How to change a span's level based on enum?

[inahga]

I'm trying to cut down on log noise due to transient errors that are automatically retried.

Scenario: I have a set of functions that return Result<_, Error> annotated with tracing::instrument(err). I would like the generated span's level to depend on the exact Error.

Here's a small illustration of the setup.

#[derive(Debug)]
pub enum Error {
    // I'd like this to log at Level::ERROR.
    Fatal,
    // I'd like this to log at level::DEBUG,
    Transient,
}

#[tracing::instrument(err(Debug))]
fn run(result: Result<(), Error>) -> Result<(), Error> {
    result
}

fn main() {
    tracing_subscriber::fmt().init();

    let _ = run(Ok(()));
    let _ = run(Err(Error::Fatal));
    let _ = run(Err(Error::Transient));
}

How can I accomplish this? I'd prefer not to abandon tracing::instrument and manage spans by hand, as it would end up being a lot of boilerplate across many functions that have the same behavior.

I was imagining such a thing would take the form of

fn error_level_fn(error: Error) -> tracing::Level {
    match error {
        Error::Fatal => tracing::Level::ERROR,
        Error::Transient => tracing::Level::DEBUG,
    }
}

#[tracing::instrument(err(Debug, level = error_level_fn))]
fn run(...) { ... }

... but I wanted to see if there was a better way before suggesting features.