //!
    //! To use either of these functions, we must first wrap our subscriber in a
    //! [`Dispatch`], a cloneable, type-erased reference to a subscriber. For
    //! example:
    //! ```rust
    //! # pub struct FooSubscriber;
    //! # use tracing_core::{
    //! #   dispatcher, Event, Metadata,
    //! #   span::{Attributes, Id, Record}
    //! # };
    //! # impl tracing_core::Subscriber for FooSubscriber {
    //! #   fn new_span(&self, _: &Attributes) -> Id { Id::from_u64(0) }
    //! #   fn record(&self, _: &Id, _: &Record) {}
    //! #   fn event(&self, _: &Event) {}
    //! #   fn record_follows_from(&self, _: &Id, _: &Id) {}
    //! #   fn enabled(&self, _: &Metadata) -> bool { false }
    //! #   fn enter(&self, _: &Id) {}
    //! #   fn exit(&self, _: &Id) {}
    //! # }
    //! # impl FooSubscriber { fn new() -> Self { FooSubscriber } }
    //! use dispatcher::Dispatch;
    //!
    //! let my_subscriber = FooSubscriber::new();
    //! let my_dispatch = Dispatch::new(my_subscriber);
    //! ```
    //! Then, we can use [`with_default`] to set our `Dispatch` as the default for
    //! the duration of a block:
    //! ```rust
    //! # pub struct FooSubscriber;
    //! # use tracing_core::{
    //! #   dispatcher, Event, Metadata,
    //! #   span::{Attributes, Id, Record}
    //! # };
    //! # impl tracing_core::Subscriber for FooSubscriber {
    //! #   fn new_span(&self, _: &Attributes) -> Id { Id::from_u64(0) }
    //! #   fn record(&self, _: &Id, _: &Record) {}
    //! #   fn event(&self, _: &Event) {}
    //! #   fn record_follows_from(&self, _: &Id, _: &Id) {}
    //! #   fn enabled(&self, _: &Metadata) -> bool { false }
    //! #   fn enter(&self, _: &Id) {}
    //! #   fn exit(&self, _: &Id) {}
    //! # }
    //! # impl FooSubscriber { fn new() -> Self { FooSubscriber } }
    //! # let my_subscriber = FooSubscriber::new();
    //! # let my_dispatch = dispatcher::Dispatch::new(my_subscriber);
    //! // no default subscriber
    //!
    //! # #[cfg(feature = "std")]
    //! dispatcher::with_default(&my_dispatch, || {
    //!     // my_subscriber is the default
    //! });
    //!
    //! // no default subscriber again
    //! ```
    //! It's important to note that `with_default` will not propagate the current
    //! thread's default subscriber to any threads spawned within the `with_default`
    //! block. To propagate the default subscriber to new threads, either use
    //! `with_default` from the new thread, or use `set_global_default`.
    //!
    //! As an alternative to `with_default`, we can use [`set_global_default`] to
    //! set a `Dispatch` as the default for all threads, for the lifetime of the
    //! program. For example:
    //! ```rust
    //! # pub struct FooSubscriber;
    //! # use tracing_core::{
    //! #   dispatcher, Event, Metadata,
    //! #   span::{Attributes, Id, Record}
    //! # };
    //! # impl tracing_core::Subscriber for FooSubscriber {
    //! #   fn new_span(&self, _: &Attributes) -> Id { Id::from_u64(0) }
    //! #   fn record(&self, _: &Id, _: &Record) {}
    //! #   fn event(&self, _: &Event) {}
    //! #   fn record_follows_from(&self, _: &Id, _: &Id) {}
    //! #   fn enabled(&self, _: &Metadata) -> bool { false }
    //! #   fn enter(&self, _: &Id) {}
    //! #   fn exit(&self, _: &Id) {}
    //! # }
    //! # impl FooSubscriber { fn new() -> Self { FooSubscriber } }
    //! # let my_subscriber = FooSubscriber::new();
    //! # let my_dispatch = dispatcher::Dispatch::new(my_subscriber);
    //! // no default subscriber
    //!
    //! dispatcher::set_global_default(my_dispatch)
    //!     // `set_global_default` will return an error if the global default
    //!     // subscriber has already been set.
    //!     .expect("global default was already set!");
    //!
    //! // `my_subscriber` is now the default
    //! ```
    //!
    //! <div class="information">
    //!     <div class="tooltip ignore" style="">ⓘ<span class="tooltiptext">Note</span></div>
    //! </div>
    //! <div class="example-wrap" style="display:inline-block">
    //! <pre class="ignore" style="white-space:normal;font:inherit;">
    //! <strong>Note</strong>:the thread-local scoped dispatcher
    //! (<a href="#fn.with_default"><code>with_default</code></a>) requires the
    //! Rust standard library. <code>no_std</code> users should use
    //! <a href="#fn.set_global_default"><code>set_global_default</code></a>
    //! instead.
    //! </pre></div>
    //!
    //! ## Accessing the Default Subscriber
    //!
    //! A thread's current default subscriber can be accessed using the
    //! [`get_default`] function, which executes a closure with a reference to the
    //! currently default `Dispatch`. This is used primarily by `tracing`
    //! instrumentation.
    //!
    //! [`Subscriber`]: struct.Subscriber.html
    //! [`with_default`]: fn.with_default.html
    //! [`set_global_default`]: fn.set_global_default.html
    //! [`get_default`]: fn.get_default.html
    //! [`Dispatch`]: struct.Dispatch.html
    use crate::{
        callsite, span,
        subscriber::{self, Subscriber},
        Event, LevelFilter, Metadata,
    };

    use crate::stdlib::{
        any::Any,
        fmt,
        sync::{
            atomic::{AtomicBool, AtomicUsize, Ordering},
            Arc, Weak,
        },
    };

    #[cfg(feature = "std")]
    use crate::stdlib::{
        cell::{Cell, RefCell, RefMut},
        error,
    };

    /// `Dispatch` trace data to a [`Subscriber`].
    ///
    /// [`Subscriber`]: trait.Subscriber.html
    #[derive(Clone)]
    pub struct Dispatch {
        subscriber: Arc<dyn Subscriber + Send + Sync>,
    }

    #[cfg(feature = "std")]
    thread_local! {
        static CURRENT_STATE: State = State {
            default: RefCell::new(Dispatch::none()),
            can_enter: Cell::new(true),
        };
    }

    static EXISTS: AtomicBool = AtomicBool::new(false);
    static GLOBAL_INIT: AtomicUsize = AtomicUsize::new(UNINITIALIZED);

    const UNINITIALIZED: usize = 0;
    const INITIALIZING: usize = 1;
    const INITIALIZED: usize = 2;

    static mut GLOBAL_DISPATCH: Option<Dispatch> = None;

    /// The dispatch state of a thread.
    #[cfg(feature = "std")]
    struct State {
        /// This thread's current default dispatcher.
        default: RefCell<Dispatch>,
        /// Whether or not we can currently begin dispatching a trace event.
        ///
        /// This is set to `false` when functions such as `enter`, `exit`, `event`,
        /// and `new_span` are called on this thread's default dispatcher, to
        /// prevent further trace events triggered inside those functions from
        /// creating an infinite recursion. When we finish handling a dispatch, this
        /// is set back to `true`.
        can_enter: Cell<bool>,
    }

    /// While this guard is active, additional calls to subscriber functions on
    /// the default dispatcher will not be able to access the dispatch context.
    /// Dropping the guard will allow the dispatch context to be re-entered.
    #[cfg(feature = "std")]
    struct Entered<'a>(&'a State);

    /// A guard that resets the current default dispatcher to the prior
    /// default dispatcher when dropped.
    #[cfg(feature = "std")]
    #[cfg_attr(docsrs, doc(cfg(feature = "std")))]
    #[derive(Debug)]
    pub struct DefaultGuard(Option<Dispatch>);

    /// Sets this dispatch as the default for the duration of a closure.
    ///
    /// The default dispatcher is used when creating a new [span] or
    /// [`Event`].
    ///
    /// <div class="information">
    ///     <div class="tooltip ignore" style="">ⓘ<span class="tooltiptext">Note</span></div>
    /// </div>
    /// <div class="example-wrap" style="display:inline-block">
    /// <pre class="ignore" style="white-space:normal;font:inherit;">
    /// <strong>Note</strong>: This function required the Rust standard library.
    /// <code>no_std</code> users should use <a href="../fn.set_global_default.html">
    /// <code>set_global_default</code></a> instead.
    /// </pre></div>
    ///
    /// [span]: ../span/index.html
    /// [`Subscriber`]: ../subscriber/trait.Subscriber.html
    /// [`Event`]: ../event/struct.Event.html
    /// [`set_global_default`]: ../fn.set_global_default.html
    #[cfg(feature = "std")]
    #[cfg_attr(docsrs, doc(cfg(feature = "std")))]
    pub fn with_default<T>(dispatcher: &Dispatch, f: impl FnOnce() -> T) -> T {
        // When this guard is dropped, the default dispatcher will be reset to the
        // prior default. Using this (rather than simply resetting after calling
        // `f`) ensures that we always reset to the prior dispatcher even if `f`
        // panics.
        let _guard = set_default(dispatcher);
        f()
    }

    /// Sets the dispatch as the default dispatch for the duration of the lifetime
    /// of the returned DefaultGuard
    ///
    /// <div class="information">
    ///     <div class="tooltip ignore" style="">ⓘ<span class="tooltiptext">Note</span></div>
    /// </div>
    /// <div class="example-wrap" style="display:inline-block">
    /// <pre class="ignore" style="white-space:normal;font:inherit;">
    /// <strong>Note</strong>: This function required the Rust standard library.
    /// <code>no_std</code> users should use <a href="../fn.set_global_default.html">
    /// <code>set_global_default</code></a> instead.
    /// </pre></div>
    ///
    /// [`set_global_default`]: ../fn.set_global_default.html
    #[cfg(feature = "std")]
    #[cfg_attr(docsrs, doc(cfg(feature = "std")))]
    #[must_use = "Dropping the guard unregisters the dispatcher."]
    pub fn set_default(dispatcher: &Dispatch) -> DefaultGuard {
        // When this guard is dropped, the default dispatcher will be reset to the
        // prior default. Using this ensures that we always reset to the prior
        // dispatcher even if the thread calling this function panics.
        State::set_default(dispatcher.clone())
    }

    /// Sets this dispatch as the global default for the duration of the entire program.
    /// Will be used as a fallback if no thread-local dispatch has been set in a thread
    /// (using `with_default`.)
    ///
    /// Can only be set once; subsequent attempts to set the global default will fail.
    /// Returns `Err` if the global default has already been set.
    ///
    ///
    /// <div class="information">
    ///     <div class="tooltip compile_fail" style="">&#x26a0; &#xfe0f;<span class="tooltiptext">Warning</span></div>
    /// </div><div class="example-wrap" style="display:inline-block"><pre class="compile_fail" style="white-space:normal;font:inherit;">
    /// <strong>Warning</strong>: In general, libraries should <em>not</em> call
    /// <code>set_global_default()</code>! Doing so will cause conflicts when
    /// executables that depend on the library try to set the default later.
    /// </pre></div>
    ///
    /// [span]: ../span/index.html
    /// [`Subscriber`]: ../subscriber/trait.Subscriber.html
    /// [`Event`]: ../event/struct.Event.html
    pub fn set_global_default(dispatcher: Dispatch) -> Result<(), SetGlobalDefaultError> {
        if GLOBAL_INIT.compare_and_swap(UNINITIALIZED, INITIALIZING, Ordering::SeqCst) == UNINITIALIZED
        {
            unsafe {
                GLOBAL_DISPATCH = Some(dispatcher);
            }
            GLOBAL_INIT.store(INITIALIZED, Ordering::SeqCst);
            EXISTS.store(true, Ordering::Release);
            Ok(())
        } else {
            Err(SetGlobalDefaultError { _no_construct: () })
        }
    }

    /// Returns true if a `tracing` dispatcher has ever been set.
    ///
    /// This may be used to completely elide trace points if tracing is not in use
    /// at all or has yet to be initialized.
    #[doc(hidden)]
    #[inline(always)]
    pub fn has_been_set() -> bool {
        EXISTS.load(Ordering::Relaxed)
    }

    /// Returned if setting the global dispatcher fails.
    #[derive(Debug)]
    pub struct SetGlobalDefaultError {
        _no_construct: (),
    }

    impl fmt::Display for SetGlobalDefaultError {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            f.pad("a global default trace dispatcher has already been set")
        }
    }

    #[cfg(feature = "std")]
    #[cfg_attr(docsrs, doc(cfg(feature = "std")))]
    impl error::Error for SetGlobalDefaultError {}

    /// Executes a closure with a reference to this thread's current [dispatcher].
    ///
    /// Note that calls to `get_default` should not be nested; if this function is
    /// called while inside of another `get_default`, that closure will be provided
    /// with `Dispatch::none` rather than the previously set dispatcher.
    ///
    /// [dispatcher]: ../dispatcher/struct.Dispatch.html
    #[cfg(feature = "std")]
    pub fn get_default<T, F>(mut f: F) -> T
    where
        F: FnMut(&Dispatch) -> T,
    {
        CURRENT_STATE
            .try_with(|state| {
                if let Some(entered) = state.enter() {
                    return f(&*entered.current());
                }

                f(&Dispatch::none())
            })
            .unwrap_or_else(|_| f(&Dispatch::none()))
    }

    /// Executes a closure with a reference to this thread's current [dispatcher].
    ///
    /// Note that calls to `get_default` should not be nested; if this function is
    /// called while inside of another `get_default`, that closure will be provided
    /// with `Dispatch::none` rather than the previously set dispatcher.
    ///
    /// [dispatcher]: ../dispatcher/struct.Dispatch.html
    #[cfg(feature = "std")]
    #[doc(hidden)]
    #[inline(never)]
    pub fn get_current<T>(f: impl FnOnce(&Dispatch) -> T) -> Option<T> {
        CURRENT_STATE
            .try_with(|state| {
                let entered = state.enter()?;
                Some(f(&*entered.current()))
            })
            .ok()?
    }

    /// Executes a closure with a reference to the current [dispatcher].
    ///
    /// [dispatcher]: ../dispatcher/struct.Dispatch.html
    #[cfg(not(feature = "std"))]
    #[doc(hidden)]
    pub fn get_current<T>(f: impl FnOnce(&Dispatch) -> T) -> Option<T> {
        let dispatch = get_global()?;
        Some(f(&dispatch))
    }

    /// Executes a closure with a reference to the current [dispatcher].
    ///
    /// [dispatcher]: ../dispatcher/struct.Dispatch.html
    #[cfg(not(feature = "std"))]
    pub fn get_default<T, F>(mut f: F) -> T
    where
        F: FnMut(&Dispatch) -> T,
    {
        if let Some(d) = get_global() {
            f(d)
        } else {
            f(&Dispatch::none())
        }
    }

    fn get_global() -> Option<&'static Dispatch> {
        if GLOBAL_INIT.load(Ordering::SeqCst) != INITIALIZED {
            return None;
        }
        unsafe {
            // This is safe given the invariant that setting the global dispatcher
            // also sets `GLOBAL_INIT` to `INITIALIZED`.
            Some(GLOBAL_DISPATCH.as_ref().expect(
                "invariant violated: GLOBAL_DISPATCH must be initialized before GLOBAL_INIT is set",
            ))
        }
    }

    pub(crate) struct Registrar(Weak<dyn Subscriber + Send + Sync>);

    impl Dispatch {
        /// Returns a new `Dispatch` that discards events and spans.
        #[inline]
        pub fn none() -> Self {
            Dispatch {
                subscriber: Arc::new(NoSubscriber),
            }
        }

        /// Returns a `Dispatch` that forwards to the given [`Subscriber`].
        ///
        /// [`Subscriber`]: ../subscriber/trait.Subscriber.html
        pub fn new<S>(subscriber: S) -> Self
        where
            S: Subscriber + Send + Sync + 'static,
        {
            let me = Dispatch {
                subscriber: Arc::new(subscriber),
            };
            callsite::register_dispatch(&me);
            me
        }

        pub(crate) fn registrar(&self) -> Registrar {
            Registrar(Arc::downgrade(&self.subscriber))
        }

        /// Registers a new callsite with this subscriber, returning whether or not
        /// the subscriber is interested in being notified about the callsite.
        ///
        /// This calls the [`register_callsite`] function on the [`Subscriber`]
        /// that this `Dispatch` forwards to.
        ///
        /// [`Subscriber`]: ../subscriber/trait.Subscriber.html
        /// [`register_callsite`]: ../subscriber/trait.Subscriber.html#method.register_callsite
        #[inline]
        pub fn register_callsite(&self, metadata: &'static Metadata<'static>) -> subscriber::Interest {
            self.subscriber.register_callsite(metadata)
        }

        /// Returns the highest [verbosity level][level] that this [`Subscriber`] will
        /// enable, or `None`, if the subscriber does not implement level-based
        /// filtering or chooses not to implement this method.
        ///
        /// This calls the [`max_level_hint`] function on the [`Subscriber`]
        /// that this `Dispatch` forwards to.
        ///
        /// [level]: ../struct.Level.html
        /// [`Subscriber`]: ../subscriber/trait.Subscriber.html
        /// [`register_callsite`]: ../subscriber/trait.Subscriber.html#method.max_level_hint
        // TODO(eliza): consider making this a public API?
        #[inline]
        pub(crate) fn max_level_hint(&self) -> Option<LevelFilter> {
            self.subscriber.max_level_hint()
        }

        /// Record the construction of a new span, returning a new [ID] for the
        /// span being constructed.
        ///
        /// This calls the [`new_span`] function on the [`Subscriber`] that this
        /// `Dispatch` forwards to.
        ///
        /// [ID]: ../span/struct.Id.html
        /// [`Subscriber`]: ../subscriber/trait.Subscriber.html
        /// [`new_span`]: ../subscriber/trait.Subscriber.html#method.new_span
        #[inline]
        pub fn new_span(&self, span: &span::Attributes<'_>) -> span::Id {
            self.subscriber.new_span(span)
        }

        /// Record a set of values on a span.
        ///
        /// This calls the [`record`] function on the [`Subscriber`] that this
        /// `Dispatch` forwards to.
        ///
        /// [`Subscriber`]: ../subscriber/trait.Subscriber.html
        /// [`record`]: ../subscriber/trait.Subscriber.html#method.record
        #[inline]
        pub fn record(&self, span: &span::Id, values: &span::Record<'_>) {
            self.subscriber.record(span, values)
        }

        /// Adds an indication that `span` follows from the span with the id
        /// `follows`.
        ///
        /// This calls the [`record_follows_from`] function on the [`Subscriber`]
        /// that this `Dispatch` forwards to.
        ///
        /// [`Subscriber`]: ../subscriber/trait.Subscriber.html
        /// [`record_follows_from`]: ../subscriber/trait.Subscriber.html#method.record_follows_from
        #[inline]
        pub fn record_follows_from(&self, span: &span::Id, follows: &span::Id) {
            self.subscriber.record_follows_from(span, follows)
        }

        /// Returns true if a span with the specified [metadata] would be
        /// recorded.
        ///
        /// This calls the [`enabled`] function on the [`Subscriber`] that this
        /// `Dispatch` forwards to.
        ///
        /// [metadata]: ../metadata/struct.Metadata.html
        /// [`Subscriber`]: ../subscriber/trait.Subscriber.html
        /// [`enabled`]: ../subscriber/trait.Subscriber.html#method.enabled
        #[inline]
        pub fn enabled(&self, metadata: &Metadata<'_>) -> bool {
            self.subscriber.enabled(metadata)
        }

        /// Records that an [`Event`] has occurred.
        ///
        /// This calls the [`event`] function on the [`Subscriber`] that this
        /// `Dispatch` forwards to.
        ///
        /// [`Event`]: ../event/struct.Event.html
        /// [`Subscriber`]: ../subscriber/trait.Subscriber.html
        /// [`event`]: ../subscriber/trait.Subscriber.html#method.event
        #[inline]
        pub fn event(&self, event: &Event<'_>) {
            self.subscriber.event(event)
        }

        /// Records that a span has been can_enter.
        ///
        /// This calls the [`enter`] function on the [`Subscriber`] that this
        /// `Dispatch` forwards to.
        ///
        /// [`Subscriber`]: ../subscriber/trait.Subscriber.html
        /// [`enter`]: ../subscriber/trait.Subscriber.html#method.enter
        #[inline]
        pub fn enter(&self, span: &span::Id) {
            self.subscriber.enter(span);
        }

        /// Records that a span has been exited.
        ///
        /// This calls the [`exit`] function on the [`Subscriber`] that this
        /// `Dispatch` forwards to.
        ///
        /// [`Subscriber`]: ../subscriber/trait.Subscriber.html
        /// [`exit`]: ../subscriber/trait.Subscriber.html#method.exit
        #[inline]
        pub fn exit(&self, span: &span::Id) {
            self.subscriber.exit(span);
        }

        /// Notifies the subscriber that a [span ID] has been cloned.
        ///
        /// This function must only be called with span IDs that were returned by
        /// this `Dispatch`'s [`new_span`] function. The `tracing` crate upholds
        /// this guarantee and any other libraries implementing instrumentation APIs
        /// must as well.
        ///
        /// This calls the [`clone_span`] function on the `Subscriber` that this
        /// `Dispatch` forwards to.
        ///
        /// [span ID]: ../span/struct.Id.html
        /// [`Subscriber`]: ../subscriber/trait.Subscriber.html
        /// [`clone_span`]: ../subscriber/trait.Subscriber.html#method.clone_span
        /// [`new_span`]: ../subscriber/trait.Subscriber.html#method.new_span
        #[inline]
        pub fn clone_span(&self, id: &span::Id) -> span::Id {
            self.subscriber.clone_span(&id)
        }

        /// Notifies the subscriber that a [span ID] has been dropped.
        ///
        /// This function must only be called with span IDs that were returned by
        /// this `Dispatch`'s [`new_span`] function. The `tracing` crate upholds
        /// this guarantee and any other libraries implementing instrumentation APIs
        /// must as well.
        ///
        /// This calls the [`drop_span`] function on the [`Subscriber`] that this
        ///  `Dispatch` forwards to.
        ///
        /// <div class="information">
        ///     <div class="tooltip compile_fail" style="">&#x26a0; &#xfe0f;<span class="tooltiptext">Warning</span></div>
        /// </div>
        /// <div class="example-wrap" style="display:inline-block"><pre class="compile_fail" style="white-space:normal;font:inherit;">
        /// <strong>Deprecated</strong>: The <a href="#method.try_close"><code>try_close</code></a>
        /// method is functionally identical, but returns <code>true</code> if the span is now closed.
        /// It should be used instead of this method.
        /// </pre></div>
        ///
        /// [span ID]: ../span/struct.Id.html
        /// [`Subscriber`]: ../subscriber/trait.Subscriber.html
        /// [`drop_span`]: ../subscriber/trait.Subscriber.html#method.drop_span
        /// [`new_span`]: ../subscriber/trait.Subscriber.html#method.new_span
        /// [`try_close`]: #method.try_close
        #[inline]
        #[deprecated(since = "0.1.2", note = "use `Dispatch::try_close` instead")]
        pub fn drop_span(&self, id: span::Id) {
            #[allow(deprecated)]
            self.subscriber.drop_span(id);
        }

        /// Notifies the subscriber that a [span ID] has been dropped, and returns
        /// `true` if there are now 0 IDs referring to that span.
        ///
        /// This function must only be called with span IDs that were returned by
        /// this `Dispatch`'s [`new_span`] function. The `tracing` crate upholds
        /// this guarantee and any other libraries implementing instrumentation APIs
        /// must as well.
        ///
        /// This calls the [`try_close`] function on the [`Subscriber`] that this
        ///  `Dispatch` forwards to.
        ///
        /// [span ID]: ../span/struct.Id.html
        /// [`Subscriber`]: ../subscriber/trait.Subscriber.html
        /// [`try_close`]: ../subscriber/trait.Subscriber.html#method.try_close
        /// [`new_span`]: ../subscriber/trait.Subscriber.html#method.new_span
        #[inline]
        pub fn try_close(&self, id: span::Id) -> bool {
            self.subscriber.try_close(id)
        }

        /// Returns a type representing this subscriber's view of the current span.
        ///
        /// This calls the [`current`] function on the `Subscriber` that this
        /// `Dispatch` forwards to.
        ///
        /// [`current`]: ../subscriber/trait.Subscriber.html#method.current
        #[inline]
        pub fn current_span(&self) -> span::Current {
            self.subscriber.current_span()
        }

        /// Returns `true` if this `Dispatch` forwards to a `Subscriber` of type
        /// `T`.
        #[inline]
        pub fn is<T: Any>(&self) -> bool {
            Subscriber::is::<T>(&*self.subscriber)
        }

        /// Returns some reference to the `Subscriber` this `Dispatch` forwards to
        /// if it is of type `T`, or `None` if it isn't.
        #[inline]
        pub fn downcast_ref<T: Any>(&self) -> Option<&T> {
            Subscriber::downcast_ref(&*self.subscriber)
        }
    }

    impl Default for Dispatch {
        /// Returns the current default dispatcher
        fn default() -> Self {
            get_default(|default| default.clone())
        }
    }

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

    impl<S> From<S> for Dispatch
    where
        S: Subscriber + Send + Sync + 'static,
    {
        #[inline]
        fn from(subscriber: S) -> Self {
            Dispatch::new(subscriber)
        }
    }

    struct NoSubscriber;
    impl Subscriber for NoSubscriber {
        #[inline]
        fn register_callsite(&self, _: &'static Metadata<'static>) -> subscriber::Interest {
            subscriber::Interest::never()
        }

        fn new_span(&self, _: &span::Attributes<'_>) -> span::Id {
            span::Id::from_u64(0xDEAD)
        }

        fn event(&self, _event: &Event<'_>) {}

        fn record(&self, _span: &span::Id, _values: &span::Record<'_>) {}

        fn record_follows_from(&self, _span: &span::Id, _follows: &span::Id) {}

        #[inline]
        fn enabled(&self, _metadata: &Metadata<'_>) -> bool {
            false
        }

        fn enter(&self, _span: &span::Id) {}
        fn exit(&self, _span: &span::Id) {}
    }

    impl Registrar {
        pub(crate) fn try_register(
            &self,
            metadata: &'static Metadata<'static>,
        ) -> Option<subscriber::Interest> {
            self.0.upgrade().map(|s| s.register_callsite(metadata))
        }

        pub(crate) fn upgrade(&self) -> Option<Dispatch> {
            self.0.upgrade().map(|subscriber| Dispatch { subscriber })
        }
    }

    // ===== impl State =====

    #[cfg(feature = "std")]
    impl State {
        /// Replaces the current default dispatcher on this thread with the provided
        /// dispatcher.Any
        ///
        /// Dropping the returned `ResetGuard` will reset the default dispatcher to
        /// the previous value.
        #[inline]
        fn set_default(new_dispatch: Dispatch) -> DefaultGuard {
            let prior = CURRENT_STATE
                .try_with(|state| {
                    state.can_enter.set(true);
                    state.default.replace(new_dispatch)
                })
                .ok();
            EXISTS.store(true, Ordering::Release);
            DefaultGuard(prior)
        }

        #[inline]
        fn enter(&self) -> Option<Entered<'_>> {
            if self.can_enter.replace(false) {
                Some(Entered(&self))
            } else {
                None
            }
        }
    }

    // ===== impl Entered =====

    #[cfg(feature = "std")]
    impl<'a> Entered<'a> {
        #[inline]
        fn current(&self) -> RefMut<'a, Dispatch> {
            let mut default = self.0.default.borrow_mut();

            if default.is::<NoSubscriber>() {
                if let Some(global) = get_global() {
                    // don't redo this call on the next check
                    *default = global.clone();
                }
            }

            default
        }
    }

    #[cfg(feature = "std")]
    impl<'a> Drop for Entered<'a> {
        #[inline]
        fn drop(&mut self) {
            self.0.can_enter.set(true);
        }
    }

    // ===== impl DefaultGuard =====

    #[cfg(feature = "std")]
    impl Drop for DefaultGuard {
        #[inline]
        fn drop(&mut self) {
            if let Some(dispatch) = self.0.take() {
                // Replace the dispatcher and then drop the old one outside
                // of the thread-local context. Dropping the dispatch may
                // lead to the drop of a subscriber which, in the process,
                // could then also attempt to access the same thread local
                // state -- causing a clash.
                let prev = CURRENT_STATE.try_with(|state| state.default.replace(dispatch));
                drop(prev)
            }
        }
    }

    #[cfg(test)]
    mod test {
        use super::*;
        #[cfg(feature = "std")]
        use crate::stdlib::sync::atomic::{AtomicUsize, Ordering};
        use crate::{
            callsite::Callsite,
            metadata::{Kind, Level, Metadata},
            subscriber::Interest,
        };

        #[test]
        fn dispatch_is() {
            let dispatcher = Dispatch::new(NoSubscriber);
            assert!(dispatcher.is::<NoSubscriber>());
        }

        #[test]
        fn dispatch_downcasts() {
            let dispatcher = Dispatch::new(NoSubscriber);
            assert!(dispatcher.downcast_ref::<NoSubscriber>().is_some());
        }

        struct TestCallsite;
        static TEST_CALLSITE: TestCallsite = TestCallsite;
        static TEST_META: Metadata<'static> = metadata! {
            name: "test",
            target: module_path!(),
            level: Level::DEBUG,
            fields: &[],
            callsite: &TEST_CALLSITE,
            kind: Kind::EVENT
        };

        impl Callsite for TestCallsite {
            fn set_interest(&self, _: Interest) {}
            fn metadata(&self) -> &Metadata<'_> {
                &TEST_META
            }
        }

        #[test]
        #[cfg(feature = "std")]
        fn events_dont_infinite_loop() {
            // This test ensures that an event triggered within a subscriber
            // won't cause an infinite loop of events.
            struct TestSubscriber;
            impl Subscriber for TestSubscriber {
                fn enabled(&self, _: &Metadata<'_>) -> bool {
                    true
                }

                fn new_span(&self, _: &span::Attributes<'_>) -> span::Id {
                    span::Id::from_u64(0xAAAA)
                }

                fn record(&self, _: &span::Id, _: &span::Record<'_>) {}

                fn record_follows_from(&self, _: &span::Id, _: &span::Id) {}

                fn event(&self, _: &Event<'_>) {
                    static EVENTS: AtomicUsize = AtomicUsize::new(0);
                    assert_eq!(
                        EVENTS.fetch_add(1, Ordering::Relaxed),
                        0,
                        "event method called twice!"
                    );
                    Event::dispatch(&TEST_META, &TEST_META.fields().value_set(&[]))
                }

                fn enter(&self, _: &span::Id) {}

                fn exit(&self, _: &span::Id) {}
            }

            with_default(&Dispatch::new(TestSubscriber), || {
                Event::dispatch(&TEST_META, &TEST_META.fields().value_set(&[]))
            })
        }

        #[test]
        #[cfg(feature = "std")]
        fn spans_dont_infinite_loop() {
            // This test ensures that a span created within a subscriber
            // won't cause an infinite loop of new spans.

            fn mk_span() {
                get_default(|current| {
                    current.new_span(&span::Attributes::new(
                        &TEST_META,
                        &TEST_META.fields().value_set(&[]),
                    ))
                });
            }

            struct TestSubscriber;
            impl Subscriber for TestSubscriber {
                fn enabled(&self, _: &Metadata<'_>) -> bool {
                    true
                }

                fn new_span(&self, _: &span::Attributes<'_>) -> span::Id {
                    static NEW_SPANS: AtomicUsize = AtomicUsize::new(0);
                    assert_eq!(
                        NEW_SPANS.fetch_add(1, Ordering::Relaxed),
                        0,
                        "new_span method called twice!"
                    );
                    mk_span();
                    span::Id::from_u64(0xAAAA)
                }

                fn record(&self, _: &span::Id, _: &span::Record<'_>) {}

                fn record_follows_from(&self, _: &span::Id, _: &span::Id) {}

                fn event(&self, _: &Event<'_>) {}

                fn enter(&self, _: &span::Id) {}

                fn exit(&self, _: &span::Id) {}
            }

            with_default(&Dispatch::new(TestSubscriber), mk_span)
        }

        #[test]
        fn default_no_subscriber() {
            let default_dispatcher = Dispatch::default();
            assert!(default_dispatcher.is::<NoSubscriber>());
        }

        #[cfg(feature = "std")]
        #[test]
        fn default_dispatch() {
            struct TestSubscriber;
            impl Subscriber for TestSubscriber {
                fn enabled(&self, _: &Metadata<'_>) -> bool {
                    true
                }

                fn new_span(&self, _: &span::Attributes<'_>) -> span::Id {
                    span::Id::from_u64(0xAAAA)
                }

                fn record(&self, _: &span::Id, _: &span::Record<'_>) {}

                fn record_follows_from(&self, _: &span::Id, _: &span::Id) {}

                fn event(&self, _: &Event<'_>) {}

                fn enter(&self, _: &span::Id) {}

                fn exit(&self, _: &span::Id) {}
            }
            let guard = set_default(&Dispatch::new(TestSubscriber));
            let default_dispatcher = Dispatch::default();
            assert!(default_dispatcher.is::<TestSubscriber>());

            drop(guard);
            let default_dispatcher = Dispatch::default();
            assert!(default_dispatcher.is::<NoSubscriber>());
        }
    }