Rust:通过工厂方法创建Struct时出现问题

采用这个最小repo(操场(，在这里我尝试将一个带有闭包的结构作为字段，并定义一些额外的工厂方法，以闭包的形式提供自定义逻辑。

#![allow(non_snake_case)]
struct FunStruct<T>
where
T: Fn(i32) -> i32
{
pub p: T,
}
fn Create<T>() -> FunStruct<T>
where
T: Fn(i32) -> i32
{
let p : T = |val| -> i32 {
return val;
};

let a = FunStruct {
p,
};

return a;
}
fn main() {
let a = Create();

println!("{}", (a.p)(5));
}

上面的代码段引发了以下错误。

error[E0308]: mismatched types
--> src/main.rs:14:17
|
10 |   fn Create<T>() -> FunStruct<T>
|             - this type parameter
...
14 |       let p : T = |val| -> i32 {
|  _____________-___^
| |             |
| |             expected due to this
15 | |         return val;
16 | |     };
| |_____^ expected type parameter `T`, found closure
|
= note: expected type parameter `T`
found closure `[closure@src/main.rs:14:17: 16:6]`
error[E0282]: type annotations needed for `FunStruct<T>`
--> src/main.rs:26:13
|
26 |     let a = Create();
|         -   ^^^^^^ cannot infer type for type parameter `T` declared on the function `Create`
|         |
|         consider giving `a` the explicit type `FunStruct<T>`, where the type parameter `T` is specified
|
= note: type must be known at this point
error: aborting due to 2 previous errors
Some errors have detailed explanations: E0282, E0308.
For more information about an error, try `rustc --explain E0282`.

然而，这似乎很好

struct FunStruct<T>
where
T: Fn(i32) -> i32
{
pub p: T,
}
fn main() {
let p = |val| -> i32 {
return val;
};

let a = FunStruct {
p
};

println!("{}", (a.p)(5));
}

一个抽象层次似乎会毁掉它。