rust - 是否可以将 Self 限制为特征中的单元结构？

Question

是否可以将 Self 限制为特征中的单元结构？

struct Error;
trait BadCreator
where
    Self: Sized, // <<< I would like to restrict Self to unit structs
{
    fn maybe_create_me(b: bool) -> Result<Self, Error> {
        match b {
            true => Ok(Self), // <<< This line doesn't compile
            false => Err(Error),
        }
    }
}

error[E0423]: expected value, found self type `Self`
 --> src/lib.rs:8:24
  |
8 |             true => Ok(Self),
  |                        ^^^^ not a value
  |
  = note: can't use `Self` as a constructor, you must use the implemented struct

我希望能够像这样使用它:

struct Foo;
impl BadCreator for Foo {}

let b: Foo = Foo::maybe_create_me(true)?;

Answer 1

不，从 Rust 1.39 开始这是不可能的。

---

老实说，你的问题对我来说没有意义。如果你想确保你可以创建一个没有任何参数的值，然后要求通过一个创建一个没有任何参数的值的函数:

trait BadCreator: Sized {
    fn create() -> Self;

    fn maybe_create_me(b: bool) -> Result<Self, Error> {
        match b {
            true => Ok(Self::create()),
            false => Err(Error),
        }
    }
}

或者使用 Default特点:

trait BadCreator: Default {
    fn maybe_create_me(b: bool) -> Result<Self, Error> {
        match b {
            true => Ok(Self::default()),
            false => Err(Error),
        }
    }
}