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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 (<code>with_default</code>)
    //! requires 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>
    //!
    //! ## 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
    #[cfg(feature = "std")]
    #[cfg_attr(docsrs, doc(cfg(feature = "std")))]
    pub use tracing_core::dispatcher::set_default;
    #[cfg(feature = "std")]
    #[cfg_attr(docsrs, doc(cfg(feature = "std")))]
    pub use tracing_core::dispatcher::with_default;
    #[cfg(feature = "std")]
    #[cfg_attr(docsrs, doc(cfg(feature = "std")))]
    pub use tracing_core::dispatcher::DefaultGuard;
    pub use tracing_core::dispatcher::{
        get_default, set_global_default, Dispatch, SetGlobalDefaultError,
    };

    /// Private API for internal use by tracing's macros.
    ///
    /// This function is *not* considered part of `tracing`'s public API, and has no
    /// stability guarantees. If you use it, and it breaks or disappears entirely,
    /// don't say we didn;'t warn you.
    #[doc(hidden)]
    pub use tracing_core::dispatcher::has_been_set;