Module tracing_core::field

Span and Event key-value data.

Spans and events may be annotated with key-value data, referred to as known as fields. These fields consist of a mapping from a key (corresponding to a &str but represented internally as an array index) to a Value.

Values and Subscribers

Subscribers consume Values as fields attached to spans or Events. The set of field keys on a given span or is defined on its Metadata. When a span is created, it provides Attributes to the Subscriber's new_span method, containing any fields whose values were provided when the span was created; and may call the Subscriber's record method with additional Records if values are added for more of its fields. Similarly, the Event type passed to the subscriber's event method will contain any fields attached to each event.

tracing represents values as either one of a set of Rust primitives (i64, u64, bool, and &str) or using a fmt::Display or fmt::Debug implementation. Subscribers are provided these primitive value types as dyn Value trait objects.

These trait objects can be formatted using fmt::Debug, but may also be recorded as typed data by calling the Value::record method on these trait objects with a visitor implementing the Visit trait. This trait represents the behavior used to record values of various types. For example, we might record integers by incrementing counters for their field names, rather than printing them.

Structs

DebugValue	A Value which serializes as a string using fmt::Debug.
DisplayValue	A Value which serializes using fmt::Display.
Empty	An empty field.
Field	An opaque key allowing O(1) access to a field in a Span's key-value data.
FieldSet	Describes the fields present on a span.
Iter	An iterator over a set of fields.
ValueSet	A set of fields and values for a span.

Traits

Value	A field value of an erased type.
Visit	Visits typed values.

Functions

debug	Wraps a type implementing fmt::Debug as a Value that can be recorded using its Debug implementation.
display	Wraps a type implementing fmt::Display as a Value that can be recorded using its Display implementation.