#trait #cast #any #upcast

macro no-std cast_trait_object_macros

Cast between trait objects using only safe Rust

4 releases

0.1.3 Jan 16, 2021
0.1.2 Nov 4, 2020
0.1.1 Sep 10, 2020
0.1.0 Sep 10, 2020

#759 in Rust patterns

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Used in cast_trait_object

MIT/Apache

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cast_trait_object

This crate offers functionality for casting between trait objects using only safe Rust and no platform specific code. If you want to downcast to concrete types instead of other trait objects then this crate can't help you, instead use something like the downcast-rs crate.

Usage

You should use the [DynCast] trait in trait bounds or as a supertrait and then do casts using the methods provided by the [DynCastExt] trait. The [DynCast] trait takes a type parameter that should be a "config" type generated by the [create_dyn_cast_config] macro, this type defines from which trait and to which trait a cast is made. Types that need to allow casting to meet the [DynCast] trait bound can then implement it via the [impl_dyn_cast] macro.

Examples

use cast_trait_object::{create_dyn_cast_config, impl_dyn_cast, DynCast, DynCastExt};

create_dyn_cast_config!(SuperToSubCast = Super => Sub);
create_dyn_cast_config!(SuperUpcast = Super => Super);
trait Super: DynCast<SuperToSubCast> + DynCast<SuperUpcast> {}
trait Sub: Super {}

struct Foo;
impl Super for Foo {}
impl Sub for Foo {}
impl_dyn_cast!(Foo as Super => Sub, Super);

let foo: &dyn Super = &Foo;
// Casting to a sub trait is fallible (the error allows us to keep using the
// `dyn Super` trait object if we want which can be important if we are casting
// movable types like `Box<dyn Trait>`):
let foo: &dyn Sub = foo.dyn_cast().ok().unwrap();
// Upcasting to a supertrait is infallible:
let foo /*: &dyn Super*/ = foo.dyn_upcast::<dyn Super>();

When implementing the [DynCast] trait via the [impl_dyn_cast] macro you can also list the created "config" types instead of the source and target traits:

impl_dyn_cast!(Foo => SuperToSubCast, SuperUpcast);

If the proc-macros feature is enabled (which it is by default) we can also use procedural attribute macros to write a little bit less boilerplate:

use cast_trait_object::{dyn_cast, dyn_upcast, DynCastExt};

#[dyn_cast(Sub)]
#[dyn_upcast]
trait Super {}
trait Sub: Super {}

struct Foo;
#[dyn_cast(Sub)]
#[dyn_upcast]
impl Super for Foo {}
impl Sub for Foo {}

Note that #[dyn_upcast] does the same as #[dyn_cast(Super)] but it is a bit clearer about intentions:

use cast_trait_object::{dyn_cast, DynCastExt};

#[dyn_cast(Super, Sub)]
trait Super {}
trait Sub: Super {}

struct Foo;
#[dyn_cast(Super, Sub)]
impl Super for Foo {}
impl Sub for Foo {}

// Upcasting still works:
let foo /*: &dyn Super*/ = foo.dyn_upcast::<dyn Super>();

How it works

How is the conversion preformed

Using the [DynCast] trait as a supertraits adds a couple of extra methods to a trait object's vtable. These methods all essentially take a pointer to the type and returns a new fat pointer which points to the wanted vtable. There are a couple of methods since we need to generate one for each type of trait object, so one for each of &dyn Trait, &mut dyn Trait, Box<dyn Trait>, Rc<dyn Trait> and Arc<dyn Trait>. Note that these methods are entirely safe Rust code, this crate doesn't use or generate any unsafe code at all.

The [DynCastExt] trait then abstracts over the different types of trait objects so that when a call is made using the dyn_cast method the compiler can inline that static method call to the correct method on the trait object.

Why "config" types

We have to generate "config" types since we need to uniquely identify each [DynCast] supertrait based on which trait it is casting from and into. Originally this was just done using two type parameters on the trait, something like DynCast<dyn Super, dyn Sub>, but that caused compile errors when they were used as a supertrait of one of the mentioned traits. So now the traits are "hidden" as associated types on a generated "config" type. To make this "config" type more ergonomic we also implement a [GetDynCastConfig] trait to easily go from the source trait and target trait to a "config" type via something like <dyn Source as GetDynCastConfig<dyn Target>>::Config. This allows the macros ([impl_dyn_cast], [dyn_cast] and [dyn_upcast]) to take traits as arguments instead of "config" types, it also makes type inference work for the [DynCastExt] trait.

How does the macros know if a type implements a "target" trait or not

When a type implementing [DynCast] for a specific config and therefore source to target trait cast the generated code must choose if the cast is going to succeed or not. We want to return Ok(value as &dyn Target) if the type implements the Target trait and Err(value as &dyn Source) if it doesn't.

We can use a clever hack to determine if the type implements the Target trait. See the impls crate's github page for how this hack works. In short the hack allows getting a const bool that is true if a type implements a trait and false otherwise.

So we could generate something like:

trait Source {}
trait Target {}

struct Foo;
impl Source for Foo {}

const IMPLEMENTS_TRAIT: bool = false /* Really should use impls!(Foo: Target) */;

impl Foo {
    fn cast(&self) -> Result<&dyn Target, &dyn Source> {
        if IMPLEMENTS_TRAIT {
            // Compile time error here:
            Ok(self)
          //   ^^^^ the trait `Target` is not implemented for `Foo`
        } else {
            Err(self)
        }
    }
}

But it fails to compile even though we will never actually run the code that coerces Foo to Target. So since coercing a type to a trait it doesn't implement is a type error we need to use our const value to affect the types in the generated code somehow.

This can be done by using the const value as a length of an array to get a type (note that a bool can be converted to a usize). Once we have a type we can use Rust's powerful type system to choose different methods based on our initial value:

struct Choose<T>(T);
impl Choose<[(); 0]> {
    fn foo(arg: usize) -> &'static str { "false" }
}
impl Choose<[(); 1]> {
    fn foo(arg: String, arg2: bool) -> bool { true }
}
// These methods have the same name but are actually totally different methods:
let foo: &'static str = Choose::<[(); 0]>::foo(1);
let foo: bool = Choose::<[(); 1]>::foo("some text".to_string(), false);

or using a trait:

trait AltChoose { type Result; }
struct A;
impl A {
    fn foo(arg: usize) -> &'static str { "false" }
}
impl AltChoose for [(); 0] { type Result = A; }
struct B;
impl B {
    fn foo(arg: String, arg2: bool) -> bool { true }
}
impl AltChoose for [(); 1] { type Result = B; }

// These methods have the same name but are actually totally different methods:
let foo: &'static str = <[(); 0] as AltChoose>::Result::foo(1);
let foo: bool = <[(); 1] as AltChoose>::Result::foo("some text".to_string(), false);

So the [impl_dyn_cast] macro works by generate a const bool that indicates if a type implements a trait and then uses that const value with one of the above hacks to determine which helper method to call when implementing the [DynCast] trait. This way the generated code doesn't call the helper method that preform the coercion to the Target trait unless the type actually implements it.

Alternatives

The intertrait crate offers similar functionality to this crate but has a totally different implementation, at least as of intertrait version 0.2.0. It uses the linkme crate to create a registry of std::any::Any type ids for types that can be cast into a certain trait object. This means it probably has some runtime overhead when it looks up a cast function in the global registry using a TypeId. It also means that it can't work on all platforms since the linkme crate needs to offer support for them. This is a limitation that this crate doesn't have.

The traitcast crate works similar to intertrait in that it has a global registry that is keyed with TypeIds. But it differs in that it uses the inventory crate to build the registry instead of the linkme crate. The inventory crate uses the ctor crate to run some code before main, something that is generally discouraged and this is something that intertrait actually mentions as an advantage to its approach.

The traitcast_core library allow for a more low level API that doesn't depend on a global registry and therefore also doesn't depend on a crate like linkme or inventory that needs platform specific support. Instead it requires that you explicitly create a registry and register all your types and their casts with it.

The downcast-rs crate offers downcasting to concrete types but not directly casting from one trait object to another trait object. So it has a different use case and both it and this crate could be useful in the same project.

References

The following GutHub issue Clean up pseudo-downcasting from VpnProvider supertrait to subtraits with better solution · Issue #21 · jamesmcm/vopono inspired this library.

License

This project is released under either:

at your choosing.

Dependencies

~0.7–1.3MB
~29K SLoC

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