Implement question mark desugaring

tests/desugar_tests.c

@@ -176,6 +176,43 @@ static void a_plus_becomes_subexpr_aa_star(void)
 	parse_free_tree_children(&t);
 }
 
+static void a_qmark_becomes_subexpr_empty_or_a(void)
+{
+	parse_term_t *terms = malloc(1 * sizeof(parse_term_t));
+	terms[0].quantifier = PARSE_QUANTIFIER_QMARK;
+	terms[0].type = PARSE_TERM_LITERAL;
+	terms[0].literal = 'a';
+	parse_sequence_t *alternatives = malloc(1 * sizeof(parse_sequence_t));
+	alternatives[0].count = alternatives[0].capacity = 1;
+	alternatives[0].contents = terms;
+	parse_tree_t t = { .count = 1, .capacity = 1, .contents = alternatives };
+
+	desugar_regex(&t);
+	ASSERT_EQ(1, t.count);
+	ASSERT_NOT_NULL(t.contents);
+	ASSERT_EQ(1, t.contents[0].count);
+	ASSERT_NOT_NULL(t.contents[0].contents);
+	ASSERT_EQ(PARSE_QUANTIFIER_NONE, t.contents[0].contents[0].quantifier);
+	ASSERT_EQ(PARSE_TERM_SUBEXPR, t.contents[0].contents[0].type);
+
+	const parse_tree_t *inner = &t.contents[0].contents[0].subexpr;
+	ASSERT_EQ(2, inner->count);
+	ASSERT_NOT_NULL(inner->contents);
+	ASSERT_EQ(1, inner->contents[0].count);
+	ASSERT_NOT_NULL(inner->contents[0].contents);
+	ASSERT_EQ(
+	    PARSE_QUANTIFIER_NONE, inner->contents[0].contents[0].quantifier);
+	ASSERT_EQ(PARSE_TERM_EMPTY, inner->contents[0].contents[0].type);
+	ASSERT_EQ(1, inner->contents[1].count);
+	ASSERT_NOT_NULL(inner->contents[1].contents);
+	ASSERT_EQ(
+	    PARSE_QUANTIFIER_NONE, inner->contents[1].contents[0].quantifier);
+	ASSERT_EQ(PARSE_TERM_LITERAL, inner->contents[1].contents[0].type);
+	ASSERT_EQ('a', inner->contents[1].contents[0].literal);
+
+	parse_free_tree_children(&t);
+}
+
 int main(void)
 {
 	TESTING_BEGIN();
@@ -185,5 +222,6 @@ int main(void)
 	a_or_b_or_c_is_unchanged();
 	subexpr_a_is_unchanged();
 	a_plus_becomes_subexpr_aa_star();
+	a_qmark_becomes_subexpr_empty_or_a();
 	return TESTING_END();
 }