Function `write_box_via_move`

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

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.