rust - 无法返回 Option 因为关联类型未调整大小

Question

我不明白为什么这段代码不能编译:

fn main() {}

trait NotWorking {
    // The associated type `Bar` must be sized
    type Bar: ?Sized;

    // Why does the compiler complain that Self::Bar is not sized?
    // I have a trait bound that says it is!
    fn notwoking() -> Option<Self::Bar>;
}

我确实有一个约束，即关联类型 Bar 必须调整大小，但编译器仍然提示它未调整大小:

error[E0277]: the trait bound `<Self as NotWorking>::Bar: std::marker::Sized` is not satisfied
  --> src/main.rs:17:5
   |
17 |     fn notwoking() -> Option<Self::Bar>;
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `<Self as NotWorking>::Bar` does not have a constant size known at compile-time
   |
   = help: the trait `std::marker::Sized` is not implemented for `<Self as NotWorking>::Bar`
   = help: consider adding a `where <Self as NotWorking>::Bar: std::marker::Sized` bound
   = note: required by `std::option::Option`

我尝试了一下，试图让它工作，并对它的工作感到惊讶(因为我们不能返回一个未确定大小的类型，我原以为它会失败):

fn main() {}

trait Working {
    type Bar;

    // Why does this work?
    // I would expect the compiler to complain the Self::Bar is not sized
    fn woking() -> Self::Bar;
}

我肯定在这里遗漏了一些重要的东西。

Answer 1

你的第二个例子是正确的。声明具有 Sized 边界的通用参数的方式是不指定任何显式边界。 Sized 的特殊之处在于默认情况下它被隐式添加为泛型参数的绑定(bind)，因为没有 Sized 的情况相对不常见。如果您不想要求通用参数在编译时具有已知大小，您可以 remove this implicit bound by writing ?Sized .

另见 documentation for Sized .