[11.x] Make mailables tappable

[kevinb1989]

I have been enjoying using the tap method from the Illuminate\Testing\TestResponse class, when I got a little logics around assertions.

The Illuminate\Mail\Mailable class contains assertion methods. It would be nice to also make it tappable.

I have the following real-life example from my organization:

use App\Mails\OrderCompleted;
use App\Models\Customer;
use App\Models\Order;

public function test_emails_containing_correct_addresses(country $countryCode)
{
    $order = Order::factory()
        ->for(Customer::factory()->create(['country' => $countryCode]))
        ->create();

    $mail = new OrderCompleted($order);

    $mail->assertHasSubject('Your Order Has Been Fulfilled!');

    match (true) {
        $countryCode === 'AU' => $mail->assertSeeInHtml('64 Barker Street REDMOND WA 6327'),
        $countryCode === 'US' => $mail->assertSeeInHtml('2142 Doctors Drive, Burbank, CA 91505'),
        $order->customer->is_from_europe => $mail->assertSeeInHtml('Gerrit van der Veenstraat 1 1077 DX  Amsterdam'),
    };
}

As a global business, we want to display correct branch address in our emails depending on the customer's location.

I would prefer to do all assertions in one go:

$mail->assertHasSubject('Your Order Has Been Fulfilled!')
    ->tap(function ($mailable) use ($countryCode, $order) {
        match (true) {
            $countryCode === 'AU' => $mailable->assertSeeInHtml('64 Barker Street REDMOND WA 6327'),
            $countryCode === 'US' => $mailable->assertSeeInHtml('2142 Doctors Drive, Burbank, CA 91505'),
            $order->customer->is_from_europe => $mailable->assertSeeInHtml('Gerrit van der Veenstraat 1 1077 DX Amsterdam'),
        };
    });