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

[Charlie Monroe]

> On Jul 19, 2016, at 8:49 AM, Brent Royal-Gordon <brent at architechies.com> wrote:
> 
>> On Jul 18, 2016, at 10:05 PM, Charlie Monroe <charlie at charliemonroe.net> wrote:
>> 
>>> 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.
>> 
>> Wouldn't this disallow access to the capacity field? That's as well defined on ManagedProtoBuffer and AFAIK can be accessed during the initialization.
> 
> Yes, which is why I suggested it should be passed in to the `initialHeader` closure too. (It's read-only anyway, so there's no loss of capability.) What it *does* prevent access to is `withUnsafeMutablePointerToHeader`, but I'm not sure how that method is supposed to work before the header's been initialized anyway.

I'd personally just remove the ManagedProtoBuffer and note in the documentation of ManagedBuffer.create that you should not read the header from within the block. I think it's a similar programming error as indexing out of bounds which is also documented to be a programming error.

I'm not sure whether we could simply eliminate ManagedBuffer altogether since looking at https://github.com/search?l=swift&q=ManagedBuffer&type=Code the only places this is used are generally just forks of apple/swift and I haven't found a single use ManagedBuffer.create in the entire stdlib, all of the Array code uses ManagedBufferPointer directly and initializes it with the unsafeBufferObject, which is some subclass of _ContiguousArrayStorageBase...

> 
> -- 
> Brent Royal-Gordon
> Architechies
>