Struct tracing::span::Span

pub struct Span { /* fields omitted */ }

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.

Methods

impl Span

pub fn new(meta: &'static Metadata<'static>, values: &ValueSet) -> 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.

pub fn new_root(meta: &'static Metadata<'static>, values: &ValueSet) -> Span

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.

pub fn child_of(
    parent: impl Into<Option<Id>>,
    meta: &'static Metadata<'static>,
    values: &ValueSet
) -> Span

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.

pub fn new_disabled(meta: &'static Metadata<'static>) -> Span

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.

pub const fn none() -> Span

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.

pub fn current() -> Span

Returns a handle to the span considered by the Subscriber to be the currrent 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.

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

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.

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, 

Visits that the field described by field has the value value.

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

Visit all the fields in the span

pub fn is_disabled(&self) -> bool

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

pub fn follows_from(&self, from: impl for<'a> Into<Option<&'a 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.

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.

Trait Implementations

impl Drop for Span

fn drop(&mut self)

Executes the destructor for this type.

impl<'a> Into<Option<&'a Id>> for &'a Span

fn into(self) -> Option<&'a Id>

Performs the conversion.

impl<'a> Into<Option<Id>> for &'a Span

fn into(self) -> Option<Id>

Performs the conversion.

impl Into<Option<Id>> for Span

fn into(self) -> Option<Id>

Performs the conversion.

impl Clone for Span

fn clone(&self) -> Span

Returns a copy of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl PartialEq<Span> for Span

fn eq(&self, other: &Self) -> bool

This method tests for self and other values to be equal, and is used by ==.

#[must_use] fn ne(&self, other: &Rhs) -> bool

This method tests for !=.

impl Debug for Span

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

Formats the value using the given formatter.

impl Hash for Span

fn hash<H: Hasher>(&self, hasher: &mut H)

Feeds this value into the given [Hasher].

fn hash_slice<H>(data: &[Self], state: &mut H) where
    H: Hasher, 

Feeds a slice of this type into the given [Hasher].