Pass a Symbol to check_name, emit_feature_err, and related functions.

src/libsyntax_ext/trace_macros.rs

@@ -1,6 +1,6 @@
 use syntax::ext::base::{self, ExtCtxt};
 use syntax::feature_gate;
-use syntax::symbol::keywords;
+use syntax::symbol::{keywords, sym};
 use syntax_pos::Span;
 use syntax::tokenstream::TokenTree;
 
@@ -10,7 +10,7 @@ pub fn expand_trace_macros(cx: &mut ExtCtxt<'_>,
                            -> Box<dyn base::MacResult + 'static> {
     if !cx.ecfg.enable_trace_macros() {
         feature_gate::emit_feature_err(&cx.parse_sess,
-                                       "trace_macros",
+                                       sym::trace_macros,
                                        sp,
                                        feature_gate::GateIssue::Language,
                                        feature_gate::EXPLAIN_TRACE_MACROS);