Rewrites standard-library calls that are other Rusty-Elixir into that Rusty-Elixir, before RustQ.Meta.Ownership and lowering see a body, so lowering keeps to forms Rust has:

• Map.new/1,2, MapSet.new/1,2, and Enum.into/2,3 into an empty collection are a comprehension collecting into it;
• Enum.group_by/2,3 is a map filled by a loop, and Enum.uniq/1 and Enum.uniq_by/2 keep the items a set has not seen;
• a generator over Enum.with_index(list) enumerates the list in place.

Where a rewrite needs what only lowering knows, it leaves an internal form for lowering to resolve: __rustq_items__(list) iterates a list as its ownership allows, and __rustq_owned__(item, list) is an item of list, copied where that costs nothing and cloned otherwise.