::ghost_lite

A lightweight implementation of ::ghost with macro_rules.

Usage

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>;
}

Caveats

1. derive should be in inner attributes (#![derive(...)]). Only the following traits are supported.

   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() {}
   

   derive in outer attributes will be directly prepended to the generated enum, which works like normal derive macros.

   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. The implementation relies on a mod name. To define multiple custom PhantomData types in the same module, you must provide custom mod name with #![mod_value_namespace = my_phantom_data].

   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. Move type generic bounds to where clause if ghost! reports error.

   Parsing tokens is limited with macro_rules, so complex type bounds are not supported.

   For example:

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

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

4. Please don't add a trailing semicolon ;.

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

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