Struct#deconstruct_keys

static VALUE
rb_struct_deconstruct_keys(VALUE s, VALUE keys)
{
    VALUE h;
    long i;

    if (NIL_P(keys)) {
        return rb_struct_to_h(s);
    }
    if (UNLIKELY(!RB_TYPE_P(keys, T_ARRAY))) {
        rb_raise(rb_eTypeError,
                 "wrong argument type %"PRIsVALUE" (expected Array or nil)",
                 rb_obj_class(keys));

    }
    if (RSTRUCT_LEN(s) < RARRAY_LEN(keys)) {
        return rb_hash_new_with_size(0);
    }
    h = rb_hash_new_with_size(RARRAY_LEN(keys));
    for (i=0; i<RARRAY_LEN(keys); i++) {
        VALUE key = RARRAY_AREF(keys, i);
        int i = rb_struct_pos(s, &key);
        if (i < 0) {
            return h;
        }
        rb_hash_aset(h, key, RSTRUCT_GET(s, i));
    }
    return h;
}