[whateveracct]

You can write safe memory management libraries in userspace with the existing extension. That can help reduce GC pressure by moving data off-heap and allow for safer & more efficient FFI bindings.

As someone who has been exploring doing gamedev in Haskell, that all is very exciting and the use-cases are constantly obvious to me.

It is entirely up to userspace to leverage this new feature. It doesn't do anything on its own outside of the type-checker. And to use it, your library will probably have to do some unsafe coercion under the hood (just like ST uses unsafePerformIO under the hood to do pure mutability.)

[dllthomas]

Right, my understanding is that in a sense it doesn't "enable new performance", because operationally speaking it's all stuff you could do anyway. What it does is move a lot of potential approaches from "absurdly unsafe" to "perfectly fine", so whatever your threshold for safety it's likely to move some speed improvements over the line if you care much about speed.