Better error messages at compile time

I just tried to write the following contract (from #7 (closed)):

    #[contracts::post(!ret.lval)]
    #[contracts::post(old(!self.lval) ==> old(self.clone()) == ret)]
    pub(super) fn rval(self) -> Self {
        unimplemented!();
    }

I got the following error message:

error: expected expression

error: aborting due to previous error

error: could not compile `rcc`.

This is so vague as to almost be useless. It doesn't have the span, line, or even file name. Additionally, expected expression doesn't really tell me anything other than 'there was an error'.