diff --git a/src/librustc_errors/json.rs b/src/librustc_errors/json.rs
index ffdff6acec5de..97b9ca924bb7c 100644
--- a/src/librustc_errors/json.rs
+++ b/src/librustc_errors/json.rs
@@ -48,7 +48,7 @@ impl JsonEmitter {
         macro_backtrace: bool,
     ) -> JsonEmitter {
         JsonEmitter {
-            dst: Box::new(io::stderr()),
+            dst: Box::new(io::BufWriter::new(io::stderr())),
             registry,
             sm: source_map,
             pretty,
@@ -104,7 +104,8 @@ impl Emitter for JsonEmitter {
             writeln!(&mut self.dst, "{}", as_pretty_json(&data))
         } else {
             writeln!(&mut self.dst, "{}", as_json(&data))
-        };
+        }
+        .and_then(|_| self.dst.flush());
         if let Err(e) = result {
             panic!("failed to print diagnostics: {:?}", e);
         }
@@ -116,7 +117,8 @@ impl Emitter for JsonEmitter {
             writeln!(&mut self.dst, "{}", as_pretty_json(&data))
         } else {
             writeln!(&mut self.dst, "{}", as_json(&data))
-        };
+        }
+        .and_then(|_| self.dst.flush());
         if let Err(e) = result {
             panic!("failed to print notification: {:?}", e);
         }