Add rule checking coalesces

[leongersen]

In a similar vein to #25, this adds a rule checking that the left side of a coalesce statement is actually valid.

[ondrejmirtes]

Again, the rule isn't added to any level. Should be level 1.

[leongersen]

@ondrejmirtes, @BackEndTea I think I've addressed your comments on this PR, let me know if there's anything else you'd like me to improve. Thanks for your reviews so far!

[BackEndTea]

How does this behave when the left side is a function call?

e.g.

$a = rand(0,1) ?? 10;

I'd expect an error here, as rand cant return null.

Also, how does this behave with 'always null'? e.g.

function (?string $a) : void
{
  if(!is_string($a) {
    $a ?? = 'foo';
  }
}

This always triggers are $a is always null here.

[leongersen]

@BackEndTea Good points. I've added support for FuncCall, as well as PropertyFetch and StaticPropertyFetch. I've also added checks for "type is always null" everywhere.

[BackEndTea]

👍 Looks pretty good to me, it even found a fault in the phpstan source code

[ondrejmirtes]

Hi, I just merged it as: 3608ded

I made some mostly cosmetic changes to the error messages, and cover more cases. The core logic is really solid and I appreciate it, it couldn't be easy to come up with it! :)

I have more work to be done I will cover myself over this weekend:

1. Reuse the same logic in existing VariableCertaintyInIssetRule.
2. Report variables on level 1 and the rest on level 4. Currently all of this is reported on level 1.
3. Create a new rule that reports the same things for isset() on level 4.

[leongersen]

@ondrejmirtes Thanks a bunch for your help on this one!