This requires introducing a new internal type `RcUninit` (and
`ArcUninit`), which can own an `RcBox<T>` without requiring it to be
initialized, sized, or a slice. This is similar to `UniqueRc`, but
`UniqueRc` doesn't support the allocator parameter, and there is no
`UniqueArc`.
detects redundant imports that can be eliminated.
for #117772 :
In order to facilitate review and modification, split the checking code and
removing redundant imports code into two PR.
