Detect failure to match an ident metavariable

Fixes #4054

gcc/rust/ChangeLog:

	* parse/rust-parse-impl.h
	(Parser::parse_identifier_or_keyword_token): Record error on
	failure.

gcc/testsuite/ChangeLog:

	* rust/compile/macros/mbe/macro-issue4054.rs: New test.

Signed-off-by: Owen Avery <powerboat9.gamer@gmail.com>

Author: powerboat9

gcc/rust/parse/rust-parse-impl.h

@@ -1053,6 +1053,7 @@ Parser<ManagedTokenSource>::parse_identifier_or_keyword_token ()
     }
   else
     {
+      add_error (Error (t->get_locus (), "expected keyword or identifier"));
       return nullptr;
     }
 }

gcc/testsuite/rust/compile/macros/mbe/macro-issue4054.rs

@@ -0,0 +1,14 @@
+#[allow(path_statements)]
+
+macro_rules! array_impl_default {
+    {$t:ident} => {
+        $t;
+        array_impl_default!{}
+    };
+    {} => {}
+}
+
+pub fn foo() {
+    let x = 12;
+    array_impl_default! {x}
+}