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    1508

    //! Spans represent periods of time in which a program was executing in a
    //! particular context.
    //!
    //! A span consists of [fields], user-defined key-value pairs of arbitrary data
    //! that describe the context the span represents, and a set of fixed attributes
    //! that describe all `tracing` spans and events. Attributes describing spans
    //! include:
    //!
    //! - An [`Id`] assigned by the subscriber that uniquely identifies it in relation
    //!   to other spans.
    //! - The span's [parent] in the trace tree.
    //! - [Metadata] that describes static characteristics of all spans
    //!   originating from that callsite, such as its name, source code location,
    //!   [verbosity level], and the names of its fields.
    //!
    //! # Creating Spans
    //!
    //! Spans are created using the [`span!`] macro. This macro is invoked with the
    //! following arguments, in order:
    //!
    //! - The [`target`] and/or [`parent`][parent] attributes, if the user wishes to
    //!   override their default values.
    //! - The span's [verbosity level]
    //! - A string literal providing the span's name.
    //! - Finally, between zero and 32 arbitrary key/value fields.
    //!
    //! [`target`]: ../struct.Metadata.html#method.target
    //!
    //! For example:
    //! ```rust
    //! use tracing::{span, Level};
    //!
    //! /// Construct a new span at the `INFO` level named "my_span", with a single
    //! /// field named answer , with the value `42`.
    //! let my_span = span!(Level::INFO, "my_span", answer = 42);
    //! ```
    //!
    //! The documentation for the [`span!`] macro provides additional examples of
    //! the various options that exist when creating spans.
    //!
    //! The [`trace_span!`], [`debug_span!`], [`info_span!`], [`warn_span!`], and
    //! [`error_span!`] exist as shorthand for constructing spans at various
    //! verbosity levels.
    //!
    //! ## Recording Span Creation
    //!
    //! The [`Attributes`] type contains data associated with a span, and is
    //! provided to the [`Subscriber`] when a new span is created. It contains
    //! the span's metadata, the ID of [the span's parent][parent] if one was
    //! explicitly set, and any fields whose values were recorded when the span was
    //! constructed. The subscriber, which is responsible for recording `tracing`
    //! data, can then store or record these values.
    //!
    //! # The Span Lifecycle
    //!
    //! ## Entering a Span
    //!
    //! A thread of execution is said to _enter_ a span when it begins executing,
    //! and _exit_ the span when it switches to another context. Spans may be
    //! entered through the [`enter`], [`entered`], and [`in_scope`] methods.
    //!
    //! The [`enter`] method enters a span, returning a [guard] that exits the span
    //! when dropped
    //! ```
    //! # use tracing::{span, Level};
    //! let my_var: u64 = 5;
    //! let my_span = span!(Level::TRACE, "my_span", my_var);
    //!
    //! // `my_span` exists but has not been entered.
    //!
    //! // Enter `my_span`...
    //! let _enter = my_span.enter();
    //!
    //! // Perform some work inside of the context of `my_span`...
    //! // Dropping the `_enter` guard will exit the span.
    //!```
    //!
    //! <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="struct.Span.html#in-asynchronous-code">the method documentation</a>
    //!     for details.
    //! </pre></div>
    //!
    //! The [`entered`] method is analogous to [`enter`], but moves the span into
    //! the returned guard, rather than borrowing it. This allows creating and
    //! entering a span in a single expression:
    //!
    //! ```
    //! # use tracing::{span, Level};
    //! // Create a span and enter it, returning a guard:
    //! let span = span!(Level::INFO, "my_span").entered();
    //!
    //! // We are now inside the span! Like `enter()`, the guard returned by
    //! // `entered()` will exit the span when it is dropped...
    //!
    //! // ...but, it can also be exited explicitly, returning the `Span`
    //! // struct:
    //! let span = span.exit();
    //! ```
    //!
    //! Finally, [`in_scope`] takes a closure or function pointer and executes it
    //! inside the span:
    //!
    //! ```
    //! # use tracing::{span, Level};
    //! let my_var: u64 = 5;
    //! let my_span = span!(Level::TRACE, "my_span", my_var = &my_var);
    //!
    //! my_span.in_scope(|| {
    //!     // perform some work in the context of `my_span`...
    //! });
    //!
    //! // Perform some work outside of the context of `my_span`...
    //!
    //! my_span.in_scope(|| {
    //!     // Perform some more work in the context of `my_span`.
    //! });
    //! ```
    //!
    //! <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>: Since entering a span takes <code>&self</code<, and
    //! <code>Span</code>s are <code>Clone</code>, <code>Send</code>, and
    //! <code>Sync</code>, it is entirely valid for multiple threads to enter the
    //! same span concurrently.
    //! </pre></div>
    //!
    //! ## Span Relationships
    //!
    //! Spans form a tree structure — unless it is a root span, all spans have a
    //! _parent_, and may have one or more _children_. When a new span is created,
    //! the current span becomes the new span's parent. The total execution time of
    //! a span consists of the time spent in that span and in the entire subtree
    //! represented by its children. Thus, a parent span always lasts for at least
    //! as long as the longest-executing span in its subtree.
    //!
    //! ```
    //! # #[macro_use] extern crate tracing;
    //! # use tracing::Level;
    //! // this span is considered the "root" of a new trace tree:
    //! span!(Level::INFO, "root").in_scope(|| {
    //!     // since we are now inside "root", this span is considered a child
    //!     // of "root":
    //!     span!(Level::DEBUG, "outer_child").in_scope(|| {
    //!         // this span is a child of "outer_child", which is in turn a
    //!         // child of "root":
    //!         span!(Level::TRACE, "inner_child").in_scope(|| {
    //!             // and so on...
    //!         });
    //!     });
    //!     // another span created here would also be a child of "root".
    //! });
    //!```
    //!
    //! In addition, the parent of a span may be explicitly specified in
    //! the `span!` macro. For example:
    //!
    //! ```rust
    //! # #[macro_use] extern crate tracing;
    //! # use tracing::Level;
    //! // Create, but do not enter, a span called "foo".
    //! let foo = span!(Level::INFO, "foo");
    //!
    //! // Create and enter a span called "bar".
    //! let bar = span!(Level::INFO, "bar");
    //! let _enter = bar.enter();
    //!
    //! // Although we have currently entered "bar", "baz"'s parent span
    //! // will be "foo".
    //! let baz = span!(parent: &foo, Level::INFO, "baz");
    //! ```
    //!
    //! A child span should typically be considered _part_ of its parent. For
    //! example, if a subscriber is recording the length of time spent in various
    //! spans, it should generally include the time spent in a span's children as
    //! part of that span's duration.
    //!
    //! In addition to having zero or one parent, a span may also _follow from_ any
    //! number of other spans. This indicates a causal relationship between the span
    //! and the spans that it follows from, but a follower is *not* typically
    //! considered part of the duration of the span it follows. Unlike the parent, a
    //! span may record that it follows from another span after it is created, using
    //! the [`follows_from`] method.
    //!
    //! As an example, consider a listener task in a server. As the listener accepts
    //! incoming connections, it spawns new tasks that handle those connections. We
    //! might want to have a span representing the listener, and instrument each
    //! spawned handler task with its own span. We would want our instrumentation to
    //! record that the handler tasks were spawned as a result of the listener task.
    //! However, we might not consider the handler tasks to be _part_ of the time
    //! spent in the listener task, so we would not consider those spans children of
    //! the listener span. Instead, we would record that the handler tasks follow
    //! from the listener, recording the causal relationship but treating the spans
    //! as separate durations.
    //!
    //! ## Closing Spans
    //!
    //! Execution may enter and exit a span multiple times before that span is
    //! _closed_. Consider, for example, a future which has an associated
    //! span and enters that span every time it is polled:
    //! ```rust
    //! # extern crate tracing;
    //! # extern crate futures;
    //! # use futures::{Future, Poll, Async};
    //! struct MyFuture {
    //!    // data
    //!    span: tracing::Span,
    //! }
    //!
    //! impl Future for MyFuture {
    //!     type Item = ();
    //!     type Error = ();
    //!
    //!     fn poll(&mut self) -> Poll<Self::Item, Self::Error> {
    //!         let _enter = self.span.enter();
    //!         // Do actual future work...
    //! # Ok(Async::Ready(()))
    //!     }
    //! }
    //! ```
    //!
    //! If this future was spawned on an executor, it might yield one or more times
    //! before `poll` returns `Ok(Async::Ready)`. If the future were to yield, then
    //! the executor would move on to poll the next future, which may _also_ enter
    //! an associated span or series of spans. Therefore, it is valid for a span to
    //! be entered repeatedly before it completes. Only the time when that span or
    //! one of its children was the current span is considered to be time spent in
    //! that span. A span which is not executing and has not yet been closed is said
    //! to be _idle_.
    //!
    //! Because spans may be entered and exited multiple times before they close,
    //! [`Subscriber`]s have separate trait methods which are called to notify them
    //! of span exits and when span handles are dropped. When execution exits a
    //! span, [`exit`] will always be called with that span's ID to notify the
    //! subscriber that the span has been exited. When span handles are dropped, the
    //! [`drop_span`] method is called with that span's ID. The subscriber may use
    //! this to determine whether or not the span will be entered again.
    //!
    //! If there is only a single handle with the capacity to exit a span, dropping
    //! that handle "closes" the span, since the capacity to enter it no longer
    //! exists. For example:
    //! ```
    //! # #[macro_use] extern crate tracing;
    //! # use tracing::Level;
    //! {
    //!     span!(Level::TRACE, "my_span").in_scope(|| {
    //!         // perform some work in the context of `my_span`...
    //!     }); // --> Subscriber::exit(my_span)
    //!
    //!     // The handle to `my_span` only lives inside of this block; when it is
    //!     // dropped, the subscriber will be informed via `drop_span`.
    //!
    //! } // --> Subscriber::drop_span(my_span)
    //! ```
    //!
    //! However, if multiple handles exist, the span can still be re-entered even if
    //! one or more is dropped. For determining when _all_ handles to a span have
    //! been dropped, `Subscriber`s have a [`clone_span`] method, which is called
    //! every time a span handle is cloned. Combined with `drop_span`, this may be
    //! used to track the number of handles to a given span — if `drop_span` has
    //! been called one more time than the number of calls to `clone_span` for a
    //! given ID, then no more handles to the span with that ID exist. The
    //! subscriber may then treat it as closed.
    //!
    //! # When to use spans
    //!
    //! As a rule of thumb, spans should be used to represent discrete units of work
    //! (e.g., a given request's lifetime in a server) or periods of time spent in a
    //! given context (e.g., time spent interacting with an instance of an external
    //! system, such as a database).
    //!
    //! Which scopes in a program correspond to new spans depend somewhat on user
    //! intent. For example, consider the case of a loop in a program. Should we
    //! construct one span and perform the entire loop inside of that span, like:
    //!
    //! ```rust
    //! # #[macro_use] extern crate tracing;
    //! # use tracing::Level;
    //! # let n = 1;
    //! let span = span!(Level::TRACE, "my_loop");
    //! let _enter = span.enter();
    //! for i in 0..n {
    //!     # let _ = i;
    //!     // ...
    //! }
    //! ```
    //! Or, should we create a new span for each iteration of the loop, as in:
    //! ```rust
    //! # #[macro_use] extern crate tracing;
    //! # use tracing::Level;
    //! # let n = 1u64;
    //! for i in 0..n {
    //!     let span = span!(Level::TRACE, "my_loop", iteration = i);
    //!     let _enter = span.enter();
    //!     // ...
    //! }
    //! ```
    //!
    //! Depending on the circumstances, we might want to do either, or both. For
    //! example, if we want to know how long was spent in the loop overall, we would
    //! create a single span around the entire loop; whereas if we wanted to know how
    //! much time was spent in each individual iteration, we would enter a new span
    //! on every iteration.
    //!
    //! [fields]: ../field/index.html
    //! [Metadata]: ../struct.Metadata.html
    //! [`Id`]: struct.Id.html
    //! [verbosity level]: ../struct.Level.html
    //! [`span!`]: ../macro.span.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
    //! [`clone_span`]: ../subscriber/trait.Subscriber.html#method.clone_span
    //! [`drop_span`]: ../subscriber/trait.Subscriber.html#method.drop_span
    //! [`exit`]: ../subscriber/trait.Subscriber.html#tymethod.exit
    //! [`Subscriber`]: ../subscriber/trait.Subscriber.html
    //! [`Attributes`]: struct.Attributes.html
    //! [`enter`]: struct.Span.html#method.enter
    //! [`entered`]: struct.Span.html#method.entered
    //! [`in_scope`]: struct.Span.html#method.in_scope
    //! [`follows_from`]: struct.Span.html#method.follows_from
    //! [guard]: struct.Entered.html
    //! [parent]: #span-relationships
    pub use tracing_core::span::{Attributes, Id, Record};

    use crate::stdlib::{
        cmp, fmt,
        hash::{Hash, Hasher},
        marker::PhantomData,
        mem,
        ops::Deref,
    };
    use crate::{
        dispatcher::{self, Dispatch},
        field, Metadata,
    };

    /// Trait implemented by types which have a span `Id`.
    pub trait AsId: crate::sealed::Sealed {
        /// Returns the `Id` of the span that `self` corresponds to, or `None` if
        /// this corresponds to a disabled span.
        fn as_id(&self) -> Option<&Id>;
    }

    /// A handle representing a span, with the capability to enter the span if it
    /// exists.
    ///
    /// If the span was rejected by the current `Subscriber`'s filter, entering the
    /// span will silently do nothing. Thus, the handle can be used in the same
    /// manner regardless of whether or not the trace is currently being collected.
    #[derive(Clone)]
    pub struct Span {
        /// A handle used to enter the span when it is not executing.
        ///
        /// If this is `None`, then the span has either closed or was never enabled.
        inner: Option<Inner>,
        /// Metadata describing the span.
        ///
        /// This might be `Some` even if `inner` is `None`, in the case that the
        /// span is disabled but the metadata is needed for `log` support.
        meta: Option<&'static Metadata<'static>>,
    }

    /// A handle representing the capacity to enter a span which is known to exist.
    ///
    /// Unlike `Span`, this type is only constructed for spans which _have_ been
    /// enabled by the current filter. This type is primarily used for implementing
    /// span handles; users should typically not need to interact with it directly.
    #[derive(Debug)]
    pub(crate) struct Inner {
        /// The span's ID, as provided by `subscriber`.
        id: Id,

        /// The subscriber that will receive events relating to this span.
        ///
        /// This should be the same subscriber that provided this span with its
        /// `id`.
        subscriber: Dispatch,
    }

    /// A guard representing a span which has been entered and is currently
    /// executing.
    ///
    /// When the guard is dropped, the span will be exited.
    ///
    /// This is returned by the [`Span::enter`] function.
    ///
    /// [`Span::enter`]: ../struct.Span.html#method.enter
    #[derive(Debug)]
    #[must_use = "once a span has been entered, it should be exited"]
    pub struct Entered<'a> {
        span: &'a Span,
    }

    /// An owned version of [`Entered`], a guard representing a span which has been
    /// entered and is currently executing.
    ///
    /// When the guard is dropped, the span will be exited.
    ///
    /// This is returned by the [`Span::entered`] function.
    ///
    /// [`Span::entered`]: super::Span::entered()
    #[derive(Debug)]
    #[must_use = "once a span has been entered, it should be exited"]
    pub struct EnteredSpan {
        span: Span,

        /// ```compile_fail
        /// use tracing::span::*;
        /// trait AssertSend: Send {}
        ///
        /// impl AssertSend for EnteredSpan {}
        /// ```
        _not_send: PhantomNotSend,
    }

    /// `log` target for all span lifecycle (creation/enter/exit/close) records.
    #[cfg(feature = "log")]
    const LIFECYCLE_LOG_TARGET: &str = "tracing::span";
    /// `log` target for span activity (enter/exit) records.
    #[cfg(feature = "log")]
    const ACTIVITY_LOG_TARGET: &str = "tracing::span::active";

    // ===== impl Span =====

    impl Span {
        /// Constructs a new `Span` with the given [metadata] and set of
        /// [field values].
        ///
        /// The new span will be constructed by the currently-active [`Subscriber`],
        /// with the current span as its parent (if one exists).
        ///
        /// After the span is constructed, [field values] and/or [`follows_from`]
        /// annotations may be added to it.
        ///
        /// [metadata]: ../metadata
        /// [`Subscriber`]: ../subscriber/trait.Subscriber.html
        /// [field values]: ../field/struct.ValueSet.html
        /// [`follows_from`]: ../struct.Span.html#method.follows_from
        pub fn new(meta: &'static Metadata<'static>, values: &field::ValueSet<'_>) -> Span {
            dispatcher::get_default(|dispatch| Self::new_with(meta, values, dispatch))
        }

        #[inline]
        #[doc(hidden)]
        pub fn new_with(
            meta: &'static Metadata<'static>,
            values: &field::ValueSet<'_>,
            dispatch: &Dispatch,
        ) -> Span {
            let new_span = Attributes::new(meta, values);
            Self::make_with(meta, new_span, dispatch)
        }

        /// Constructs a new `Span` as the root of its own trace tree, with the
        /// given [metadata] and set of [field values].
        ///
        /// After the span is constructed, [field values] and/or [`follows_from`]
        /// annotations may be added to it.
        ///
        /// [metadata]: ../metadata
        /// [field values]: ../field/struct.ValueSet.html
        /// [`follows_from`]: ../struct.Span.html#method.follows_from
        pub fn new_root(meta: &'static Metadata<'static>, values: &field::ValueSet<'_>) -> Span {
            dispatcher::get_default(|dispatch| Self::new_root_with(meta, values, dispatch))
        }

        #[inline]
        #[doc(hidden)]
        pub fn new_root_with(
            meta: &'static Metadata<'static>,
            values: &field::ValueSet<'_>,
            dispatch: &Dispatch,
        ) -> Span {
            let new_span = Attributes::new_root(meta, values);
            Self::make_with(meta, new_span, dispatch)
        }

        /// Constructs a new `Span` as child of the given parent span, with the
        /// given [metadata] and set of [field values].
        ///
        /// After the span is constructed, [field values] and/or [`follows_from`]
        /// annotations may be added to it.
        ///
        /// [metadata]: ../metadata
        /// [field values]: ../field/struct.ValueSet.html
        /// [`follows_from`]: ../struct.Span.html#method.follows_from
        pub fn child_of(
            parent: impl Into<Option<Id>>,
            meta: &'static Metadata<'static>,
            values: &field::ValueSet<'_>,
        ) -> Span {
            let mut parent = parent.into();
            dispatcher::get_default(move |dispatch| {
                Self::child_of_with(Option::take(&mut parent), meta, values, dispatch)
            })
        }

        #[inline]
        #[doc(hidden)]
        pub fn child_of_with(
            parent: impl Into<Option<Id>>,
            meta: &'static Metadata<'static>,
            values: &field::ValueSet<'_>,
            dispatch: &Dispatch,
        ) -> Span {
            let new_span = match parent.into() {
                Some(parent) => Attributes::child_of(parent, meta, values),
                None => Attributes::new_root(meta, values),
            };
            Self::make_with(meta, new_span, dispatch)
        }

        /// Constructs a new disabled span with the given `Metadata`.
        ///
        /// This should be used when a span is constructed from a known callsite,
        /// but the subscriber indicates that it is disabled.
        ///
        /// Entering, exiting, and recording values on this span will not notify the
        /// `Subscriber` but _may_ record log messages if the `log` feature flag is
        /// enabled.
        #[inline(always)]
        pub fn new_disabled(meta: &'static Metadata<'static>) -> Span {
            Self {
                inner: None,
                meta: Some(meta),
            }
        }

        /// Constructs a new span that is *completely disabled*.
        ///
        /// This can be used rather than `Option<Span>` to represent cases where a
        /// span is not present.
        ///
        /// Entering, exiting, and recording values on this span will do nothing.
        #[inline(always)]
        pub const fn none() -> Span {
            Self {
                inner: None,
                meta: None,
            }
        }

        /// Returns a handle to the span [considered by the `Subscriber`] to be the
        /// current span.
        ///
        /// If the subscriber indicates that it does not track the current span, or
        /// that the thread from which this function is called is not currently
        /// inside a span, the returned span will be disabled.
        ///
        /// [considered by the `Subscriber`]: ../subscriber/trait.Subscriber.html#method.current
        pub fn current() -> Span {
            dispatcher::get_default(|dispatch| {
                if let Some((id, meta)) = dispatch.current_span().into_inner() {
                    let id = dispatch.clone_span(&id);
                    Self {
                        inner: Some(Inner::new(id, dispatch)),
                        meta: Some(meta),
                    }
                } else {
                    Self::none()
                }
            })
        }

        fn make_with(
            meta: &'static Metadata<'static>,
            new_span: Attributes<'_>,
            dispatch: &Dispatch,
        ) -> Span {
            let attrs = &new_span;
            let id = dispatch.new_span(attrs);
            let inner = Some(Inner::new(id, dispatch));

            let span = Self {
                inner,
                meta: Some(meta),
            };

            if_log_enabled! { *meta.level(), {
                let target = if attrs.is_empty() {
                    LIFECYCLE_LOG_TARGET
                } else {
                    meta.target()
                };
                span.log(target, level_to_log!(*meta.level()), format_args!("++ {}{}", meta.name(), FmtAttrs(attrs)));
            }}

            span
        }

        /// Enters this span, returning a guard that will exit the span when dropped.
        ///
        /// If this span is enabled by the current subscriber, then this function will
        /// call [`Subscriber::enter`] with the span's [`Id`], and dropping the guard
        /// will call [`Subscriber::exit`]. If the span is disabled, this does
        /// nothing.
        ///
        /// # In Asynchronous Code
        ///
        /// **Warning**: in asynchronous code that uses [async/await syntax][syntax],
        /// `Span::enter` should be used very carefully or avoided entirely. Holding
        /// the drop guard returned by `Span::enter` across `.await` points will
        /// result in incorrect traces.
        ///
        /// For example,
        ///
        /// ```
        /// # use tracing::info_span;
        /// # async fn some_other_async_function() {}
        /// async fn my_async_function() {
        ///     let span = info_span!("my_async_function");
        ///
        ///     // THIS WILL RESULT IN INCORRECT TRACES
        ///     let _enter = span.enter();
        ///     some_other_async_function().await;
        ///
        ///     // ...
        /// }
        /// ```
        ///
        /// The drop guard returned by `Span::enter` exits the span when it is
        /// dropped. When an async function or async block yields at an `.await`
        /// point, the current scope is _exited_, but values in that scope are
        /// **not** dropped (because the async block will eventually resume
        /// execution from that await point). This means that _another_ task will
        /// begin executing while _remaining_ in the entered span. This results in
        /// an incorrect trace.
        ///
        /// Instead of using `Span::enter` in asynchronous code, prefer the
        /// following:
        ///
        /// * To enter a span for a synchronous section of code within an async
        ///   block or function, prefer [`Span::in_scope`]. Since `in_scope` takes a
        ///   synchronous closure and exits the span when the closure returns, the
        ///   span will always be exited before the next await point. For example:
        ///   ```
        ///   # use tracing::info_span;
        ///   # async fn some_other_async_function(_: ()) {}
        ///   async fn my_async_function() {
        ///       let span = info_span!("my_async_function");
        ///
        ///       let some_value = span.in_scope(|| {
        ///           // run some synchronous code inside the span...
        ///       });
        ///
        ///       // This is okay! The span has already been exited before we reach
        ///       // the await point.
        ///       some_other_async_function(some_value).await;
        ///
        ///       // ...
        ///   }
        ///   ```
        /// * For instrumenting asynchronous code, `tracing` provides the
        ///   [`Future::instrument` combinator][instrument] for
        ///   attaching a span to a future (async function or block). This will
        ///   enter the span _every_ time the future is polled, and exit it whenever
        ///   the future yields.
        ///
        ///   `Instrument` can be used with an async block inside an async function:
        ///   ```ignore
        ///   # use tracing::info_span;
        ///   use tracing::Instrument;
        ///
        ///   # async fn some_other_async_function() {}
        ///   async fn my_async_function() {
        ///       let span = info_span!("my_async_function");
        ///       async move {
        ///          // This is correct! If we yield here, the span will be exited,
        ///          // and re-entered when we resume.
        ///          some_other_async_function().await;
        ///
        ///          //more asynchronous code inside the span...
        ///
        ///       }
        ///         // instrument the async block with the span...
        ///         .instrument(span)
        ///         // ...and await it.
        ///         .await
        ///   }
        ///   ```
        ///
        ///   It can also be used to instrument calls to async functions at the
        ///   callsite:
        ///   ```ignore
        ///   # use tracing::debug_span;
        ///   use tracing::Instrument;
        ///
        ///   # async fn some_other_async_function() {}
        ///   async fn my_async_function() {
        ///       let some_value = some_other_async_function()
        ///          .instrument(debug_span!("some_other_async_function"))
        ///          .await;
        ///
        ///       // ...
        ///   }
        ///   ```
        ///
        /// * The [`#[instrument]` attribute macro][attr] can automatically generate
        ///   correct code when used on an async function:
        ///
        ///   ```ignore
        ///   # async fn some_other_async_function() {}
        ///   #[tracing::instrument(level = "info")]
        ///   async fn my_async_function() {
        ///
        ///       // This is correct! If we yield here, the span will be exited,
        ///       // and re-entered when we resume.
        ///       some_other_async_function().await;
        ///
        ///       // ...
        ///
        ///   }
        ///   ```
        ///
        /// [syntax]: https://rust-lang.github.io/async-book/01_getting_started/04_async_await_primer.html
        /// [`Span::in_scope`]: #method.in_scope
        /// [instrument]: https://docs.rs/tracing/latest/tracing/trait.Instrument.html
        /// [attr]: ../../attr.instrument.html
        ///
        /// # Examples
        ///
        /// ```
        /// #[macro_use] extern crate tracing;
        /// # use tracing::Level;
        /// let span = span!(Level::INFO, "my_span");
        /// let guard = span.enter();
        ///
        /// // code here is within the span
        ///
        /// drop(guard);
        ///
        /// // code here is no longer within the span
        ///
        /// ```
        ///
        /// Guards need not be explicitly dropped:
        ///
        /// ```
        /// #[macro_use] extern crate tracing;
        /// fn my_function() -> String {
        ///     // enter a span for the duration of this function.
        ///     let span = trace_span!("my_function");
        ///     let _enter = span.enter();
        ///
        ///     // anything happening in functions we call is still inside the span...
        ///     my_other_function();
        ///
        ///     // returning from the function drops the guard, exiting the span.
        ///     return "Hello world".to_owned();
        /// }
        ///
        /// fn my_other_function() {
        ///     // ...
        /// }
        /// ```
        ///
        /// Sub-scopes may be created to limit the duration for which the span is
        /// entered:
        ///
        /// ```
        /// #[macro_use] extern crate tracing;
        /// let span = info_span!("my_great_span");
        ///
        /// {
        ///     let _enter = span.enter();
        ///
        ///     // this event occurs inside the span.
        ///     info!("i'm in the span!");
        ///
        ///     // exiting the scope drops the guard, exiting the span.
        /// }
        ///
        /// // this event is not inside the span.
        /// info!("i'm outside the span!")
        /// ```
        ///
        /// [`Subscriber::enter`]: ../subscriber/trait.Subscriber.html#method.enter
        /// [`Subscriber::exit`]: ../subscriber/trait.Subscriber.html#method.exit
        /// [`Id`]: ../struct.Id.html
        #[inline]
        pub fn enter(&self) -> Entered<'_> {
            self.do_enter();
            Entered { span: self }
        }

        /// Enters this span, consuming it and returning a [guard][`EnteredSpan`]
        /// that will exit the span when dropped.
        ///
        /// If this span is enabled by the current subscriber, then this function will
        /// call [`Subscriber::enter`] with the span's [`Id`], and dropping the guard
        /// will call [`Subscriber::exit`]. If the span is disabled, this does
        /// nothing.
        ///
        /// This is similar to the [`Span::enter`] method, except that it moves the
        /// span by value into the returned guard, rather than borrowing it.
        /// Therefore, this method can be used to create and enter a span in a
        /// single expression, without requiring a `let`-binding. For example:
        ///
        /// ```
        /// # use tracing::info_span;
        /// let _span = info_span!("something_interesting").entered();
        /// ```
        /// rather than:
        /// ```
        /// # use tracing::info_span;
        /// let span = info_span!("something_interesting");
        /// let _e = span.enter();
        /// ```
        ///
        /// Furthermore, `entered` may be used when the span must be stored in some
        /// other struct or be passed to a function while remaining entered.
        ///
        /// <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;">
        ///
        /// **Note**: The returned [`EnteredSpan`] guard does not
        /// implement `Send`. Dropping the guard will exit *this* span,
        /// and if the guard is sent to another thread and dropped there, that thread may
        /// never have entered this span. Thus, `EnteredSpan`s should not be sent
        /// between threads.
        ///
        /// </pre></div>
        ///
        /// **Warning**: in asynchronous code that uses [async/await syntax][syntax],
        /// [`Span::entered`] should be used very carefully or avoided entirely. Holding
        /// the drop guard returned by `Span::entered` across `.await` points will
        /// result in incorrect traces. See the documentation for the
        /// [`Span::enter`] method for details.
        ///
        /// [syntax]: https://rust-lang.github.io/async-book/01_getting_started/04_async_await_primer.html
        ///
        /// # Examples
        ///
        /// The returned guard can be [explicitly exited][EnteredSpan::exit],
        /// returning the un-entered span:
        ///
        /// ```
        /// # use tracing::{Level, span};
        /// let span = span!(Level::INFO, "doing_something").entered();
        ///
        /// // code here is within the span
        ///
        /// // explicitly exit the span, returning it
        /// let span = span.exit();
        ///
        /// // code here is no longer within the span
        ///
        /// // enter the span again
        /// let span = span.entered();
        ///
        /// // now we are inside the span once again
        /// ```
        ///
        /// Guards need not be explicitly dropped:
        ///
        /// ```
        /// # use tracing::trace_span;
        /// fn my_function() -> String {
        ///     // enter a span for the duration of this function.
        ///     let span = trace_span!("my_function").entered();
        ///
        ///     // anything happening in functions we call is still inside the span...
        ///     my_other_function();
        ///
        ///     // returning from the function drops the guard, exiting the span.
        ///     return "Hello world".to_owned();
        /// }
        ///
        /// fn my_other_function() {
        ///     // ...
        /// }
        /// ```
        ///
        /// Since the [`EnteredSpan`] guard can dereference to the [`Span`] itself,
        /// the span may still be accessed while entered. For example:
        ///
        /// ```rust
        /// # use tracing::info_span;
        /// use tracing::field;
        ///
        /// // create the span with an empty field, and enter it.
        /// let span = info_span!("my_span", some_field = field::Empty).entered();
        ///
        /// // we can still record a value for the field while the span is entered.
        /// span.record("some_field", &"hello world!");
        /// ```
        ///
        /// [`Subscriber::enter`]: ../subscriber/trait.Subscriber.html#method.enter
        /// [`Subscriber::exit`]: ../subscriber/trait.Subscriber.html#method.exit
        /// [`Id`]: ../struct.Id.html
        #[inline]
        pub fn entered(self) -> EnteredSpan {
            self.do_enter();
            EnteredSpan {
                span: self,
                _not_send: PhantomNotSend,
            }
        }

        #[inline]
        fn do_enter(&self) {
            if let Some(inner) = self.inner.as_ref() {
                inner.subscriber.enter(&inner.id);
            }

            if_log_enabled! { crate::Level::TRACE, {
                if let Some(ref meta) = self.meta {
                    self.log(ACTIVITY_LOG_TARGET, log::Level::Trace, format_args!("-> {}", meta.name()));
                }
            }}
        }

        // Called from [`Entered`] and [`EnteredSpan`] drops.
        //
        // Running this behaviour on drop rather than with an explicit function
        // call means that spans may still be exited when unwinding.
        #[inline]
        fn do_exit(&self) {
            if let Some(inner) = self.inner.as_ref() {
                inner.subscriber.exit(&inner.id);
            }

            if_log_enabled! { crate::Level::TRACE, {
                if let Some(ref _meta) = self.meta {
                    self.log(ACTIVITY_LOG_TARGET, log::Level::Trace, format_args!("<- {}", _meta.name()));
                }
            }}
        }

        /// Executes the given function in the context of this span.
        ///
        /// If this span is enabled, then this function enters the span, invokes `f`
        /// and then exits the span. If the span is disabled, `f` will still be
        /// invoked, but in the context of the currently-executing span (if there is
        /// one).
        ///
        /// Returns the result of evaluating `f`.
        ///
        /// # Examples
        ///
        /// ```
        /// # #[macro_use] extern crate tracing;
        /// # use tracing::Level;
        /// let my_span = span!(Level::TRACE, "my_span");
        ///
        /// my_span.in_scope(|| {
        ///     // this event occurs within the span.
        ///     trace!("i'm in the span!");
        /// });
        ///
        /// // this event occurs outside the span.
        /// trace!("i'm not in the span!");
        /// ```
        ///
        /// Calling a function and returning the result:
        /// ```
        /// # use tracing::{info_span, Level};
        /// fn hello_world() -> String {
        ///     "Hello world!".to_owned()
        /// }
        ///
        /// let span = info_span!("hello_world");
        /// // the span will be entered for the duration of the call to
        /// // `hello_world`.
        /// let a_string = span.in_scope(hello_world);
        ///
        pub fn in_scope<F: FnOnce() -> T, T>(&self, f: F) -> T {
            let _enter = self.enter();
            f()
        }

        /// Returns a [`Field`](../field/struct.Field.html) for the field with the
        /// given `name`, if one exists,
        pub fn field<Q: ?Sized>(&self, field: &Q) -> Option<field::Field>
        where
            Q: field::AsField,
        {
            self.metadata().and_then(|meta| field.as_field(meta))
        }

        /// Returns true if this `Span` has a field for the given
        /// [`Field`](../field/struct.Field.html) or field name.
        #[inline]
        pub fn has_field<Q: ?Sized>(&self, field: &Q) -> bool
        where
            Q: field::AsField,
        {
            self.field(field).is_some()
        }

        /// Records that the field described by `field` has the value `value`.
        ///
        /// This may be used with [`field::Empty`] to declare fields whose values
        /// are not known when the span is created, and record them later:
        /// ```
        /// 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.
        /// // (note that the field name is passed as a string slice)
        /// span.record("parting", &"goodbye world!");
        /// ```
        /// However, it may also be used to record a _new_ value for a field whose
        /// value was already recorded:
        /// ```
        /// use tracing::info_span;
        /// # fn do_something() -> Result<(), ()> { Err(()) }
        ///
        /// // Initially, let's assume that our attempt to do something is going okay...
        /// let span = info_span!("doing_something", is_okay = true);
        /// let _e = span.enter();
        ///
        /// match do_something() {
        ///     Ok(something) => {
        ///         // ...
        ///     }
        ///     Err(_) => {
        ///         // Things are no longer okay!
        ///         span.record("is_okay", &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>: The fields associated with a span are part of its
        /// <a href="../struct.Metadata.html"><code>Metadata</code></a>.
        /// The <a href="../struct.Metadata.html"><code>Metadata</code></a>. describing a particular
        /// span is constructed statically when the span is created and cannot be extended later to
        /// add new fields. Therefore, you cannot record a value for a field that was not specified
        /// when the span was created:</pre></div>
        ///
        /// ```
        /// 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, you try to record a value for a new field, `new_field`, which was not
        /// // declared as `Empty` or populated when you created `span`.
        /// // You won't get any error, but the assignment will have no effect!
        /// span.record("new_field", &"interesting_value_you_really_need");
        ///
        /// // Instead, all fields that may be recorded after span creation should be declared up front,
        /// // using field::Empty when a value is not known, as we did for `parting`.
        /// // This `record` call will indeed replace field::Empty with "you will be remembered".
        /// span.record("parting", &"you will be remembered");
        /// ```
        ///
        /// [`field::Empty`]: ../field/struct.Empty.html
        /// [`Metadata`]: ../struct.Metadata.html
        pub fn record<Q: ?Sized, V>(&self, field: &Q, value: &V) -> &Self
        where
            Q: field::AsField,
            V: field::Value,
        {
            if let Some(ref meta) = self.meta {
                if let Some(field) = field.as_field(meta) {
                    self.record_all(
                        &meta
                            .fields()
                            .value_set(&[(&field, Some(value as &dyn field::Value))]),
                    );
                }
            }

            self
        }

        /// Records all the fields in the provided `ValueSet`.
        pub fn record_all(&self, values: &field::ValueSet<'_>) -> &Self {
            let record = Record::new(values);
            if let Some(ref inner) = self.inner {
                inner.record(&record);
            }

            if let Some(ref _meta) = self.meta {
                if_log_enabled! { *_meta.level(), {
                    let target = if record.is_empty() {
                        LIFECYCLE_LOG_TARGET
                    } else {
                        _meta.target()
                    };
                    self.log(target, level_to_log!(*_meta.level()), format_args!("{}{}", _meta.name(), FmtValues(&record)));
                }}
            }

            self
        }

        /// Returns `true` if this span was disabled by the subscriber and does not
        /// exist.
        ///
        /// See also [`is_none`].
        ///
        /// [`is_none`]: #method.is_none
        #[inline]
        pub fn is_disabled(&self) -> bool {
            self.inner.is_none()
        }

        /// Returns `true` if this span was constructed by [`Span::none`] and is
        /// empty.
        ///
        /// If `is_none` returns `true` for a given span, then [`is_disabled`] will
        /// also return `true`. However, when a span is disabled by the subscriber
        /// rather than constructed by `Span::none`, this method will return
        /// `false`, while `is_disabled` will return `true`.
        ///
        /// [`Span::none`]: #method.none
        /// [`is_disabled`]: #method.is_disabled
        #[inline]
        pub fn is_none(&self) -> bool {
            self.is_disabled() && self.meta.is_none()
        }

        /// Indicates that the span with the given ID has an indirect causal
        /// relationship with this span.
        ///
        /// 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 this span is disabled, or the resulting follows-from relationship
        /// would be invalid, this function will do nothing.
        ///
        /// # Examples
        ///
        /// Setting a `follows_from` relationship with a `Span`:
        /// ```
        /// # use tracing::{span, Id, Level, Span};
        /// let span1 = span!(Level::INFO, "span_1");
        /// let span2 = span!(Level::DEBUG, "span_2");
        /// span2.follows_from(span1);
        /// ```
        ///
        /// Setting a `follows_from` relationship with the current span:
        /// ```
        /// # use tracing::{span, Id, Level, Span};
        /// let span = span!(Level::INFO, "hello!");
        /// span.follows_from(Span::current());
        /// ```
        ///
        /// Setting a `follows_from` relationship with a `Span` reference:
        /// ```
        /// # use tracing::{span, Id, Level, Span};
        /// let span = span!(Level::INFO, "hello!");
        /// let curr = Span::current();
        /// span.follows_from(&curr);
        /// ```
        ///
        /// Setting a `follows_from` relationship with an `Id`:
        /// ```
        /// # use tracing::{span, Id, Level, Span};
        /// let span = span!(Level::INFO, "hello!");
        /// let id = span.id();
        /// span.follows_from(id);
        /// ```
        pub fn follows_from(&self, from: impl Into<Option<Id>>) -> &Self {
            if let Some(ref inner) = self.inner {
                if let Some(from) = from.into() {
                    inner.follows_from(&from);
                }
            }
            self
        }

        /// Returns this span's `Id`, if it is enabled.
        pub fn id(&self) -> Option<Id> {
            self.inner.as_ref().map(Inner::id)
        }

        /// Returns this span's `Metadata`, if it is enabled.
        pub fn metadata(&self) -> Option<&'static Metadata<'static>> {
            self.meta
        }

        #[cfg(feature = "log")]
        #[inline]
        fn log(&self, target: &str, level: log::Level, message: fmt::Arguments<'_>) {
            if let Some(ref meta) = self.meta {
                if level_to_log!(*meta.level()) <= log::max_level() {
                    let logger = log::logger();
                    let log_meta = log::Metadata::builder().level(level).target(target).build();
                    if logger.enabled(&log_meta) {
                        if let Some(ref inner) = self.inner {
                            logger.log(
                                &log::Record::builder()
                                    .metadata(log_meta)
                                    .module_path(meta.module_path())
                                    .file(meta.file())
                                    .line(meta.line())
                                    .args(format_args!("{}; span={}", message, inner.id.into_u64()))
                                    .build(),
                            );
                        } else {
                            logger.log(
                                &log::Record::builder()
                                    .metadata(log_meta)
                                    .module_path(meta.module_path())
                                    .file(meta.file())
                                    .line(meta.line())
                                    .args(message)
                                    .build(),
                            );
                        }
                    }
                }
            }
        }

        /// Invokes a function with a reference to this span's ID and subscriber.
        ///
        /// if this span is enabled, the provided function is called, and the result is returned.
        /// If the span is disabled, the function is not called, and this method returns `None`
        /// instead.
        pub fn with_subscriber<T>(&self, f: impl FnOnce((&Id, &Dispatch)) -> T) -> Option<T> {
            self.inner
                .as_ref()
                .map(|inner| f((&inner.id, &inner.subscriber)))
        }
    }

    impl cmp::PartialEq for Span {
        fn eq(&self, other: &Self) -> bool {
            match (&self.meta, &other.meta) {
                (Some(this), Some(that)) => {
                    this.callsite() == that.callsite() && self.inner == other.inner
                }
                _ => false,
            }
        }
    }

    impl Hash for Span {
        fn hash<H: Hasher>(&self, hasher: &mut H) {
            self.inner.hash(hasher);
        }
    }

    impl fmt::Debug for Span {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            let mut span = f.debug_struct("Span");
            if let Some(ref meta) = self.meta {
                span.field("name", &meta.name())
                    .field("level", &meta.level())
                    .field("target", &meta.target());

                if let Some(ref inner) = self.inner {
                    span.field("id", &inner.id());
                } else {
                    span.field("disabled", &true);
                }

                if let Some(ref path) = meta.module_path() {
                    span.field("module_path", &path);
                }

                if let Some(ref line) = meta.line() {
                    span.field("line", &line);
                }

                if let Some(ref file) = meta.file() {
                    span.field("file", &file);
                }
            } else {
                span.field("none", &true);
            }

            span.finish()
        }
    }

    impl<'a> Into<Option<&'a Id>> for &'a Span {
        fn into(self) -> Option<&'a Id> {
            self.inner.as_ref().map(|inner| &inner.id)
        }
    }

    impl<'a> Into<Option<Id>> for &'a Span {
        fn into(self) -> Option<Id> {
            self.inner.as_ref().map(Inner::id)
        }
    }

    impl Into<Option<Id>> for Span {
        fn into(self) -> Option<Id> {
            self.inner.as_ref().map(Inner::id)
        }
    }

    impl Drop for Span {
        fn drop(&mut self) {
            if let Some(Inner {
                ref id,
                ref subscriber,
            }) = self.inner
            {
                subscriber.try_close(id.clone());
            }

            if let Some(ref _meta) = self.meta {
                if_log_enabled! { crate::Level::TRACE, {
                    self.log(
                        LIFECYCLE_LOG_TARGET,
                        log::Level::Trace,
                        format_args!("-- {}", _meta.name()),
                    );
                }}
            }
        }
    }

    // ===== impl Inner =====

    impl Inner {
        /// Indicates that the span with the given ID has an indirect causal
        /// relationship with this span.
        ///
        /// 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 this span is disabled, this function will do nothing. Otherwise, it
        /// returns `Ok(())` if the other span was added as a precedent of this
        /// span, or an error if this was not possible.
        fn follows_from(&self, from: &Id) {
            self.subscriber.record_follows_from(&self.id, &from)
        }

        /// Returns the span's ID.
        fn id(&self) -> Id {
            self.id.clone()
        }

        fn record(&self, values: &Record<'_>) {
            self.subscriber.record(&self.id, values)
        }

        fn new(id: Id, subscriber: &Dispatch) -> Self {
            Inner {
                id,
                subscriber: subscriber.clone(),
            }
        }
    }

    impl cmp::PartialEq for Inner {
        fn eq(&self, other: &Self) -> bool {
            self.id == other.id
        }
    }

    impl Hash for Inner {
        fn hash<H: Hasher>(&self, state: &mut H) {
            self.id.hash(state);
        }
    }

    impl Clone for Inner {
        fn clone(&self) -> Self {
            Inner {
                id: self.subscriber.clone_span(&self.id),
                subscriber: self.subscriber.clone(),
            }
        }
    }

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

    impl EnteredSpan {
        /// Exits this span, returning the underlying [`Span`].
        #[inline]
        pub fn exit(mut self) -> Span {
            // One does not simply move out of a struct with `Drop`.
            let span = mem::replace(&mut self.span, Span::none());
            span.do_exit();
            span
        }
    }

    impl Deref for EnteredSpan {
        type Target = Span;

        #[inline]
        fn deref(&self) -> &Span {
            &self.span
        }
    }

    impl<'a> Drop for Entered<'a> {
        #[inline]
        fn drop(&mut self) {
            self.span.do_exit()
        }
    }

    impl Drop for EnteredSpan {
        #[inline]
        fn drop(&mut self) {
            self.span.do_exit()
        }
    }

    /// Technically, `EnteredSpan` _can_ implement both `Send` *and*
    /// `Sync` safely. It doesn't, because it has a `PhantomNotSend` field,
    /// specifically added in order to make it `!Send`.
    ///
    /// Sending an `EnteredSpan` guard between threads cannot cause memory unsafety.
    /// However, it *would* result in incorrect behavior, so we add a
    /// `PhantomNotSend` to prevent it from being sent between threads. This is
    /// because it must be *dropped* on the same thread that it was created;
    /// otherwise, the span will never be exited on the thread where it was entered,
    /// and it will attempt to exit the span on a thread that may never have entered
    /// it. However, we still want them to be `Sync` so that a struct holding an
    /// `Entered` guard can be `Sync`.
    ///
    /// Thus, this is totally safe.
    #[derive(Debug)]
    struct PhantomNotSend {
        ghost: PhantomData<*mut ()>,
    }
    #[allow(non_upper_case_globals)]
    const PhantomNotSend: PhantomNotSend = PhantomNotSend { ghost: PhantomData };

    /// # Safety
    ///
    /// Trivially safe, as `PhantomNotSend` doesn't have any API.
    unsafe impl Sync for PhantomNotSend {}

    #[cfg(feature = "log")]
    struct FmtValues<'a>(&'a Record<'a>);

    #[cfg(feature = "log")]
    impl<'a> fmt::Display for FmtValues<'a> {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            let mut res = Ok(());
            let mut is_first = true;
            self.0.record(&mut |k: &field::Field, v: &dyn fmt::Debug| {
                res = write!(f, "{} {}={:?}", if is_first { ";" } else { "" }, k, v);
                is_first = false;
            });
            res
        }
    }

    #[cfg(feature = "log")]
    struct FmtAttrs<'a>(&'a Attributes<'a>);

    #[cfg(feature = "log")]
    impl<'a> fmt::Display for FmtAttrs<'a> {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            let mut res = Ok(());
            let mut is_first = true;
            self.0.record(&mut |k: &field::Field, v: &dyn fmt::Debug| {
                res = write!(f, "{} {}={:?}", if is_first { ";" } else { "" }, k, v);
                is_first = false;
            });
            res
        }
    }

    #[cfg(test)]
    mod test {
        use super::*;

        trait AssertSend: Send {}
        impl AssertSend for Span {}

        trait AssertSync: Sync {}
        impl AssertSync for Span {}
        impl AssertSync for Entered<'_> {}
        impl AssertSync for EnteredSpan {}
    }