::macro_rules_attribute

Use declarative macros in attribute or derive position.

macro_rules! my_fancy_decorator { /* … */ }

#[apply(my_fancy_decorator!)]
struct Foo { /* … */ }

macro_rules! MyFancyDerive { /* … */ }

#[derive(MyFancyDerive!)]  /* using this crate's `#[derive]` attribute */
struct Foo { /* … */ }

Motivation

Click to see

macro_rules! macros can be extremely powerful, but their call-site ergonomics are sometimes not great, especially when decorating item definitions.

Indeed, compare:

foo! {
    struct Struct {
        some_field: SomeType,
    }
}

to:

#[foo]
struct Struct {
    some_field: SomeType,
}

1. The former does not scale well, since it leads to rightward drift and "excessive" braces.

2. But on the other hand, the latter requires setting up a dedicated crate for the compiler, a proc-macro crate. And 99% of the time this will pull the ::syn and ::quote dependencies, which have a non-negligible compile-time overhead (the first time they are compiled).

   • note: these crates are a wonderful piece of technology, and can lead to extremely powerful macros. When the logic of the macro is so complicated that it requires a recursive tt muncher when implemented as a macro_rules! macro, it is definitely time to be using a procedural macro.

     Anything involving ident generation / derivation, for instance, will very often require procedural macros, unless it is simple enough for ::paste to handle it.

---

Solution

With this crate's #[apply] and #[derive] attributes, it is now possible to use proc_macro_attribute syntax to apply a macro_rules! macro:

#[macro_use]
extern crate macro_rules_attribute;

macro_rules! foo {
    // …
    # ( $($tt:tt)* ) => ()
}

macro_rules! Bar {
    // …
    # ( $($tt:tt)* ) => ()
}

#[apply(foo)]
#[derive(Debug, Bar!)]
struct Struct {
    some_field: SomeType,
}
#
# fn main() {}

without even depending on ::quote, ::syn or ::proc-macro2, for fast compile times.

Examples

Click to see

Nicer derives

#[macro_use]
extern crate macro_rules_attribute;

// Easily define shorthand aliases for "derive groups"
derive_alias! {
    #[derive(Eq!)] = #[derive(Eq, PartialEq)];
    #[derive(Ord!)] = #[derive(Ord, PartialOrd, Eq!)];
    #[derive(Copy!)] = #[derive(Copy, Clone)];
    #[derive(StdDerives!)] = #[derive(Debug, Copy!, Default, Ord!, Hash)];
}

/// Strongly-typed newtype wrapper around a `usize`, to be used for `PlayerId`s.
#[derive(StdDerives!, Into!, From!)]
pub
struct PlayerId /* = */ (
    pub usize,
);

// You can also fully define your own derives using `macro_rules!` syntax
// (handling generic type definitions may be the only finicky thing, though…)
macro_rules! Into {(
    $( #[$attr:meta] )*
    $pub:vis
    struct $NewType:ident (
        $(#[$field_attr:meta])*
        $field_pub:vis
        $Inner:ty $(,

        $($rest:tt)* )?
    );
) => (
    impl ::core::convert::Into<$Inner> for $NewType {
        #[inline]
        fn into (self: $NewType)
          -> $Inner
        {
            self.0
        }
    }
)} use Into;

macro_rules! From {(
    $( #[$attr:meta] )*
    $pub:vis
    struct $NewType:ident (
        $(#[$field_attr:meta])*
        $field_pub:vis
        $Inner:ty $(,

        $(#[$other_field_attr:meta])*
        $other_field_pub:vis
        $Rest:ty )* $(,)?
    );
) => (
    impl ::core::convert::From<$Inner> for $NewType {
        #[inline]
        fn from (inner: $Inner)
          -> Self
        {
            Self(inner, $($Rest::default),*)
        }
    }
)} use From;
#
# fn main() {}

Have a -lite version of a proc-macro dependency that thus requires unergonomic macro_rules!?

Say you are writing a (pervasive and yet) tiny dependency within the async ecosystem.

• By virtue of working with async, you'll most probably need to deal with pin-projections, and thence, with ::pin-project.

• But by virtue of being (pervasive and yet) tiny, you don't want to depend on the quote / proc-macro2 / syn heavyweight¹ troika/trinity/triumvirate of more advanced proc-macro crates.

Hence why you may reach for something such as ::pin-project-lite, and its pin_project! macro_rules!-based polyfill of the former's #[pin_project] attribute.

But this suddenly hinders the ergonomics of your type definitions, and, worse, would not be composable whenever the pattern were to be repeated for some other functionality (e.g., say a cell_project! similar macro).

Say no more! Time to #[apply] our neat trick:

#[macro_use]
extern crate macro_rules_attribute;

use {
    ::core::pin::{
        Pin,
    },
    ::pin_project_lite::{
        pin_project,
    },
};

#[apply(pin_project!)]
struct Struct<T, U> {
    #[pin]
    pinned: T,
    unpinned: U,
}

impl<T, U> Struct<T, U> {
    fn method(self: Pin<&mut Self>) {
        let this = self.project();
        let _: Pin<&mut T> = this.pinned; // Pinned reference to the field
        let _: &mut U = this.unpinned; // Normal reference to the field
    }
}
#
# fn main() {}

More ergonomic lazy_static!s

Say you had something like:

# use Sync as Logic;
#
static MY_GLOBAL: &dyn Logic = &Vec::<i32>::new();

and now you want to change the value of that MY_GLOBAL to something that isn't const-constructible, and yet would like to minimize the churn in doing so.

// (For those unaware of it, leaking memory to initialize a lazy static is
// a completely fine pattern, since it only occurs once, and thus, a bounded
// amount of times).
static MY_GLOBAL: &dyn Logic = Box::leak(Box::new(vec![42, 27])); // Error: not `const`!

You could directly use a lazy_static! or a OnceCell, but then the definition of your static will now appear noisier than it needs be. It's time for attribute-position polish!

First, define the helper around, say, OnceCell's Lazy type:

macro_rules! lazy_init {(
    $( #[$attrs:meta] )*
    $pub:vis
    static $NAME:ident: $Ty:ty = $init_value:expr ;
) => (
    $( #[$attrs] )*
    $pub
    static $NAME : ::once_cell::sync::Lazy<$Ty> =
        ::once_cell::sync::Lazy::new(|| $init_value)
    ;
)} pub(in crate) use lazy_init;

and now it is time to use it!:

# use Sync as Logic;
#
#[macro_use]
extern crate macro_rules_attribute;

#[apply(lazy_init)]
static MY_GLOBAL: &dyn Logic = Box::leak(Box::new(vec![42, 27]));
#
# macro_rules! lazy_init {(
#     $( #[$attrs:meta] )*
#     $pub:vis
#     static $NAME:ident : $Ty:ty = $init_value:expr ;
# ) => (
#     $( #[$attrs] )*
#     $pub
#     static $NAME : ::once_cell::sync::Lazy<$Ty> =
#         ::once_cell::sync::Lazy::new(|| $init_value)
#     ;
# )} use lazy_init;
#
# fn main() {}

Debugging

An optional compilation feature, "verbose-expansions" can be used to print at compile-time the exact output of each macro invocation from this crate:

[dependencies]
macro_rules_attribute.version = "..."
macro_rules_attribute.features = ["verbose-expansions"]

Features

derive aliases

# fn main() {}
#[macro_use]
extern crate macro_rules_attribute;

derive_alias! {
    #[derive(Ord!)] = #[derive(PartialEq, Eq, PartialOrd, Ord)];
}

#[derive(Debug, Clone, Copy, Ord!)]
struct Foo {
    // …
}

• See derive_alias! and #[derive] for more info.

cfg aliases

Click to see

# fn main() {}
#[macro_use]
extern crate macro_rules_attribute;

attribute_alias! {
    #[apply(complex_cfg!)] = #[cfg(
        any(
            any(
                foo,
                feature = "bar",
            ),
            all(
                target_os = "fenestrations",
                not(target_arch = "Pear"),
            ),
        ),
    )];
}

#[apply(complex_cfg!)]
mod some_item { /* … */ }

Not using #[macro_use] extern crate macro_rules_attribute

Click to see

If you are allergic to #[macro_use] unscoped / globally-preluded semantics, you may not be fond of having to use:

#[macro_use]
extern crate macro_rules_attribute;
# fn main() {}

like this documentation pervasively does.

In that case, know that you may very well stick to using use imports:

use ::macro_rules_attribute::{derive, derive_alias, /* … */};
// or even
use ::macro_rules_attribute::*;

derive_alias! {
    #[derive(Copy!)] = #[derive(Clone, Copy)];
}

#[derive(Copy!)]
struct Foo;

or even inlining the fully qualified paths (but note that the …_alias! macros still take unqualified paths inside the definitions):

::macro_rules_attribute::derive_alias! {
    #[derive(Copy!)] = #[derive(Clone, Copy)];
}

#[::macro_rules_attribute::derive(Copy!)]
struct Foo;

I personally find these approaches too noisy to be worth it, despite the so gained "namespace purity", hence my not using that pattern across the rest of the examples.

Footnotes

1. (note that only syn with the "full" features would be the truly heavyweight party) ↩