Any Trait and Downcasting

// Copyright 2025 Google LLC
// SPDX-License-Identifier: Apache-2.0

use std::any::Any;

#[derive(Debug)]
pub struct ThisImplementsAny;

fn take_any(dyn_any: &dyn Any) {
    // We can get a unique identifier for the type.
    dbg!(dyn_any.type_id());

    // We can check if our object is a particular type.
    dbg!(dyn_any.is::<ThisImplementsAny>());

    // We can attempt to downcast to a concrete type.
    if let Some(concrete) = dyn_any.downcast_ref::<ThisImplementsAny>() {
        dbg!(concrete);
    }
}

fn main() {
    take_any(&ThisImplementsAny);
    take_any(&123);
    take_any(&"A string");
}

By default, trait objects cannot be downcasted to their concrete type. This is because they do not have any runtime type information that would allow us to determine what the concrete type is, and Rust won’t allow us to blindly cast to a type that may not be correct.
The Any trait allows us to downcast values back from dyn values into concrete values by adding the necessary runtime type information.
This is an auto trait: like Send/Sync/Sized, it is automatically implemented for any type that meets specific criteria.
The criteria for Any is that a type is 'static. That is, the type does not contain any non-'static lifetimes within it.
Any offers two related behaviors: downcasting, and runtime checking of types being the same.

In the example above, we see the ability to downcast from Any into ThisImplementsAny automatically.

We also see Any::is being used to check to see what type the value is.
Any does not implement reflection for a type, it just adds the runtime type information necessary to safely determine the concrete type at runtime.
You can add downcasting support to your own traits by using Any as a supertrait.
This also works for generics! We can use Any as a trait bound in our generic code, and then “downcast” to a concrete type. This is uncommon, though, as generics already have full type information and access to all functionality provided by traits.

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Any Trait and Downcasting