Linter to make properties and methods only accessible through `$this`

[lexidor]

final class Example {
  public function __construct(
    private int $private,
    <<ThisOnly>> private int $thisOnly,
  ) {}

  private function priv(): void {}
  <<ThisOnly>>
  private function thisMethod(): void {}
)

Within the body of this class, $this->thisOnly and $this->thisMethod() would be valid. expression->thisOnly and expression->thisMethod() would be an error.

There is no need to check the type of expression. There is a small chance that thisOnly is a public property on some unrelated class, but this would be a known false positive.

The ThisOnly attribute name is a placeholder. Feel free to suggest a better one. Important, this Linter should work on any attribute matching the name. It would be a shame if hhast became a dependency of a project because it needs the attribute. Hhast is a dev dependency.

facebook/hhvm#7574