| > Why would a stack frame want to move ownership to its callee Rust's system of ownership and borrowing effectively lets you hand out "permissions" for data access. The owner gets the maximum permissions, including the ability to hand out references, which grant lesser permissions. In some cases these permissions are useful for performance, yes. The owner has the permission to eagerly destroy something to instantly free up memory. It also has the permission to "move out" data, which allows you to avoid making unnecessary copies. But it's useful for other reasons too. For example, threads don't follow a stack discipline; a callee is not guaranteed to terminate before the caller returns, so passing ownership of data sent to another thread is important for correctness. And naturally, the ability to pass ownership to higher stack frames (from callee to caller) is also necessary for correctness. In practice, people write functions that need the least permissions necessary. It's overwhelmingly common for callees to take references rather than taking ownership, because what they're doing just doesn't require ownership. |