具有大小超特征的特征仍存在错误“std::marker::Sized is not successment”[重复]

我有以下代码:

use std::collections::HashMap;

trait T: Sized {}

struct A;

impl T for A {}

fn main() {
    let h: HashMap<String, T>;
}

但编译器抱怨:

error[E0277]: the trait bound `T: std::marker::Sized` is not satisfied
  --> src\main.rs:10:12
   |
10 |     let h: HashMap<String, T>;
   |            ^^^^^^^^^^^^^^^^^^ `T` does not have a constant size known at compile-time
   |
   = help: the trait `std::marker::Sized` is not implemented for `T`
   = note: required by `std::collections::HashMap`

error[E0038]: the trait `T` cannot be made into an object
  --> src\main.rs:10:12
   |
10 |     let h: HashMap<String, T>;
   |            ^^^^^^^^^^^^^^^^^^ the trait `T` cannot be made into an object
   |
   = note: the trait cannot require that `Self : Sized`

我不理解错误信息,因为我已经标记了我的特质 T 像 Sized . 我错过什么了吗?