Kickstart the inner usage of macro_metavar_expr

compiler/rustc_expand/src/expand.rs

@@ -83,9 +83,7 @@ macro_rules! ast_fragments {
                 }
                 match self {
                     $($(AstFragment::$Kind(ast) => ast.extend(placeholders.iter().flat_map(|id| {
-                        // We are repeating through arguments with `many`, to do that we have to
-                        // mention some macro variable from those arguments even if it's not used.
-                        macro _repeating($flat_map_ast_elt) {}
+                        ${ignore(flat_map_ast_elt)}
                         placeholder(AstFragmentKind::$Kind, *id, None).$make_ast()
                     })),)?)*
                     _ => panic!("unexpected AST fragment kind")