Missing feature backtrace for examples/size.rs

Author: Yamakaky

examples/size.rs

@@ -21,10 +21,20 @@ fn main() {
     println!("      ErrorKind::Msg: {}", size_of_val(&msg));
     println!("        String: {}", size_of::<String>());
     println!("    State: {}", size_of::<error_chain::State>());
-    let state = error_chain::State {
-        next_error: None,
-        backtrace: None,
-    };
-    println!("      State.next_error: {}", size_of_val(&state.next_error));
-    println!("      State.backtrace: {}", size_of_val(&state.backtrace));
+    #[cfg(feature = "backtrace")]
+    {
+        let state = error_chain::State {
+            next_error: None,
+            backtrace: None,
+        };
+        println!("      State.next_error: {}", size_of_val(&state.next_error));
+        println!("      State.backtrace: {}", size_of_val(&state.backtrace));
+    }
+    #[cfg(not(feature = "backtrace"))]
+    {
+        let state = error_chain::State {
+            next_error: None,
+        };
+        println!("      State.next_error: {}", size_of_val(&state.next_error));
+    }
 }