Fix false positive in unused-output-variable (#1398)

Comprehension term vars are never unused, and should not be
flagged.

Fixes #1353

Signed-off-by: Anders Eknert <anders@styra.com>

bundle/regal/rules/bugs/unused-output-variable/unused_output_variable.rego

@@ -30,6 +30,7 @@ report contains violation if {
 	not ast.var_in_head(input.rules[to_number(rule_index)].head, var.value)
 	not ast.var_in_call(ast.function_calls, rule_index, var.value)
 	not _ref_base_vars[rule_index][var.value]
+	not _comprehension_term_vars[rule_index][var.value]
 
 	# this is by far the most expensive condition to check, so only do
 	# so when all other conditions apply
@@ -52,3 +53,12 @@ _ref_base_vars[rule_index][term.value] contains term if {
 
 	not ast.is_wildcard(term)
 }
+
+_comprehension_term_vars[rule_index] contains var.value if {
+	some rule_index, comprehensions in ast.found.comprehensions
+	some comprehension in comprehensions
+
+	only_head := object.remove(comprehension.value, ["body"])
+
+	some var in ast.find_term_vars(only_head)
+}

bundle/regal/rules/bugs/unused-output-variable/unused_output_variable_test.rego

@@ -176,3 +176,24 @@ test_success_not_output_variable_argument if {
 	r := rule.report with input as module
 	r == set()
 }
+
+test_success_not_unused_comprehension_term if {
+	module := ast.with_rego_v1(`success if {x | input[x]}`)
+
+	r := rule.report with input as module
+	r == set()
+}
+
+test_success_not_unused_comprehension_term_complex if {
+	module := ast.with_rego_v1(`success if {[x, y] | input[x][y]}`)
+
+	r := rule.report with input as module
+	r == set()
+}
+
+test_success_not_unused_comprehension_term_key_value if {
+	module := ast.with_rego_v1(`success if {x: y | input[x][y]}`)
+
+	r := rule.report with input as module
+	r == set()
+}