56cd04af5c
Ensure that queries only return Copy types. This should pervent the perf footgun of returning a result with an expensive `Clone` impl (like a `Vec` of a hash map). I went for the stupid solution of allocating on an arena everything that was not `Copy`. Some query results could be made Copy easily, but I did not really investigate.
Please read the rustc-dev-guide chapter on Backend Agnostic Codegen.