`impl ToTokens for SpannedValue` is bogus

[H2CO3]

I just stumbled upon this:

impl<T> ToTokens for SpannedValue<T> {
    fn to_tokens(&self, tokens: &mut TokenStream) {
        let mut group = proc_macro2::Group::new(proc_macro2::Delimiter::None, Default::default());
        group.set_span(self.span);
        tokens.append(group);
    }
}

I.e., SpannedValue<T> always expands to an empty group. For something that's supposed to be a transparent wrapper, this is very surprising behavior. It should instead be:

impl<T: ToTokens> ToTokens for SpannedValue<T> {
    fn to_tokens(&self, tokens: &mut TokenStream) {
        let mut group = proc_macro2::Group::new(proc_macro2::Delimiter::None, self.value.to_token_stream());
        group.set_span(self.span);
        tokens.append(group);
    }
}

I do realize you may be abusing the ToTokens impl as a getter for the span through the Spanned blanket impl, but that's no good. You can provide a separate .span() method instead.