min-specialization

Rust's specialization feature allows you to provide a default implementation of a trait for generic types and then specialize it for specific types. This feature is currently unstable and only available on the nightly version of Rust.

This crate emulates Rust's #[feature(min_specialization)] unstable feature on stable Rust.

Example

Annotate a module with #[specialization]. Inside it, write a trait, one blanket implementation that provides the default behaviour (its specializable methods are marked default), and any number of concrete implementations that override those methods for specific types:

use min_specialization::specialization;

#[specialization]
mod inner {
    #[allow(unused)]
    trait Trait<U> {
        type Ty;
        fn number(_: U) -> Self::Ty;
    }

    impl<T, U> Trait<U> for T {
        type Ty = usize;
        default fn number(_: U) -> Self::Ty {
            0
        }
    }

    impl<U> Trait<U> for () {
        fn number(_: U) -> Self::Ty {
            1
        }
    }
}

A more complex example

A blanket impl can have several specializations, including ones for borrowed types such as &str (which TypeId-based dispatch normally forbids, since it requires 'static). Associated types are defined once by the blanket impl and shared by every specialization — only methods are specialized. Dispatch happens at run time and picks the most specific matching impl:

use min_specialization::specialization;

#[specialization]
mod describe {
    pub trait Describe {
        type Out;
        fn describe(&self) -> Self::Out;
    }

    // The blanket impl defines the associated type and the `default` method
    // used by every type that is not specialized below.
    impl<T> Describe for T {
        type Out = &'static str;
        default fn describe(&self) -> Self::Out {
            "something else"
        }
    }

    // Specializations override the method for concrete types...
    impl Describe for bool {
        fn describe(&self) -> Self::Out {
            "a boolean"
        }
    }
    impl Describe for i32 {
        fn describe(&self) -> Self::Out {
            "an integer"
        }
    }

    // ...including borrowed types.
    impl Describe for &str {
        fn describe(&self) -> Self::Out {
            "a string slice"
        }
    }
}

use describe::Describe;

assert_eq!(true.describe(), "a boolean");         // specialized
assert_eq!(7i32.describe(), "an integer");        // specialized
assert_eq!("hi".describe(), "a string slice");    // specialized (borrowed type)
assert_eq!(2.5f64.describe(), "something else");  // blanket default

Specialization also supports generic methods (including const generics), generic associated types (GATs), trait lifetime parameters, and non-trivial argument patterns; see tests for more.