This seems a bit long and it's not completely accurate either (it accepts classes with __toString() even if they don't implement Stringable). The other alternative I considered was just string, but that might be a bit confusing to users. Is there anything better?

Let's try __stringandstringable. I can see you're already using new StringAlwaysAcceptingObjectWithToStringType in a related place.

@schlndh @ondrejmirtes After enabling this rule I noticed a lot of errors related to string|null types.

Personally, I think replacing those sprintf call arguments with $value ?? '' feels like making a computer happy. As it does exactly the same as what sprintf does for %s.

See https://3v4l.org/TlHU9#v8.4.13

I know this is a strict rule, but wonder if this was really intentional or something that should be reconsidered?

I feel like it's useful that PHPStan lets you know you're potentially passing null to sprintf. You might want to deal with null differently than what sprintf does.

I intentionally implemented it like that to match my understanding of "strict". However, I encountered the same thing: hundreds of issues like this, most of which I don't care about (though I'm fine with letting them all fall into the baseline). So I would be open to changing it. I'm just not sure what's the best way to go about it.

Strict-rules do allow $null . $string and "{$null}", so I guess this wouldn't have to be as strict either (unless it's allowed by omission rather than intentionally). At the very least it's a bit inconsistent.

On the other hand, there are third-party rules (e.g. shipmonk rules) which do prohibit it, so clearly there is also interest in the fully strict version.

Another thing to consider is whether it should be allowed only in %s or also other placehoders?

Strict-rules do allow $null . $string and "{$null}", so I guess this wouldn't have to be as strict either (unless it's allowed by omission rather than intentionally). At the very least it's a bit inconsistent.

This is an excellent point.

Another thing to consider is whether it should be allowed only in %s or also other placehoders?

I would say only for %s.

An idea: string|null should be reported only on level 8+, basicallh RuleLevelHelper should be utilized.

Unfortunately, that won't help me (level 9 🙈)

I would say only for %s.

To expand on that point: %d + null make 0, which is confusable with, well, passing an actual 0 integer. 😄 So for that reason I wouldn't allow it either.