Lib.rs

Casting from Any

In the standard library, the std::any::Any trait comes with downcast methods which let you cast from an Any trait object to a concrete type.

let x: i32 = 7;
let y: &dyn std::any::Any = &x;

// Cast to i32 succeeds because x: i32
assert_eq!(y.downcast_ref::<i32>(), Some(&7));
// Cast to f32 fails
assert_eq!(y.downcast_ref::<f32>(), None);

However, it is not possible to downcast to a trait object.

trait Foo {
fn foo(&self) -> i32;
}

struct A {
x: i32
}

impl Foo for A {
fn foo(&self) -> i32 {
self.x
}
}

let x = A { x: 7 };
let y: &dyn std::any::Any = &x;

// This cast is not possible, because it is only possible to cast to types that
// are Sized. Among other things, this precludes trait objects.
let z: Option<&dyn Foo> = y.downcast_ref();

Traitcast

This library provides a way of casting between different trait objects.

use traitcast::{TraitcastFrom, Traitcast};

// Extending `TraitcastFrom` is optional. This allows `Foo` objects themselves
// to be cast to other trait objects. If you do not extend `TraitcastFrom`,
// then Foo may only be cast into, not out of.
trait Foo: TraitcastFrom {
fn foo(&self) -> i32;
}

trait Bar: TraitcastFrom {
fn bar(&mut self) -> i32;
}

struct A {
x: i32
}

// No implementation of TraitcastFrom is necessary, because it is covered by
// the blanket impl for any sized type with a static lifetime.
impl Foo for A {
fn foo(&self) -> i32 {
self.x
}
}

impl Bar for A {
fn bar(&mut self) -> i32 {
self.x *= 2;
self.x
}
}

// Register the traits.

// For each struct that implements each trait, register the implementation.
traitcast::traitcast!(struct A: Foo, Bar);

fn main() {
let mut x = A { x: 7 };

{
let x: &dyn Foo = &x;
// Test whether x is of a type that implements Bar.
assert!(traitcast::implements_trait::<dyn Foo, dyn Bar>(x));
}

{
let x: &dyn Bar = &x;
// Cast an immutable reference using the `cast_ref` method (via the
// `Traitcast` trait, which is blanket implemented for all pairs of
// traits that may be cast between).
let x: &dyn Foo = x.cast_ref().unwrap();
assert_eq!(x.foo(), 7);

// We can also cast using the top-level `cast_ref` function, which can
// be more convenient when type arguments cannot be inferred.
assert!(traitcast::cast_ref::<dyn Foo, dyn Bar>(x).is_some());
}

{
let x: &mut dyn Foo = &mut x;
// Cast a mutable reference using the `cast_mut` method
let x: &mut dyn Bar = x.cast_mut().unwrap();
assert_eq!(x.bar(), 14);
}

{
// We can cast from `Any` too!
let y: Box<dyn std::any::Any> = Box::new(x);
// Cast a boxed reference
let z: Box<dyn Foo> = y.cast_box().unwrap();
assert_eq!(z.foo(), 14);
}
}