pub trait Engine {
    fn add(a: usize, b: usize) -> usize;

    #[cfg(feature = "gpu")]
    fn gpu_add(a: usize, b: usize) -> usize {
        // Fallback implementation
        Self::add(a, b)
    }
}
pub struct MyEngine {}

impl Engine for MyEngine {
    fn add(a: usize, b: usize) -> usize {
        a + b
    }

    #[cfg(feature = "gpu")]
    fn gpu_add(a: usize, b: usize) -> usize {
        println!("fast gpu add");
        a + b
    }
}

pub struct OtherEngine {}

impl Engine for OtherEngine {
    fn add(a: usize, b: usize) -> usize {
        a + b
    }

    // Uses fallback implementation in the gpu case
}

pub fn use_engine<T: Engine>(engine: T) {
    #[cfg(feature = "gpu")]
    let res = T::gpu_add(1, 1);

    #[cfg(not(feature = "gpu"))]
    let res = T::add(1, 1);
}