# tracing_core - Rust
API documentation for the Rust `tracing_core` crate.
Keywords: rust, rustlang, rust-lang, tracing_core

# Crate tracing_core [−] [src]

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 `Event`s.

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

- `Dispatch` allows spans and events to be dispatched to `Subscriber`s.

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` 1.40+

## Usage

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`:

      [dependencies]
      tracing-core = { version = "0.1.17", default-features = false }

  Note:`tracing-core`'s `no_std` support requires `liballoc`.

## Supported Rust Versions

Tracing is built against the latest stable release. The minimum supported version is 1.40. 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 1.45, the minimum supported version will not be increased past 1.42, 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.

## Re-exports

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

## Modules

callsite | Callsites represent the source locations from which spans or events originate.
dispatcher | Dispatches trace events to `Subscriber`s.
event | Events represent single points in time during the execution of a program.
field | Span and `Event` key-value data.
metadata | Metadata describing trace data.
span | Spans represent periods of time in the execution of a program.
subscriber | Subscribers collect and record trace data.

## Macros

identify_callsite | Statically constructs an `Identifier` for the provided `Callsite`.
metadata | Statically constructs new span metadata.

## Structs

Dispatch | `Dispatch` trace data to a `Subscriber`.
Event | `Event`s represent single points in time where something occurred during the execution of a program.
Field | An opaque key allowing O(1) access to a field in a `Span`'s key-value data.
Level | Describes the level of verbosity of a span or event.
LevelFilter | A filter comparable to a verbosity `Level`.
Metadata | Metadata describing a span or event.
Once | 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()`.

## Traits

Callsite | Trait implemented by callsites.
Subscriber | Trait representing the functions required to collect trace data.