private Optional<ValidationEvent> validateBuiltIn(
            ServiceShape serviceShape,
            String builtInName,
            FromSourceLocation source,
            String... eventIdSuffixes
    ) {
        if (!EndpointRuleSet.hasBuiltIn(builtInName)) {
            return Optional.of(error(serviceShape,
                    source,
                    String.format(
                            "The `%s` built-in used is not registered, valid built-ins: %s",
                            builtInName,
                            EndpointRuleSet.getKeyString()),
                    String.join(".", Arrays.asList(eventIdSuffixes))));
        }
        return Optional.empty();
    }
Validates that the built-in's specified on parameters are supported.
smithy-rules-engine/src/main/java/software/amazon/smithy/rulesengine/validators/RuleSetBuiltInValidator.java