How to pass around large immutable data? Copy-on-Write?

[Araq]

Note that the compiler does **not** perform "lent" inference and the existing 
"sink" inference had to be disabled -- separate compilation put a spoke in the 
wheel. Fortunately the resulting language is still perfectly usable and no new 
type modifiers are planned either, so it doesn't get worse than that. (Famous 
last words...)