Change error to warning on normal-derivable traits

[Niedzwiedzw]

I'm trying to write something quickly to test it using macro that runs your macro and it'd be annoying to fix. Also I think warning should be sufficient of a punishment?

derive-where/src/error.rs

Line 51 in 61ce435

pub fn use_case(span: Span) -> syn::Error {

[ModProg]

Puh... currently all you can do for warning is generate some code and set it to deprecated...

But we could consider. Once the new proc macro diagnostics are a thing I'd be for this 100% but currently I'm not sure.

@daxpedda what do you think?

[daxpedda]

I'm also absolutely in favor of demoting these kind of errors to warnings. I remember that we tried this at the beginning, but were really unable to make it work reliably.

So AFAICT the problem here is implementation.