🛈 Note: This is pre-release documentation for the upcoming tracing 0.2.0 ecosystem.

For the release documentation, please see docs.rs, instead.

Crate tracing_core

source ·
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.

  • Collect, 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 collectors.

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 1.63+


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.

Collector 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 collector implementations.

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

§no_std Support

In embedded systems and other bare-metal applications, tracing-core can be used without requiring the Rust standard library, although some features are disabled.

The dependency on the standard library is controlled by two crate feature flags, “std”, which enables the dependency on libstd, and “alloc”, which enables the dependency on liballoc (and is enabled by the “std” feature). These features are enabled by default, but no_std users can disable them using:

# Cargo.toml
tracing-core = { version = "0.2", default-features = false }

To enable liballoc but not std, use:

# Cargo.toml
tracing-core = { version = "0.2", default-features = false, features = ["alloc"] }

When both the “std” and “alloc” feature flags are disabled, tracing-core will not make any dynamic memory allocations at runtime, and does not require a global memory allocator.

The “alloc” feature is required to enable the Dispatch::new function, which requires dynamic memory allocation to construct a collector trait object at runtime. When liballoc is disabled, new Dispatchs may still be created from &'static dyn Collect references, using Dispatch::from_static.

The “std” feature is required to enable the following features:

All other features of tracing-core should behave identically with and without std and alloc.

§Crate Feature Flags

The following crate feature flags are available:

  • std: Depend on the Rust standard library (enabled by default).
  • alloc: Depend on liballoc (enabled by “std”).

§Supported Rust Versions

Tracing is built against the latest stable release. The minimum supported version is 1.63. 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::collect::Interest;
  • pub use self::metadata::Kind;


  • Callsites represent the source locations from which spans or events originate.
  • Collectors collect and record trace data.
  • Dispatches trace events to Collects.
  • 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.



  • Dispatch trace data to a Collect.
  • 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.


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