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    //! A scoped, structured logging and diagnostics system.
    //!
    //! # Overview
    //!
    //! `tracing` is a framework for instrumenting Rust programs to collect
    //! structured, event-based diagnostic information.
    //!
    //! In asynchronous systems like Tokio, interpreting traditional log messages can
    //! often be quite challenging. Since individual tasks are multiplexed on the same
    //! thread, associated events and log lines are intermixed making it difficult to
    //! trace the logic flow. `tracing` expands upon logging-style diagnostics by
    //! allowing libraries and applications to record structured events with additional
    //! information about *temporality* and *causality* — unlike a log message, a span
    //! in `tracing` has a beginning and end time, may be entered and exited by the
    //! flow of execution, and may exist within a nested tree of similar spans. In
    //! addition, `tracing` spans are *structured*, with the ability to record typed
    //! data as well as textual messages.
    //!
    //! The `tracing` crate provides the APIs necessary for instrumenting libraries
    //! and applications to emit trace data.
    //!
    //! *Compiler support: [requires `rustc` 1.42+][msrv]*
    //!
    //! [msrv]: #supported-rust-versions
    //! # Core Concepts
    //!
    //! The core of `tracing`'s API is composed of _spans_, _events_ and
    //! _subscribers_. We'll cover these in turn.
    //!
    //! ## Spans
    //!
    //! To record the flow of execution through a program, `tracing` introduces the
    //! concept of [spans]. Unlike a log line that represents a _moment in
    //! time_, a span represents a _period of time_ with a beginning and an end. When a
    //! program begins executing in a context or performing a unit of work, it
    //! _enters_ that context's span, and when it stops executing in that context,
    //! it _exits_ the span. The span in which a thread is currently executing is
    //! referred to as that thread's _current_ span.
    //!
    //! For example:
    //! ```
    //! use tracing::{span, Level};
    //! # fn main() {
    //! let span = span!(Level::TRACE, "my_span");
    //! // `enter` returns a RAII guard which, when dropped, exits the span. this
    //! // indicates that we are in the span for the current lexical scope.
    //! let _enter = span.enter();
    //! // perform some work in the context of `my_span`...
    //! # }
    //!```
    //!
    //! The [`span` module][span]'s documentation provides further details on how to
    //! use spans.
    //!
    //! <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 asynchronous code that uses async/await syntax,
    //!     <code>Span::enter</code> may produce incorrect traces if the returned drop
    //!     guard is held across an await point. See
    //!     <a href="span/struct.Span.html#in-asynchronous-code">the method documentation</a>
    //!     for details.
    //! </pre></div>
    //!
    //! ## Events
    //!
    //! An [`Event`] represents a _moment_ in time. It signifies something that
    //! happened while a trace was being recorded. `Event`s are comparable to the log
    //! records emitted by unstructured logging code, but unlike a typical log line,
    //! an `Event` may occur within the context of a span.
    //!
    //! For example:
    //! ```
    //! use tracing::{event, span, Level};
    //!
    //! # fn main() {
    //! // records an event outside of any span context:
    //! event!(Level::INFO, "something happened");
    //!
    //! let span = span!(Level::INFO, "my_span");
    //! let _guard = span.enter();
    //!
    //! // records an event within "my_span".
    //! event!(Level::DEBUG, "something happened inside my_span");
    //! # }
    //!```
    //!
    //! In general, events should be used to represent points in time _within_ a
    //! span — a request returned with a given status code, _n_ new items were
    //! taken from a queue, and so on.
    //!
    //! The [`Event` struct][`Event`] documentation provides further details on using
    //! events.
    //!
    //! ## Subscribers
    //!
    //! As `Span`s and `Event`s occur, they are recorded or aggregated by
    //! implementations of the [`Subscriber`] trait. `Subscriber`s are notified
    //! when an `Event` takes place and when a `Span` is entered or exited. These
    //! notifications are represented by the following `Subscriber` trait methods:
    //!
    //! + [`event`][Subscriber::event], called when an `Event` takes place,
    //! + [`enter`], called when execution enters a `Span`,
    //! + [`exit`], called when execution exits a `Span`
    //!
    //! In addition, subscribers may implement the [`enabled`] function to _filter_
    //! the notifications they receive based on [metadata] describing each `Span`
    //! or `Event`. If a call to `Subscriber::enabled` returns `false` for a given
    //! set of metadata, that `Subscriber` will *not* be notified about the
    //! corresponding `Span` or `Event`. For performance reasons, if no currently
    //! active subscribers express interest in a given set of metadata by returning
    //! `true`, then the corresponding `Span` or `Event` will never be constructed.
    //!
    //! # Usage
    //!
    //! First, add this to your `Cargo.toml`:
    //!
    //! ```toml
    //! [dependencies]
    //! tracing = "0.1"
    //! ```
    //!
    //! *Compiler support: requires rustc 1.39+*
    //!
    //! ## Recording Spans and Events
    //!
    //! Spans and events are recorded using macros.
    //!
    //! ### Spans
    //!
    //! The [`span!`] macro expands to a [`Span` struct][`Span`] which is used to
    //! record a span. The [`Span::enter`] method on that struct records that the
    //! span has been entered, and returns a [RAII] guard object, which will exit
    //! the span when dropped.
    //!
    //! For example:
    //!
    //! ```rust
    //! use tracing::{span, Level};
    //! # fn main() {
    //! // Construct a new span named "my span" with trace log level.
    //! let span = span!(Level::TRACE, "my span");
    //!
    //! // Enter the span, returning a guard object.
    //! let _enter = span.enter();
    //!
    //! // Any trace events that occur before the guard is dropped will occur
    //! // within the span.
    //!
    //! // Dropping the guard will exit the span.
    //! # }
    //! ```
    //!
    //! The [`#[instrument]`][instrument] attribute provides an easy way to
    //! add `tracing` spans to functions. A function annotated with `#[instrument]`
    //! will create and enter a span with that function's name every time the
    //! function is called, with arguments to that function will be recorded as
    //! fields using `fmt::Debug`.
    //!
    //! For example:
    //! ```ignore
    //! # // this doctest is ignored because we don't have a way to say
    //! # // that it should only be run with cfg(feature = "attributes")
    //! use tracing::{Level, event, instrument};
    //!
    //! #[instrument]
    //! pub fn my_function(my_arg: usize) {
    //!     // This event will be recorded inside a span named `my_function` with the
    //!     // field `my_arg`.
    //!     event!(Level::INFO, "inside my_function!");
    //!     // ...
    //! }
    //! # fn main() {}
    //! ```
    //!
    //!
    //! You can find more examples showing how to use this crate [here][examples].
    //!
    //! [RAII]: https://github.com/rust-unofficial/patterns/blob/master/patterns/RAII.md
    //! [examples]: https://github.com/tokio-rs/tracing/tree/master/examples
    //!
    //! ### Events
    //!
    //! [`Event`]s are recorded using the [`event!`] macro:
    //!
    //! ```rust
    //! # fn main() {
    //! use tracing::{event, Level};
    //! event!(Level::INFO, "something has happened!");
    //! # }
    //! ```
    //!
    //! ## Using the Macros
    //!
    //! The [`span!`] and [`event!`] macros as well as the `#[instrument]` attribute
    //! use fairly similar syntax, with some exceptions.
    //!
    //! ### Configuring Attributes
    //!
    //! Both macros require a [`Level`] specifying the verbosity of the span or
    //! event. Optionally, the [target] and [parent span] may be overridden. If the
    //! target and parent span are not overridden, they will default to the
    //! module path where the macro was invoked and the current span (as determined
    //! by the subscriber), respectively.
    //!
    //! For example:
    //!
    //! ```
    //! # use tracing::{span, event, Level};
    //! # fn main() {
    //! span!(target: "app_spans", Level::TRACE, "my span");
    //! event!(target: "app_events", Level::INFO, "something has happened!");
    //! # }
    //! ```
    //! ```
    //! # use tracing::{span, event, Level};
    //! # fn main() {
    //! let span = span!(Level::TRACE, "my span");
    //! event!(parent: &span, Level::INFO, "something has happened!");
    //! # }
    //! ```
    //!
    //! The span macros also take a string literal after the level, to set the name
    //! of the span.
    //!
    //! ### Recording Fields
    //!
    //! Structured fields on spans and events are specified using the syntax
    //! `field_name = field_value`. Fields are separated by commas.
    //!
    //! ```
    //! # use tracing::{event, Level};
    //! # fn main() {
    //! // records an event with two fields:
    //! //  - "answer", with the value 42
    //! //  - "question", with the value "life, the universe and everything"
    //! event!(Level::INFO, answer = 42, question = "life, the universe, and everything");
    //! # }
    //! ```
    //!
    //! As shorthand, local variables may be used as field values without an
    //! assignment, similar to [struct initializers]. For example:
    //!
    //! ```
    //! # use tracing::{span, Level};
    //! # fn main() {
    //! let user = "ferris";
    //!
    //! span!(Level::TRACE, "login", user);
    //! // is equivalent to:
    //! span!(Level::TRACE, "login", user = user);
    //! # }
    //!```
    //!
    //! Field names can include dots, but should not be terminated by them:
    //! ```
    //! # use tracing::{span, Level};
    //! # fn main() {
    //! let user = "ferris";
    //! let email = "ferris@rust-lang.org";
    //! span!(Level::TRACE, "login", user, user.email = email);
    //! # }
    //!```
    //!
    //! Since field names can include dots, fields on local structs can be used
    //! using the local variable shorthand:
    //! ```
    //! # use tracing::{span, Level};
    //! # fn main() {
    //! # struct User {
    //! #    name: &'static str,
    //! #    email: &'static str,
    //! # }
    //! let user = User {
    //!     name: "ferris",
    //!     email: "ferris@rust-lang.org",
    //! };
    //! // the span will have the fields `user.name = "ferris"` and
    //! // `user.email = "ferris@rust-lang.org"`.
    //! span!(Level::TRACE, "login", user.name, user.email);
    //! # }
    //!```
    //!
    //! Fields with names that are not Rust identifiers, or with names that are Rust reserved words,
    //! may be created using quoted string literals. However, this may not be used with the local
    //! variable shorthand.
    //! ```
    //! # use tracing::{span, Level};
    //! # fn main() {
    //! // records an event with fields whose names are not Rust identifiers
    //! //  - "guid:x-request-id", containing a `:`, with the value "abcdef"
    //! //  - "type", which is a reserved word, with the value "request"
    //! span!(Level::TRACE, "api", "guid:x-request-id" = "abcdef", "type" = "request");
    //! # }
    //!```
    //!
    //! The `?` sigil is shorthand that specifies a field should be recorded using
    //! its [`fmt::Debug`] implementation:
    //! ```
    //! # use tracing::{event, Level};
    //! # fn main() {
    //! #[derive(Debug)]
    //! struct MyStruct {
    //!     field: &'static str,
    //! }
    //!
    //! let my_struct = MyStruct {
    //!     field: "Hello world!"
    //! };
    //!
    //! // `my_struct` will be recorded using its `fmt::Debug` implementation.
    //! event!(Level::TRACE, greeting = ?my_struct);
    //! // is equivalent to:
    //! event!(Level::TRACE, greeting = tracing::field::debug(&my_struct));
    //! # }
    //! ```
    //!
    //! The `%` sigil operates similarly, but indicates that the value should be
    //! recorded using its [`fmt::Display`] implementation:
    //! ```
    //! # use tracing::{event, Level};
    //! # fn main() {
    //! # #[derive(Debug)]
    //! # struct MyStruct {
    //! #     field: &'static str,
    //! # }
    //! #
    //! # let my_struct = MyStruct {
    //! #     field: "Hello world!"
    //! # };
    //! // `my_struct.field` will be recorded using its `fmt::Display` implementation.
    //! event!(Level::TRACE, greeting = %my_struct.field);
    //! // is equivalent to:
    //! event!(Level::TRACE, greeting = tracing::field::display(&my_struct.field));
    //! # }
    //! ```
    //!
    //! The `%` and `?` sigils may also be used with local variable shorthand:
    //!
    //! ```
    //! # use tracing::{event, Level};
    //! # fn main() {
    //! # #[derive(Debug)]
    //! # struct MyStruct {
    //! #     field: &'static str,
    //! # }
    //! #
    //! # let my_struct = MyStruct {
    //! #     field: "Hello world!"
    //! # };
    //! // `my_struct.field` will be recorded using its `fmt::Display` implementation.
    //! event!(Level::TRACE, %my_struct.field);
    //! # }
    //! ```
    //!
    //! Additionally, a span may declare fields with the special value [`Empty`],
    //! which indicates that that the value for that field does not currently exist
    //! but may be recorded later. For example:
    //!
    //! ```
    //! use tracing::{trace_span, field};
    //!
    //! // Create a span with two fields: `greeting`, with the value "hello world", and
    //! // `parting`, without a value.
    //! let span = trace_span!("my_span", greeting = "hello world", parting = field::Empty);
    //!
    //! // ...
    //!
    //! // Now, record a value for parting as well.
    //! span.record("parting", &"goodbye world!");
    //! ```
    //!
    //! Note that a span may have up to 32 fields. The following will not compile:
    //!
    //! ```rust,compile_fail
    //! # use tracing::Level;
    //! # fn main() {
    //! let bad_span = span!(
    //!     Level::TRACE,
    //!     "too many fields!",
    //!     a = 1, b = 2, c = 3, d = 4, e = 5, f = 6, g = 7, h = 8, i = 9,
    //!     j = 10, k = 11, l = 12, m = 13, n = 14, o = 15, p = 16, q = 17,
    //!     r = 18, s = 19, t = 20, u = 21, v = 22, w = 23, x = 24, y = 25,
    //!     z = 26, aa = 27, bb = 28, cc = 29, dd = 30, ee = 31, ff = 32, gg = 33
    //! );
    //! # }
    //! ```
    //!
    //! Finally, events may also include human-readable messages, in the form of a
    //! [format string][fmt] and (optional) arguments, **after** the event's
    //! key-value fields. If a format string and arguments are provided,
    //! they will implicitly create a new field named `message` whose value is the
    //! provided set of format arguments.
    //!
    //! For example:
    //!
    //! ```
    //! # use tracing::{event, Level};
    //! # fn main() {
    //! let question = "the answer to the ultimate question of life, the universe, and everything";
    //! let answer = 42;
    //! // records an event with the following fields:
    //! // - `question.answer` with the value 42,
    //! // - `question.tricky` with the value `true`,
    //! // - "message", with the value "the answer to the ultimate question of life, the
    //! //    universe, and everything is 42."
    //! event!(
    //!     Level::DEBUG,
    //!     question.answer = answer,
    //!     question.tricky = true,
    //!     "the answer to {} is {}.", question, answer
    //! );
    //! # }
    //! ```
    //!
    //! Specifying a formatted message in this manner does not allocate by default.
    //!
    //! [struct initializers]: https://doc.rust-lang.org/book/ch05-01-defining-structs.html#using-the-field-init-shorthand-when-variables-and-fields-have-the-same-name
    //! [target]: struct.Metadata.html#method.target
    //! [parent span]: span/struct.Attributes.html#method.parent
    //! [determined contextually]: span/struct.Attributes.html#method.is_contextual
    //! [`fmt::Debug`]: https://doc.rust-lang.org/std/fmt/trait.Debug.html
    //! [`fmt::Display`]: https://doc.rust-lang.org/std/fmt/trait.Display.html
    //! [fmt]: https://doc.rust-lang.org/std/fmt/#usage
    //! [`Empty`]: field/struct.Empty.html
    //!
    //! ### Shorthand Macros
    //!
    //! `tracing` also offers a number of macros with preset verbosity levels.
    //! The [`trace!`], [`debug!`], [`info!`], [`warn!`], and [`error!`] behave
    //! similarly to the [`event!`] macro, but with the [`Level`] argument already
    //! specified, while the corresponding [`trace_span!`], [`debug_span!`],
    //! [`info_span!`], [`warn_span!`], and [`error_span!`] macros are the same,
    //! but for the [`span!`] macro.
    //!
    //! These are intended both as a shorthand, and for compatibility with the [`log`]
    //! crate (see the next section).
    //!
    //! [`span!`]: macro.span.html
    //! [`event!`]: macro.event.html
    //! [`trace!`]: macro.trace.html
    //! [`debug!`]: macro.debug.html
    //! [`info!`]: macro.info.html
    //! [`warn!`]: macro.warn.html
    //! [`error!`]: macro.error.html
    //! [`trace_span!`]: macro.trace_span.html
    //! [`debug_span!`]: macro.debug_span.html
    //! [`info_span!`]: macro.info_span.html
    //! [`warn_span!`]: macro.warn_span.html
    //! [`error_span!`]: macro.error_span.html
    //! [`Level`]: struct.Level.html
    //!
    //! ### For `log` Users
    //!
    //! Users of the [`log`] crate should note that `tracing` exposes a set of
    //! macros for creating `Event`s (`trace!`, `debug!`, `info!`, `warn!`, and
    //! `error!`) which may be invoked with the same syntax as the similarly-named
    //! macros from the `log` crate. Often, the process of converting a project to
    //! use `tracing` can begin with a simple drop-in replacement.
    //!
    //! Let's consider the `log` crate's yak-shaving example:
    //!
    //! ```rust,ignore
    //! use std::{error::Error, io};
    //! use tracing::{debug, error, info, span, warn, Level};
    //!
    //! // the `#[tracing::instrument]` attribute creates and enters a span
    //! // every time the instrumented function is called. The span is named after the
    //! // the function or method. Parameters passed to the function are recorded as fields.
    //! #[tracing::instrument]
    //! pub fn shave(yak: usize) -> Result<(), Box<dyn Error + 'static>> {
    //!     // this creates an event at the DEBUG level with two fields:
    //!     // - `excitement`, with the key "excitement" and the value "yay!"
    //!     // - `message`, with the key "message" and the value "hello! I'm gonna shave a yak."
    //!     //
    //!     // unlike other fields, `message`'s shorthand initialization is just the string itself.
    //!     debug!(excitement = "yay!", "hello! I'm gonna shave a yak.");
    //!     if yak == 3 {
    //!         warn!("could not locate yak!");
    //!         // note that this is intended to demonstrate `tracing`'s features, not idiomatic
    //!         // error handling! in a library or application, you should consider returning
    //!         // a dedicated `YakError`. libraries like snafu or thiserror make this easy.
    //!         return Err(io::Error::new(io::ErrorKind::Other, "shaving yak failed!").into());
    //!     } else {
    //!         debug!("yak shaved successfully");
    //!     }
    //!     Ok(())
    //! }
    //!
    //! pub fn shave_all(yaks: usize) -> usize {
    //!     // Constructs a new span named "shaving_yaks" at the TRACE level,
    //!     // and a field whose key is "yaks". This is equivalent to writing:
    //!     //
    //!     // let span = span!(Level::TRACE, "shaving_yaks", yaks = yaks);
    //!     //
    //!     // local variables (`yaks`) can be used as field values
    //!     // without an assignment, similar to struct initializers.
    //!     let _span = span!(Level::TRACE, "shaving_yaks", yaks).entered();
    //!
    //!     info!("shaving yaks");
    //!
    //!     let mut yaks_shaved = 0;
    //!     for yak in 1..=yaks {
    //!         let res = shave(yak);
    //!         debug!(yak, shaved = res.is_ok());
    //!
    //!         if let Err(ref error) = res {
    //!             // Like spans, events can also use the field initialization shorthand.
    //!             // In this instance, `yak` is the field being initalized.
    //!             error!(yak, error = error.as_ref(), "failed to shave yak!");
    //!         } else {
    //!             yaks_shaved += 1;
    //!         }
    //!         debug!(yaks_shaved);
    //!     }
    //!
    //!     yaks_shaved
    //! }
    //! ```
    //!
    //! ## In libraries
    //!
    //! Libraries should link only to the `tracing` crate, and use the provided
    //! macros to record whatever information will be useful to downstream
    //! consumers.
    //!
    //! ## In executables
    //!
    //! In order to record trace events, executables have to use a `Subscriber`
    //! implementation compatible with `tracing`. A `Subscriber` implements a
    //! way of collecting trace data, such as by logging it to standard output.
    //!
    //! This library does not contain any `Subscriber` implementations; these are
    //! provided by [other crates](#related-crates).
    //!
    //! The simplest way to use a subscriber is to call the [`set_global_default`]
    //! function:
    //!
    //! ```
    //! extern crate tracing;
    //! # pub struct FooSubscriber;
    //! # use tracing::{span::{Id, Attributes, Record}, Metadata};
    //! # impl tracing::Subscriber for FooSubscriber {
    //! #   fn new_span(&self, _: &Attributes) -> Id { Id::from_u64(0) }
    //! #   fn record(&self, _: &Id, _: &Record) {}
    //! #   fn event(&self, _: &tracing::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 }
    //! # }
    //! # fn main() {
    //!
    //! let my_subscriber = FooSubscriber::new();
    //! tracing::subscriber::set_global_default(my_subscriber)
    //!     .expect("setting tracing default failed");
    //! # }
    //! ```
    //!
    //! <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>
    //!
    //! This subscriber will be used as the default in all threads for the
    //! remainder of the duration of the program, similar to setting the logger
    //! in the `log` crate.
    //!
    //! In addition, the default subscriber can be set through using the
    //! [`with_default`] function. This follows the `tokio` pattern of using
    //! closures to represent executing code in a context that is exited at the end
    //! of the closure. For example:
    //!
    //! ```rust
    //! # pub struct FooSubscriber;
    //! # use tracing::{span::{Id, Attributes, Record}, Metadata};
    //! # impl tracing::Subscriber for FooSubscriber {
    //! #   fn new_span(&self, _: &Attributes) -> Id { Id::from_u64(0) }
    //! #   fn record(&self, _: &Id, _: &Record) {}
    //! #   fn event(&self, _: &tracing::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 }
    //! # }
    //! # fn main() {
    //!
    //! let my_subscriber = FooSubscriber::new();
    //! # #[cfg(feature = "std")]
    //! tracing::subscriber::with_default(my_subscriber, || {
    //!     // Any trace events generated in this closure or by functions it calls
    //!     // will be collected by `my_subscriber`.
    //! })
    //! # }
    //! ```
    //!
    //! This approach allows trace data to be collected by multiple subscribers
    //! within different contexts in the program. Note that the override only applies to the
    //! currently executing thread; other threads will not see the change from with_default.
    //!
    //! Any trace events generated outside the context of a subscriber will not be collected.
    //!
    //! Once a subscriber has been set, instrumentation points may be added to the
    //! executable using the `tracing` crate's macros.
    //!
    //! ## `log` Compatibility
    //!
    //! The [`log`] crate provides a simple, lightweight logging facade for Rust.
    //! While `tracing` builds upon `log`'s foundation with richer structured
    //! diagnostic data, `log`'s simplicity and ubiquity make it the "lowest common
    //! denominator" for text-based logging in Rust — a vast majority of Rust
    //! libraries and applications either emit or consume `log` records. Therefore,
    //! `tracing` provides multiple forms of interoperability with `log`: `tracing`
    //! instrumentation can emit `log` records, and a compatibility layer enables
    //! `tracing` [`Subscriber`]s to consume `log` records as `tracing` [`Event`]s.
    //!
    //! ### Emitting `log` Records
    //!
    //! This crate provides two feature flags, "log" and "log-always", which will
    //! cause [spans] and [events] to emit `log` records. When the "log" feature is
    //! enabled, if no `tracing` `Subscriber` is active, invoking an event macro or
    //! creating a span with fields will emit a `log` record. This is intended
    //! primarily for use in libraries which wish to emit diagnostics that can be
    //! consumed by applications using `tracing` *or* `log`, without paying the
    //! additional overhead of emitting both forms of diagnostics when `tracing` is
    //! in use.
    //!
    //! Enabling the "log-always" feature will cause `log` records to be emitted
    //! even if a `tracing` `Subscriber` _is_ set. This is intended to be used in
    //! applications where a `log` `Logger` is being used to record a textual log,
    //! and `tracing` is used only to record other forms of diagnostics (such as
    //! metrics, profiling, or distributed tracing data). Unlike the "log" feature,
    //! libraries generally should **not** enable the "log-always" feature, as doing
    //! so will prevent applications from being able to opt out of the `log` records.
    //!
    //! See [here][flags] for more details on this crate's feature flags.
    //!
    //! The generated `log` records' messages will be a string representation of the
    //! span or event's fields, and all additional information recorded by `log`
    //! (target, verbosity level, module path, file, and line number) will also be
    //! populated. Additionally, `log` records are also generated when spans are
    //! entered, exited, and closed. Since these additional span lifecycle logs have
    //! the potential to be very verbose, and don't include additional fields, they
    //! will always be emitted at the `Trace` level, rather than inheriting the
    //! level of the span that generated them. Furthermore, they are are categorized
    //! under a separate `log` target, "tracing::span" (and its sub-target,
    //! "tracing::span::active", for the logs on entering and exiting a span), which
    //! may be enabled or disabled separately from other `log` records emitted by
    //! `tracing`.
    //!
    //! ### Consuming `log` Records
    //!
    //! The [`tracing-log`] crate provides a compatibility layer which
    //! allows a `tracing` [`Subscriber`] to consume `log` records as though they
    //! were `tracing` [events]. This allows applications using `tracing` to record
    //! the logs emitted by dependencies using `log` as events within the context of
    //! the application's trace tree. See [that crate's documentation][log-tracer]
    //! for details.
    //!
    //! [log-tracer]: https://docs.rs/tracing-log/latest/tracing_log/#convert-log-records-to-tracing-events
    //!
    //! ## Related Crates
    //!
    //! In addition to `tracing` and `tracing-core`, the [`tokio-rs/tracing`] repository
    //! contains several additional crates designed to be used with the `tracing` ecosystem.
    //! This includes a collection of `Subscriber` implementations, as well as utility
    //! and adapter crates to assist in writing `Subscriber`s and instrumenting
    //! applications.
    //!
    //! In particular, the following crates are likely to be of interest:
    //!
    //!  - [`tracing-futures`] provides a compatibility layer with the `futures`
    //!    crate, allowing spans to be attached to `Future`s, `Stream`s, and `Executor`s.
    //!  - [`tracing-subscriber`] provides `Subscriber` implementations and
    //!    utilities for working with `Subscriber`s. This includes a [`FmtSubscriber`]
    //!    `FmtSubscriber` for logging formatted trace data to stdout, with similar
    //!    filtering and formatting to the [`env_logger`] crate.
    //!  - [`tracing-log`] provides a compatibility layer with the [`log`] crate,
    //!    allowing log messages to be recorded as `tracing` `Event`s within the
    //!    trace tree. This is useful when a project using `tracing` have
    //!    dependencies which use `log`. Note that if you're using
    //!    `tracing-subscriber`'s `FmtSubscriber`, you don't need to depend on
    //!    `tracing-log` directly.
    //!  - [`tracing-appender`] provides utilities for outputting tracing data,
    //!     including a file appender and non blocking writer.
    //!
    //! Additionally, there are also several third-party crates which are not
    //! maintained by the `tokio` project. These include:
    //!
    //!  - [`tracing-timing`] implements inter-event timing metrics on top of `tracing`.
    //!    It provides a subscriber that records the time elapsed between pairs of
    //!    `tracing` events and generates histograms.
    //!  - [`tracing-opentelemetry`] provides a subscriber for emitting traces to
    //!    [OpenTelemetry]-compatible distributed tracing systems.
    //!  - [`tracing-honeycomb`] Provides a layer that reports traces spanning multiple machines to [honeycomb.io]. Backed by [`tracing-distributed`].
    //!  - [`tracing-distributed`] Provides a generic implementation of a layer that reports traces spanning multiple machines to some backend.
    //!  - [`tracing-actix`] provides `tracing` integration for the `actix` actor
    //!    framework.
    //!  - [`tracing-gelf`] implements a subscriber for exporting traces in Greylog
    //!    GELF format.
    //!  - [`tracing-coz`] provides integration with the [coz] causal profiler
    //!    (Linux-only).
    //!  - [`tracing-bunyan-formatter`] provides a layer implementation that reports events and spans
    //!    in [bunyan] format, enriched with timing information.
    //!  - [`tracing-wasm`] provides a `Subscriber`/`Layer` implementation that reports
    //!    events and spans via browser `console.log` and [User Timing API (`window.performance`)].
    //!  - [`tide-tracing`] provides a [tide] middleware to trace all incoming requests and responses.
    //!  - [`test-env-log`] takes care of initializing `tracing` for tests, based on
    //!    environment variables with an `env_logger` compatible syntax.
    //!  - [`tracing-unwrap`] provides convenience methods to report failed unwraps
    //!    on `Result` or `Option` types to a `Subscriber`.
    //!  - [`diesel-tracing`] provides integration with [`diesel`] database connections.
    //!  - [`tracing-tracy`] provides a way to collect [Tracy] profiles in instrumented
    //!    applications.
    //!  - [`tracing-elastic-apm`] provides a layer for reporting traces to [Elastic APM].
    //!
    //! If you're the maintainer of a `tracing` ecosystem crate not listed above,
    //! please let us know! We'd love to add your project to the list!
    //!
    //! [`tracing-opentelemetry`]: https://crates.io/crates/tracing-opentelemetry
    //! [OpenTelemetry]: https://opentelemetry.io/
    //! [`tracing-honeycomb`]: https://crates.io/crates/tracing-honeycomb
    //! [`tracing-distributed`]: https://crates.io/crates/tracing-distributed
    //! [honeycomb.io]: https://www.honeycomb.io/
    //! [`tracing-actix`]: https://crates.io/crates/tracing-actix
    //! [`tracing-gelf`]: https://crates.io/crates/tracing-gelf
    //! [`tracing-coz`]: https://crates.io/crates/tracing-coz
    //! [coz]: https://github.com/plasma-umass/coz
    //! [`tracing-bunyan-formatter`]: https://crates.io/crates/tracing-bunyan-formatter
    //! [bunyan]: https://github.com/trentm/node-bunyan
    //! [`tracing-wasm`]: https://docs.rs/tracing-wasm
    //! [User Timing API (`window.performance`)]: https://developer.mozilla.org/en-US/docs/Web/API/User_Timing_API
    //! [`tide-tracing`]: https://crates.io/crates/tide-tracing
    //! [tide]: https://crates.io/crates/tide
    //! [`test-env-log`]: https://crates.io/crates/test-env-log
    //! [`tracing-unwrap`]: https://docs.rs/tracing-unwrap
    //! [`diesel`]: https://crates.io/crates/diesel
    //! [`diesel-tracing`]: https://crates.io/crates/diesel-tracing
    //! [`tracing-tracy`]: https://crates.io/crates/tracing-tracy
    //! [Tracy]: https://github.com/wolfpld/tracy
    //! [`tracing-elastic-apm`]: https://crates.io/crates/tracing-elastic-apm
    //! [Elastic APM]: https://www.elastic.co/apm
    //!
    //! <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>: Some of these ecosystem crates are currently
    //! unreleased and/or in earlier stages of development. They may be less stable
    //! than <code>tracing</code> and <code>tracing-core</code>.
    //! </pre></div>
    //!
    //! ## Crate Feature Flags
    //!
    //! The following crate feature flags are available:
    //!
    //! * A set of features controlling the [static verbosity level].
    //! * `log`: causes trace instrumentation points to emit [`log`] records as well
    //!   as trace events, if a default `tracing` subscriber has not been set. This
    //!   is intended for use in libraries whose users may be using either `tracing`
    //!   or `log`.
    //! * `log-always`: Emit `log` records from all `tracing` spans and events, even
    //!   if a `tracing` subscriber has been set. This should be set only by
    //!   applications which intend to collect traces and logs separately; if an
    //!   adapter is used to convert `log` records into `tracing` events, this will
    //!   cause duplicate events to occur.
    //! * `attributes`: Includes support for the `#[instrument]` attribute.
    //!   This is on by default, but does bring in the `syn` crate as a dependency,
    //!   which may add to the compile time of crates that do not already use it.
    //! * `std`: Depend on the Rust standard library (enabled by default).
    //!
    //!   `no_std` users may disable this feature with `default-features = false`:
    //!
    //!   ```toml
    //!   [dependencies]
    //!   tracing = { version = "0.1.25", default-features = false }
    //!   ```
    //!
    //! <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>: <code>tracing</code>'s <code>no_std</code> support
    //! requires <code>liballoc</code>.
    //! </pre></div>
    //!
    //! ## Supported Rust Versions
    //!
    //! Tracing is built against the latest stable release. The minimum supported
    //! version is 1.42. The current Tracing version is not guaranteed to build on
    //! Rust versions earlier than the minimum supported version.
    //!
    //! Tracing follows the same compiler support policies as the rest of the Tokio
    //! project. The current stable Rust compiler and the three most recent minor
    //! versions before it will always be supported. For example, if the current
    //! stable compiler version is 1.45, the minimum supported version will not be
    //! increased past 1.42, three minor versions prior. Increasing the minimum
    //! supported compiler version is not considered a semver breaking change as
    //! long as doing so complies with this policy.
    //!
    //! [`log`]: https://docs.rs/log/0.4.6/log/
    //! [span]: mod@span
    //! [spans]: mod@span
    //! [`Span`]: span::Span
    //! [`in_scope`]: span::Span::in_scope
    //! [event]: Event
    //! [events]: Event
    //! [`Subscriber`]: subscriber::Subscriber
    //! [Subscriber::event]: subscriber::Subscriber::event
    //! [`enter`]: subscriber::Subscriber::enter
    //! [`exit`]: subscriber::Subscriber::exit
    //! [`enabled`]: subscriber::Subscriber::enabled
    //! [metadata]: Metadata
    //! [`field::display`]: field::display
    //! [`field::debug`]: field::debug
    //! [`set_global_default`]: subscriber::set_global_default
    //! [`with_default`]: subscriber::with_default
    //! [`tokio-rs/tracing`]: https://github.com/tokio-rs/tracing
    //! [`tracing-futures`]: https://crates.io/crates/tracing-futures
    //! [`tracing-subscriber`]: https://crates.io/crates/tracing-subscriber
    //! [`tracing-log`]: https://crates.io/crates/tracing-log
    //! [`tracing-timing`]: https://crates.io/crates/tracing-timing
    //! [`tracing-appender`]: https://crates.io/crates/tracing-appender
    //! [`env_logger`]: https://crates.io/crates/env_logger
    //! [`FmtSubscriber`]: https://docs.rs/tracing-subscriber/latest/tracing_subscriber/fmt/struct.Subscriber.html
    //! [static verbosity level]: level_filters/index.html#compile-time-filters
    //! [instrument]: https://docs.rs/tracing-attributes/latest/tracing_attributes/attr.instrument.html
    //! [flags]: #crate-feature-flags
    #![cfg_attr(not(feature = "std"), no_std)]
    #![cfg_attr(docsrs, feature(doc_cfg), deny(broken_intra_doc_links))]
    #![doc(html_root_url = "https://docs.rs/tracing/0.1.25")]
    #![doc(
        html_logo_url = "https://raw.githubusercontent.com/tokio-rs/tracing/master/assets/logo-type.png",
        issue_tracker_base_url = "https://github.com/tokio-rs/tracing/issues/"
    )]
    #![warn(
        missing_debug_implementations,
        missing_docs,
        rust_2018_idioms,
        unreachable_pub,
        bad_style,
        const_err,
        dead_code,
        improper_ctypes,
        non_shorthand_field_patterns,
        no_mangle_generic_items,
        overflowing_literals,
        path_statements,
        patterns_in_fns_without_body,
        private_in_public,
        unconditional_recursion,
        unused,
        unused_allocation,
        unused_comparisons,
        unused_parens,
        while_true
    )]

    #[cfg(not(feature = "std"))]
    extern crate alloc;

    #[macro_use]
    extern crate cfg_if;

    #[cfg(feature = "log")]
    #[doc(hidden)]
    pub use log;

    // Somehow this `use` statement is necessary for us to re-export the `core`
    // macros on Rust 1.26.0. I'm not sure how this makes it work, but it does.
    #[allow(unused_imports)]
    #[doc(hidden)]
    use tracing_core::*;

    #[doc(inline)]
    pub use self::instrument::Instrument;
    pub use self::{dispatcher::Dispatch, event::Event, field::Value, subscriber::Subscriber};

    #[doc(hidden)]
    pub use self::span::Id;

    #[doc(hidden)]
    pub use tracing_core::{
        callsite::{self, Callsite},
        metadata,
    };
    pub use tracing_core::{event, Level, Metadata};

    #[doc(inline)]
    pub use self::span::Span;
    #[cfg(feature = "attributes")]
    #[cfg_attr(docsrs, doc(cfg(feature = "attributes")))]
    #[doc(inline)]
    pub use tracing_attributes::instrument;

    #[macro_use]
    mod macros;

    pub mod dispatcher;
    pub mod field;
    /// Attach a span to a `std::future::Future`.
    pub mod instrument;
    pub mod level_filters;
    pub mod span;
    pub(crate) mod stdlib;
    pub mod subscriber;

    #[doc(hidden)]
    pub mod __macro_support {
        pub use crate::callsite::Callsite;
        use crate::stdlib::sync::atomic::{AtomicUsize, Ordering};
        use crate::{subscriber::Interest, Metadata};
        use tracing_core::Once;

        /// Callsite implementation used by macro-generated code.
        ///
        /// /!\ WARNING: This is *not* a stable API! /!\
        /// This type, and all code contained in the `__macro_support` module, is
        /// a *private* API of `tracing`. It is exposed publicly because it is used
        /// by the `tracing` macros, but it is not part of the stable versioned API.
        /// Breaking changes to this module may occur in small-numbered versions
        /// without warning.
        #[derive(Debug)]
        pub struct MacroCallsite {
            interest: AtomicUsize,
            meta: &'static Metadata<'static>,
            registration: Once,
        }

        impl MacroCallsite {
            /// Returns a new `MacroCallsite` with the specified `Metadata`.
            ///
            /// /!\ WARNING: This is *not* a stable API! /!\
            /// This method, and all code contained in the `__macro_support` module, is
            /// a *private* API of `tracing`. It is exposed publicly because it is used
            /// by the `tracing` macros, but it is not part of the stable versioned API.
            /// Breaking changes to this module may occur in small-numbered versions
            /// without warning.
            pub const fn new(meta: &'static Metadata<'static>) -> Self {
                Self {
                    interest: AtomicUsize::new(0xDEADFACED),
                    meta,
                    registration: Once::new(),
                }
            }

            /// Registers this callsite with the global callsite registry.
            ///
            /// If the callsite is already registered, this does nothing.
            ///
            /// /!\ WARNING: This is *not* a stable API! /!\
            /// This method, and all code contained in the `__macro_support` module, is
            /// a *private* API of `tracing`. It is exposed publicly because it is used
            /// by the `tracing` macros, but it is not part of the stable versioned API.
            /// Breaking changes to this module may occur in small-numbered versions
            /// without warning.
            #[inline(never)]
            // This only happens once (or if the cached interest value was corrupted).
            #[cold]
            pub fn register(&'static self) -> Interest {
                self.registration
                    .call_once(|| crate::callsite::register(self));
                match self.interest.load(Ordering::Relaxed) {
                    0 => Interest::never(),
                    2 => Interest::always(),
                    _ => Interest::sometimes(),
                }
            }

            /// Returns the callsite's cached Interest, or registers it for the
            /// first time if it has not yet been registered.
            ///
            /// /!\ WARNING: This is *not* a stable API! /!\
            /// This method, and all code contained in the `__macro_support` module, is
            /// a *private* API of `tracing`. It is exposed publicly because it is used
            /// by the `tracing` macros, but it is not part of the stable versioned API.
            /// Breaking changes to this module may occur in small-numbered versions
            /// without warning.
            #[inline]
            pub fn interest(&'static self) -> Interest {
                match self.interest.load(Ordering::Relaxed) {
                    0 => Interest::never(),
                    1 => Interest::sometimes(),
                    2 => Interest::always(),
                    _ => self.register(),
                }
            }

            pub fn is_enabled(&self, interest: Interest) -> bool {
                interest.is_always()
                    || crate::dispatcher::get_default(|default| default.enabled(self.meta))
            }

            #[inline]
            #[cfg(feature = "log")]
            pub fn disabled_span(&self) -> crate::Span {
                crate::Span::new_disabled(self.meta)
            }

            #[inline]
            #[cfg(not(feature = "log"))]
            pub fn disabled_span(&self) -> crate::Span {
                crate::Span::none()
            }
        }

        impl Callsite for MacroCallsite {
            fn set_interest(&self, interest: Interest) {
                let interest = match () {
                    _ if interest.is_never() => 0,
                    _ if interest.is_always() => 2,
                    _ => 1,
                };
                self.interest.store(interest, Ordering::SeqCst);
            }

            #[inline(always)]
            fn metadata(&self) -> &Metadata<'static> {
                &self.meta
            }
        }
    }

    mod sealed {
        pub trait Sealed {}
    }