Fix a test and address review comment

src/librustc_resolve/lib.rs

@@ -1200,7 +1200,7 @@ pub struct Resolver<'a> {
     pub found_unresolved_macro: bool,
 
     // List of crate local macros that we need to warn about as being unused.
-    // Right now this only includes macro_rules! macros, and 2.0 macros.
+    // Right now this only includes macro_rules! macros, and macros 2.0.
     unused_macros: FxHashSet<DefId>,
 
     // Maps the `Mark` of an expansion to its containing module or block.

src/test/compile-fail/feature-gate-decl_macro.rs

@@ -8,6 +8,8 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+#![allow(unused_macros)]
+
 macro m() {} //~ ERROR `macro` is experimental (see issue #39412)
 //~| HELP add #![feature(decl_macro)] to the crate attributes to enable