I wasn’t talking about Psalm - I was talking about Hakana, a Rust project that informs much of Mago’s type-checking architecture. Hakana was designed to analyse Hack, a fork of PHP that doesn’t support PHP magic methods and so is much easier to analyse statically.

The big challenge to typechecking PHP is that it’s essentially two different languages — there’s typechecking code like Laravel that makes heavy use of magic methods (effectively impossible statically) and typechecking code that doesn’t (very doable).

Psalm was mostly designed for the latter, whereas PHPStan excelled at the former with dynamic analysis in plugins like Larastan.

That dynamic analysis — where you have to run some PHP code to make sense of a PHP codebase — is another big reason a Rust-based tool will have a hard time becoming popular in the PHP ecosystem.