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authorDaniel Baumann <daniel.baumann@progress-linux.org>2024-04-07 09:22:09 +0000
committerDaniel Baumann <daniel.baumann@progress-linux.org>2024-04-07 09:22:09 +0000
commit43a97878ce14b72f0981164f87f2e35e14151312 (patch)
tree620249daf56c0258faa40cbdcf9cfba06de2a846 /third_party/rust/async-trait/src/lib.rs
parentInitial commit. (diff)
downloadfirefox-43a97878ce14b72f0981164f87f2e35e14151312.tar.xz
firefox-43a97878ce14b72f0981164f87f2e35e14151312.zip
Adding upstream version 110.0.1.upstream/110.0.1upstream
Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>
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+//! [![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;
+//! ```
+
+#![allow(
+ clippy::default_trait_access,
+ clippy::doc_markdown,
+ clippy::explicit_auto_deref,
+ 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 bound;
+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))
+}