Ensure all quote! tokens have the default Span

[alexcrichton]

Apparently parse can return some... "flavorful" spans! This resets all
spans coming out of parse back to that of Default::default to ensure
that we're spanned correctly. This also just so happens to fix an error
I was seeing locally, although I have no idea why.

[dtolnay]

I would prefer to fix this somewhere other than quote if possible. It looks like these are the two impls involved: 1 2 both of which just call into libproc_macro. Is it possible to apply this fix in libproc_macro? If not, is it possible to fix in those impls in proc-macro2?

[alexcrichton]

Hm ok thinking on this more this may actually be the right fix I think. I believe the span information coming out is otherwise correct as we're basically stringifying a token at compile time and then parsing it at runtime. The span information points to that token itself (and sort of a source file that just has the one token in it). I think in general the tokens produced by quote! have the Span::default() value (created explicitly in a few locations for things like parentheses) and then this is just touching up other tokens like operators and identifiers.

In that sense I'm not entirely sure where else the fix would go? I don't think we'd want to change parse for that reason b/c you may not always want parse to return the default span

[alexcrichton]

Er wait, this fixed one thing locally and broke tons of others, now I have no idea what's going on... Should certainly hold off on merging!

[alexcrichton]

Ok so I made a much longer writeup with my findings here about the use case I was working with. This fix is not enough in that we also needed to use call_site instead of default to get things working. In the meantime I'm going to close this while I wait for more info on that issue.

Sorry for the noise!