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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<T: Any>(t: &T) {}

fn main() {
    let is_an_any = ThisImplementsAny;
    take_any(&is_an_any);

    let dyn_any: &dyn Any = &is_an_any;
    dbg!(dyn_any.type_id());
    dbg!(dyn_any.is::<ThisImplementsAny>());
    let is_downcast: Option<&ThisImplementsAny> = dyn_any.downcast_ref();
    dbg!(is_downcast);
}

  • The Any trait allows us to downcast values back from dyn values into concrete values.

  • 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, this is all you can do with Any.