I was on the rust mailing list for a little while about a year ago. Around that time there was discussion of this exact subject. Most of the discussion was around exactly how to break parameters beyond a certain length and so forth, but it was there.
They're actually RFCs. And the results are in a repository. I'm on mobile or I'd link you. But they generally codify already-existing style, rather than mandating it, although sometimes we just need a decision to tear down a bikeshed...