The docs for as_mut_ptr: https://doc.rust-lang.org/stable/std/mem/union.MaybeUninit.h...

> Incorrect usage of this method:

  let mut x = MaybeUninit::<Vec<u32>>::uninit();
  let x_vec = unsafe { &mut *x.as_mut_ptr() };
  // We have created a reference to an uninitialized vector! This is undefined behavior.

Also, above, it explicitly describes the intended API for partially initializing a struct: https://doc.rust-lang.org/stable/std/mem/union.MaybeUninit.h...