[swift-evolution] [Pitch] Eliminate `ManagedProtoBuffer`

[Brent Royal-Gordon]

> On Jul 18, 2016, at 4:13 PM, Erik Eckstein via swift-evolution <swift-evolution at swift.org> wrote:
> 
> The reason why `ManagedProtoBuffer` (the base class of `ManagedBuffer`) exists is to give the users an extra bit of type safety inside of the closure `initialHeader` passed to `ManagedBuffer.create()`.
...
> This closure receives the `ManagedBuffer` instance and returns the initial value that is stored in the buffer (the header part of the buffer).  We are passing the `ManagedBuffer` as `ManagedProtoBuffer` to prevent the closure from reading the uninitialized `value` property. This property is defined in `ManagedBuffer` but not in `ManagedProtoBuffer`.
...
> This proposal suggests removing `ManagedProtoBuffer` in order to simplify the API.
> It means that  `ManagedBuffer` would not be derived from `ManagedProtoBuffer` and all methods from `ManagedProtoBuffer` would be moved to `ManagedBuffer`.
> The closure `initialHeader` would receive a  `ManagedBuffer` instead of a `ManagedProtoBuffer`.

I haven't used `ManagedBuffer`, but would it make sense to change the signature of `initialHeader` to `@noescape (elements: UnsafeMutablePointer<Element>, capacity: Int) -> Header` and then effectively run it inside a `withUnsafeMutablePointerToElements()` call? That would prevent access to the uninitialized `header` field while also allowing us to eliminate the `ManagedProtoBuffer` type.

-- 
Brent Royal-Gordon
Architechies