Add rust tests for lookahead iterator

cli/src/tests/language_test.rs

@@ -0,0 +1,41 @@
+use super::helpers::fixtures::get_language;
+use tree_sitter::Parser;
+
+#[test]
+fn test_lookahead_iterator() {
+    let mut parser = Parser::new();
+    let language = get_language("rust");
+    parser.set_language(language).unwrap();
+
+    let tree = parser.parse("struct Stuff {}", None).unwrap();
+
+    let mut cursor = tree.walk();
+
+    assert!(cursor.goto_first_child()); // struct
+    assert!(cursor.goto_first_child()); // struct keyword
+
+    let next_state = cursor.node().next_parse_state();
+    assert_ne!(next_state, 0);
+    assert_eq!(
+        next_state,
+        language.next_state(cursor.node().parse_state(), cursor.node().grammar_id())
+    );
+    assert!((next_state as usize) < language.parse_state_count());
+    assert!(cursor.goto_next_sibling()); // type_identifier
+    assert_eq!(next_state, cursor.node().parse_state());
+    assert_eq!(cursor.node().grammar_name(), "identifier");
+    assert_ne!(cursor.node().grammar_id(), cursor.node().kind_id());
+
+    let expected_symbols = ["identifier", "block_comment", "line_comment"];
+    let lookahead = language.lookahead_iterator(next_state).unwrap();
+    assert_eq!(lookahead.language(), language);
+    assert!(lookahead.iter_names().eq(expected_symbols));
+
+    lookahead.reset_state(next_state);
+    assert!(lookahead.iter_names().eq(expected_symbols));
+
+    lookahead.reset(&language, next_state);
+    assert!(lookahead
+        .map(|s| language.node_kind_for_id(s).unwrap())
+        .eq(expected_symbols));
+}

cli/src/tests/mod.rs

@@ -2,6 +2,7 @@ mod corpus_test;
 mod github_issue_test;
 mod helpers;
 mod highlight_test;
+mod language_test;
 mod node_test;
 mod parser_test;
 mod pathological_test;