# `::macro_rules_attribute`
Use declarative macros in attribute or derive position.
```rust ,ignore
macro_rules! my_fancy_decorator { /* … */ }
#[apply(my_fancy_decorator!)]
struct Foo { /* … */ }
```
```rust ,ignore
macro_rules! MyFancyDerive { /* … */ }
#[derive(MyFancyDerive!)] /* using this crate's `#[derive]` attribute */
struct Foo { /* … */ }
```
[](
https://github.com/danielhenrymantilla/macro_rules_attribute-rs)
[](
https://crates.io/crates/macro_rules_attribute)
[](
https://docs.rs/macro_rules_attribute)
[](
https://gist.github.com/danielhenrymantilla/9b59de4db8e5f2467ed008b3c450527b)
[](
https://github.com/rust-secure-code/safety-dance/)
[](
https://github.com/danielhenrymantilla/macro_rules_attribute-rs/blob/master/LICENSE-ZLIB)
[](
https://github.com/danielhenrymantilla/macro_rules_attribute-rs/actions)
## 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:
```rust ,ignore
foo! {
struct Struct {
some_field: SomeType,
}
}
```
to:
```rust ,ignore
#[foo]
struct Struct {
some_field: SomeType,
}
```
1. The former does not scale well, since it leads to **rightward drift and
"excessive" braces**.
1. 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 `proc`edural
macro.
Anything involving `ident` generation / derivation, for instance, will very
often require `proc`edural macros, unless it is simple enough for
[`::paste`] to handle it.
___
## Solution
#\[[apply]\] and #\[[derive]\]
attributes, it is now possible to use `proc_macro_attribute` syntax to apply a
`macro_rules!` macro:
```rust
#[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**.
[`::paste`]: https://docs.rs/paste
[`::proc-macro2`]: https://docs.rs/proc_macro2
[`::syn`]: https://docs.rs/syn
[`::quote`]: https://docs.rs/quote
[`::pin-project`]: https://docs.rs/pin-project
[`::pin-project-lite`]: https://docs.rs/pin-project-lite
# Examples
Click to see
### Nicer derives
```rust
#[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` heavyweight[^only_full_syn_is_heavy]
troika/trinity/triumvirate of more advanced proc-macro crates.
[^only_full_syn_is_heavy]: (note that only `syn` with the `"full"` features would be the truly heavyweight party)
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:
```rust
#[macro_use]
extern crate macro_rules_attribute;
use {
::core::pin::{
Pin,
},
::pin_project_lite::{
pin_project,
},
};
#[apply(pin_project!)]
struct Struct {
#[pin]
pinned: T,
unpinned: U,
}
impl Struct {
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:
```rust
# use Sync as Logic;
#
static MY_GLOBAL: &dyn Logic = &Vec::::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.
```rust ,compile_fail
// (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:
```rust
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!:
```rust
# 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() {}
```
#\[[derive]\] for more info.
### `cfg` aliases
Click to see
```rust
# 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 { /* … */ }
```
Click to see
If you are allergic to `#[macro_use]` unscoped / globally-preluded semantics,
you may not be fond of having to use:
```rust
#[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:
```rust
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):
```rust
::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.