genericsrustselftype-annotation

How do I call an associated function on a generic type without providing the generic type?

I have function that is part of a struct (for context reasons) which does not take the self parameter. Furthermore, the struct itself takes a generic parameter T with some trait restrictions:

trait SomeRestriction {}
struct SomeStruct<T: SomeRestriction>(T);

impl<T: SomeRestriction> SomeStruct<T> {
    fn some_function_that_does_not_take_self() -> String {
        todo!()
    }
}

I want to write a test and I would like to avoid giving that function the self parameter since mocking the object with some generic struct parameter is a lot of effort for that small function and test.

I do it for the other tests since it is necessary there, but I would like to avoid it wherever I can.

I tried to call it like this:

let some_string = SomeStruct::some_function_that_does_not_take_self();

but it will ask me for a type annotation even though it is not needed.

Is there a way to call it without mocking the struct or removing the function from the struct implementation?

Solution

  • Is there a way to call it without mocking the struct or removing the function from the struct implementation?

    No. For all Rust knows SomeStruct<T>::some_function_that_does_not_take_self is completely distinct for each T. They could have different behavior, too, consider this:

    use core::fmt::Debug;

#[derive(Debug, Default)] struct A;
#[derive(Debug, Default)] struct B;

struct C<T> { t: T }
impl<T: Debug + Default> C<T> {
    fn foo() { println!("{:?}", T::default()) }
}

fn main() {
    C::<A>::foo(); // Prints 'A'.
    C::<B>::foo(); // Prints 'B'.
}