ObjectSpace::WeakMap#keys

static VALUE
wmap_keys(VALUE self)
{
    struct weakmap *w;
    TypedData_Get_Struct(self, struct weakmap, &weakmap_type, w);

    VALUE ary = rb_ary_new();
    wmap_foreach(w, wmap_keys_i, (st_data_t)ary);

    return ary;
}