Here is a link to a playground
这是代码（为了可读性而剥离）：

#[async_trait]
impl <'a, FUNC, CONTEXT, PARAM, RETURN, FUT> StepWithParams<CONTEXT, PARAM, RETURN> for FUNC 
where 
    FUNC: FnOnce(&'a PARAM) -> FUT + Send + Sync, 
    FUT: Future<Output = Result<RETURN, ErrTypes>> + Send + Sync,
    PARAM: Send + Sync,
    CONTEXT: Send + Sync,
    RETURN: Send + Sync,
{

    async fn run_user_step (self, request_context: &'a CONTEXT) -> Result<RETURN, ErrTypes> {
        let param = PARAM::from_context(request_context);
        self(param).await
    }
}

字符串
完整版本链接在操场上。编译此将生成编译器错误，如下所示：

error[E0207]: the type parameter `FUT` is not constrained by the impl trait, self type, or predicates
  --> src/main.rs:22:41
   |
22 | impl <'a, FUNC, CONTEXT, PARAM, RETURN, FUT> StepWithParams<CONTEXT, PARAM, RETURN> for FUNC 
   |                                         ^^^ unconstrained type parameter

For more information about this error, try `rustc --explain E0207`.

型
但是，泛型参数不是明显受到约束吗？