Add roc_tracing crate and initial traces in mono

crates/tracing/src/lib.rs

@@ -0,0 +1,42 @@
+//! Utilities for turning on tracing in user-facing or test executables of the Roc compiler.
+//!
+//! Tracing always writes to stderr, and is controlled with the ROC_LOG environment variable.
+//!
+//! Rather than using the Rust `tracing` crate (or any other tracing crate) directly,
+//! you should use the exposed members of `roc_tracing` for your tracing needs.
+//! This enables us to easily modify the tracing infrastructure without inducing sweeping changes.
+//!
+//! Tracing is only turned on in debug builds. Use the provided [setup_tracing] macro to turn on
+//! tracing at an executable's entry point.
+
+/// Sets up tracing of a Roc executable.
+///
+/// This macro should only be invoked at an executable's entry point.
+/// Tracing will only be enabled in debug builds.
+/// Tracing is controlled with the `ROC_LOG` environment variable. See [directive-syntax] for the filtering directive syntax.
+///
+/// [directive-syntax]: https://docs.rs/tracing-subscriber/latest/tracing_subscriber/filter/struct.EnvFilter.html#directives
+#[macro_export]
+macro_rules! setup_tracing {
+    () => {
+        if cfg!(debug_assertions) {
+            $crate::setup_tracing();
+        }
+    };
+}
+
+pub use tracing::debug;
+pub use tracing::info;
+
+const ENV_FILTER: &str = "ROC_LOG";
+
+use tracing_subscriber::{fmt, prelude::*, EnvFilter, Layer, Registry};
+
+#[doc(hidden)]
+pub fn setup_tracing() {
+    let stderr_layer = fmt::Layer::default()
+        .with_writer(std::io::stderr)
+        .with_filter(EnvFilter::from_env(ENV_FILTER));
+
+    Registry::default().with(stderr_layer).init();
+}