::ghost_lite

使用 macro_rules 的轻量级 ::ghost 实现。

用法

use ghost_lite::ghost;

ghost! {
    /// `ghost` macro defines a custom `PhantomData`,
    /// which can be used as both a type and a value.
    pub struct MyPhantomData<T>
}

fn main() {
    let _: MyPhantomData<i32> = MyPhantomData;
    let _ = MyPhantomData::<&str>;
}

注意事项

1. derive 应在内部属性中（#![derive(...)]）。仅支持以下特性。

   ghost_lite::ghost! {
       #![derive(Clone, Copy, Default, Hash, PartialOrd, Ord, PartialEq, Eq, Debug)]
       pub struct MyPhantom<T>
   }
   
   /// `String` is not copy, but `MyPhantom` is always Copy, like `PhantomData`
   fn test() -> impl Copy {
       MyPhantom::<String>
   }
   # fn main() {}
   

   外部属性中的 derive 将直接添加到生成的 enum，这与正常的 derive 宏类似。

   ghost_lite::ghost! {
       /// MyPhantom is `Clone` and `Copy` only if T is `Clone` and `Copy`
       #[derive(Clone, Copy)]
       pub struct MyPhantom<T>
   }
   
   /// `String` is not copy, so `MyPhantom` is not Copy
   fn test() -> impl Copy {
       MyPhantom::<String>
   }
   # fn main() {}
   

2. 实现依赖于模块名称。要在同一模块中定义多个自定义 PhantomData 类型，必须使用 #![mod_value_namespace = my_phantom_data] 提供自定义模块名称。

   ghost_lite::ghost! {
       struct MyPhantomData1<T>
   }
   
   ghost_lite::ghost! {
       struct MyPhantomData2<T>
   }
   # fn main() {}
   

   ghost_lite::ghost! {
       struct MyPhantomData1<T>
   }
   
   ghost_lite::ghost! {
       #![mod_value_namespace = my_phantom_data_2]
       struct MyPhantomData2<T>
   }
   # fn main() {}
   

3. 如果 ghost! 报错，则将移动类型泛型约束移动到 where 子句。

   由于 macro_rules 的解析限制，不支持复杂类型约束。

   例如

   ghost_lite::ghost! {
       struct MyPhantomData<T: Clone + PartialEq>
   }
   # fn main() {}
   

   ghost_lite::ghost! {
       struct MyPhantomData<T> where T: Clone + PartialEq
   }
   # fn main() {}
   

4. 请勿添加尾随分号 ;。

   ghost_lite::ghost! {
       struct MyPhantomData<T>;
   }
   # fn main() {}
   

   ghost_lite::ghost! {
       struct MyPhantomData<T>
   }
   # fn main() {}