Expressing Rust Move-Only Types in Swift

[nvzqz]

In Rust, types that don't implement Copy are considered to be move-only and can only be copied via Clone or other means.

In Swift, there's currently no notion of moving a value. Reassignments of a value result in an implicit copy. For class types or anything that contains a class member, this will increment the strong reference count.

There seems to be three approaches to this:

1. Retrofit a Move protocol that gives automagical move semantics.

   Pros:

   • Doesn't require Swift users to rewrite any existing code.

   • Allows for deinit on non-class types.

   Cons:

   • Wouldn't work in the context of generics. Implicit copying would still be the default there unless a generic type is specified as Move. Rust's move-only types work in generics because all types are assumed to be move-only unless a generic type is specified as Copy.

2. Introduce a Copy protocol and assume move-only otherwise. Essentially, just copy Rust.

   Pros:

   • Allows for easier FFI when Swift types cross over into Rust-land.

   • Fine-grained control over move semantics types.

   • Works in the context of generics out-of-the-box.

   Cons:

   • Results in significant added complexity, increasing the learning curve. Implicit copying helps keep Swift succinct and expressive.

   • Massive source compatibility break. Requires stdlib and Swift users to rewrite all implicit copies and opt into Copy where appropriate.

3. Introduce a special Move<T> wrapper type that gives a type move semantics.

   Pros:

   • Allows for basic move-only types.

   Cons:

   • Doesn't gain much. Really only makes existing code more verbose.