Stabilize C string literals

compiler/rustc_builtin_macros/src/concat_bytes.rs

@@ -19,8 +19,8 @@ fn invalid_type_err(
     let snippet = cx.sess.source_map().span_to_snippet(span).ok();
     match ast::LitKind::from_token_lit(token_lit) {
         Ok(ast::LitKind::CStr(_, _)) => {
-            // FIXME(c_str_literals): should concatenation of C string literals
-            // include the null bytes in the end?
+            // Avoid ambiguity in handling of terminal `NUL` by refusing to
+            // concatenate C string literals as bytes.
             cx.emit_err(errors::ConcatCStrLit { span: span });
         }
         Ok(ast::LitKind::Char(_)) => {