add an explanatory comment

src/libsyntax/parse/parser.rs

@@ -6151,6 +6151,11 @@ impl<'a> Parser<'a> {
     pub fn submod_path_from_attr(attrs: &[Attribute], dir_path: &Path) -> Option<PathBuf> {
         if let Some(s) = attr::first_attr_value_str_by_name(attrs, "path") {
             let s = s.as_str();
+
+            // On windows, the base path might have the form
+            // `\\?\foo\bar` in which case it does not tolerate
+            // mixed `/` and `\` separators, so canonicalize
+            // `/` to `\`.
             #[cfg(windows)]
             let s = s.replace("/", "\\");
             Some(dir_path.join(s))