Generics for InRule

[timmattison]

I just got bitten by the fact that this code doesn't work how most people would expect:

	optionsList := []string{"a", "b", "c"}

	// I've always felt that this was dangerous
	assert.Error(t, Validate("a", In(optionsList)))

I've made some changes that make this behavior stay the same (validating a list like this will return an error) but makes it more ergonomic to simply pass the list as a variadic like this:

	optionsList := []string{"a", "b", "c"}

	// I've always felt that this was dangerous
	assert.Error(t, Validate("a", In(optionsList)))

	// This is better, no need to convert to interface
	assert.NoError(t, Validate("a", In(optionsList...)))

Without this I had been using a helper function like this:

func validationIn[T any](input []T) validation.InRule {
	interfaceValues := make([]interface{}, len(input))

	for i, v := range input {
		interfaceValues[i] = v
	}

	return validation.In(interfaceValues...)
}

But considering everyone would need to add this function if they want to validate against a list I think it would be nice to have it incorporated into the library.

[samlown]

This is a great idea! Thanks. I appreciate this is from a while back, sorry! I'll see if I can get this merged ASAP.

[samlown]

Very nice. I had an error in some of my code where I was using the validation.In() method without any parameters, but the fix of course was to switch that to a validation.Empty.

Thanks!