[13.x] Add BackedEnum support to SimpleMessage mailer method

[julesjanssen]

Adds Enum support to the mailer() method on Illuminate\Notifications\Messages\SimpleMessage (and therefore MailMessage), matching the behavior already present in Illuminate\Mail\MailManager::mailer().

This allows a dedicated mailer to be selected using an enum when composing a mail notification, instead of a hard-coded string:

enum Mailer: string
{
    case Postmark = 'postmark';
    case Ses = 'ses';
}

public function toMail(object $notifiable): MailMessage
{
    return (new MailMessage)                                                                                                                                                
        ->mailer(Mailer::Postmark)
        ->subject('Welcome')
        ->line('Thanks for signing up!');
}

Plain strings continue to work as before — the value is resolved through the enum_value() helper.

[jackbayliss]

If this is to match MailManager::mailer, shouldnt it be UnitEnum? 👀

[julesjanssen]

Hmm. You're right, but how is that even useful?
How / why would I use this method with a non-backed Enum?

[shaedrich]

enum_value() returns

• BackedEnum::$value
• UnitEnum::$name
• the string value passed directly

[julesjanssen]

Makes sense. Thank you.
I updated the PR.

[taylorotwell]

I've got Enum PR fatigue.