Expand description

Utilities for implementing and composing tracing subscribers.

tracing is a framework for instrumenting Rust programs to collect scoped, structured, and async-aware diagnostics. The Subscriber trait represents the functionality necessary to collect this trace data. This crate contains tools for composing subscribers out of smaller units of behaviour, and batteries-included implementations of common subscriber functionality.

tracing-subscriber is intended for use by both Subscriber authors and application authors using tracing to instrument their applications.

Compiler support: requires rustc 1.63+

Layers and Filters

The most important component of the tracing-subscriber API is the Layer trait, which provides a composable abstraction for building Subscribers. Like the Subscriber trait, a Layer defines a particular behavior for collecting trace data. Unlike Subscribers, which implement a complete strategy for how trace data is collected, Layers provide modular implementations of specific behaviors. Therefore, they can be composed together to form a Subscriber which is capable of recording traces in a variety of ways. See the layer module’s documentation for details on using Layers.

In addition, the Filter trait defines an interface for filtering what spans and events are recorded by a particular layer. This allows different Layers to handle separate subsets of the trace data emitted by a program. See the documentation on per-layer filtering for more information on using Filters.

Included Subscribers

The following Subscribers are provided for application authors:

  • fmt - Formats and logs tracing data (requires the fmt feature flag)

Feature Flags

  • std: Enables APIs that depend on the Rust standard library (enabled by default).
  • alloc: Depend on liballoc (enabled by “std”).
  • env-filter: Enables the EnvFilter type, which implements filtering similar to the env_logger crate. Requires “std”.
  • fmt: Enables the fmt module, which provides a subscriber implementation for printing formatted representations of trace events. Enabled by default. Requires “registry” and “std”.
  • ansi: Enables fmt support for ANSI terminal colors. Enabled by default.
  • registry: enables the registry module. Enabled by default. Requires “std”.
  • json: Enables fmt support for JSON output. In JSON output, the ANSI feature does nothing. Requires “fmt” and “std”.
  • local-time: Enables local time formatting when using the time crate’s timestamp formatters with the fmt subscriber.

Optional Dependencies

  • tracing-log: Enables better formatting for events emitted by log macros in the fmt subscriber. Enabled by default.
  • time: Enables support for using the time crate for timestamp formatting in the fmt subscriber.
  • smallvec: Causes the EnvFilter type to use the smallvec crate (rather than Vec) as a performance optimization. Enabled by default.
  • parking_lot: Use the parking_lot crate’s RwLock implementation rather than the Rust standard library’s implementation.

no_std Support

In embedded systems and other bare-metal applications, tracing can be used without requiring the Rust standard library, although some features are disabled. Although most of the APIs provided by tracing-subscriber, such as fmt and EnvFilter, require the standard library, some functionality, such as the Layer trait, can still be used in no_std environments.

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-subscriber = { version = "0.3", default-features = false }

Additional APIs are available when liballoc is available. To enable liballoc but not std, use:

# Cargo.toml
tracing-subscriber = { version = "0.3", default-features = false, features = ["alloc"] }

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:

  • valuable: Enables support for serializing values recorded using the valuable crate as structured JSON in the format::Json formatter.
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 1.63. 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 fmt::fmt;
    fmt and std
  • pub use fmt::Subscriber as FmtSubscriber;
    fmt and std
  • pub use filter::EnvFilter;
    env-filter and std
  • pub use layer::Layer;
  • pub use registry::Registry;
    registry and std


  • Utilities for working with fields and field visitors.
  • Layers that control which spans and events are enabled by the wrapped subscriber.
  • fmtfmt and std
    A Subscriber for formatting and logging tracing data.
  • The Layer trait, a composable abstraction for building Subscribers.
  • The tracing-subscriber prelude.
  • Storage for span data shared by multiple Layers.
  • Wrapper for a Layer to allow it to be dynamically reloaded.
  • Extension traits and other utilities to make working with subscribers more ergonomic.