created_at2024-05-02 22:08:19.970929
updated_at2024-05-02 22:08:19.970929
descriptionCast from `dyn Any` to other trait objects, with no_std, no alloc support
Robert J├Ârdens (jordens)



Cast from dyn Any to other trait objects

  • no_std no alloc support
  • No proc macros


crosstrait = "0.1"

Then use the register!{ Type => Trait } declarative macro and the [Cast] traits.

For embedded, the linker needs to be informed of the type registry.


use core::any::Any;
use crosstrait::{Cast, Castable, CastableRef, register, REGISTRY};

// Some example traits to play with
use core::{fmt::{Debug, Formatter, Write}, ops::{AddAssign, SubAssign}};

// Add types and trait implementations in the default global registy
// Implementation status is verified at compile time
register! { i32 => dyn Debug }

// Registering foreign types and traits works fine
// Serialization/deserialization of `dyn Any` is a major use case
// register! { i32 => dyn erased_serde::Serialize }

// Check for trait impl registration on concrete type
assert!(i32::castable::<dyn Debug>());

// Check for trait impl registration on Any
let any: &dyn Any = &42i32;
assert!(any.castable::<dyn Debug>());

// SubAssign<i32> is impl'd for i32 but not registered
assert!(!any.castable::<dyn SubAssign<i32>>());

// Cast ref
let a: &dyn Debug = any.cast().unwrap();
println!("42 = {a:?}");

// Cast mut
let mut value = 5i32;
let any: &mut dyn Any = &mut value;
let v: &mut dyn AddAssign<i32> = any.cast().unwrap();
*v += 3;
assert_eq!(value, 5 + 3);

// Cast Box
let any: Box<dyn Any> = Box::new(0i32);
let _: Box<dyn Debug> = any.cast().unwrap();

// Cast Rc
use std::rc::Rc;
let any: Rc<dyn Any> = Rc::new(0i32);
let _: Rc<dyn Debug> = any.cast().unwrap();

// Cast Arc
use std::sync::Arc;
let any: Arc<dyn Any + Sync + Send> = Arc::new(0i32);
let _: Arc<dyn Debug> = any.cast().unwrap();

// Explicit registry usage
let any: &dyn Any = &0i32;
let _: &dyn Debug = REGISTRY.cast_ref(any).unwrap();

// Autotraits and type/const generics are distinct
let a: Option<&(dyn Debug + Sync)> = any.cast();

// Registration can happen anywhere in any order in any downstream crate
register! { i32 => dyn AddAssign<i32> }

// If a type is not Send + Sync, it can't cast as Arc. `no_arc` accounts for that
register! { Formatter => dyn Write, no_arc }

Related crates

  • intertrait: similar goals, std
  • miniconf: provides several ways to get dyn Any from nodes in heterogeneous nested data structures, no_std, no alloc
  • erased_serde: Serialize/Serializer/Deserializer trait objects
  • linkme: linker magic used to build distributed static type registry


Registry size on no_std

Currently the size of the global registry on no_std is fixed and arbitrarily set to 128 entries.

Auto traits

Since adding any combination of auto traits (in particular Send, Sync, Unpin) to a trait results in a distinct trait, all relevant combinations of traits plus auto traits needs to be registered explicitly.

Global registry

A custom non-static [Registry] can be built and used explicitly but the Cast traits will not use it.

Commit count: 1017

cargo fmt