How to implement specialized versions of a generic function?

Question

I'd like to have multiple versions of a function optimized for type of its arguments, and Rust call appropriate one depending on context.

In my case all arguments have the same type, and all are equivalent, so it'd rather avoid having a self argument.

I've tried this code:

trait Foo<T> { 
    fn foo(a: T, b: T, c: T); 
}

impl Foo<i32> {
    fn foo(a: i32, b: i32, c: i32) {}
}

impl Foo<i16> {
    fn foo(a: i16, b: i16, c: i16) {}
}

fn main() {
    Foo::foo(1i32,2,3);
    Foo::foo(1i16,2,3);
}

but Rust requires type annotations:

error: type annotations required: cannot resolve _ : Foo<i32> [E0283]

Can I avoid providing type annotations at the call site? If I have to, how to do it?

Any current specialisation of a generic function is a quirk that probably isn't actually meant to work (i.e. it's a bug). [True specialisation](https://github.com/rust-lang/rfcs/pull/1210) is a proposed but not implemented feature. — huon, Dec 26 '15 at 12:27

Vladimir Matveev · Accepted Answer · 2015-12-26T20:13:33.727

Remember that you always implement a trait for something. Therefore, trait implementation must always contain for clause:

impl SomeTrait for Something

If there is no for, then it is not a trait implementation. In your case impl Foo<i32> is not an implementation of Foo for i32 or whatever you think it is; it is an inherent method declaration clause on the bare trait object type Foo<i32>.

What you actually want is possible to do using Self type parameter:

trait Foo { 
    fn foo(a: Self, b: Self, c: Self); 
}

impl Foo for i32 {
    fn foo(a: i32, b: i32, c: i32) {}
}

impl Foo for i16 {
    fn foo(a: i16, b: i16, c: i16) {}
}

fn main() {
    Foo::foo(1i32,2,3);
    Foo::foo(1i16,2,3);
}

This code works.

Note that now Foo is implemented for a certain type. The type a trait is implemented for is available via the implicit Self type parameter, and you can see how it is used in foo() declaration.

How to implement specialized versions of a generic function?

1 Answers1