Issue 3223: make unsafe scopes tighter

src/arena.rs

@@ -496,14 +496,13 @@ impl Arena {
     }
 }
 
-unsafe fn drop_slab_in_place(value: NonNull<AllocSlab>, tag: ArenaHeaderTag) {
-    unsafe {
+unsafe fn drop_slab_in_place(value: NonNull<AllocSlab>, tag: ArenaHeaderTag) {    
         macro_rules! drop_typed_slab_in_place {
             ($payload: ty, $value: expr) => {
                 <$payload as ArenaAllocated>::dealloc($value.cast::<TypedAllocSlab<$payload>>())
             };
         }
-
+    unsafe {
         match tag {
             ArenaHeaderTag::Integer => {
                 drop_typed_slab_in_place!(Integer, value);