Rust invoke trait method on generic type parameter

Question

Suppose I have a rust trait that contains a function that does not take a &self parameter. Is there a way for me to call this function based on a generic type parameter of the concrete type that implements that trait? For example, in the get_type_id function below, how do I successfully call the type_id() function for the CustomType trait?

pub trait TypeTrait {
    fn type_id() -> u16;
}

pub struct CustomType {
    // fields...
}

impl TypeTrait for CustomType {
    fn type_id() -> u16 { 0 }
}

pub fn get_type_id<T : TypeTrait>() {
    // how?
}

Thanks!

Answer 1

Unfortunately, this isn't currently possible. It used to be, based on a implementation detail, however that was removed in favor of eventually implementing a proper way of doing this.

When it is eventually implemented, it may end up looking something like this: TypeTrait::<for T>::type_id(), however there is, currently, no syntax set in stone.

This is a known case and one that is fully intended to be supported, it is just unfortunate that it currently is not possible.

The full discussion about this topic (called associated methods) is here: https://github.com/mozilla/rust/issues/6894 and here: https://github.com/mozilla/rust/issues/8888

Answer 2

As Aatch mentioned, this isn't currently possible. A workaround is to use a dummy parameter to specify the type of Self:

pub trait TypeTrait {
    fn type_id(_: Option<Self>) -> u16;
}

pub struct CustomType {
    // fields...
}

impl TypeTrait for CustomType {
    fn type_id(_: Option<CustomType>) -> u16 { 0 }
}

pub fn get_type_id<T : TypeTrait>() {
    let type_id = TypeTrait::type_id(None::<T>);
}