Quite often, uses of std::mem::uninitialized() end up in unsound code.
Therefore, the MaybeUninit union has been added to std::mem and std::mem::uninitialized() is being deprecated.
However, MaybeUninit has been added quite recently.
Sometimes you might want to support older versions of Rust as well.
Here is where maybe-uninit comes in: it supports ALL stable Rust versions (MSRV: Rust 1.0.0).
Sadly, a feature-complete implementation of MaybeUninit is not possible on stable Rust.
Therefore, the library offers the guarantees of MaybeUninit in a staged fashion:
Rust 1.36.0 onward: MaybeUninit implementation of Rust stable is being re-exported
Rust 1.22.x - 1.35.0: No panicing on uninhabited types,
unsoundness when used with types like bool or enums.
However, there is protection from accidentially Droping e.g. during unwind!
Rust 1.20.x - 1.21.x: No support for Copy/Clone of MaybeUninit<T>,
even if T impls Copy or even Clone.
Rust 1.6.x - 1.20.x: No Drop protection: now,
MaybeUninit is basically like std::mem::uninitialized().
Rust 1.0.x - 1.5.x: No support for no_std (it got stabilized in 1.6.0).