Rust tracing basic setup

I spent ages yesterday trying to figure out how to use Rust's tracing library (and tracing-subscriber) for a very simple logging use-case.

I wanted to use tracing because a) it's rapidly becoming standard everywhere and b) I was using libraries that used it.

I found it very hard to find a simple example that either takes its log level from the RUST_LOG environment variable, or uses a non-trivial default. Here's what I came up with:

$ cargo add tracing
$ cargo add tracing-subscriber --features env-filter

use tracing_subscriber::FmtSubscriber;

fn main() {
    const DEFAULT_LOGGING: &str = "myprogram=info,warn";

    let rust_log = std::env::var("RUST_LOG")
        .ok()
        .and_then(|s| if s.is_empty() { None } else { Some(s) })
        .unwrap_or_else(|| DEFAULT_LOGGING.to_owned());

    tracing::subscriber::set_global_default(
        FmtSubscriber::builder().with_env_filter(rust_log).finish(),
    )
    .expect("tracing setup failed");

    // The rest of my program here ...
}

The myprogram=info,warn part sets the log level of my project to info and any libraries I use to warn. You'll need to replace myprogram with the name of your project.

This sets a default logging level which can be overridden by launching the program with e.g. RUST_LOG=debug ./myprogram to set the level to debug everywhere or e.g. RUST_LOG="myproject=debug,warn" ./myprogram to do something more clever.

(The format of these comma-separated lists of directives are kind of documented at tracing_fmt::Builder::with_env_filter and EnvFilter.)

Once that's done I can log using code like this:

use tracing::{debug, info};

pub fn myfun() {
    let x = 3;
    info!("The number is {x}");
    debug!("DEBUG ONLY");
}

If anyone knows a simpler way to achieve these results, let me know. I spent a long time fiddling with EnvFilter::with_default_directive and similar stuff before I decided I had to check the contents of RUST_LOG manually if I wanted a more complex default (not a single directive).

Side note: wow, tracing_subscriber is hard to use! I couldn't find any simple examples - links welcome.