diff --git a/src/filemap.rs b/src/filemap.rs
index f41915b8cff5..64c892a09297 100644
--- a/src/filemap.rs
+++ b/src/filemap.rs
@@ -31,14 +31,14 @@ pub fn append_newlines(file_map: &mut FileMap) {
     }
 }
 
-pub fn write_all_files<T>(file_map: &FileMap, mut out: T, config: &Config) -> Result<(), io::Error>
+pub fn write_all_files<T>(file_map: &FileMap, out: &mut T, config: &Config) -> Result<(), io::Error>
     where T: Write
 {
-    output_header(&mut out, config.write_mode).ok();
+    output_header(out, config.write_mode).ok();
     for filename in file_map.keys() {
-        try!(write_file(&file_map[filename], filename, &mut out, config));
+        try!(write_file(&file_map[filename], filename, out, config));
     }
-    output_footer(&mut out, config.write_mode).ok();
+    output_footer(out, config.write_mode).ok();
 
     Ok(())
 }
diff --git a/src/lib.rs b/src/lib.rs
index 3443db338cfd..a559c23006bd 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -432,8 +432,8 @@ pub fn run(file: &Path, config: &Config) {
     let mut result = format(file, config);
 
     print!("{}", fmt_lines(&mut result, config));
-    let out = stdout();
-    let write_result = filemap::write_all_files(&result, out, config);
+    let mut out = stdout();
+    let write_result = filemap::write_all_files(&result, &mut out, config);
 
     if let Err(msg) = write_result {
         println!("Error writing files: {}", msg);