1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
//! [![github]](https://github.com/dtolnay/async-trait) [![crates-io]](https://crates.io/crates/async-trait) [![docs-rs]](https://docs.rs/async-trait)
//!
//! [github]: https://img.shields.io/badge/github-8da0cb?style=for-the-badge&labelColor=555555&logo=github
//! [crates-io]: https://img.shields.io/badge/crates.io-fc8d62?style=for-the-badge&labelColor=555555&logo=rust
//! [docs-rs]: https://img.shields.io/badge/docs.rs-66c2a5?style=for-the-badge&labelColor=555555&logoColor=white&logo=data:image/svg+xml;base64,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
//!
//! <br>
//!
//! <h5>Type erasure for async trait methods</h5>
//!
//! The initial round of stabilizations for the async/await language feature in
//! Rust 1.39 did not include support for async fn in traits. Trying to include
//! an async fn in a trait produces the following error:
//!
//! ```compile_fail
//! trait MyTrait {
//!     async fn f() {}
//! }
//! ```
//!
//! ```text
//! error[E0706]: trait fns cannot be declared `async`
//!  --> src/main.rs:4:5
//!   |
//! 4 |     async fn f() {}
//!   |     ^^^^^^^^^^^^^^^
//! ```
//!
//! This crate provides an attribute macro to make async fn in traits work.
//!
//! Please refer to [*why async fn in traits are hard*][hard] for a deeper
//! analysis of how this implementation differs from what the compiler and
//! language hope to deliver in the future.
//!
//! [hard]: https://smallcultfollowing.com/babysteps/blog/2019/10/26/async-fn-in-traits-are-hard/
//!
//! <br>
//!
//! # Example
//!
//! This example implements the core of a highly effective advertising platform
//! using async fn in a trait.
//!
//! The only thing to notice here is that we write an `#[async_trait]` macro on
//! top of traits and trait impls that contain async fn, and then they work.
//!
//! ```
//! use async_trait::async_trait;
//!
//! #[async_trait]
//! trait Advertisement {
//!     async fn run(&self);
//! }
//!
//! struct Modal;
//!
//! #[async_trait]
//! impl Advertisement for Modal {
//!     async fn run(&self) {
//!         self.render_fullscreen().await;
//!         for _ in 0..4u16 {
//!             remind_user_to_join_mailing_list().await;
//!         }
//!         self.hide_for_now().await;
//!     }
//! }
//!
//! struct AutoplayingVideo {
//!     media_url: String,
//! }
//!
//! #[async_trait]
//! impl Advertisement for AutoplayingVideo {
//!     async fn run(&self) {
//!         let stream = connect(&self.media_url).await;
//!         stream.play().await;
//!
//!         // Video probably persuaded user to join our mailing list!
//!         Modal.run().await;
//!     }
//! }
//! #
//! # impl Modal {
//! #     async fn render_fullscreen(&self) {}
//! #     async fn hide_for_now(&self) {}
//! # }
//! #
//! # async fn remind_user_to_join_mailing_list() {}
//! #
//! # struct Stream;
//! # async fn connect(_media_url: &str) -> Stream { Stream }
//! # impl Stream {
//! #     async fn play(&self) {}
//! # }
//! ```
//!
//! <br><br>
//!
//! # Supported features
//!
//! It is the intention that all features of Rust traits should work nicely with
//! #\[async_trait\], but the edge cases are numerous. Please file an issue if
//! you see unexpected borrow checker errors, type errors, or warnings. There is
//! no use of `unsafe` in the expanded code, so rest assured that if your code
//! compiles it can't be that badly broken.
//!
//! > &#9745;&emsp;Self by value, by reference, by mut reference, or no self;<br>
//! > &#9745;&emsp;Any number of arguments, any return value;<br>
//! > &#9745;&emsp;Generic type parameters and lifetime parameters;<br>
//! > &#9745;&emsp;Associated types;<br>
//! > &#9745;&emsp;Having async and non-async functions in the same trait;<br>
//! > &#9745;&emsp;Default implementations provided by the trait;<br>
//! > &#9745;&emsp;Elided lifetimes;<br>
//! > &#9745;&emsp;Dyn-capable traits.<br>
//!
//! <br>
//!
//! # Explanation
//!
//! Async fns get transformed into methods that return `Pin<Box<dyn Future +
//! Send + 'async>>` and delegate to a private async freestanding function.
//!
//! For example the `impl Advertisement for AutoplayingVideo` above would be
//! expanded as:
//!
//! ```
//! # const IGNORE: &str = stringify! {
//! impl Advertisement for AutoplayingVideo {
//!     fn run<'async>(
//!         &'async self,
//!     ) -> Pin<Box<dyn core::future::Future<Output = ()> + Send + 'async>>
//!     where
//!         Self: Sync + 'async,
//!     {
//!         async fn run(_self: &AutoplayingVideo) {
//!             /* the original method body */
//!         }
//!
//!         Box::pin(run(self))
//!     }
//! }
//! # };
//! ```
//!
//! <br><br>
//!
//! # Non-threadsafe futures
//!
//! Not all async traits need futures that are `dyn Future + Send`. To avoid
//! having Send and Sync bounds placed on the async trait methods, invoke the
//! async trait macro as `#[async_trait(?Send)]` on both the trait and the impl
//! blocks.
//!
//! <br>
//!
//! # Elided lifetimes
//!
//! Be aware that async fn syntax does not allow lifetime elision outside of `&`
//! and `&mut` references. (This is true even when not using #\[async_trait\].)
//! Lifetimes must be named or marked by the placeholder `'_`.
//!
//! Fortunately the compiler is able to diagnose missing lifetimes with a good
//! error message.
//!
//! ```compile_fail
//! # use async_trait::async_trait;
//! #
//! type Elided<'a> = &'a usize;
//!
//! #[async_trait]
//! trait Test {
//!     async fn test(not_okay: Elided, okay: &usize) {}
//! }
//! ```
//!
//! ```text
//! error[E0726]: implicit elided lifetime not allowed here
//!  --> src/main.rs:9:29
//!   |
//! 9 |     async fn test(not_okay: Elided, okay: &usize) {}
//!   |                             ^^^^^^- help: indicate the anonymous lifetime: `<'_>`
//! ```
//!
//! The fix is to name the lifetime or use `'_`.
//!
//! ```
//! # use async_trait::async_trait;
//! #
//! # type Elided<'a> = &'a usize;
//! #
//! #[async_trait]
//! trait Test {
//!     // either
//!     async fn test<'e>(elided: Elided<'e>) {}
//! # }
//! # #[async_trait]
//! # trait Test2 {
//!     // or
//!     async fn test(elided: Elided<'_>) {}
//! }
//! ```
//!
//! <br><br>
//!
//! # Dyn traits
//!
//! Traits with async methods can be used as trait objects as long as they meet
//! the usual requirements for dyn -- no methods with type parameters, no self
//! by value, no associated types, etc.
//!
//! ```
//! # use async_trait::async_trait;
//! #
//! #[async_trait]
//! pub trait ObjectSafe {
//!     async fn f(&self);
//!     async fn g(&mut self);
//! }
//!
//! # const IGNORE: &str = stringify! {
//! impl ObjectSafe for MyType {...}
//!
//! let value: MyType = ...;
//! # };
//! #
//! # struct MyType;
//! #
//! # #[async_trait]
//! # impl ObjectSafe for MyType {
//! #     async fn f(&self) {}
//! #     async fn g(&mut self) {}
//! # }
//! #
//! # let value: MyType = MyType;
//! let object = &value as &dyn ObjectSafe;  // make trait object
//! ```
//!
//! The one wrinkle is in traits that provide default implementations of async
//! methods. In order for the default implementation to produce a future that is
//! Send, the async_trait macro must emit a bound of `Self: Sync` on trait
//! methods that take `&self` and a bound `Self: Send` on trait methods that
//! take `&mut self`. An example of the former is visible in the expanded code
//! in the explanation section above.
//!
//! If you make a trait with async methods that have default implementations,
//! everything will work except that the trait cannot be used as a trait object.
//! Creating a value of type `&dyn Trait` will produce an error that looks like
//! this:
//!
//! ```text
//! error: the trait `Test` cannot be made into an object
//!  --> src/main.rs:8:5
//!   |
//! 8 |     async fn cannot_dyn(&self) {}
//!   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
//! ```
//!
//! For traits that need to be object safe and need to have default
//! implementations for some async methods, there are two resolutions. Either
//! you can add Send and/or Sync as supertraits (Send if there are `&mut self`
//! methods with default implementations, Sync if there are `&self` methods with
//! default implementations) to constrain all implementors of the trait such that
//! the default implementations are applicable to them:
//!
//! ```
//! # use async_trait::async_trait;
//! #
//! #[async_trait]
//! pub trait ObjectSafe: Sync {  // added supertrait
//!     async fn can_dyn(&self) {}
//! }
//! #
//! # struct MyType;
//! #
//! # #[async_trait]
//! # impl ObjectSafe for MyType {}
//! #
//! # let value = MyType;
//!
//! let object = &value as &dyn ObjectSafe;
//! ```
//!
//! or you can strike the problematic methods from your trait object by
//! bounding them with `Self: Sized`:
//!
//! ```
//! # use async_trait::async_trait;
//! #
//! #[async_trait]
//! pub trait ObjectSafe {
//!     async fn cannot_dyn(&self) where Self: Sized {}
//!
//!     // presumably other methods
//! }
//! #
//! # struct MyType;
//! #
//! # #[async_trait]
//! # impl ObjectSafe for MyType {}
//! #
//! # let value = MyType;
//!
//! let object = &value as &dyn ObjectSafe;
//! ```

#![allow(
    clippy::default_trait_access,
    clippy::doc_markdown,
    clippy::if_not_else,
    clippy::items_after_statements,
    clippy::module_name_repetitions,
    clippy::shadow_unrelated,
    clippy::similar_names,
    clippy::too_many_lines
)]

extern crate proc_macro;

mod args;
mod expand;
mod lifetime;
mod parse;
mod receiver;

use crate::args::Args;
use crate::expand::expand;
use crate::parse::Item;
use proc_macro::TokenStream;
use quote::quote;
use syn::parse_macro_input;

#[proc_macro_attribute]
pub fn async_trait(args: TokenStream, input: TokenStream) -> TokenStream {
    let args = parse_macro_input!(args as Args);
    let mut item = parse_macro_input!(input as Item);
    expand(&mut item, args.local);
    TokenStream::from(quote!(#item))
}