Allow parser user code blocks in tree mode

spec/test_parser_user_code_tree.c

@@ -0,0 +1,39 @@
+#include "testparser.h"
+#include <assert.h>
+#include <string.h>
+#include "testutils.h"
+
+int main()
+{
+    char const * input = "ab";
+    p_context_t * context;
+    context = p_context_new((uint8_t const *)input, strlen(input));
+    assert_eq(P_SUCCESS, p_parse(context));
+
+    /* The parser user code recorded values accessed via $$, $1, and $2 while
+     * the tree node for the Start rule was being formed. */
+    assert_eq(3, context->start_n_fields);
+    assert_eq(11, context->start_a_value);
+    assert_eq(11, context->a_value);
+    assert_eq(22, context->b_value);
+    assert_eq(TOKEN_b, context->b_token);
+
+    /* The empty-matched rule C has a null $$ tree node, and its field in the
+     * Start node is null as well. */
+    assert_eq(1, context->c_is_null);
+    assert_eq(1, context->c_field_is_null);
+
+    /* Field aliases reference the same component tree nodes as the positional
+     * references. */
+    assert_eq(11, context->alias_a_value);
+    assert_eq(22, context->alias_b_value);
+
+    Start * start = p_result(context);
+    assert(start->pA != NULL);
+    assert(start->pB != NULL);
+    assert(start->pC == NULL);
+    p_tree_delete(start);
+    p_context_delete(context);
+
+    return 0;
+}