There is some work ongoing to improve this situation in Swift 4.0. Take/copy/destroy operations for enums are now outlined and shared between call sites, and a further improvement is in the works to pass large structs indirectly even when they are otherwise loadable (no address-only fields).

Another recent improvement is copy-on-write existentials. There's no code size win here, but it improves runtime performance by avoiding copying the payload when passing existentials by value.