https://forums.swift.org/t/why-does-the-withunsafemutablebufferpointer-closure-take-an-inout-parameter/6794/11 フォーラム検索したら似たような指摘がありました。

Why does the withUnsafeMutableBufferPointer closure take an inout ...

A lot of the MutableCollection implementation is in protocol extensions (swapAt, for example.) Should an additional version be written just for the Unsafe*BufferPointer types? Makes sense to me, given the examples above. It doesn’t seem to be a high priority task though, ...

1:33 AM

As far as I can see, MutableCollection defines four mutating methods: swapAt(), sort(), reverse(), and partition(). Implementing those as non-mutating methods of UnsafeMutableBufferPointer seems natural to me.