I would rather be explicit here than allowing nothing and prepending an assumed `=`. At the macro-expansion phase, I think that all output constraints without `=` and `+` should be disallowed (and have a note explaining why). It could very easily be that someone forgot a semicolon to make these an input constraint.

Conversely, we should probably be validating that input constraints don't have `=` or `+` for similar reasons (one too many colons by accident).