返回泛型类型的 Rust 函数

问：

use std::marker::PhantomData;

fn main() {
    pub type KG = u32;

    #[derive(Eq, PartialEq, Debug, Clone, Copy)]
    pub struct Pound(pub u32);

    impl From<Pound> for KG {
        fn from(p: Pound) -> Self {
            p.0 / 45
        } 
    }
    
    impl From<KG> for Pound {
        fn from(k: KG) -> Self {
            Self(k * 45)
        }
    }

    pub trait Matter {
        /// The output unit of the energy density.
        type Output: Into<KG> + From<KG>;
    
        /// The amount of mass contained in a single unit of matter.
        fn density() -> Self::Output;
    }


    pub struct Iron;
    impl Matter for Iron {
        type Output = Pound;
        fn density() -> Self::Output {
            Pound::from(7800)
        }
    }

    pub struct MatterContainer<M: Matter> {
        volume: u32,
        _marker: PhantomData<M>,
    }

    impl<M: Matter> MatterContainer<M> {
        pub fn new(volume: u32) -> Self {
            Self {
                volume,
                _marker: Default::default(),
            }
        }
    }

    pub trait GetWeight<M: Matter> {
        /// Get the weight of Matter in `m`
        /// This method must be provided as it will be the same in all implementations.
        fn get_weight(&self, m: MatterContainer<M>) -> <M as Matter>::Output {
            todo!()
        }
    }
}

我正在尝试编写一个代码，其中get_weight函数通常通过将物质的密度乘以容器的体积来获取物质容器的重量，但是对于不同的物质，密度以不同的单位给出（在这个例子中，铁密度以磅为单位，但对于其他材料，例如它可以是盎司（为了简单起见，这里没有实现给定问题）），返回类型是泛型 Output 类型。如何在不更改get_weight函数签名的情况下实现它？ 谢谢