Crate tracing_core

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Expand description

Core primitives for tracing.

tracing is a framework for instrumenting Rust programs to collect structured, event-based diagnostic information. This crate defines the core primitives of tracing.

This crate provides:

  • span::Id identifies a span within the execution of a program.

  • Event represents a single event within a trace.

  • Subscriber, the trait implemented to collect trace data.

  • Metadata and Callsite provide information describing spans and Events.

  • Field, FieldSet, Value, and ValueSet represent the structured data attached to a span.

  • Dispatch allows spans and events to be dispatched to Subscribers.

In addition, it defines the global callsite registry and per-thread current dispatcher which other components of the tracing system rely on.

Compiler support: requires rustc 1.56+


Application authors will typically not use this crate directly. Instead, they will use the tracing crate, which provides a much more fully-featured API. However, this crate’s API will change very infrequently, so it may be used when dependencies must be very stable.

Subscriber implementations may depend on tracing-core rather than tracing, as the additional APIs provided by tracing are primarily useful for instrumenting libraries and applications, and are generally not necessary for Subscriber implementations.

The tokio-rs/tracing repository contains less stable crates designed to be used with the tracing ecosystem. It includes a collection of Subscriber implementations, as well as utility and adapter crates.

§Crate Feature Flags

The following crate feature flags are available:

  • std: Depend on the Rust standard library (enabled by default).

    no_std users may disable this feature with default-features = false:

    tracing-core = { version = "0.1.22", default-features = false }

    Note:tracing-core’s no_std support requires liballoc.

§Unstable Features

These feature flags enable unstable features. The public API may break in 0.1.x releases. To enable these features, the --cfg tracing_unstable must be passed to rustc when compiling.

The following unstable feature flags are currently available:

§Enabling Unstable Features

The easiest way to set the tracing_unstable cfg is to use the RUSTFLAGS env variable when running cargo commands:

RUSTFLAGS="--cfg tracing_unstable" cargo build

Alternatively, the following can be added to the .cargo/config file in a project to automatically enable the cfg flag for that project:

rustflags = ["--cfg", "tracing_unstable"]

§Supported Rust Versions

Tracing is built against the latest stable release. The minimum supported version is 1.56. The current Tracing version is not guaranteed to build on Rust versions earlier than the minimum supported version.

Tracing follows the same compiler support policies as the rest of the Tokio project. The current stable Rust compiler and the three most recent minor versions before it will always be supported. For example, if the current stable compiler version is 1.69, the minimum supported version will not be increased past 1.66, three minor versions prior. Increasing the minimum supported compiler version is not considered a semver breaking change as long as doing so complies with this policy.


  • pub use self::metadata::Kind;
  • pub use self::subscriber::Interest;


  • Callsites represent the source locations from which spans or events originate.
  • Dispatches trace events to Subscribers.
  • Events represent single points in time during the execution of a program.
  • Span and Event key-value data.
  • Metadata describing trace data.
  • Spans represent periods of time in the execution of a program.
  • Collectors collect and record trace data.



  • Dispatch trace data to a Subscriber.
  • Events represent single points in time where something occurred during the execution of a program.
  • An opaque key allowing O(1) access to a field in a Span’s key-value data.
  • Describes the level of verbosity of a span or event.
  • A filter comparable to a verbosity Level.
  • Metadata describing a span or event.
  • A synchronization primitive which can be used to run a one-time global initialization. Useful for one-time initialization for FFI or related functionality. This type can only be constructed with Once::new().


  • Trait implemented by callsites.
  • Trait representing the functions required to collect trace data.