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

pub trait GpuEngine: Engine {
    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
    }
}

impl GpuEngine for MyEngine {
    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
impl GpuEngine for OtherEngine {}

#[cfg(not(eature = "gpu-engine"))]
pub fn use_engine<T: Engine>(engine: T) {
    // do sth cool with that engine
    let res =  T::add(1, 1);
}

#[cfg(feature = "gpu-engine")]
pub fn use_engine<T: GpuEngine>(engine: T) {
    // do sth cool with that gpu engine
    let res = T::gpu_add(1, 1);
}