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Afaik Swift is pretty close to using Rc and Arc everywhere. Does swift do some more escape analysis than Rust?

• janef0421 4y ago
  Swift only uses reference counting when working with objects; Structs are optimised using copy-on-write. There's currently work on implementing move and ownership semantics, similar to Rust, but opt-in rather than by default.
  • meindnoch 4y ago
    Ummm… No. Swift structs are just like C structs (from a memory perspective). The copy-on-write thing is implemented manually by storing a private refcounted object in your struct. See the implementation of Array for example: https://github.com/apple/swift/blob/main/stdlib/public/core/Array.swift https://github.com/apple/swift/blob/main/stdlib/public/core/... There’s no magical copy-on-write mechanism at the language level.
    • jshier 4y ago
      I don't know why this is downvoted, it's true. There is no automatic copy on write optimization in Swift. It's a manual optimization that expensive types like Array implement manually.
      • meindnoch 4y ago
        >I don't know why this is downvoted, it's true. I know, right? Some people are pretty insecure…
• manmal 4y ago
  I haven’t read that much on Rust, but that should be pretty much it. Swift has an emphasis on value types, which often are stored on the stack, but only up to a certain size. Copy-on-write makes this feasible - often, those value types are passed by reference until a write actually occurs, but that’s opaque to the dev. Value types can have pointers to reference-counted types - if a value type is passed/copied, any pointer it owns is retained (weirdly enough, they claim that value types have no „lifetime“, but at some point those refs have to be released - we just have to trust the compiler in this).