add tracing

src/source.rs

@@ -102,6 +102,11 @@ pub struct SourceFile {
 impl SourceFile {
     /// Panics if `path` is a directory path instead of a file path.
     pub fn open_from(path: Arc<Path>, options: OpenOptions) -> Result<Self, IoError> {
+        trace!(
+            "SourceFile::open_from(path={:?}, options={:?})",
+            path,
+            options.options(),
+        );
         assert!(path.file_name().is_some());
 
         let file = Arc::new(Mutex::new(options.open(&*path)?));