spandoc

Attribute macro that transforms doc comments in functions into tracing spans.

Details

All doc comments intended to be transformed into spans must begin with SPANDOC: :

use spandoc::spandoc;
use tracing::info;

#[spandoc]
fn foo() {
    /// SPANDOC: this will be converted into a span
    info!("event 1");

    /// this will be ignored and produce a warning for an unused doc comment
    info!("event 2");
}

The spans that are created by spandoc are explicitly scoped to the expression they're associated with.

use spandoc::spandoc;
use tracing::info;

#[spandoc]
fn main() {
    tracing_subscriber::fmt::init();
    let local = 4;

    /// SPANDOC: Emit a tracing info event {?local}
    info!("event 1");

    info!("event 2");
}

Running the above example will produce the following output

spandoc on  await-support [!+] is 📦 v0.1.3 via 🦀 v1.44.1
❯ cargo run --example scoped
    Finished dev [unoptimized + debuginfo] target(s) in 0.03s
     Running `target/debug/examples/scoped`
Jul 09 12:42:43.691  INFO main::comment{local=4 text=Emit a tracing info event}: scoped: event 1
Jul 09 12:42:43.691  INFO scoped: event 2

Local variables can be associated with the generated spans by adding a trailing block to the doc comment. The syntax for fields in the span is the same as in tracing.

use spandoc::spandoc;
use tracing::info;

#[spandoc]
fn foo() {
    let path = "fake.txt";
    /// SPANDOC: going to load config {?path}
    info!("event 1");

    /// this will be ignored and produce a warning for an unused doc comment
    info!("event 2");
}

When applied to expressions that contain awaits spandoc will correctly use instrument() and exit/enter the span when suspending and resuming the future. If there are multiple await expressions inside of the annotated expression it will instrument each expression with the same span. The macro will not recurse into async blocks.

use std::future::Future;
use spandoc::spandoc;
use tracing::info;

fn make_liz() -> impl Future<Output = Result<(), ()>> {
    info!("this will be printed in the span from `clever_girl`");

    liz()
}

async fn liz() -> Result<(), ()> {
    info!("this will also be printed in the span from `clever_girl`");

    // return a result so we can call map outside of the scope of the future
    Ok(())
}

#[spandoc]
async fn clever_girl() {
    // This span will be entered before the await, exited correctly when the
    // future suspends, and instrument the future returned from `liz` with
    // `tracing-futures`
    /// clever_girl async span
    make_liz().await.map(|()| info!("this will also be printed in the span"));
}

License