Ownership adoption case study: building a wrapper type

[Andrew_Trick]

str1.myMutableExt.mutatingMethod() // sorry, but not working

We always planned to fix this but haven't committed to a solution. @John_McCall alluded the "re-borrowing" approach here: `exclusive` parameter ownership modifier - #23 by John_McCall

While I don't think it's been pitched yet, I like everything about it... except the name :) It should be called the "exclusive-copy" approach.

The Mutable/Immutable type dichotomy is annoying but pretty fundamental to Swift ownership as @John_McCall explained here:

Safe element ownership for lifetime-restricted buffer types

The Swift standard library currently provides some buffer types: UnsafeBufferPointer and so on. Other than being well-typed about the elements they store, these types are essentially completely unsafe: not only do they not represent ownership of the underlying memory, they also don't provide any static information about the ownership of the values stored in that memory; in fact, there might not even be any values in that memory because it could be totally uninitialized (or partially uninitializ…

I’ll leave it to others to suggest struct-extension alternatives.