Rust return different structs those implement same trait

impl_trait_test.rs

struct Test0;
struct Test1;

trait A {}

impl A for Test0 {}
impl A for Test1 {}

//failed
/*fn test(a: i32) -> impl A {
    if a < 1 {
        return Test0;
    }
    return Test1;
}*/

//all below works
fn test1(a: i32) -> Box<dyn A> {
    if a < 1 {
        return Box::new(Test0);
    }
    return Box::new(Test1);
}

fn test2(a: i32) -> impl A {
    return Test1;
}

fn test3(a: i32) -> Box<A> {
    if a < 1 {
        return Box::new(Test0);
    }
    return Box::new(Test1);
}

fn main() {}