add a comment

src/tools/miri/test-cargo-miri/test-local-crate-detection/src/main.rs

@@ -1,3 +1,5 @@
 fn main() {
+    // Make sure we detect all crates from this workspace as "local".
+    // The env var is set during the "build" so we can use `env!` to access it directly.
     println!("{}", env!("MIRI_LOCAL_CRATES"));
 }