Add test of ineffective TokenStream size_hint

Author: dtolnay

tests/test.rs

@@ -547,6 +547,13 @@ fn default_tokenstream_is_empty() {
     assert!(default_token_stream.is_empty());
 }
 
+#[test]
+fn tokenstream_size_hint() {
+    let tokens = "a b (c d) e".parse::<TokenStream>().unwrap();
+
+    assert_eq!(tokens.into_iter().size_hint(), (0, None)); // FIXME
+}
+
 #[test]
 fn tuple_indexing() {
     // This behavior may change depending on https://github.com/rust-lang/rust/pull/71322