> I could theoretically transform any span into pointing at the entire string, though. But I don't know how to do that

Yes, that's exactly what I meant. (Use a Folder overriding `new_span`.)

> It can't refer to the #[default] string itself, because the parsed string doesn't necessarily map one-to-one with the string written in the source

If you were interested in doing sub-spans into the string, anything that has an error does correspond to some sequence of characters in the as-written source (i.e. if the `42i` mismatched types, then you could have a length-3 span pointing to just that, the fact it's surrounded by `\t`s etc. is irrelevant).

> but what about errors raised by the parsing itself

Yeah, not sure about this. In any case, we can improve the error messages in some cases so we should.