There was a talk at the Copenhagen rust group a few months ago about someone using Rust for GraphQL in production, but I don't think it was ever added to the agenda for that month, so I can't find it. You could write to the email listed on every entry and ask if they remember who and which company it was.
As for the four listed options, I would probably go for rocket or warp. Hyper is relatively low level and I haven't heard of anyone using Iron in a long time.