Function write_box_via_move 

pub fn write_box_via_move<T>(
    b: Box<MaybeUninit<T>>,
    x: T,
) -> Box<MaybeUninit<T>>

🔬This is a nightly-only experimental API. (liballoc_internals)

Writes x into b.

This is needed for vec!, which can’t afford any extra copies of the argument (or else debug builds regress), has to be written fully as a call chain without let (or else this breaks inference of e.g. unsizing coercions), and can’t use an unsafe block as that would then also include the user-provided $x.