This library provides direct casting among trait objects implemented by a type.
In Rust, a trait object for a sub-trait of [std::any::Any] can be downcast to a concrete type at runtime
if the type is known. But no direct casting between two trait objects (i.e. without involving the concrete type
of the backing value) are possible (even no coercion from a trait object for a trait to that for its super-trait yet).
With this crate, any trait object for a sub-trait of [CastFrom] can be cast directly to a trait object
for another trait implemented by the underlying type if the target traits are registered beforehand
with the macros provided by this crate.
```rust use intertrait::; use intertrait::cast::;
struct Data;
trait Source: CastFrom {}
trait Greet { fn greet(&self); }
impl Greet for Data { fn greet(&self) { println!("Hello"); } }
impl Source for Data {}
fn main() {
let data = Data;
let source: &dyn Source = &data;
let greet = source.cast::
Target traits must be explicitly designated beforehand. There are three ways of doing it:
#[cast_to] to impl itemThe trait implemented is designated as a target trait.
```rust use intertrait::*;
struct Data; trait Greet { fn greet(&self); }
impl Greet for Data { fn greet(&self) { println!("Hello"); } } ```
#[cast_to(Trait)] to type definitionFor the type, the traits specified as arguments to the #[cast_to(...)] attribute are designated as target traits.
```rust use intertrait::*;
trait Greet { fn greet(&self); }
impl Greet for Data { fn greet(&self) { println!("Hello"); } }
struct Data; ```
castable_to!(Type => Trait1, Trait2)For the type, the traits following : are designated as target traits.
```rust use intertrait::*;
struct Data; trait Greet { fn greet(&self); } impl Greet for Data { fn greet(&self) { println!("Hello"); } } // Only in an item position due to the current limitation in the stable Rust. // https://github.com/rust-lang/rust/pull/68717 castable_to!(Data => Greet, std::fmt::Debug);
fn main() {} ```
Arc Supportstd::sync::Arc is unique in that it implement downcast method only on dyn Any + Send + Sync + 'static'.
To use withArc`, the following steps should be taken:
CastFromSync] instead of [CastFrom][sync] flag to #[cast_to] and castable_to! as follows:
ignore
#[cast_to([sync])]
#[cast_to([sync] Trait1, Trait2)]
castable_to!(Type => [sync] Trait, Trait2);
First of all, [CastFrom] trait makes it possible to retrieve an object of [std::any::Any]
from an object for a sub-trait of [CastFrom].
And the macros provided by intertrait generates trampoline functions for downcasting a trait object
for [std::any::Any] back to its concrete type and then creating a trait object for the target trait from it.
Those trampoline functions are aggregated into a global registry
using linkme crate, which involves no (generally discouraged)
life-before-main trick. The registry is keyed with a pair of [TypeId]s, which are those of the concrete type
backing a trait object for a sub-trait of [CastFrom] and the target trait (the actual implementation
is a bit different here, but conceptually so).
In the course, it doesn't rely on any unstable Rust implementation details such as the layout of trait objects that may be changed in the future.
intertrait has taken much of its core ideas from the great traitcast crate.
Licensed under either of
at your option.
Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the work by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.