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    //! Subscribers collect and record trace data.
    use crate::{span, Event, LevelFilter, Metadata};

    use crate::stdlib::any::{Any, TypeId};

    /// Trait representing the functions required to collect trace data.
    ///
    /// Crates that provide implementations of methods for collecting or recording
    /// trace data should implement the `Subscriber` interface. This trait is
    /// intended to represent fundamental primitives for collecting trace events and
    /// spans — other libraries may offer utility functions and types to make
    /// subscriber implementations more modular or improve the ergonomics of writing
    /// subscribers.
    ///
    /// A subscriber is responsible for the following:
    /// - Registering new spans as they are created, and providing them with span
    ///   IDs. Implicitly, this means the subscriber may determine the strategy for
    ///   determining span equality.
    /// - Recording the attachment of field values and follows-from annotations to
    ///   spans.
    /// - Filtering spans and events, and determining when those filters must be
    ///   invalidated.
    /// - Observing spans as they are entered, exited, and closed, and events as
    ///   they occur.
    ///
    /// When a span is entered or exited, the subscriber is provided only with the
    /// [ID] with which it tagged that span when it was created. This means
    /// that it is up to the subscriber to determine whether and how span _data_ —
    /// the fields and metadata describing the span — should be stored. The
    /// [`new_span`] function is called when a new span is created, and at that
    /// point, the subscriber _may_ choose to store the associated data if it will
    /// be referenced again. However, if the data has already been recorded and will
    /// not be needed by the implementations of `enter` and `exit`, the subscriber
    /// may freely discard that data without allocating space to store it.
    ///
    /// ## Overriding default impls
    ///
    /// Some trait methods on `Subscriber` have default implementations, either in
    /// order to reduce the surface area of implementing `Subscriber`, or for
    /// backward-compatibility reasons. However, many subscribers will likely want
    /// to override these default implementations.
    ///
    /// The following methods are likely of interest:
    ///
    /// - [`register_callsite`] is called once for each callsite from which a span
    ///   event may originate, and returns an [`Interest`] value describing whether or
    ///   not the subscriber wishes to see events or spans from that callsite. By
    ///   default, it calls [`enabled`], and returns `Interest::always()` if
    ///   `enabled` returns true, or `Interest::never()` if enabled returns false.
    ///   However, if the subscriber's interest can change dynamically at runtime,
    ///   it may want to override this function to return `Interest::sometimes()`.
    ///   Additionally, subscribers which wish to perform a behaviour once for each
    ///   callsite, such as allocating storage for data related to that callsite,
    ///   can perform it in `register_callsite`.
    /// - [`clone_span`] is called every time a span ID is cloned, and [`try_close`]
    ///   is called when a span ID is dropped. By default, these functions do
    ///   nothing. However, they can be used to implement reference counting for
    ///   spans, allowing subscribers to free storage for span data and to determine
    ///   when a span has _closed_ permanently (rather than being exited).
    ///   Subscribers which store per-span data or which need to track span closures
    ///   should override these functions together.
    ///
    /// [ID]: ../span/struct.Id.html
    /// [`new_span`]: trait.Subscriber.html#method.new_span
    /// [`register_callsite`]: trait.Subscriber.html#method.register_callsite
    /// [`Interest`]: struct.Interest.html
    /// [`enabled`]: trait.Subscriber.html#method.enabled
    /// [`clone_span`]: trait.Subscriber.html#method.clone_span
    /// [`try_close`]: trait.Subscriber.html#method.try_close
    pub trait Subscriber: 'static {
        // === Span registry methods ==============================================

        /// Registers a new callsite with this subscriber, returning whether or not
        /// the subscriber is interested in being notified about the callsite.
        ///
        /// By default, this function assumes that the subscriber's [filter]
        /// represents an unchanging view of its interest in the callsite. However,
        /// if this is not the case, subscribers may override this function to
        /// indicate different interests, or to implement behaviour that should run
        /// once for every callsite.
        ///
        /// This function is guaranteed to be called at least once per callsite on
        /// every active subscriber. The subscriber may store the keys to fields it
        /// cares about in order to reduce the cost of accessing fields by name,
        /// preallocate storage for that callsite, or perform any other actions it
        /// wishes to perform once for each callsite.
        ///
        /// The subscriber should then return an [`Interest`], indicating
        /// whether it is interested in being notified about that callsite in the
        /// future. This may be `Always` indicating that the subscriber always
        /// wishes to be notified about the callsite, and its filter need not be
        /// re-evaluated; `Sometimes`, indicating that the subscriber may sometimes
        /// care about the callsite but not always (such as when sampling), or
        /// `Never`, indicating that the subscriber never wishes to be notified about
        /// that callsite. If all active subscribers return `Never`, a callsite will
        /// never be enabled unless a new subscriber expresses interest in it.
        ///
        /// `Subscriber`s which require their filters to be run every time an event
        /// occurs or a span is entered/exited should return `Interest::sometimes`.
        /// If a subscriber returns `Interest::sometimes`, then its' [`enabled`] method
        /// will be called every time an event or span is created from that callsite.
        ///
        /// For example, suppose a sampling subscriber is implemented by
        /// incrementing a counter every time `enabled` is called and only returning
        /// `true` when the counter is divisible by a specified sampling rate. If
        /// that subscriber returns `Interest::always` from `register_callsite`, then
        /// the filter will not be re-evaluated once it has been applied to a given
        /// set of metadata. Thus, the counter will not be incremented, and the span
        /// or event that corresponds to the metadata will never be `enabled`.
        ///
        /// `Subscriber`s that need to change their filters occasionally should call
        /// [`rebuild_interest_cache`] to re-evaluate `register_callsite` for all
        /// callsites.
        ///
        /// Similarly, if a `Subscriber` has a filtering strategy that can be
        /// changed dynamically at runtime, it would need to re-evaluate that filter
        /// if the cached results have changed.
        ///
        /// A subscriber which manages fanout to multiple other subscribers
        /// should proxy this decision to all of its child subscribers,
        /// returning `Interest::never` only if _all_ such children return
        /// `Interest::never`. If the set of subscribers to which spans are
        /// broadcast may change dynamically, the subscriber should also never
        /// return `Interest::Never`, as a new subscriber may be added that _is_
        /// interested.
        ///
        /// # Notes
        /// This function may be called again when a new subscriber is created or
        /// when the registry is invalidated.
        ///
        /// If a subscriber returns `Interest::never` for a particular callsite, it
        /// _may_ still see spans and events originating from that callsite, if
        /// another subscriber expressed interest in it.
        ///
        /// [filter]: #method.enabled
        /// [metadata]: ../metadata/struct.Metadata.html
        /// [`Interest`]: struct.Interest.html
        /// [`enabled`]: #method.enabled
        /// [`rebuild_interest_cache`]: ../callsite/fn.rebuild_interest_cache.html
        fn register_callsite(&self, metadata: &'static Metadata<'static>) -> Interest {
            if self.enabled(metadata) {
                Interest::always()
            } else {
                Interest::never()
            }
        }

        /// Returns true if a span or event with the specified [metadata] would be
        /// recorded.
        ///
        /// By default, it is assumed that this filter needs only be evaluated once
        /// for each callsite, so it is called by [`register_callsite`] when each
        /// callsite is registered. The result is used to determine if the subscriber
        /// is always [interested] or never interested in that callsite. This is intended
        /// primarily as an optimization, so that expensive filters (such as those
        /// involving string search, et cetera) need not be re-evaluated.
        ///
        /// However, if the subscriber's interest in a particular span or event may
        /// change, or depends on contexts only determined dynamically at runtime,
        /// then the `register_callsite` method should be overridden to return
        /// [`Interest::sometimes`]. In that case, this function will be called every
        /// time that span or event occurs.
        ///
        /// [metadata]: ../metadata/struct.Metadata.html
        /// [interested]: struct.Interest.html
        /// [`Interest::sometimes`]: struct.Interest.html#method.sometimes
        /// [`register_callsite`]: #method.register_callsite
        fn enabled(&self, metadata: &Metadata<'_>) -> bool;

        /// 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.
        ///
        /// If this method returns a [`Level`][level], it will be used as a hint to
        /// determine the most verbose level that will be enabled. This will allow
        /// spans and events which are more verbose than that level to be skipped
        /// more efficiently. Subscribers which perform filtering are strongly
        /// encouraged to provide an implementation of this method.
        ///
        /// If the maximum level the subscriber will enable can change over the
        /// course of its lifetime, it is free to return a different value from
        /// multiple invocations of this method. However, note that changes in the
        /// maximum level will **only** be reflected after the callsite [`Interest`]
        /// cache is rebuilt, by calling the [`callsite::rebuild_interest_cache`][rebuild]
        /// function. Therefore, if the subscriber will change the value returned by
        /// this method, it is responsible for ensuring that
        /// [`rebuild_interest_cache`][rebuild] is called after the value of the max
        /// level changes.
        ///
        /// [level]: ../struct.Level.html
        /// [`Interest`]: struct.Interest.html
        /// [rebuild]: ../callsite/fn.rebuild_interest_cache.html
        fn max_level_hint(&self) -> Option<LevelFilter> {
            None
        }

        /// Visit the construction of a new span, returning a new [span ID] for the
        /// span being constructed.
        ///
        /// The provided [`Attributes`] contains any field values that were provided
        /// when the span was created. The subscriber may pass a [visitor] to the
        /// `Attributes`' [`record` method] to record these values.
        ///
        /// IDs are used to uniquely identify spans and events within the context of a
        /// subscriber, so span equality will be based on the returned ID. Thus, if
        /// the subscriber wishes for all spans with the same metadata to be
        /// considered equal, it should return the same ID every time it is given a
        /// particular set of metadata. Similarly, if it wishes for two separate
        /// instances of a span with the same metadata to *not* be equal, it should
        /// return a distinct ID every time this function is called, regardless of
        /// the metadata.
        ///
        /// Note that the subscriber is free to assign span IDs based on whatever
        /// scheme it sees fit. Any guarantees about uniqueness, ordering, or ID
        /// reuse are left up to the subscriber implementation to determine.
        ///
        /// [span ID]: ../span/struct.Id.html
        /// [`Attributes`]: ../span/struct.Attributes.html
        /// [visitor]: ../field/trait.Visit.html
        /// [`record` method]: ../span/struct.Attributes.html#method.record
        fn new_span(&self, span: &span::Attributes<'_>) -> span::Id;

        // === Notification methods ===============================================

        /// Record a set of values on a span.
        ///
        /// This method will be invoked when value is recorded on a span.
        /// Recording multiple values for the same field is possible,
        /// but the actual behaviour is defined by the subscriber implementation.
        ///
        /// Keep in mind that a span might not provide a value
        /// for each field it declares.
        ///
        /// The subscriber is expected to provide a [visitor] to the `Record`'s
        /// [`record` method] in order to record the added values.
        ///
        /// # Example
        ///  "foo = 3" will be recorded when [`record`] is called on the
        /// `Attributes` passed to `new_span`.
        /// Since values are not provided for the `bar` and `baz` fields,
        /// the span's `Metadata` will indicate that it _has_ those fields,
        /// but values for them won't be recorded at this time.
        ///
        /// ```rust,ignore
        /// #[macro_use]
        /// extern crate tracing;
        ///
        /// let mut span = span!("my_span", foo = 3, bar, baz);
        ///
        /// // `Subscriber::record` will be called with a `Record`
        /// // containing "bar = false"
        /// span.record("bar", &false);
        ///
        /// // `Subscriber::record` will be called with a `Record`
        /// // containing "baz = "a string""
        /// span.record("baz", &"a string");
        /// ```
        ///
        /// [visitor]: ../field/trait.Visit.html
        /// [`record`]: ../span/struct.Attributes.html#method.record
        /// [`record` method]: ../span/struct.Record.html#method.record
        fn record(&self, span: &span::Id, values: &span::Record<'_>);

        /// Adds an indication that `span` follows from the span with the id
        /// `follows`.
        ///
        /// This relationship differs somewhat from the parent-child relationship: a
        /// span may have any number of prior spans, rather than a single one; and
        /// spans are not considered to be executing _inside_ of the spans they
        /// follow from. This means that a span may close even if subsequent spans
        /// that follow from it are still open, and time spent inside of a
        /// subsequent span should not be included in the time its precedents were
        /// executing. This is used to model causal relationships such as when a
        /// single future spawns several related background tasks, et cetera.
        ///
        /// If the subscriber has spans corresponding to the given IDs, it should
        /// record this relationship in whatever way it deems necessary. Otherwise,
        /// if one or both of the given span IDs do not correspond to spans that the
        /// subscriber knows about, or if a cyclical relationship would be created
        /// (i.e., some span _a_ which proceeds some other span _b_ may not also
        /// follow from _b_), it may silently do nothing.
        fn record_follows_from(&self, span: &span::Id, follows: &span::Id);

        /// Records that an [`Event`] has occurred.
        ///
        /// This method will be invoked when an Event is constructed by
        /// the `Event`'s [`dispatch` method]. For example, this happens internally
        /// when an event macro from `tracing` is called.
        ///
        /// The key difference between this method and `record` is that `record` is
        /// called when a value is recorded for a field defined by a span,
        /// while `event` is called when a new event occurs.
        ///
        /// The provided `Event` struct contains any field values attached to the
        /// event. The subscriber may pass a [visitor] to the `Event`'s
        /// [`record` method] to record these values.
        ///
        /// [`Event`]: ../event/struct.Event.html
        /// [visitor]: ../field/trait.Visit.html
        /// [`record` method]: ../event/struct.Event.html#method.record
        /// [`dispatch` method]: ../event/struct.Event.html#method.dispatch
        fn event(&self, event: &Event<'_>);

        /// Records that a span has been entered.
        ///
        /// When entering a span, this method is called to notify the subscriber
        /// that the span has been entered. The subscriber is provided with the
        /// [span ID] of the entered span, and should update any internal state
        /// tracking the current span accordingly.
        ///
        /// [span ID]: ../span/struct.Id.html
        fn enter(&self, span: &span::Id);

        /// Records that a span has been exited.
        ///
        /// When exiting a span, this method is called to notify the subscriber
        /// that the span has been exited. The subscriber is provided with the
        /// [span ID] of the exited span, and should update any internal state
        /// tracking the current span accordingly.
        ///
        /// Exiting a span does not imply that the span will not be re-entered.
        ///
        /// [span ID]: ../span/struct.Id.html
        fn exit(&self, span: &span::Id);

        /// Notifies the subscriber that a [span ID] has been cloned.
        ///
        /// This function is guaranteed to only be called with span IDs that were
        /// returned by this subscriber's `new_span` function.
        ///
        /// Note that the default implementation of this function this is just the
        /// identity function, passing through the identifier. However, it can be
        /// used in conjunction with [`try_close`] to track the number of handles
        /// capable of `enter`ing a span. When all the handles have been dropped
        /// (i.e., `try_close` has been called one more time than `clone_span` for a
        /// given ID), the subscriber may assume that the span will not be entered
        /// again. It is then free to deallocate storage for data associated with
        /// that span, write data from that span to IO, and so on.
        ///
        /// For more unsafe situations, however, if `id` is itself a pointer of some
        /// kind this can be used as a hook to "clone" the pointer, depending on
        /// what that means for the specified pointer.
        ///
        /// [span ID]: ../span/struct.Id.html
        /// [`try_close`]: trait.Subscriber.html#method.try_close
        fn clone_span(&self, id: &span::Id) -> span::Id {
            id.clone()
        }

        /// **This method is deprecated.**
        ///
        /// Using `drop_span` may result in subscribers composed using
        /// `tracing-subscriber` crate's `Layer` trait from observing close events.
        /// Use [`try_close`] instead.
        ///
        /// The default implementation of this function does nothing.
        ///
        /// [`try_close`]: trait.Subscriber.html#method.try_close
        #[deprecated(since = "0.1.2", note = "use `Subscriber::try_close` instead")]
        fn drop_span(&self, _id: span::Id) {}

        /// Notifies the subscriber that a [`span ID`] has been dropped, and returns
        /// `true` if there are now 0 IDs that refer to that span.
        ///
        /// Higher-level libraries providing functionality for composing multiple
        /// subscriber implementations may use this return value to notify any
        /// "layered" subscribers that this subscriber considers the span closed.
        ///
        /// The default implementation of this method calls the subscriber's
        /// [`drop_span`] method and returns `false`. This means that, unless the
        /// subscriber overrides the default implementation, close notifications
        /// will never be sent to any layered subscribers. In general, if the
        /// subscriber tracks reference counts, this method should be implemented,
        /// rather than `drop_span`.
        ///
        /// This function is guaranteed to only be called with span IDs that were
        /// returned by this subscriber's `new_span` function.
        ///
        /// It's guaranteed that if this function has been called once more than the
        /// number of times `clone_span` was called with the same `id`, then no more
        /// handles that can enter the span with that `id` exist. This means that it
        /// can be used in conjunction with [`clone_span`] to track the number of
        /// handles capable of `enter`ing a span. When all the handles have been
        /// dropped (i.e., `try_close` has been called one more time than
        /// `clone_span` for a given ID), the subscriber may assume that the span
        /// will not be entered again, and should return `true`. It is then free to
        /// deallocate storage for data associated with that span, write data from
        /// that span to IO, and so on.
        ///
        /// **Note**: since this function is called when spans are dropped,
        /// implementations should ensure that they are unwind-safe. Panicking from
        /// inside of a `try_close` function may cause a double panic, if the span
        /// was dropped due to a thread unwinding.
        ///
        /// [span ID]: ../span/struct.Id.html
        /// [`clone_span`]: trait.Subscriber.html#method.clone_span
        /// [`drop_span`]: trait.Subscriber.html#method.drop_span
        fn try_close(&self, id: span::Id) -> bool {
            #[allow(deprecated)]
            self.drop_span(id);
            false
        }

        /// Returns a type representing this subscriber's view of the current span.
        ///
        /// If subscribers track a current span, they should override this function
        /// to return [`Current::new`] if the thread from which this method is
        /// called is inside a span, or [`Current::none`] if the thread is not
        /// inside a span.
        ///
        /// By default, this returns a value indicating that the subscriber
        /// does **not** track what span is current. If the subscriber does not
        /// implement a current span, it should not override this method.
        ///
        /// [`Current::new`]: ../span/struct.Current.html#tymethod.new
        /// [`Current::none`]: ../span/struct.Current.html#tymethod.none
        fn current_span(&self) -> span::Current {
            span::Current::unknown()
        }

        // === Downcasting methods ================================================

        /// If `self` is the same type as the provided `TypeId`, returns an untyped
        /// `*const` pointer to that type. Otherwise, returns `None`.
        ///
        /// If you wish to downcast a `Subscriber`, it is strongly advised to use
        /// the safe API provided by [`downcast_ref`] instead.
        ///
        /// This API is required for `downcast_raw` to be a trait method; a method
        /// signature like [`downcast_ref`] (with a generic type parameter) is not
        /// object-safe, and thus cannot be a trait method for `Subscriber`. This
        /// means that if we only exposed `downcast_ref`, `Subscriber`
        /// implementations could not override the downcasting behavior
        ///
        /// This method may be overridden by "fan out" or "chained" subscriber
        /// implementations which consist of multiple composed types. Such
        /// subscribers might allow `downcast_raw` by returning references to those
        /// component if they contain components with the given `TypeId`.
        ///
        /// # Safety
        ///
        /// The [`downcast_ref`] method expects that the pointer returned by
        /// `downcast_raw` is non-null and points to a valid instance of the type
        /// with the provided `TypeId`. Failure to ensure this will result in
        /// undefined behaviour, so implementing `downcast_raw` is unsafe.
        ///
        /// [`downcast_ref`]: #method.downcast_ref
        unsafe fn downcast_raw(&self, id: TypeId) -> Option<*const ()> {
            if id == TypeId::of::<Self>() {
                Some(self as *const Self as *const ())
            } else {
                None
            }
        }
    }

    impl dyn Subscriber {
        /// Returns `true` if this `Subscriber` is the same type as `T`.
        pub fn is<T: Any>(&self) -> bool {
            self.downcast_ref::<T>().is_some()
        }

        /// Returns some reference to this `Subscriber` value if it is of type `T`,
        /// or `None` if it isn't.
        pub fn downcast_ref<T: Any>(&self) -> Option<&T> {
            unsafe {
                let raw = self.downcast_raw(TypeId::of::<T>())?;
                if raw.is_null() {
                    None
                } else {
                    Some(&*(raw as *const _))
                }
            }
        }
    }

    /// Indicates a [`Subscriber`]'s interest in a particular callsite.
    ///
    /// `Subscriber`s return an `Interest` from their [`register_callsite`] methods
    /// in order to determine whether that span should be enabled or disabled.
    ///
    /// [`Subscriber`]: ../trait.Subscriber.html
    /// [`register_callsite`]: ../trait.Subscriber.html#method.register_callsite
    #[derive(Clone, Debug)]
    pub struct Interest(InterestKind);

    #[derive(Copy, Clone, Debug, Eq, PartialEq, Ord, PartialOrd)]
    enum InterestKind {
        Never = 0,
        Sometimes = 1,
        Always = 2,
    }

    impl Interest {
        /// Returns an `Interest` indicating that the subscriber is never interested
        /// in being notified about a callsite.
        ///
        /// If all active subscribers are `never()` interested in a callsite, it will
        /// be completely disabled unless a new subscriber becomes active.
        #[inline]
        pub fn never() -> Self {
            Interest(InterestKind::Never)
        }

        /// Returns an `Interest` indicating the subscriber is sometimes interested
        /// in being notified about a callsite.
        ///
        /// If all active subscribers are `sometimes` or `never` interested in a
        /// callsite, the currently active subscriber will be asked to filter that
        /// callsite every time it creates a span. This will be the case until a new
        /// subscriber expresses that it is `always` interested in the callsite.
        #[inline]
        pub fn sometimes() -> Self {
            Interest(InterestKind::Sometimes)
        }

        /// Returns an `Interest` indicating the subscriber is always interested in
        /// being notified about a callsite.
        ///
        /// If any subscriber expresses that it is `always()` interested in a given
        /// callsite, then the callsite will always be enabled.
        #[inline]
        pub fn always() -> Self {
            Interest(InterestKind::Always)
        }

        /// Returns `true` if the subscriber is never interested in being notified
        /// about this callsite.
        #[inline]
        pub fn is_never(&self) -> bool {
            match self.0 {
                InterestKind::Never => true,
                _ => false,
            }
        }

        /// Returns `true` if the subscriber is sometimes interested in being notified
        /// about this callsite.
        #[inline]
        pub fn is_sometimes(&self) -> bool {
            match self.0 {
                InterestKind::Sometimes => true,
                _ => false,
            }
        }

        /// Returns `true` if the subscriber is always interested in being notified
        /// about this callsite.
        #[inline]
        pub fn is_always(&self) -> bool {
            match self.0 {
                InterestKind::Always => true,
                _ => false,
            }
        }

        /// Returns the common interest between these two Interests.
        ///
        /// If both interests are the same, this propagates that interest.
        /// Otherwise, if they differ, the result must always be
        /// `Interest::sometimes` --- if the two subscribers differ in opinion, we
        /// will have to ask the current subscriber what it thinks, no matter what.
        pub(crate) fn and(self, rhs: Interest) -> Self {
            if self.0 == rhs.0 {
                self
            } else {
                Interest::sometimes()
            }
        }
    }