Function field

pub fn field<P: IntoFieldPredicate>(
    name: &'static str,
    matches: P,
) -> FieldPredicate<P::Predicate>

Creates a predicate for a particular field of a CapturedSpan or CapturedEvent.

Arguments

The argument of this function is essentially a predicate for the TracedValue of the field. It may be:

• bool, i64, i128, u64, u128, f64, &str: will be compared to the TracedValue using the corresponding PartialEq implementation.
• A predicate produced by the value() function.
• Any Predicate for TracedValue. To bypass Rust orphaning rules, the predicate must be enclosed in square brackets (i.e., a one-value array).

Examples

let storage = SharedStorage::default();
let subscriber = Registry::default().with(CaptureLayer::new(&storage));
tracing::subscriber::with_default(subscriber, || {
    tracing::info_span!("compute", arg = 5_i32).in_scope(|| {
        tracing::info!("done");
    });
});

let storage = storage.lock();
// All of these access the single captured span.
let spans = storage.scan_spans();
let _ = spans.single(&field("arg", [always()]));
let _ = spans.single(&field("arg", 5_i64));
let _ = spans.single(&field("arg", value(gt(3_i64))));