Fix typo in #[must_use] message

src/librustc_lint/unused.rs

@@ -183,7 +183,7 @@ fn check_must_use(
             for attr in cx.tcx.get_attrs(def_id).iter() {
                 if attr.check_name("must_use") {
                     let msg = format!("unused {}`{}`{} that must be used",
-                        descr_pre_path, descr_post_path, cx.tcx.item_path_str(def_id));
+                        descr_pre_path, cx.tcx.item_path_str(def_id), descr_post_path);
                     let mut err = cx.struct_span_lint(UNUSED_MUST_USE, sp, &msg);
                     // check for #[must_use = "..."]
                     if let Some(note) = attr.value_str() {