MetaEntry: split get_metaslab by intent

src/backend/address_space_core.h

@@ -125,7 +125,7 @@ namespace snmalloc
         const MetaEntry& t = Pagemap::template get_metaentry<false>(
           local_state, address_cast(base));
         return capptr::Chunk<FreeChunk>(
-          reinterpret_cast<FreeChunk*>(t.get_metaslab()));
+          reinterpret_cast<FreeChunk*>(t.get_metaslab_no_remote()));
       }
 
       return base->next;

src/mem/localalloc.h

@@ -534,8 +534,8 @@ namespace snmalloc
         UNUSED(size);
 #  endif
 
-        ChunkRecord* slab_record =
-          reinterpret_cast<ChunkRecord*>(entry.get_metaslab());
+        auto slab_record =
+          static_cast<ChunkRecord*>(entry.get_metaslab_no_remote());
 
         SNMALLOC_ASSERT(
           address_cast(slab_record->meta_common.chunk) == address_cast(p_tame));

src/mem/metaslab.h

@@ -243,8 +243,24 @@ namespace snmalloc
         pointer_offset(reinterpret_cast<uintptr_t>(remote), sizeclass);
     }
 
+    /**
+     * Return the Metaslab field as a void*, guarded by an assert that there is
+     * no remote that owns this chunk.
+     */
+    [[nodiscard]] SNMALLOC_FAST_PATH void* get_metaslab_no_remote() const
+    {
+      SNMALLOC_ASSERT(get_remote() == nullptr);
+      return static_cast<void*>(meta);
+    }
+
+    /**
+     * Return the Metaslab metadata associated with this chunk, guarded by an
+     * assert that this chunk is being used as a slab (i.e., has an associated
+     * owning allocator).
+     */
     [[nodiscard]] SNMALLOC_FAST_PATH Metaslab* get_metaslab() const
     {
+      SNMALLOC_ASSERT(get_remote() != nullptr);
       return meta;
     }