How to get mutable struct from boxed trait

Question

I'd like to modify data in a struct based on a trait that is boxed. The following code prints the value but gives me "cannot mutably borrow immutable field" when I try to change it or "cannot borrow as mutable" when calling its function.

My plan is to have a vector of Ai each containing the AiData derived struct and then iterate over them, set some data in it and call the tick() function.

use std::any::Any;

pub trait AiData {
    fn tick(&mut self);
    fn as_any(&self) -> &Any;
}

pub struct Ai {
    pub ai_data: Box<AiData>,
}

impl Ai {
    pub fn new(ai_data: Box<AiData>) -> Ai {
        Ai { ai_data: ai_data }
    }
}

pub struct TestAi {
    pub index: u8,
}

impl TestAi {
    pub fn new() -> TestAi {
        TestAi { index: 1 }
    }
}

impl AiData for TestAi {
    fn tick(&mut self) {
        println!("tick");
    }

    fn as_any(&self) -> &Any {
        self
    }
}

fn main() {
    let ai_data: TestAi = TestAi::new();
    let ai: Ai = Ai::new(Box::new(ai_data));

    let b: &TestAi = match ai.ai_data.as_any().downcast_ref::<TestAi>() {
        Some(b) => b,
        None => panic!("&a isn't a B!"),
    };
    println!("{:?}", b.index);

    b.tick();
    b.index = 2;
}

error[E0596]: cannot borrow immutable borrowed content `*b` as mutable
  --> src/main.rs:48:5
   |
48 |     b.tick();
   |     ^ cannot borrow as mutable

error[E0594]: cannot assign to immutable field `b.index`
  --> src/main.rs:49:5
   |
49 |     b.index = 2;
   |     ^^^^^^^^^^^ cannot mutably borrow immutable field

What leads you to believe that you can get a mutable reference from this immutable variable? Have you already read [*The Rust Programming Language*](https://doc.rust-lang.org/book/second-edition/), the free official book which explains Rust concepts like mutability? — Shepmaster, Feb 12 '18 at 18:26

Shepmaster · Accepted Answer · 2018-02-12T18:59:45.147

How to get mutable struct from boxed trait

You cannot get a struct from the boxed trait object. You can get a reference to the struct, however.

As explained in The Rust Programming Language's chapter on variables and mutability, mutability is a property of the binding. Additionally, as described in the chapter on references and borrowing, a mutable reference (&mut T) is distinct from an immutable reference (&T). Based on these two points, you cannot get a mutable reference from an immutable variable¹.

The code has:

An immutable variable
An immutable reference to that variable
Calls Any::downcast_ref, which returns an immutable reference

When you fix all of those, the code works:

use std::any::Any;

pub trait AiData {
    fn tick(&mut self);
    fn as_any_mut(&mut self) -> &mut Any;
}

pub struct Ai {
    pub ai_data: Box<AiData>,
}

impl Ai {
    pub fn new(ai_data: Box<AiData>) -> Ai {
        Ai { ai_data }
    }
}

pub struct TestAi {
    pub index: u8,
}

impl TestAi {
    pub fn new() -> TestAi {
        TestAi { index: 1 }
    }
}

impl AiData for TestAi {
    fn tick(&mut self) {
        println!("tick");
    }

    fn as_any_mut(&mut self) -> &mut Any {
        self
    }
}

fn main() {
    let ai_data = TestAi::new();
    let mut ai = Ai::new(Box::new(ai_data));

    let b = ai.ai_data
        .as_any_mut()
        .downcast_mut::<TestAi>()
        .expect("&a isn't a B!");
    println!("{:?}", b.index);

    b.tick();
    b.index = 2;
}

¹ You can read about interior mutability which actually does allow you to get a mutable reference from an immutable variable, at the expense of introducing runtime checks to prevent aliasing.

How to get mutable struct from boxed trait

1 Answers1