Crate macro_attr_2018

This crate provides the macro_attr! macro that enables the use of custom, macro-based attributes and derivations.

The macro_attr! macro should be used to wrap an entire single item (enum, struct, etc.) declaration, including its attributes (both derive and others). All attributes and derivations which whose names end with ! will be assumed to be implemented by macros, and treated accordingly.

use macro_attr_2018::macro_attr;

// Define some traits to be derived.

trait TypeName {
    fn type_name() -> &'static str;
}

trait ReprType {
    type Repr;
}

// Define macros which derive implementations of these macros.

macro_rules! TypeName {
    // We can support any kind of item we want.
    (() $vis:vis enum $name:ident $($tail:tt)+) => { TypeName! { @impl $name } };
    (() $vis:vis struct $name:ident $($tail:tt)+) => { TypeName! { @impl $name } };

    // Inner rule to cut down on repetition.
    (@impl $name:ident) => {
        impl TypeName for $name {
            fn type_name() -> &'static str { stringify!($name) }
        }
    };
}

macro_rules! ReprType {
    // Note that we use a "derivation argument" here for the `$repr` type.
    (($repr:ty) $vis:vis enum $name:ident $($tail:tt)+) => {
        impl ReprType for $name {
            type Repr = $repr;
        }
    };
}

// Derive.

macro_attr! {
    #[derive(TypeName!, ReprType!(u16))]
    #[repr(u16)]
    enum SomeEnum { A, B, C, D }
}

assert_eq!(SomeEnum::type_name(), "SomeEnum");
assert_eq!(SomeEnum::A as <SomeEnum as ReprType>::Repr, 0u16);

Macros

macro_attr

When given an item definition, including its attributes, this macro parses said attributes and dispatches any derivations suffixed with ! to user-defined macros. This allows multiple macros to process the same item.