Rust supports a logging facade, which we can include using the following dependency in Cargo.toml

[dependencies]
log = "0.4.28"

Now in our main.rs we can use the various levels of logging like this

use log::{info, warn, error, trace, debug, LevelFilter};

fn main() {
    debug!("Debug log message");
    trace!("Trace log message");
    info!("Info log message");
    warn!("Warning log message");
    error!("Error log message");
}

If you run this, nothing will be output because we need to add a logging provider.

One simple provider is env_logger which will log to standard out. To include, add the following to the Cargo.toml dependencies

env_logger = "0.11.8"

We’ll need to add the use clause

use env_logger::{Builder, Env};

and then we need to initialise the env_logger, we can use the following at the start of the main function

env_logger::init();

This will only output ERROR messages, we can change the log level using the environment variable like this

RUST_LOG=trace

Alternatively we can set the environment variable within code by replace the environment variable and the env_logger::init(); line with

let env = Env::default().filter_or("RUST_LOG", "trace");
Builder::from_env(env).init();

or we can set in code instead using

Builder::new()
   .filter_level(LevelFilter::Trace)
   .init();