Cell: get, get_mut, set RefCell: get_mut, borrow, borrow_mut
ただしget_mutはそれぞれ https://doc.rust-lang.org/std/cell/struct.Cell.html#method.get_mut > this method expects self to be mutable, which is generally not the case when using a Cell. https://doc.rust-lang.org/std/cell/struct.RefCell.html#method.get_mut > this method expects self to be mutable, which is generally not the case when using a RefCell. とあり通常の用途とやらで考えると
