ast::Node::accept() not const

[benh]

Any reason that ast::Node::accept() is not const?

It would be great to create a nested visitor while visiting and we can't call accept() on any nested nodes because we always have const pointers or references to them.

[swolchok]

It would be great to create a nested visitor while visiting

yes, yes it would! :)

Making accept const seems reasonable from the perspective of GraphQL execution, given that we don't support any kind of AST editing right now anyway. If we end up with tools that process GraphQL queries written in C++ that want to edit the AST in-place for efficiency reasons, we can worry about adding non-const accept and some way to mutate AST nodes later.

I'll have to think about it a little more when it's not late at night, but nothing else comes to mind off the top of my head.