I needed this, so I wrote it. You might need it too, so here it is: delegate-trait on crates.io
To quote the readme
This crate lets library authors provide downstream crates with automatic
implementations of (possibly many) traits by implementing a single
"delegate" trait.
For example, a library author could provide some traits that are expected
to be commonly implemented:
trait Surface { fn area(&self) -> f64; } trait Body { fn volume(&self) -> f64; } To allow these traits to be automatically implemented based on delegate
traits that they or other library authors write later, they can provide
blanket implementations like this:
use delegate_trait::DelegateProvider; impl<T> Surface for T where T: DelegateProvider, T::Target: Surface, { #[inline] fn area(&self) -> f64 { self.delegate_ref().area() } } impl<T> Body for T where T: DelegateProvider, T::Target: Body, { #[inline] fn volume(&self) -> f64 { self.delegate_ref().volume() } } A possibly different crate can then create traits that can provide automatic implementations of these:
use delegate_trait::delegate_trait; #[delegate_trait(CubicDelegate)] pub trait Cubic { fn side(&self) -> f64; fn surface_area(&self) -> f64 { let s = self.side(); s*s*6.0 } fn body_volume(&self) -> f64 { let s = self.side(); s*s*s } } impl Surface for dyn CubicDelegate { #[inline] fn area(&self) -> f64 { self.surface_area() } } impl Body for dyn CubicDelegate { #[inline] fn volume(&self) -> f64 { self.body_volume() } } When applied to a trait, as above, the #[delegate_trait(CubicDelegate)] attribute just
declares an empty subtrait pub trait CubicDelegate: Cubic. This is provided as a separate
trait to use in DelegateProvider to avoid conflicts. There is no reason to use
a dyn CubicDelegate for any other purpose.
Now, any type can pick up the common traits for free:
use delegate_trait::delegate_trait; #[delegate_trait(CubicDelegate)] pub struct UnitCube {} impl Cubic for UnitCube { fn side(&self) -> f64 { 1.0 } } fn get_unit_cube_surface() -> impl Surface { // UnitCube implements Surface for free UnitCube{} } fn get_unit_cube_body() -> impl Body { // UnitCube implements Body for free UnitCube{} } When used on a struct, union, or enum, as above, the #[delegate_trait(CubicDelegate)] attribute implements the CubicDelegate marker subtrait on the struct, as well as the DelegateProvider<Target = dyn CubicDelegate> trait, which allows the blanket implementations of Surface and Body to apply.
Since Cubic is a supertrait of CubicDelegate, it must then be implemented for the struct.
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