Add debug tracing to FilePathMapping::map_prefix

Author: michaelwoerister

compiler/rustc_span/src/source_map.rs

@@ -1098,28 +1098,43 @@ impl FilePathMapping {
     /// The return value is the remapped path and a boolean indicating whether
     /// the path was affected by the mapping.
     pub fn map_prefix(&self, path: PathBuf) -> (PathBuf, bool) {
-        // NOTE: We are iterating over the mapping entries from last to first
-        //       because entries specified later on the command line should
-        //       take precedence.
-        for &(ref from, ref to) in self.mapping.iter().rev() {
-            if let Ok(rest) = path.strip_prefix(from) {
-                let remapped = if rest.as_os_str().is_empty() {
-                    // This is subtle, joining an empty path onto e.g. `foo/bar` will
-                    // result in `foo/bar/`, that is, there'll be an additional directory
-                    // separator at the end. This can lead to duplicated directory separators
-                    // in remapped paths down the line.
-                    // So, if we have an exact match, we just return that without a call
-                    // to `Path::join()`.
-                    to.clone()
-                } else {
-                    to.join(rest)
-                };
+        if path.as_os_str().is_empty() {
+            return (path, false);
+        }
 
-                return (remapped, true);
+        return remap_path_prefix(&self.mapping, path);
+
+        #[instrument(level = "debug", skip(mapping))]
+        fn remap_path_prefix(mapping: &[(PathBuf, PathBuf)], path: PathBuf) -> (PathBuf, bool) {
+            // NOTE: We are iterating over the mapping entries from last to first
+            //       because entries specified later on the command line should
+            //       take precedence.
+            for &(ref from, ref to) in mapping.iter().rev() {
+                debug!("Trying to apply {:?} => {:?}", from, to);
+
+                if let Ok(rest) = path.strip_prefix(from) {
+                    let remapped = if rest.as_os_str().is_empty() {
+                        // This is subtle, joining an empty path onto e.g. `foo/bar` will
+                        // result in `foo/bar/`, that is, there'll be an additional directory
+                        // separator at the end. This can lead to duplicated directory separators
+                        // in remapped paths down the line.
+                        // So, if we have an exact match, we just return that without a call
+                        // to `Path::join()`.
+                        to.clone()
+                    } else {
+                        to.join(rest)
+                    };
+                    debug!("Match - remapped {:?} => {:?}", path, remapped);
+
+                    return (remapped, true);
+                } else {
+                    debug!("No match - prefix {:?} does not match {:?}", from, path);
+                }
             }
-        }
 
-        (path, false)
+            debug!("Path {:?} was not remapped", path);
+            (path, false)
+        }
     }
 
     fn map_filename_prefix(&self, file: &FileName) -> (FileName, bool) {