Struct tracing::span::EnteredSpan

#[must_use = "once a span has been entered, it should be exited"]pub struct EnteredSpan { /* fields omitted */ }

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.

Implementations

impl EnteredSpan

pub fn exit(self) -> Span

Exits this span, returning the underlying Span.

Methods from Deref<Target = Span>

pub fn enter(&self) -> Entered<'_>

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, 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,

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:

  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 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:

  use tracing::Instrument;
  
  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:

  use tracing::Instrument;
  
  async fn my_async_function() {
      let some_value = some_other_async_function()
         .instrument(debug_span!("some_other_async_function"))
         .await;
  
      // ...
  }

• The #[instrument] attribute macro can automatically generate correct code when used on an 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;
  
      // ...
    
  }

Examples

#[macro_use] extern crate tracing;
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!")

pub fn in_scope<F: FnOnce() -> T, T>(&self, f: F) -> T

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

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:

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 field<Q: ?Sized>(&self, field: &Q) -> Option<Field> where
    Q: AsField, 

Returns a Field for the field with the given name, if one exists,

pub fn has_field<Q: ?Sized>(&self, field: &Q) -> bool where
    Q: AsField, 

Returns true if this Span has a field for the given Field or field name.

pub fn record<Q: ?Sized, V>(&self, field: &Q, value: &V) -> &Self where
    Q: AsField,
    V: Value, 

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;

// 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);
    }
}

Note: The fields associated with a span are part of its
Metadata.
The Metadata. 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:

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");

pub fn record_all(&self, values: &ValueSet<'_>) -> &Self

Records all the fields in the provided ValueSet.

pub fn is_disabled(&self) -> bool

Returns true if this span was disabled by the subscriber and does not exist.

See also is_none.

pub fn is_none(&self) -> bool

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.

pub fn follows_from(&self, from: impl Into<Option<Id>>) -> &Self

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:

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:

let span = span!(Level::INFO, "hello!");
span.follows_from(Span::current());

Setting a follows_from relationship with a Span reference:

let span = span!(Level::INFO, "hello!");
let curr = Span::current();
span.follows_from(&curr);

Setting a follows_from relationship with an Id:

let span = span!(Level::INFO, "hello!");
let id = span.id();
span.follows_from(id);

pub fn id(&self) -> Option<Id>

Returns this span's Id, if it is enabled.

pub fn metadata(&self) -> Option<&'static Metadata<'static>>

Returns this span's Metadata, if it is enabled.

pub fn with_subscriber<T>(
    &self,
    f: impl FnOnce((&Id, &Dispatch)) -> T
) -> Option<T>

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.

Trait Implementations

impl Debug for EnteredSpan

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl Deref for EnteredSpan

type Target = Span

The resulting type after dereferencing.

fn deref(&self) -> &Span

Dereferences the value.

impl Drop for EnteredSpan

fn drop(&mut self)

Executes the destructor for this type.