move 发生是因为 value 的类型为 `RefCell<…>`，它没有实现 `Copy` 特征

Question

UPDATE My real problem was caused by my IDE having auto-imported use std::borrow::{Borrow, BorrowMut}; . With this line, the accepted answer also doesn't compile . The solution was removing the line.

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I'm getting the following error message:

15 |     instance_context.into_inner().instances = Some(vec![String::from("abc")]);
   |     ^^^^^^^^^^^^^^^^ move occurs because value has type `RefCell<InstanceContext>`, which does not implement the `Copy` trait

and I have no idea why, or how to fix the code.

Playground :

#![allow(dead_code)]
#![allow(unused_variables)]

use std::rc::Rc;
use std::cell::RefCell;

struct InstanceContext {
    id: i32,
    instances: Option<Vec<String>>,
}

fn main() {
    let instance_context = Rc::new(RefCell::new(InstanceContext { id: 5, instances: None }));
    // clojures are created that use instance_context, which does not yet have 'instances' set
    instance_context.into_inner().instances = Some(vec![String::from("abc")]);
}

Answer 1

into_inner() method will consume the RefCell instance to give you the wrapped value.

To achieve that, it requires you to have ownership over the instance of RefCell . But you don't have that as it is within Rc and unless you consume the Rc as well to get ownership of RefCell , you can't call into_inner() .

---

In your code, due to deref coersion, you get immutable ref to the inside RefCell so you can only call methods that accept &self .

If you want mutate the contents inside RefCell , you can do it as follows:

#![allow(dead_code)]
#![allow(unused_variables)]

use std::rc::Rc;
use std::cell::RefCell;

struct InstanceContext {
    id: i32,
    instances: Option<Vec<String>>,
}

fn main() {
    let instance_context = Rc::new(RefCell::new(InstanceContext { id: 5, instances: None }));
    instance_context.borrow_mut().instances = Some(vec![String::from("abc")]);
}

Playground

borrow_mut() takes an immutable ref to RefCell instance and lets you get a mutable ref to the wrapped value with which you can mutate the contents of the wrapped value.