diff --git a/src/librustc/driver/driver.rs b/src/librustc/driver/driver.rs
index f6aab6ef477..d3ed0dbd99e 100644
--- a/src/librustc/driver/driver.rs
+++ b/src/librustc/driver/driver.rs
@@ -660,7 +660,7 @@ fn write_out_deps(sess: &Session,
         _ => return,
     };
 
-    let result = (|| {
+    let result = (|| -> io::IoResult<()> {
         // Build a list of files used to compile the output and
         // write Makefile-compatible dependency rules
         let files: Vec<String> = sess.codemap().files.borrow()