Attribute Macro tracing::instrument

#[instrument]

This is supported on feature="attributes" only.

Instruments a function to create and enter a tracing span every time the function is called.

The generated span's name will be the name of the function. Any arguments to that function will be recorded as fields using fmt::Debug. To skip recording a function's or method's argument, pass the argument's name to the skip argument on the #[instrument] macro. For example, skip can be used when an argument to an instrumented function does not implement fmt::Debug, or to exclude an argument with a verbose or costly Debug implementation. Note that:

• multiple argument names can be passed to skip.
• arguments passed to skip do not need to implement fmt::Debug.

You can also pass additional fields (key-value pairs with arbitrary data) to the generated span. This is achieved using the fields argument on the #[instrument] macro. You can use a string, integer or boolean literal as a value for each field. The name of the field must be a single valid Rust identifier, nested (dotted) field names are not supported.

Note that overlap between the names of fields and (non-skipped) arguments will result in a compile error.

Examples

Instrumenting a function:

#[instrument]
pub fn my_function(my_arg: usize) {
    // This event will be recorded inside a span named `my_function` with the
    // field `my_arg`.
    tracing::info!("inside my_function!");
    // ...
}

Setting the level for the generated span:

#[instrument(level = "debug")]
pub fn my_function() {
    // ...
}

Overriding the generated span's name:

#[instrument(name = "my_name")]
pub fn my_function() {
    // ...
}

Overriding the generated span's target:

#[instrument(target = "my_target")]
pub fn my_function() {
    // ...
}

To skip recording an argument, pass the argument's name to the skip:

struct NonDebug;

#[instrument(skip(non_debug))]
fn my_function(arg: usize, non_debug: NonDebug) {
    // ...
}

To add an additional context to the span, you can pass key-value pairs to fields:

#[instrument(fields(foo="bar", id=1, show=true))]
fn my_function(arg: usize) {
    // ...
}

If the function returns a Result<T, E> and E implements std::fmt::Display, you can add err to emit error events when the function returns Err:

#[instrument(err)]
fn my_function(arg: usize) -> Result<(), std::io::Error> {
    Ok(())
}

If tracing_futures is specified as a dependency in Cargo.toml, async fns may also be instrumented:

#[instrument]
pub async fn my_function() -> Result<(), ()> {
    // ...
}

It also works with async-trait (a crate that allows defining async functions in traits, something not currently possible in Rust), and hopefully most libraries that exhibit similar behaviors:

use async_trait::async_trait;

#[async_trait]
pub trait Foo {
    async fn foo(&self, arg: usize);
}

#[derive(Debug)]
struct FooImpl(usize);

#[async_trait]
impl Foo for FooImpl {
    #[instrument(fields(value = self.0, tmp = std::any::type_name::<Self>()))]
    async fn foo(&self, arg: usize) {}
}

An interesting note on this subject is that references to the Self type inside the fields argument are only allowed when the instrumented function is a method aka. the function receives self as an argument. For example, this will not work because it doesn't receive self:

use async_trait::async_trait;

#[async_trait]
pub trait Bar {
    async fn bar();
}

#[derive(Debug)]
struct BarImpl(usize);

#[async_trait]
impl Bar for BarImpl {
    #[instrument(fields(tmp = std::any::type_name::<Self>()))]
    async fn bar() {}
}

Instead, you should manually rewrite any Self types as the type for which you implement the trait: #[instrument(fields(tmp = std::any::type_name::<Bar>()))].