How can I downcast from Box to a trait object type?

Question

pub struct WidgetWrap {
    // ...
    widget: RefCell>,
}

At some point I want to cast Box to B

Answer 1

The original code works as-is in at least Rust 1.25; presumably there was a bug or limitation in the compiler that has since been fixed:

use std::{any::Any, cell::RefCell};

trait WidgetTrait {}

fn example(widget: RefCell>) {
    let mut cell = widget.borrow_mut();
    let _w = cell.downcast_mut::>();
}

As pointed out by trentcl, that doesn't mean that this does what you want:

This compiles, I admit, but it cannot succeed when the content of the box is an object that implements WidgetTrait, because it downcasts specifically to Box. You'd have to create a Box> for it to work. (Put another way, the type parameter of downcast_mut is the unboxed type T, not Box.)

use std::{any::Any, cell::RefCell};

trait WidgetTrait {
    fn demo(&mut self);
}

fn example(widget: RefCell>) {
    let mut cell = widget.borrow_mut();
    match cell.downcast_mut::>() {
        Some(w) => w.demo(),
        None => println!("Not here!"),
    }
}

struct Alpha(u8);
impl WidgetTrait for Alpha {
    fn demo(&mut self) {
        self.0 += 1;
        dbg!(self.0);
    }
}

fn main() {
    let b: Box = Box::new(Alpha(0));
    let r = RefCell::new(b);
    example(r);
}

Not here!

Boxing it twice:

fn main() {
    let b: Box = Box::new(Alpha(0));
    let b2: Box = Box::new(b);
    let r = RefCell::new(b2);
    example(r);
}

[src/main.rs:19] self.0 = 1

See also:

• How to get a reference to a concrete type from a trait object?