Rust Any Part 2: As-Any Hack

written on January 07, 2022

Yesterday I wrote about how to use the Any type in Rust to implement extension maps. One thing that was brought up as a response is that it's hard to implement Debug for the extension map as Any does not implement Debug. The challenge is that unfortunately in Rust you cannot do Box<dyn Any + Debug>. However there are ways around it.

The Simplifed Problem

Let's say you want to have a wrapper around a boxed any that you can debug. This is what we basically want to accomplish:

#[derive(Debug)]
struct AnyBox(Box<dyn Any + Debug>);

If we were to compile this, the compiler doesn't come back happy:

error[E0225]: only auto traits can be used as additional traits in a trait object
 --> src/main.rs:9:29
  |
9 | struct AnyBox(Box<dyn Any + Debug>);
  |                       ---   ^^^^^ additional non-auto trait
  |                       |
  |                       first non-auto trait
  |
  = help: consider creating a new trait with all of these as supertraits and
    using that trait here instead: `trait NewTrait: Any + Debug {}`

Supertraits

Thankfully the compiler actually tells is what the solution is: we need to create a new super trait that we can use. However it leaves us a bit in the dark of how to do this successfully. Basically we want to implement our super trait for all types implementing the individual traits. Something like this:

#[derive(Debug)]
struct AnyBox(Box<dyn DebugAny>);

trait DebugAny: Any + Debug {}

impl<T: Any + Debug + 'static> DebugAny for T {}

This will in fact compile and you will be able to construct such a box, but what will not work is downcasting:

fn main() {
    let any_box = AnyBox(Box::new(42i32));
    dbg!(any_box.0.downcast_ref::<i32>());
}

The compiler will tell us that our value in the anybox has no method named downcast_ref:

error[E0599]: no method named `downcast_ref` found for struct
  `Box<(dyn DebugAny + 'static)>` in the current scope
  --> src/main.rs:15:20
   |
15 |     dbg!(any_box.0.downcast_ref::<i32>());
   |                    ^^^^^^^^^^^^ method not found in `Box<(dyn DebugAny + 'static)>`

The reason for this is that a Box<dyn DebugAny> unfortunately is not a Box<dyn Any> and as such we don't get the methods from it that we need. So how do we fix this? The simplest method is the “as any” pattern where we implement a method on our DebugAny trait that upcasts into an Any. This looks like this:

trait DebugAny: Any + Debug {
    fn as_any(&self) -> &dyn Any;
    fn as_any_mut(&mut self) -> &mut dyn Any;
}

impl<T: Any + Debug + 'static> DebugAny for T {
    fn as_any(&self) -> &dyn Any { self }
    fn as_any_mut(&mut self) -> &mut dyn Any { self }
}

Now we still can't downcast_ref on the box, but we can take our value, call as_any on it, retrieve a &dyn Any and then go to town:

fn main() {
    let any_box = AnyBox(Box::new(42i32));
    dbg!(any_box.0.as_any().downcast_ref::<i32>());
    dbg!(&any_box);
}

Except if we run it, we get None. What's going on?

[src/main.rs:23] any_box.0.as_any().downcast_ref::<i32>() = None

The answer to this riddle has to do with how the method resolution works and blanket implementations. When we invoke as_any on Box<dyn DebugAny> we're not looking through the box, we're in fact invoking as_any on the Box<dyn DebugAny> itself since the box also implements our DebugAny now. So how do we reach through the box? By dereferencing it.

fn main() {
    let any_box = AnyBox(Box::new(42i32));
    dbg!((*any_box.0).as_any().downcast_ref::<i32>());
    dbg!(&any_box);
}

And now we get what we expect:

[src/main.rs:23] (*any_box.0).as_any().downcast_ref::<i32>() = Some(
    42,
)
[src/main.rs:24] &any_box = AnyBox(
    42,
)

Debuggable Extension Map

These learnings we can now take back to building an extension map which can be debug printed. Let's take the non sync extension map from last time and modify it so we can debug print it:

use std::any::{Any, TypeId};
use std::cell::{Ref, RefCell, RefMut};
use std::collections::HashMap;
use std::fmt::Debug;

trait DebugAny: Any + Debug {
    fn as_any(&self) -> &dyn Any;
    fn as_any_mut(&mut self) -> &mut dyn Any;
}

impl<T: Any + Debug + 'static> DebugAny for T {
    fn as_any(&self) -> &dyn Any { self }
    fn as_any_mut(&mut self) -> &mut dyn Any { self }
}

#[derive(Default, Debug)]
pub struct Extensions {
    map: RefCell<HashMap<TypeId, Box<dyn DebugAny>>>,
}

impl Extensions {
    pub fn insert<T: Debug + 'static>(&self, value: T) {
        self.map
            .borrow_mut()
            .insert(TypeId::of::<T>(), Box::new(value));
    }

    pub fn get<T: Default + Debug + 'static>(&self) -> Ref<'_, T> {
        self.ensure::<T>();
        Ref::map(self.map.borrow(), |m| {
            m.get(&TypeId::of::<T>())
                .and_then(|b| (**b).as_any().downcast_ref())
                .unwrap()
        })
    }

    pub fn get_mut<T: Default + Debug + 'static>(&self) -> RefMut<'_, T> {
        self.ensure::<T>();
        RefMut::map(self.map.borrow_mut(), |m| {
            m.get_mut(&TypeId::of::<T>())
                .and_then(|b| (**b).as_any_mut().downcast_mut())
                .unwrap()
        })
    }

    fn ensure<T: Default + Debug + 'static>(&self) {
        if self.map.borrow().get(&TypeId::of::<T>()).is_none() {
            self.insert(T::default());
        }
    }
}

Adding some stuff into the map and debug printing it makes it output something like this now:

[src/main.rs:63] &extensions = Extensions {
    map: RefCell {
        value: {
            TypeId {
                t: 13431306602944299956,
            }: 42,
        },
    },
}

In this case I placed a 32bit integer 42 in the map and we can see that it prints out the type id of that as key, and 42 as value.

Retaining Type Names

If we want to retain the original type name and not just type ID we could change our TypeId key for a custom type which also stores the original type name. This could be accomplished by creating a wrapper for our TypeId which uses std::any::type_name internally:

use std::any::{TypeId, type_name};
use std::hash::{Hash, Hasher};
use std::fmt::{self, Debug};

pub struct TypeKey(TypeId, &'static str);

impl TypeKey {
    pub fn of<T: 'static>() -> TypeKey {
        TypeKey(TypeId::of::<T>(), type_name::<T>())
    }
}

impl Hash for TypeKey {
    fn hash<H: Hasher>(&self, state: &mut H) {
        self.0.hash(state);
    }
}

impl PartialEq for TypeKey {
    fn eq(&self, other: &Self) -> bool {
        self.0 == other.0
    }
}

impl Eq for TypeKey {}

impl Debug for TypeKey {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "{}", self.1)
    }
}

Now we can replace our use of TypeId with TypeKey in the extension map and our debug output looks like this instead:

[src/main.rs:90] &extensions = Extensions {
    map: RefCell {
        value: {
            i32: 42,
            alloc::vec::Vec<i32>: [
                1,
                2,
                3,
            ],
        },
    },
}

Note that i additionally inserted a Vec<i32> into the map to get some more extra output.

This entry was tagged rust