Add Hack-like option to forget property assignments after function call

[muglug]

This is a feature request for a feature I don't think many are interested in. Still, worth thinking about.

class X {
  /** @var ?int **/
  private $x;

  public function getX(): int {
    $this->x = 5;

    $this->modifyX();

    return $this->x;
  }

  private function modifyX(): void {
    $this->x = null;
  }
}

Hack forbids the above code (though to see it you have to change the property type to one hack understands). Psalm, by default, allows the above code, but a config flag makes it follow Hack's behaviour.

[ondrejmirtes]

How do you do it? Do you recursively inspect the called methods bodies what properties do they touch?

[muglug]

Oh I don't do anything sophisticated. Just as Hack does, I remove all the cached types for properties on all objects in scope after every function call, method call or new.

Meaning this code also fails when that config is on (ditto in Hack):

class X {
  /** @var ?int **/
  private $x;

  public function getX(): int {
    $this->x = 5;

    $this->modifyX();

    return $this->x;
  }

  private function modifyX(): void {
    $this->x = 7;
  }
}

[Fuco1]

Flow for javascript does something similar as well and I find it quite annoying at times. But at least it forces you to write "pure" code and not mess with stuff by side effect which is not a bad thing.

[muglug]

Yeah, I don’t find the option all that useful (enabling it for Psalm’s own codebase produces 100s of false positives), but we have it enabled for a transactions-related repo that needs to be bulletproof.

[muglug]

Just a note that Psalm now does a bit more inference, so the above code doesn’t need a special config flag to work.

At the scanning step (where Psalm gathers docblocks and other metadata about the code) it checks to see if a method contains $this->{some property} = ... and treats that method as one that resets that property back to an unknown state.

Happens here: vimeo/psalm@85ae8f9#diff-b75faef2d9b16277900cfa9edff604d5R469-R484

and here: vimeo/psalm@85ae8f9#diff-11f0b4d80267531254d3fdd0fbc67248R1305-R1308

[ondrejmirtes]

The assignment is now forgotten when calling void function and also function that's annotated with @phpstan-impure.

[github-actions]

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