[SR-11300] Provide a RawPointer version of ManagedBuffer

[Lukasa]

Previous ID	SR-11300
Radar	rdar://problem/54262180
Original Reporter	@Lukasa
Type	Improvement

Additional Detail from JIRA

Votes	2
Component/s	Standard Library
Labels	Improvement
Assignee	None
Priority	Medium

md5: f9e37d1629f65a324f493c1dffb093ff

Issue Description:

ManagedBuffer is useful for tail-allocating arrays of data, avoiding an extra level of indirection and saving allocations in the process.

However, some C APIs use "variable length" data structures, usually in the form of a zero-length array declared at the back of the structure, and ask their users to malloc appropriately-sized buffers for them. This doesn't import well into Swift, requiring us to use the "class holding an UnsafePointer" method of CoW-boxing, incurring an extra indirection in order to use the data behind the pointer.

We could resolve this if we had a variation of ManagedBuffer that gives the user an UnsafeMutableRawPointer to the tail storage, instead of a typed pointer. That would allow the user to bind and initialise the memory as they see fit, supporting variable length data structures.

[Lukasa]

@swift-ci create

[belkadan]

cc @atrick

[atrick]

Thanks for filing this. It isn't the first time I've seen someone want this.

In the meantime, I think you can create a typed `ManagedBuffer<Uint8>` and downcast from `UnsafePointer<UInt8>` to `UnsafeRawPointer` whenever you use it, possibly with a wrapper type.

I also notice that `ManagedBuffer` vends a standalone pointer instead of a buffer pointer which is not best practice.