Buffer types should utilize Memory<T> and implement IPinnable, IMemoryOwner<T> et.al

[allisterb]

Since Memory<T> and related types and the System.Memory package is now generally available on all .NET platforms, all jemalloc.NET API types like FixedBuffer<T> should utilize these types.

[antonfirsov]

Also be aware that the API shape of System.Memory went through significant changes. Eg. IRetainable does no longer exist, implementing reference counting is optional.