[Request for Feedback] Partial Consumption of Fields of NonCopyable Types

[Michael_Gottesman]

@ksluder

@Michael_Gottesman

We ban partial consumption of noncopyable types with deinits since when a field
is partially consumed, we are allowing for the type to be destroyed in
parts.

Can this restriction be relaxed if the type is “made whole” before the borrow ends? Otherwise one still couldn’t do reasonable things like:

struct Queue<T> : ~Copyable {
  var buf = UnmanagerBuffer<T>()
  mutating func append(_ element: T) {
    buf.append(element)
  }

  // contract: you must free the UnmanagedBuffer yourself
  mutating func drain() -> UnmanagedBuffer<T> {
    // avoid copying or retaining buf
    let buffer = consume buf
    buf = UnmanagedBuffer<T>()
    return buf
  }

  deinit { buf.free() }
}

You can still do that today by consuming self like I mentioned above:

struct Queue<T> : ~Copyable {
  var buf = UnmanagerBuffer<T>()
  mutating func append(_ element: T) {
    buf.append(element)
  }

  // contract: you must free the UnmanagedBuffer yourself
  mutating func drain() -> UnmanagedBuffer<T> {
    // avoid copying or retaining buf
    let buffer = (consume self).buf
    self = Queue<T>(UnmanagedBuffer<T>())
    return buffer
  }

  deinit { buf.free() }
}

... but I get your larger point that just like it is nice to have the expressivity of not having to reconstruct self. For instance, consider if self's initializers have a bunch of arguments or if self only has non-trivial inits with business logic that you don't want to have to turn off when just trivially reconstructing self.