| <!DOCTYPE html><html lang="en"><head><meta charset="utf-8"><meta name="viewport" content="width=device-width, initial-scale=1.0"><meta name="generator" content="rustdoc"><meta name="description" content="Source of the Rust file `/root/.cargo/registry/src/github.com-1ecc6299db9ec823/async-trait-0.1.68/src/lib.rs`."><meta name="keywords" content="rust, rustlang, rust-lang"><title>lib.rs - source</title><link rel="preload" as="font" type="font/woff2" crossorigin href="../../SourceSerif4-Regular.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../FiraSans-Regular.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../FiraSans-Medium.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../SourceCodePro-Regular.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../SourceSerif4-Bold.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../SourceCodePro-Semibold.ttf.woff2"><link rel="stylesheet" href="../../normalize.css"><link rel="stylesheet" href="../../rustdoc.css" id="mainThemeStyle"><link rel="stylesheet" href="../../ayu.css" disabled><link rel="stylesheet" href="../../dark.css" disabled><link rel="stylesheet" href="../../light.css" id="themeStyle"><script id="default-settings" ></script><script src="../../storage.js"></script><script defer src="../../source-script.js"></script><script defer src="../../source-files.js"></script><script defer src="../../main.js"></script><noscript><link rel="stylesheet" href="../../noscript.css"></noscript><link rel="alternate icon" type="image/png" href="../../favicon-16x16.png"><link rel="alternate icon" type="image/png" href="../../favicon-32x32.png"><link rel="icon" type="image/svg+xml" href="../../favicon.svg"></head><body class="rustdoc source"><!--[if lte IE 11]><div class="warning">This old browser is unsupported and will most likely display funky things.</div><![endif]--><nav class="sidebar"><a class="sidebar-logo" href="../../async_trait/index.html"><div class="logo-container"><img class="rust-logo" src="../../rust-logo.svg" alt="logo"></div></a></nav><main><div class="width-limiter"><nav class="sub"><a class="sub-logo-container" href="../../async_trait/index.html"><img class="rust-logo" src="../../rust-logo.svg" alt="logo"></a><form class="search-form"><div class="search-container"><span></span><input class="search-input" name="search" autocomplete="off" spellcheck="false" placeholder="Click or press ‘S’ to search, ‘?’ for more options…" type="search"><div id="help-button" title="help" tabindex="-1"><a href="../../help.html">?</a></div><div id="settings-menu" tabindex="-1"><a href="../../settings.html" title="settings"><img width="22" height="22" alt="Change settings" src="../../wheel.svg"></a></div></div></form></nav><section id="main-content" class="content"><div class="example-wrap"><pre class="src-line-numbers"><span id="1">1</span> |
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| </pre><pre class="rust"><code><span class="doccomment">//! [![github]](https://github.com/dtolnay/async-trait)&ensp;[![crates-io]](https://crates.io/crates/async-trait)&ensp;[![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&logo=docs.rs |
| //! |
| //! <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; |
| //! ``` |
| |
| </span><span class="attribute">#![doc(html_root_url = <span class="string">"https://docs.rs/async-trait/0.1.68"</span>)] |
| #![allow( |
| clippy::default_trait_access, |
| clippy::doc_markdown, |
| clippy::explicit_auto_deref, |
| clippy::if_not_else, |
| clippy::items_after_statements, |
| clippy::match_like_matches_macro, |
| clippy::module_name_repetitions, |
| clippy::shadow_unrelated, |
| clippy::similar_names, |
| clippy::too_many_lines |
| )] |
| |
| </span><span class="kw">extern crate </span>proc_macro; |
| |
| <span class="kw">mod </span>args; |
| <span class="kw">mod </span>bound; |
| <span class="kw">mod </span>expand; |
| <span class="kw">mod </span>lifetime; |
| <span class="kw">mod </span>parse; |
| <span class="kw">mod </span>receiver; |
| <span class="kw">mod </span>verbatim; |
| |
| <span class="kw">use </span><span class="kw">crate</span>::args::Args; |
| <span class="kw">use </span><span class="kw">crate</span>::expand::expand; |
| <span class="kw">use </span><span class="kw">crate</span>::parse::Item; |
| <span class="kw">use </span>proc_macro::TokenStream; |
| <span class="kw">use </span>quote::quote; |
| <span class="kw">use </span>syn::parse_macro_input; |
| |
| <span class="attribute">#[proc_macro_attribute] |
| </span><span class="kw">pub fn </span>async_trait(args: TokenStream, input: TokenStream) -> TokenStream { |
| <span class="kw">let </span>args = <span class="macro">parse_macro_input!</span>(args <span class="kw">as </span>Args); |
| <span class="kw">let </span><span class="kw-2">mut </span>item = <span class="macro">parse_macro_input!</span>(input <span class="kw">as </span>Item); |
| expand(<span class="kw-2">&mut </span>item, args.local); |
| TokenStream::from(<span class="macro">quote!</span>(#item)) |
| } |
| </code></pre></div> |
| </section></div></main><div id="rustdoc-vars" data-root-path="../../" data-current-crate="async_trait" data-themes="ayu,dark,light" data-resource-suffix="" data-rustdoc-version="1.66.0-nightly (5c8bff74b 2022-10-21)" ></div></body></html> |
