Crate rustc_log

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This crate allows tools to enable rust logging without having to magically match rustc’s tracing crate version.

For example if someone is working on rustc_ast and wants to write some minimal code against it to run in a debugger, with access to the debug! logs emitted by rustc_ast, that can be done by writing:

rustc_ast = { path = "../rust/compiler/rustc_ast" }
rustc_log = { path = "../rust/compiler/rustc_log" }
rustc_span = { path = "../rust/compiler/rustc_span" }
fn main() {

    let edition = rustc_span::edition::Edition::Edition2021;
    rustc_span::create_session_globals_then(edition, || {
        /* ... */

Now LOG=debug cargo +nightly run will run your minimal and show rustc’s debug logging. In a workflow like this, one might also add std::env::set_var("LOG", "debug") to the top of main so that cargo +nightly run by itself is sufficient to get logs.

The reason rustc_log is a tiny separate crate, as opposed to exposing the same things in rustc_driver only, is to enable the above workflow. If you had to depend on rustc_driver in order to turn on rustc’s debug logs, that’s an enormously bigger dependency tree; every change you make to rustc_ast (or whichever piece of the compiler you are interested in) would involve rebuilding all the rest of rustc up to rustc_driver in order to run your Whereas by depending only on rustc_log and the few crates you are debugging, you can make changes inside those crates and quickly run to read the debug logs.


  • The values of all the environment variables that matter for configuring a logger. Errors are explicitly preserved so that we can share error handling.