use std::fmt::Debug;

    // struct A{};

    trait A {
        fn get_some(t: u8) -> Box<dyn Debug> {
            match t {
                0 => {
                    Box::new("asdaasd")
                },
                _ =>{
                    Box::new(32)
                }
            }
        }
    }

    let some = A::get_some(0);
    let some = A::get_some(1);

error[E0283]: type annotations needed
  --> src/main.rs:84:16
   |
72 |         fn get_some(t: u8) -> Box<dyn Debug> {
   |         ------------------------------------ required by `A::get_some`
...
84 |     let some = A::get_some(0);
   |                ^^^^^^^^^^^ cannot infer type
   |
   = note: cannot satisfy `_: A`

需要将trait A改为 struct A才可以

    use std::fmt::Debug;

    struct A{};

    impl A {
        fn get_some(t: u8) -> Box<dyn Debug> {
            match t {
                0 => {
                    Box::new("asdaasd")
                },
                _ =>{
                    Box::new(32)
                }
            }
        }
    }

    let some = A::get_some(0);
    let some = A::get_some(1);

看了半天没发现有什么问题啊，求大神解答