tracing/

lib.rs

//! 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` 1.65+][msrv]*
//!
//! [msrv]: #supported-rust-versions
//! # Core Concepts
//!
//! The core of `tracing`'s API is composed of _spans_, _events_ and
//! _collectors_. 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][mod@span]'s documentation provides further details on how to
//! use spans.
//!
//! <div class="example-wrap" style="display:inline-block"><pre class="compile_fail" style="white-space:normal;font:inherit;">
//!
//!  **Warning**: In asynchronous code that uses async/await syntax,
//!  `Span::enter` may produce incorrect traces if the returned drop
//!  guard is held across an await point. See
//!  [the method documentation][Span#in-asynchronous-code] 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.
//!
//! ## Collectors
//!
//! As `Span`s and `Event`s occur, they are recorded or aggregated by
//! implementations of the [`Collect`] trait. Collectors are notified
//! when an `Event` takes place and when a `Span` is entered or exited. These
//! notifications are represented by the following `Collect` trait methods:
//!
//! + [`event`][Collect::event], called when an `Event` takes place,
//! + [`enter`], called when execution enters a `Span`,
//! + [`exit`], called when execution exits a `Span`
//!
//! In addition, collectors may implement the [`enabled`] function to _filter_
//! the notifications they receive based on [metadata] describing each `Span`
//! or `Event`. If a call to `Collect::enabled` returns `false` for a given
//! set of metadata, that collector will *not* be notified about the
//! corresponding `Span` or `Event`. For performance reasons, if no currently
//! active collectors 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"
//! ```
//!
//! ## 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() {}
//! ```
//!
//! For functions which don't have built-in tracing support and can't have
//! the `#[instrument]` attribute applied (such as from an external crate),
//! the [`Span` struct][`Span`] has a [`in_scope()` method][`in_scope`]
//! which can be used to easily wrap synchronous code in a span.
//!
//! For example:
//! ```rust
//! use tracing::info_span;
//!
//! # fn doc() -> Result<(), ()> {
//! # mod serde_json {
//! #    pub(crate) fn from_slice(buf: &[u8]) -> Result<(), ()> { Ok(()) }
//! # }
//! # let buf: [u8; 0] = [];
//! let json = info_span!("json.parse").in_scope(|| serde_json::from_slice(&buf))?;
//! # let _ = json; // suppress unused variable warning
//! # Ok(())
//! # }
//! ```
//!
//! You can find more examples showing how to use this crate [here][examples].
//!
//! [RAII]: https://github.com/rust-unofficial/patterns/blob/main/src/patterns/behavioural/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 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 collector), 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 (as above).  In the case of the event macros, the name of the event can
//! be overridden (the default is `event file:line`) using the `name:` specifier.
//!
//! ```
//! # use tracing::{span, event, Level};
//! # fn main() {
//! span!(Level::TRACE, "my span");
//! event!(name: "some_info", Level::INFO, "something has happened!");
//! # }
//! ```
//!
//! ### 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");
//! # }
//!```
//!
//! Constant expressions can also be used as field names. Constants
//! must be enclosed in curly braces (`{}`) to indicate that the *value*
//! of the constant is to be used as the field name, rather than the
//! constant's name. For example:
//! ```
//! # use tracing::{span, Level};
//! # fn main() {
//! const RESOURCE_NAME: &str = "foo";
//! // this span will have the field `foo = "some_id"`
//! span!(Level::TRACE, "get", { RESOURCE_NAME } = "some_id");
//! # }
//!```
//!
//! 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!");
//! ```
//!
//! 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 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]: Metadata::target()
//! [parent span]: span::Attributes::parent()
//! [determined contextually]: span::Attributes::is_contextual()
//! [`fmt::Debug`]: std::fmt::Debug
//! [`fmt::Display`]: std::fmt::Display
//! [fmt]: std::fmt#usage
//! [`Empty`]: field::Empty
//!
//! ### 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!`]: span!
//! [`event!`]: event!
//! [`trace!`]: trace!
//! [`debug!`]: debug!
//! [`info!`]: info!
//! [`warn!`]: warn!
//! [`error!`]: error!
//! [`trace_span!`]: trace_span!
//! [`debug_span!`]: debug_span!
//! [`info_span!`]: info_span!
//! [`warn_span!`]: warn_span!
//! [`error_span!`]: error_span!
//!
//! ### 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 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);
//!     let _enter = span.enter();
//!
//!     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 initialized.
//!             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 collector
//! implementation compatible with `tracing`. A collector implements a
//! way of collecting trace data, such as by logging it to standard output.
//!
//! This library does not contain any `Collect` implementations; these are
//! provided by [other crates](#related-crates).
//!
//! The simplest way to use a collector is to call the [`set_global_default`]
//! function:
//!
//! ```
//! # pub struct FooCollector;
//! # use tracing::{span::{Id, Attributes, Record}, Metadata};
//! # use tracing_core::span::Current;
//! # impl tracing::Collect for FooCollector {
//! #   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) {}
//! #   fn current_span(&self) -> Current { Current::unknown() }
//! # }
//! # impl FooCollector {
//! #   fn new() -> Self { FooCollector }
//! # }
//! # fn main() {
//!
//! # #[cfg(feature = "alloc")]
//! let my_collector = FooCollector::new();
//! # #[cfg(feature = "alloc")]
//! tracing::collect::set_global_default(my_collector)
//!     .expect("setting tracing default failed");
//! # }
//! ```
//!
//! <div class="information">
//! </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 collector 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 collector 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 FooCollector;
//! # use tracing::{span::{Id, Attributes, Record}, Metadata};
//! # use tracing_core::span::Current;
//! # impl tracing::Collect for FooCollector {
//! #   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) {}
//! #   fn current_span(&self) -> Current { Current::unknown() }
//! # }
//! # impl FooCollector {
//! #   fn new() -> Self { FooCollector }
//! # }
//! # fn main() {
//!
//! let my_collector = FooCollector::new();
//! # #[cfg(feature = "std")]
//! tracing::collect::with_default(my_collector, || {
//!     // Any trace events generated in this closure or by functions it calls
//!     // will be collected by `my_collector`.
//! })
//! # }
//! ```
//!
//! This approach allows trace data to be collected by multiple collectors
//! 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 collector will not be collected.
//!
//! Once a collector 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` [`Collect`]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` collector 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` collector _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 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` collector 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.