Custom collection constraint

[theofidry]

Q	A
Bug report?	no
Feature request?	yes
BC Break report?	no
RFC?	no
Version/Branch	1.6.0

As of now constraints can be applied as such:

AddUserInput:
    type: input-object
    config:
        fields:
            username:
                type: String!
                validation: # using an explicit list of constraints
                    - NotBlank: ~
                    - Length: { min: 6, max: 32 }
            password:
                type: String!
                validation: App\Entity\User::$password # using a link
            email:
                type: String!
                validation: # mixing both
                    link: App\Entity\User::$email
                    constraints:
                        - Email: ~

User:
    type: input-object
    config:
        validation:
            - Callback: [App\Validation\UserValidator, validate]
        fields:
            username:
                type: String!
            password:
                type: String!
            email:
                type: String!

Maybe it is more of a personal thing but I find easier to sometimes define a custom constraint (potentially with its own validator) in plain PHP code. In other words, I have a Collection constraint AddUserInput, which I validate in my resolver:

$violations = $this->validator->validate($input, new AddUserInput());

if (count($violations) > 0) {
    throw new ArgumentsValidationException($violations);
}

And whilst it works, I wish I could just do this:

AddUserInput:
    type: input-object
    config:
        validation
            - App\GraphQL\Validation\AddUserInput: ~
        fields:
            username:
                type: String!
            password:
                type: String!
            email:
                type: String!

Current this will not work because it is a Collection constraint which cannot be applied to a class. However in usage, it does not really need to be a class. Do you have any idea how we could make this work?