How to Specify the Unused Return Type of a Generic Function?

February 27, 2023

I have a function which returns a struct with PhantomData, where I discard the result. An example of this looks like:

use std::marker::PhantomData;

struct Foo<T> {
    marker: PhantomData<T>,
}

fn foo<T>() -> Foo<T> {
    // something useful
    Foo {
        marker: PhantomData,
    }
}

fn main() {
    let _ = foo();
}

This code produces this error:

error[E0282]: type annotations needed for `Foo<T>`
  --> src/main.rs:15:9
   |
15 |     let _ = foo();
   |         ^
   |
help: consider giving this pattern a type, where the type for type parameter `T` is specified
   |
15 |     let _: Foo<T> = foo();
   |          ++++++++

What is the best type to assign here? (does it not matter?)