Allow parameters and constants in top level rule (#347)

The `top-level-iteration` is meant to detect rules that have iteration
without function body. Referencing parameters or constants should be
okay without a function body. This change allows that.

bundle/regal/rules/bugs/top_level_iteration.rego

@@ -15,14 +15,29 @@ report contains violation if {
 	rule.head.value.type == "ref"
 
 	last := regal.last(rule.head.value.value)
-	last.type == "var"
 
-	illegal_value_ref(last.value)
+	last.type == "var"
+	illegal_value_ref(last.value, rule)
 
 	violation := result.fail(rego.metadata.chain(), result.location(rule.head))
 }
 
 _rule_names := {ast.name(rule) | some rule in input.rules}
 
-# regal ignore:external-reference
-illegal_value_ref(value) if not value in _rule_names
+_arg_names(rule) := {arg.value | some arg in rule.head.args}
+
+_path(loc) := concat(".", {l.value | some l in loc})
+
+illegal_value_ref(value, rule) if {
+	# regal ignore:external-reference
+	not value in _rule_names
+	not is_arg_or_input(value, rule)
+}
+
+is_arg_or_input(value, rule) if {
+	value in _arg_names(rule)
+}
+
+is_arg_or_input(value, rule) if {
+	startswith(_path(value), "input.")
+}

bundle/regal/rules/bugs/top_level_iteration_test.rego

@@ -48,3 +48,13 @@ test_success_top_level_input_ref if {
 	r := rule.report with input as ast.with_future_keywords(`x := input.foo.bar[input.y]`)
 	r == set()
 }
+
+test_success_top_level_const if {
+	r := rule.report with input as ast.with_future_keywords(`x := input.foo.bar[4]`)
+	r == set()
+}
+
+test_success_top_level_param if {
+	r := rule.report with input as ast.with_future_keywords(`x(y) := input.foo.bar[y]`)
+	r == set()
+}