--- original
+++ modified
@@ -56,6 +56,7 @@
 use style::str::char_is_whitespace;
 use style::stylesheets::{DocumentStyleSheet, Stylesheet};
 use style::stylist::Stylist;
+#[cfg(debug_assertions)]
 use style::thread_state::{self, ThreadState};
 use style::values::computed::Overflow;
 use style_traits::CSSPixel;