Cell<T> 与 RefCell<T> 的内部可变性

#[derive(Debug)]
struct People {
    name: String,
    age: u8,
}

fn main(){
    let p = People{name: "roadup".to_string(), age: 18};
    let a = &p;
    a.age = 19;
    println!("People: {:?}", a);
}

error[E0594]: cannot assign to `a.age`, which is behind a `&` reference
  --> src/main.rs:10:5
   |
9  |     let a = &p;
   |             -- help: consider changing this to be a mutable reference: `&mut p`
10 |     a.age = 19;
   |     ^^^^^^^^^^ `a` is a `&` reference, so the data it refers to cannot be written

use std::cell::{Cell, RefCell};

#[derive(Debug)]
struct People {
    name: String,
    age: Cell<u8>,
    height: RefCell<u8>
}

fn main(){
    let p = People{
        name: "roadup".to_string(), 
        age: Cell::new(18), 
        height: RefCell::new(170)
    };
    
    let a = &p;
    a.age.set(19);
    *a.height.borrow_mut() = 180;
    println!("People: {:#?}", a);
}

People: People {
    name: "roadup",
    age: Cell {
        value: 19,
    },
    height: RefCell {
        value: 180,
    },
}

pub struct Cell<T: ?Sized> {
    value: UnsafeCell<T>,
}

type BorrowFlag = isize;

pub struct RefCell<T: ?Sized> {
    borrow: Cell<BorrowFlag>,
    // Stores the location of the earliest currently active borrow.
    // This gets updated whenever we go from having zero borrows
    // to having a single borrow. When a borrow occurs, this gets included
    // in the generated `BorrowError/`BorrowMutError`
    #[cfg(feature = "debug_refcell")]
    borrowed_at: Cell<Option<&'static crate::panic::Location<'static>>>,
    value: UnsafeCell<T>,
}

impl <T> Cell<T> where T : Copy {

    const fn new(value: T): Cell<T>;

    fn get(&self) -> T // return a copy
    
    fn set(&self, val: T) // replace with val
}