如何在 Rust 中提供泛型 struct 的实现

类型参数<T: SomeTrait>应该紧跟在impl关键字之后:

impl<T: SomeTrait> MyStruct<T> {
    fn new(t: T) -> Self {
        MyStruct { value: t }
    }
}

如果impl<...>中的类型和约束列表过长，可以使用where语法并单独列出约束:

impl<T> MyStruct<T>
where
    T: SomeTrait,
{
    fn new(t: T) -> Self {
        MyStruct { value: t }
    }
}

请注意Self的用法，这是impl块中MyStruct<T>的快捷方式.

Remarks

1. this answer中解释了为什么需要impl<T>.从本质上讲，它归结为一个事实，即impl<T> MyStruct<T>和impl MyStruct<T>都是有效的，但意味着不同的东西.

2. 当您将new移动到impl块中时，应该删除多余的类型参数，否则 struct 的接口将变得不可用，如下例所示:

   // trait SomeTrait and struct MyStruct as above
   // [...]
   
   impl<T> MyStruct<T>
   where
       T: SomeTrait,
   {
       fn new<S: SomeTrait>(t: S) -> MyStruct<S> {
           MyStruct { value: t }
       }
   }
   
   impl SomeTrait for u64 {}
   impl SomeTrait for u128 {}
   
   fn main() {
       // just a demo of problematic code, don't do this!
       let a: MyStruct<u128> = MyStruct::<u64>::new::<u128>(1234);
       //                                 ^
       //                                 |
       //        This is an irrelevant type
       //        that cannot be inferred. Not only will the compiler
       //        force you to provide an irrelevant type, it will also
       //        not prevent you from passing incoherent junk as type
       //        argument, as this example demonstrates. This happens 
       //        because `S` and `T` are completely unrelated.
   }