Rust needs a safe abstraction over uninitialized memory

language design
98

In light of @RalfJung's comment here:

The discussion quickly moved to freeze as that is usually the primitive people ask for here. arbitrary() is compatible with what I laid out for inline asm blocks so I am confident it is compatible with the rest of the compiler.

Maybe we should do an informal poll as to whether people would prefer an API like:

fn arbitrary<T: FromBytes>() -> T;

or

fn freeze<T: FromBytes>(data: MaybeUninit<T>) -> T;

The difference being that freeze() must preserve data that was already initialized.