fn mem::transmute: Replace with sound `Arc::from_raw(Arc::into_raw(…

……) as *const _)` (#1361)

As @Darksonn [pointed
out](#1360 (comment)),
using `mem::transmute` on an `Arc` is unsound since it's not
`#[repr(transparent)]`, and we need to go through `Arc::into_raw` and
`Arc::from_raw` with a ptr cast instead.

src/disjoint_mut.rs

@@ -1195,13 +1195,15 @@ impl<T> FromIterator<T> for DisjointMutArcSlice<T> {
             let arc_slice = iter.into_iter().collect::<Arc<[_]>>();
 
             // Do our best to check that `DisjointMut` is in fact `#[repr(transparent)]`.
-            type A = Vec<u8>; // Some concrete sized type.
-            const _: () = assert!(mem::size_of::<DisjointMut<A>>() == mem::size_of::<A>());
-            const _: () = assert!(mem::align_of::<DisjointMut<A>>() == mem::align_of::<A>());
+            const {
+                type A = Vec<u8>; // Some concrete sized type.
+                assert!(mem::size_of::<DisjointMut<A>>() == mem::size_of::<A>());
+                assert!(mem::align_of::<DisjointMut<A>>() == mem::align_of::<A>());
+            }
 
             // SAFETY: When `#[cfg(not(debug_assertions))]`, `DisjointMut` is `#[repr(transparent)]`,
             // containing only an `UnsafeCell`, which is also `#[repr(transparent)]`.
-            unsafe { mem::transmute::<Arc<[_]>, Arc<DisjointMut<[_]>>>(arc_slice) }
+            unsafe { Arc::from_raw(Arc::into_raw(arc_slice) as *const DisjointMut<[_]>) }
         };
         Self { inner }
     }