Isn't the style rule that multi-line blocks are always do/end? And should the formatter enforce this?

Uh well my personal style certainly isn't, the formatter doesn't seem to enforce it and https://crystal-lang.org/docs/conventions/coding_style.html doesn't mention anything about it.

The ruby style guide discourages it for the reason that multi-line brace blocks encourage chaining methods on the return values of multi-line blocks. And I tend to agree with it. I'm not sure what the core team thinks though.

I roughly follow Weirich's rule personally, with the variation of communicating whether I care about the call's return value rather than the block's. Most of the time that matches up but this is one of the cases where it doesn't.

In my opinion, the stdlib should stick to one rule, and that rule should be easy to enforce.