I propose @{Attr} for the attributes syntax.

Pro:

• no bc breaks
• end delimiter
• some similarity to docblock annotations (same es @@, @: and @[])
• looks less like an array (compared to @[Attr] and #[Attr])
• for me it fits well to the grouped use syntax (use App\{Foo, Bar, Baz as Qux}), where {} is already used for a comma separated list

Con:

• no forward compatibility
• not used by any other language
• the grouped multiline variant looks a bit like a silenced block

@{
  ORM\Entity,
  ORM\Table("user")
}
class User
{
    @{ORM\Id, ORM\Column("integer"), ORM\GeneratedValue}
    private $id;
    @{ORM\Column("string", ORM\Column::UNIQUE)}
    @{Assert\Email(["message" => "The email '{{ value }}' is not a valid email."])}
    private $email;
}

@beberlei Is it possible to include this in https://wiki.php.net/rfc/shorter_attribute_syntax_change? Since there will be a STV anyway, I think it doesn't hurt to have one more option to choose from.