Allow per test rules in neverallow_test.go

Makes testing individual rules easier by allowing them to be specified
per test rather than having to add them to the global defaults.

Bug: 138428610
Test: m nothing
Change-Id: Ic65a55dee2a02b6d33254753c047295dd5804408

android/neverallow.go

@@ -177,7 +177,7 @@ func neverallowMutator(ctx BottomUpMutatorContext) {
 
 	osClass := ctx.Module().Target().Os.Class
 
-	for _, r := range neverallows {
+	for _, r := range neverallowRules(ctx.Config()) {
 		n := r.(*rule)
 		if !n.appliesToPath(dir) {
 			continue
@@ -551,3 +551,19 @@ func matchValue(value reflect.Value, check func(string) bool) bool {
 
 	panic("Can't handle type: " + value.Kind().String())
 }
+
+var neverallowRulesKey = NewOnceKey("neverallowRules")
+
+func neverallowRules(config Config) []Rule {
+	return config.Once(neverallowRulesKey, func() interface{} {
+		// No test rules were set by setTestNeverallowRules, use the global rules
+		return neverallows
+	}).([]Rule)
+}
+
+// Overrides the default neverallow rules for the supplied config.
+//
+// For testing only.
+func setTestNeverallowRules(config Config, testRules []Rule) {
+	config.Once(neverallowRulesKey, func() interface{} { return testRules })
+}