How to mutably borrow a boxed pointer when iterating over a `Vec>`?

Question

I have a custom trait like this:

pub struct SomeObject;

pub trait Renderer {
    fn render(&self) -> Vec<SomeObject>;
}

and a module with multiple structs implementing said trait. The module exposes a single function:

mod renderer {
    use super::*;
    pub fn all() -> Vec<Box<Renderer>> {
        vec![]
    }
}

which gets used in the main function like so:

fn main() {
    let renderers = renderer::all();

    loop {
        // ...

        let objects: Vec<SomeObject> = renderers.iter().flat_map(|r| r.render()).collect();

        // ...
    }
}

That's working nicely so far.

I now need to change the render() method to take a mutable reference to self. Doing so gives the following error:

error[E0596]: cannot borrow immutable `Box` content `**r` as mutable
  --> src/main.rs:20:74
   |
20 |             let objects: Vec<SomeObject> = renderers.iter().flat_map(|r| r.render()).collect();
   |                                                                          ^ cannot borrow as mutable

How can I rectify this? The renderers instance is not used passed to any other place, so I should be able to borrow mutably within the scope of this line, at least according to my intuition.