Generic function that will return a type based on the annotations.

generic_function.rs

#[derive(Debug)]
struct MyError(String);

trait Poke {
    fn aw();
}

struct Lovely;

impl Poke for Lovely {
    fn aw() {
        println!("Awww lovely...!");
    }
}

impl std::default::Default for Lovely {
    fn default() -> Self {
        Self {}
    }
}

struct Hurtly;

impl Poke for Hurtly {
    fn aw() {
        println!("Aww it hurts!");
    }
}

impl std::default::Default for Hurtly {
    fn default() -> Self {
        Self {}
    }
}

fn main() -> Result<(), MyError> {
    let _: Lovely = poke_something()?;
    let _: Hurtly = poke_something()?;

    Ok(())
}

fn poke_something<T: Poke + Default>() -> Result<T, MyError> {
    T::aw();
    Ok(T::default())
}