Add compile fail tests for LookaheadIterator

cli/src/lib.rs

@@ -14,3 +14,7 @@ pub mod wasm;
 
 #[cfg(test)]
 mod tests;
+
+// To run compile fail tests
+#[cfg(doctest)]
+mod tests;

cli/src/tests/language_test.rs

@@ -39,3 +39,57 @@ fn test_lookahead_iterator() {
         .map(|s| language.node_kind_for_id(s).unwrap())
         .eq(expected_symbols));
 }
+
+#[test]
+fn test_lookahead_iterator_modifiable_only_by_mut() {
+    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);
+
+    let mut lookahead = language.lookahead_iterator(next_state).unwrap();
+    let _ = lookahead.next();
+
+    let mut names = lookahead.iter_names();
+    let _ = names.next();
+}
+
+/// It doesn't allowed to use lookahead iterator by shared ref:
+///     error[E0596]: cannot borrow `lookahead` as mutable, as it is not declared as mutable
+/// ```compile_fail
+/// use tree_sitter::{Parser, Language};
+/// let mut parser = Parser::new();
+/// let language = unsafe { Language::from_raw(std::ptr::null()) };
+/// let tree = parser.parse("", None).unwrap();
+/// let mut cursor = tree.walk();
+/// let next_state = cursor.node().next_parse_state();
+/// let lookahead = language.lookahead_iterator(next_state).unwrap();
+/// let _ = lookahead.next();
+/// ```
+
+/// It doesn't allowed to use lookahead names iterator by shared ref:
+///     error[E0596]: cannot borrow `names` as mutable, as it is not declared as mutable
+/// ```compile_fail
+/// use tree_sitter::{Parser, Language};
+/// let mut parser = Parser::new();
+/// let language = unsafe { Language::from_raw(std::ptr::null()) };
+/// let tree = parser.parse("", None).unwrap();
+/// let mut cursor = tree.walk();
+/// let next_state = cursor.node().next_parse_state();
+/// if let Some(mut lookahead) = language.lookahead_iterator(next_state) {
+///     let _ = lookahead.next();
+///     let names = lookahead.iter_names();
+///     let _ = names.next();
+/// }
+/// ```
+
+fn _dummy() {}