为什么具有 const 泛型布尔值的方法不能调用为 true 和 false 实现的方法？

Question

此代码完美运行 (playground)：

struct MyStruct<const B: bool>;

impl MyStruct<false> {
    pub fn bar() {
        println!("false");
    }
}
impl MyStruct<true> {
    pub fn bar() {
        println!("true");
    }
}

impl MyStruct<false> {
    pub fn foo() {
        MyStruct::<false>::bar()
    }
}
impl MyStruct<true> {
    pub fn foo() {
        MyStruct::<true>::bar()
    }
}

fn main() {
    MyStruct::<false>::foo();
    MyStruct::<true>::foo();
}

结果是：

false
true

另一方面，此代码将失败 (playground)：

struct MyStruct<const B: bool>;

impl MyStruct<false> {
    pub fn bar() {
        println!("false");
    }
}
impl MyStruct<true> {
    pub fn bar() {
        println!("true");
    }
}

impl<const B: bool> MyStruct<B> {
    pub fn foo() {
        MyStruct::<B>::bar()
    }
}

fn main() {
    MyStruct::<false>::foo();
    MyStruct::<true>::foo();
}

导致：

error[E0599]: no function or associated item named `bar` found for struct `MyStruct<B>` in the current scope
  --> src/main.rs:16:24
   |
1  | struct MyStruct<const B: bool>;
   | ------------------------------- function or associated item `bar` not found for this
...
16 |         MyStruct::<B>::bar()
   |                        ^^^ function or associated item not found in `MyStruct<B>`
   |
   = note: the function or associated item was found for
           - `MyStruct<false>`
           - `MyStruct<true>`

对于无限值类型，我可以理解这个错误，但为什么是布尔值？

有办法克服这个问题吗？

Answer 1

必须使用特征和泛型类型条件 (playground):

struct MyStruct<const B: bool>;

trait Bar {
    fn bar();
}

impl Bar for MyStruct<false> {
    fn bar() {
        println!("false");
    }
}
impl Bar for MyStruct<true> {
    fn bar() {
        println!("true");
    }
}

impl<const B: bool> MyStruct<B>
where
    MyStruct<B>: Bar,
{
    pub fn foo() {
        MyStruct::<B>::bar()
    }
}

fn main() {
    MyStruct::<false>::foo();
    MyStruct::<true>::foo();
}

为什么具有 const 泛型布尔值的方法不能调用为 true 和 false 实现的方法？

Why can a method with a const generic boolean not call a method that is implemented for both true and false?

generics

implementation

constants

rust