How to fix "cannot infer type" error when implementing methods for a generic struct that return specific types?

Question

I am learning to make functions with generic types but I have been stuck for a few hours with this error that says cannot infer type for type parameter 'X'. It is assumed that in my implementation I have already defined the types that X and Y will have.

pub struct Generate<X, Y> {
    pub data_type: X,
    pub data_config: Y,
}

impl<X, Y> Generate<X, Y> {
    pub fn new_bi() -> Generate<u16, String> {
        Generate {
            data_type: 10,
            data_config: String::from("Hello"),
        }
    }
    pub fn new_an() -> Generate<String, u16> {
        Generate {
            data_type: String::from("Hello"),
            data_config: 10,
        }
    }
}

fn main() {
    let y = Generate::new_bi(); // ERROR HERE
    let y: Generate<u16, String> = Generate::new_bi(); // I tried this but it didn't work
}

error[E0282]: type annotations needed
  --> src/lib.rs:22:13
   |
22 |     let y = Generate::new_bi(); // ERROR HERE
   |             ^^^^^^^^^^^^^^^^ cannot infer type for type parameter `X`

error[E0282]: type annotations needed
  --> src/lib.rs:23:36
   |
23 |     let y: Generate<u16, String> = Generate::new_bi(); // I tried this but it didn't work
   |                                    ^^^^^^^^^^^^^^^^ cannot infer type for type parameter `X`

Answer 1

You can make specific implementations depending on the types:

pub struct Generate<X, Y> {
    pub data_type: X,
    pub data_config: Y,
}

impl Generate<u16, String> {
    pub fn new_bi() -> Generate<u16, String> {
        Generate {
            data_type: 10,
            data_config: String::from("Hello"),
        }
    }
}

impl Generate<String, u16> {
    pub fn new_an() -> Generate<String, u16> {
        Generate {
            data_type: String::from("Hello"),
            data_config: 10,
        }
    }
}

fn main() {
    let y = Generate::new_bi();
    let y: Generate<u16, String> = Generate::new_bi();
    let x = Generate::new_an();
}

Playground