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0.2.4 | Jun 14, 2024 |
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0.2.3 | Aug 3, 2022 |
0.2.1 | Jul 18, 2022 |
0.2.0 | Jun 24, 2022 |
0.1.1 | Jun 21, 2022 |
#82 in Procedural macros
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Used in 27 crates
(26 directly)
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2.5K
SLoC
Maybe-Async-Cfg Procedure Macro
Why bother writing similar code twice for blocking and async code?
When implementing both sync and async versions of API in a crate, most API of the two version are almost the same except for some async/await keyword.
maybe-async-cfg
help unifying async and sync implementation by procedural macro.
- Write async code with normal
async
,await
, and letmaybe_async_cfg
handles thoseasync
andawait
when you need a blocking code. - Add
maybe
attributes and specify feature-based conditions under which sync or async code should be generated. - Use
only_if
(orremove_if
) to keep code in specified version if necessary.
The maybe
procedural macro can be applied to the following codes:
- use declaration
- trait declaration
- trait implementation
- function definition
- struct and enum definition
- modules
RECOMMENDATION: Enable resolver ver2 in your crate, which is introduced in Rust 1.51. If not, two crates in dependency with conflict version (one async and another blocking) can fail compilation.
Motivation
The async/await language feature alters the async world of rust. Comparing with the map/and_then style, now the async code really resembles sync version code.
In many crates, the async and sync version of crates shares the same API, but the minor difference that all async code must be awaited prevent the unification of async and sync code. In other words, we are forced to write an async and an sync implementation respectively.
Macros in Detail
To use maybe-async-cfg
, we must know which block of codes is only used on sync implementation,
and which on async. These two versions of the implementation should share the same function
signatures except for async/await keywords.
Use maybe
macro for code that is the same in both async and sync versions except for
async/await keywords. Specify in the macro parameters the conditions (based on features) under
which async and/or sync versions of the code should appear.
-
attribute macro
maybe
Offers a unified way to provide sync and async conversion on demand depending on features, enabled for your crate, with async first policy.
[dependencies] maybe_async_cfg = "0.2" [features] use_sync = [] use_async = []
In this and all the following examples, we use two features. But you can use any conditions that are convenient for you, for example, replacing
feature="use_sync"
withnot(feature="use_async")
everywhere. Feel free,maybe-async-cfg
does not analyze the conditions in any way, just substituting them as is.Add the
maybe
attribute before all the items that need to be changed in different versions of the code (sync or async).Want to keep async code? Specify the
async
parameter with the condition (based on features) when your code should be async.Wanna convert async code to sync? Specify the
sync
parameter with the condition when the sync code should be generated.#[maybe_async_cfg::maybe( idents(Foo), sync(feature="use_sync"), async(feature="use_async") )] struct Struct { f: Foo, }
After conversion:
#[cfg(feature="use_sync")] struct StructSync { f: FooSync, } #[cfg(feature="use_async")] struct StructAsync { f: FooAsync, }
-
procedural macro
content
The
content
macro allows you to specify common parameters for manymaybe
macros. Use the internaldefault
attribute with the required parameters inside thecontent
macro.maybe_async_cfg::content!{ #![maybe_async_cfg::default( idents(Foo, Bar), )] #[maybe_async_cfg::maybe( sync(feature="use_sync"), async(feature="use_async") )] struct Struct { f: Foo, } #[maybe_async_cfg::maybe( sync(feature="use_sync"), async(feature="use_async") )] async fn func(b: Bar) { todo!() } } // content!
After conversion:
#[cfg(feature="use_sync")] struct StructSync { f: FooSync, } #[cfg(feature="use_async")] struct StructAsync { f: FooAsync, } #[cfg(feature="use_sync")] fn func_sync(b: BarSync) { todo!() } #[cfg(feature="use_async")] async fn func_async(b: BarAsync) { todo!() }
Doctests
When writing doctests, you can mark them as applicable only in the corresponding code version.
To do this, specify only_if(
VARIANT_KEY)
in the doctest attributes. Then in all other
versions of the code, this doctest will be replaced with an empty string.
#[maybe_async_cfg::maybe(
idents(Foo),
sync(feature="use_sync"),
async(feature="use_async")
)]
/// This is a structure.
/// ```rust, only_if(sync)
/// let s = StructSync{ f: FooSync::new() };
/// ```
/// ```rust, only_if(async)
/// let s = StructAsync{ f: FooAsync::new().await };
/// ```
struct Struct {
f: Foo,
}
After conversion:
#[cfg(feature="use_sync")]
/// This is a structure.
/// ```rust, only_if(sync)
/// let s = StructSync{ f: FooSync::new() };
/// ```
///
struct StructSync {
f: FooSync,
}
#[cfg(feature="use_async")]
/// This is a structure.
///
/// ```rust, only_if(async)
/// let s = StructAsync{ f: FooAsync::new().await };
/// ```
struct StructAsync {
f: FooAsync,
}
Examples
rust client for services
When implementing rust client for any services, like awz3. The higher level API of async and sync version is almost the same, such as creating or deleting a bucket, retrieving an object and etc.
The example service_client
is a proof of concept that maybe_async_cfg
can actually free us
from writing almost the same code for sync and async. We can toggle between a sync AWZ3 client
and async one by is_sync
feature gate when we add maybe-async-cfg
to dependency.
Acknowledgements
This crate is a redesigned fork of these wonderful crates:
Thanks!
License
MIT
Dependencies
~1.3–1.7MB
~41K SLoC