Struct Context

pub struct Context<'a, S> { /* private fields */ }

Represents information about the current context provided to subscribers by the wrapped collector.

To access stored data keyed by a span ID, implementors of the Subscribe trait should ensure that the Collect type parameter is also bound by the LookupSpan:

use tracing::Collect;
use tracing_subscriber::{Subscribe, registry::LookupSpan};

pub struct MySubscriber;

impl<C> Subscribe<C> for MySubscriber
where
    C: Collect + for<'a> LookupSpan<'a>,
{
    // ...
}

Implementations

impl<'a, C> Context<'a, C>

where C: Collect,

pub fn current_span(&self) -> Current

Returns the wrapped collector’s view of the current span.

pub fn enabled(&self, metadata: &Metadata<'_>) -> bool

Returns whether the wrapped collector would enable the current span.

pub fn event(&self, event: &Event<'_>)

Records the provided event with the wrapped collector.

Notes

• The collector is free to expect that the event’s callsite has been registered, and may panic or fail to observe the event if this is not the case. The tracing crate’s macros ensure that all events are registered, but if the event is constructed through other means, the user is responsible for ensuring that register_callsite has been called prior to calling this method.
• This does not call enabled on the inner collector. If the caller wishes to apply the wrapped collector’s filter before choosing whether to record the event, it may first call Context::enabled to check whether the event would be enabled. This allows subscriberss to elide constructing the event if it would not be recorded.

pub fn event_span(&self, event: &Event<'_>) -> Option<SpanRef<'_, C>>

where C: for<'lookup> LookupSpan<'lookup>,

Returns a SpanRef for the parent span of the given Event, if it has a parent.

If the event has an explicitly overridden parent, this method returns a reference to that span. If the event’s parent is the current span, this returns a reference to the current span, if there is one. If this returns None, then either the event’s parent was explicitly set to None, or the event’s parent was defined contextually, but no span is currently entered.

Compared to Context::current_span and Context::lookup_current, this respects overrides provided by the Event.

Compared to Event::parent, this automatically falls back to the contextual span, if required.

use tracing::{Event, Collect};
use tracing_subscriber::{
    subscribe::{Context, Subscribe},
    prelude::*,
    registry::LookupSpan,
};

struct PrintingSubscriber;
impl<C> Subscribe<C> for PrintingSubscriber
where
    C: Collect + for<'lookup> LookupSpan<'lookup>,
{
    fn on_event(&self, event: &Event, ctx: Context<C>) {
        let span = ctx.event_span(event);
        println!("Event in span: {:?}", span.map(|s| s.name()));
    }
}

tracing::collect::with_default(tracing_subscriber::registry().with(PrintingSubscriber), || {
    tracing::info!("no span");
    // Prints: Event in span: None

    let span = tracing::info_span!("span");
    tracing::info!(parent: &span, "explicitly specified");
    // Prints: Event in span: Some("span")

    let _guard = span.enter();
    tracing::info!("contextual span");
    // Prints: Event in span: Some("span")
});

Note: This requires the wrapped collector to implement the
LookupSpan trait.
See the documentation on Context's
declaration for details.

pub fn metadata(&self, id: &Id) -> Option<&'static Metadata<'static>>

where C: for<'lookup> LookupSpan<'lookup>,

Returns metadata for the span with the given id, if it exists.

If this returns None, then no span exists for that ID (either it has closed or the ID is invalid).

pub fn span(&self, id: &Id) -> Option<SpanRef<'_, C>>

where C: for<'lookup> LookupSpan<'lookup>,

Returns stored data for the span with the given id, if it exists.

If this returns None, then no span exists for that ID (either it has closed or the ID is invalid).

Note: This requires the wrapped collector to implement the
LookupSpan trait.
See the documentation on Context's
declaration for details.

pub fn exists(&self, id: &Id) -> bool

where C: for<'lookup> LookupSpan<'lookup>,

Returns true if an active span exists for the given Id.

Note: This requires the wrapped collector to implement the
LookupSpan trait.
See the documentation on Context's
declaration for details.

pub fn lookup_current(&self) -> Option<SpanRef<'_, C>>

where C: for<'lookup> LookupSpan<'lookup>,

Returns stored data for the span that the wrapped collector considers to be the current.

If this returns None, then we are not currently within a span.

Note: This requires the wrapped collector to implement the
LookupSpan trait.
See the documentation on Context's
declaration for details.

pub fn span_scope(&self, id: &Id) -> Option<Scope<'_, C>>

where C: for<'lookup> LookupSpan<'lookup>,

Returns an iterator over the stored data for all the spans in the current context, starting with the specified span and ending with the root of the trace tree.

Note: This returns the spans in reverse order (from leaf to root). Use
Scope::from_root
in case root-to-leaf ordering is desired.

Note: This requires the wrapped collector to implement the
LookupSpan trait.
See the documentation on Context's
declaration for details.

pub fn event_scope(&self, event: &Event<'_>) -> Option<Scope<'_, C>>

where C: for<'lookup> LookupSpan<'lookup>,

Returns an iterator over the stored data for all the spans in the current context, starting with the parent span of the specified event, and ending with the root of the trace tree and ending with the current span.

Note: Compared to scope this
returns the spans in reverse order (from leaf to root). Use
Scope::from_root
in case root-to-leaf ordering is desired.

Note: This requires the wrapped collector to implement the
LookupSpan trait.
See the documentation on Context's
declaration for details.

Trait Implementations

impl<'a, S> Clone for Context<'a, S>

fn clone(&self) -> Self

Returns a copy of the value.

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

Performs copy-assignment from source.

impl<'a, S: Debug> Debug for Context<'a, S>

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

Formats the value using the given formatter.