Allow shapes to carry zero embedded capacity

rb_shape_verify_consistency asserted that every checked object has room
for at least one embedded field.

The default GC always satisfies this because its smallest slot is 32
bytes so this assertion never fires. MMTk gives an empty imemo_fields a
16-byte slot, so its shape carries zero capacity bits, and boot failed
on the first class freeze.

Instead of forcing MMTk to have a minimum object size, we should just
allow shapes to support a capacity of 0.

gc/mmtk/mmtk.c

@@ -704,15 +704,6 @@ rb_mmtk_align_obj_size(size_t object_size)
     return (object_size + MMTk_MIN_OBJ_ALIGN - 1) & ~((size_t)MMTk_MIN_OBJ_ALIGN - 1);
 }
 
-static inline size_t
-rb_mmtk_total_obj_size(size_t payload_size)
-{
-    if (payload_size < sizeof(struct RBasic) + sizeof(VALUE)) {
-        payload_size = sizeof(struct RBasic) + sizeof(VALUE);
-    }
-    return rb_mmtk_align_obj_size(payload_size + sizeof(VALUE));
-}
-
 bool
 rb_gc_impl_zjit_new_obj_fastpath(void *objspace_ptr, size_t alloc_size, VALUE flags, VALUE klass,
                                  struct rb_gc_zjit_fastpath *fastpath)
@@ -720,7 +711,7 @@ rb_gc_impl_zjit_new_obj_fastpath(void *objspace_ptr, size_t alloc_size, VALUE fl
 #if USE_ZJIT
     struct objspace *objspace = objspace_ptr;
 
-    size_t total_size = rb_mmtk_total_obj_size(alloc_size);
+    size_t total_size = rb_mmtk_align_obj_size(alloc_size + sizeof(VALUE));
     size_t object_size = total_size - sizeof(VALUE);
     size_t value_size_shift = sizeof(VALUE) == 8 ? 3 : 2;
 
@@ -1035,7 +1026,7 @@ rb_gc_impl_new_obj(void *objspace_ptr, void *cache_ptr, VALUE klass, VALUE flags
     }
 
     // Layout: [hidden size header (sizeof(VALUE))][payload (alloc_size)]
-    size_t total_size = rb_mmtk_total_obj_size(alloc_size);
+    size_t total_size = rb_mmtk_align_obj_size(alloc_size + sizeof(VALUE));
     size_t object_size = total_size - sizeof(VALUE);
     MMTk_AllocationSemantics semantics = total_size > objspace->max_non_los_default_alloc_bytes
         ? MMTK_ALLOCATION_SEMANTICS_LOS
@@ -1093,7 +1084,7 @@ rb_gc_impl_size_slot_size(void *objspace_ptr, size_t size)
         rb_bug("rb_gc_impl_size_slot_size: size too large (size=%"PRIuSIZE")", size);
     }
 
-    return rb_mmtk_total_obj_size(size) - sizeof(VALUE);
+    return rb_mmtk_align_obj_size(size + sizeof(VALUE)) - sizeof(VALUE);
 }
 
 bool

shape.c

@@ -1343,7 +1343,6 @@ rb_shape_verify_consistency(VALUE obj, shape_id_t shape_id)
 
     if (rb_shape_verify_capacity_consistency_p(obj)) {
         attr_index_t shape_id_capacity = rb_shape_embedded_capacity(shape_id);
-        RUBY_ASSERT(shape_id_capacity > 0);
 
         size_t shape_id_slot_size = shape_id_capacity * sizeof(VALUE) + sizeof(struct RBasic);
         size_t actual_slot_size = rb_gc_obj_slot_size(obj);