add #[allow(unused)] for compatibility

src/fs/file.rs

@@ -95,6 +95,9 @@ impl<'dir> File<'dir> {
     /// against a pre-compiled list of extensions which are known to only exist
     /// within ASCII, so it’s alright.
     fn ext(path: &Path) -> Option<String> {
+        #[allow(unused)]
+        use std::ascii::AsciiExt;
+
         let name = match path.file_name() {
             Some(f) => f.to_string_lossy().to_string(),
             None => return None,