[8.x] Add ValidatorAwareRule interface

[bastien-phi]

In #36960 Nuno added a DataAwareRule interface which allows a Rule to have access to the data under validation. This is so great !

Unfortunately, it does not allows the Rule to gain access to :

• 1 - does the current field already have validation errors ?
• 2 - what are the other rules applied to the current field ?
• 3 - does other fields has validation errors ?
• 4 - what are the rules applied to other fields ?

Why do we need this ? I will give it some examples but possibilities are limitless

1 - If we want to check some model existence with some custom condition
Actually

$rules = [
   'user_id' => [
        'required',
        'integer',
        Rule::exists(User::class)
        ->using(fn ($query) => (new User)->newEloquentBuilder($query)->whereIsAuthor())
    ]
]

or

$rules = ['user_id' => ['required', 'integer', 'bail', AuthorExists::class]];

class AuthorExists implements Rule 
{
    public function passes($attribute, $value)
    {
        return User::where('id', $value)->whereIsAuthor()->exists();
    }
}

Here the bail is required to ensure that AuthorExists::passes is not executed with unsafe data.
After

$rules = ['user_id' => ['required', 'integer', AuthorExists::class]];

class AuthorExists implements Rule, ValidatorAwareRule
{
    private $validator;

    public function setValidator($validator)
    {
        $this->validator = $validator;

        return $this;
    }


    public function passes($attribute, $value)
    {
        if ($this->validator->errors()->has($attribute)) {
            return true; 
        }

        return User::where('id', $value)->whereIsAuthor()->exists();
    }
}

The bail here in included directly in the rule, which matches what is done with exists rule (see https://github.com/laravel/framework/blob/8.x/src/Illuminate/Validation/Validator.php#L657 and https://github.com/laravel/framework/blob/8.x/src/Illuminate/Validation/Validator.php#L733)

2 - We may want the rule applied in different ways depending on the previous defined rules.
Which is the way that size, min, ... actually work.
Another use case would be to fetch the date_format in order to have create a DateTime and make assertions about this :

class Monday implements Rule, ValidatorAwareRule
{
    public function passes($attribute, $value)
    {
        if (!$this->validator->hasRule($attribute, 'DateFormat')) {
             throw new Exception("The $attribute must be validated with date_format.");
        }
        if ($this->validator->errors()->has($attribute)) {
            return true;
        }
        // extract  format.
        // this part would be easier if Validator::getRule($attribute, $rules) was public but not in scope of this PR
        [, $params] = collect($v->getRules()[$attribute])
                                    ->map(fn ($rule) => ValidationRuleParser::parse($rule))
                                    ->first(fn ($items) => $items[0] === 'DateFormat'); 
        
         return Date::createFromFormat($params[0], '2021-05-21')->dayOfWeek === Carbon::MONDAY;
    }
}

3 - 4 - Imagine we are building a booking service and we want to validate that the location is available a given day.

With this PR we could simply do something like this :

$rules = [
    'date' => ['required', 'date_format:Y-m-d'],
    'location_id' => ['required', 'integer', Available::class],
];

class Available implements Rule, ValidatorAwareRule 
{
    public function passes($attribute, $value)
    {
        if ($this->validator->errors()->hasAny(['date', $attribute])) {
            return true; 
        }

        return Location::where('id', $value)->whereAvailableOn($this->validator->getData()['date'])->exists();
    }
}

TL;DR : this PR adds only a few lines in the framework but I believe opens up great possibilities and power to custom Rules

[bastien-phi]

Thanks !