Implement Trait with generic type on all types but string

[Getrusty]

Hello Rust Community,

I would like to know if I can achieve the below

trait Something<T> { }


impl Something<String> for SomeStruct {}
impl Something<All rust types other than String> for SomeStruct{}

I would like to include all rust variable types: i32, u32, Option, ... etc

I tried wrapping in Any but my trait is "async" so its panicking.

[jofas]

Unfortunately this is not possible right now (nor anytime soon I think). If you were to do something like:

trait Something<T> { }

struct SomeStruct;

impl Something<String> for SomeStruct {}
impl<T> Something<T> for SomeStruct{}

you'll get an error because there are conflicting implementations for Something<String>. The solution for this is called trait specialization. You can find the RFC is here: 1210-impl-specialization - The Rust RFC Book and the tracking issue here: Tracking issue for specialization (RFC 1210) · Issue #31844 · rust-lang/rust · GitHub. Note that specialization right now is unsound, so it is discouraged to be used. But there is the #![feature(min_specialization)] on nightly you could try:

#![feature(min_specialization)]

trait Something<T> { 
    fn ty() -> String;
}

struct SomeStruct;

impl Something<String> for SomeStruct {
    fn ty() -> String {
        "String".to_owned()
    }
}

impl<T> Something<T> for SomeStruct {
    default fn ty() -> String {
        "T".to_owned()
    }
}


fn main() {
    assert_eq!(<SomeStruct as Something<String>>::ty(), "String".to_owned());
    assert_eq!(<SomeStruct as Something<bool>>::ty(), "T".to_owned());
}

Playground.