Document 'async' attributes.

core/codegen/src/lib.rs

@@ -4,7 +4,7 @@
 #![doc(html_favicon_url = "https://rocket.rs/images/favicon.ico")]
 #![doc(html_logo_url = "https://rocket.rs/images/logo-boxed.png")]
 
-#![warn(rust_2018_idioms)]
+#![warn(rust_2018_idioms, missing_docs)]
 
 //! # Rocket - Code Generation
 //!
@@ -377,11 +377,100 @@ pub fn async_test(args: TokenStream, input: TokenStream) -> TokenStream {
     emit!(attribute::entry::async_test_attribute(args, input))
 }
 
+/// Retrofits `async fn` support in `main` functions.
+///
+/// A `main` `async fn` function decorated with `#[rocket::main]` is transformed
+/// into a regular `main` function that internally initalizes a Rocket-specific
+/// tokio runtime and runs the attributed `async fn` inside of it:
+///
+/// ```rust,no_run
+/// #[rocket::main]
+/// async fn main() -> Result<(), rocket::Error> {
+///     rocket::build()
+///         .ignite().await?
+///         .launch().await
+/// }
+/// ```
+///
+/// It should be used only when inspection of an ignited instance of `Rocket` is
+/// required, or when the return value of `launch()` is to be inspected:
+///
+/// ```rust,no_run
+/// #[rocket::main]
+/// async fn main() -> Result<(), rocket::Error> {
+///     let rocket = rocket::build().ignite().await?;
+///     println!("Hello, Rocket: {:?}", rocket);
+///
+///     let result = rocket.launch().await;
+///     println!("The server shutdown: {:?}", result);
+///
+///     result
+/// }
+/// ```
+///
+/// For all other cases, use [`#[launch]`](launch) instead.
+///
+/// The function attributed with `#[rocket::main]` _must_ be `async` and _must_
+/// be called `main`. Violation of either results in a compile-time error.
 #[proc_macro_attribute]
 pub fn main(args: TokenStream, input: TokenStream) -> TokenStream {
     emit!(attribute::entry::main_attribute(args, input))
 }
 
+/// Generates a `main` function that launches a returned `Rocket<Build>`.
+///
+/// When applied to a function that returns a `Rocket<Build>` instance,
+/// `#[launch]` automatically initializes an `async` runtime and
+/// launches the function's returned instance:
+///
+/// ```rust,no_run
+/// # use rocket::launch;
+/// use rocket::{Rocket, Build};
+///
+/// #[launch]
+/// fn rocket() -> Rocket<Build> {
+///     rocket::build()
+/// }
+/// ```
+///
+/// This generates code equivalent to the following:
+///
+/// ```rust,no_run
+/// # use rocket::{Rocket, Build};
+/// # fn rocket() -> Rocket<Build> {
+/// #     rocket::build()
+/// # }
+/// #
+/// #[rocket::main]
+/// async fn main() {
+///     // Recall that an uninspected `Error` will cause a pretty-printed panic,
+///     // so rest assured failures do not go undetected when using `#[launch]`.
+///     let _ = rocket().launch().await;
+/// }
+/// ```
+///
+/// To avoid needing to import _any_ items in the common case, the `launch`
+/// attribute will infer a return type written as `_` as `Rocket<Build>`:
+///
+/// ```rust,no_run
+/// # use rocket::launch;
+/// #[launch]
+/// fn rocket() -> _ {
+///     rocket::build()
+/// }
+/// ```
+///
+/// The attributed function may be `async`:
+///
+/// ```rust,no_run
+/// # use rocket::launch;
+/// # async fn some_async_work() {}
+/// #[launch]
+/// async fn rocket() -> _ {
+///     some_async_work().await;
+///     rocket::build()
+/// }
+/// ```
 #[proc_macro_attribute]
 pub fn launch(args: TokenStream, input: TokenStream) -> TokenStream {
     emit!(attribute::entry::launch_attribute(args, input))

core/lib/src/lib.rs

@@ -179,7 +179,30 @@ pub fn custom<T: figment::Provider>(provider: T) -> Rocket<Build> {
 
 /// Retrofits support for `async fn` in trait impls and declarations.
 ///
-/// See [`async_trait`](mod@async_trait) for full details.
+/// Any trait declaration or trait `impl` decorated with `#[async_trait]` is
+/// retrofitted with support for `async fn`s:
+///
+/// ```rust
+/// # use rocket::*;
+/// #[async_trait]
+/// trait MyAsyncTrait {
+///     async fn do_async_work();
+/// }
+///
+/// #[async_trait]
+/// impl MyAsyncTrait for () {
+///     async fn do_async_work() { /* .. */ }
+/// }
+/// ```
+///
+/// All `impl`s for a trait declared with `#[async_trait]` must themselves be
+/// decorated with `#[async_trait]`. Many of Rocket's traits, such as
+/// [`FromRequest`](crate::request::FromRequest) and
+/// [`Fairing`](crate::fairing::Fairing) are `async`. As such, implementations
+/// of said traits must be decorated with `#[async_trait]`. See the individual
+/// trait docs for trait-specific details.
+///
+/// For more details on `#[async_trait]`, see [`async_trait`](mod@async_trait).
 #[doc(inline)]
 pub use async_trait::async_trait;
 

core/lib/src/rocket.rs

@@ -54,7 +54,8 @@ use crate::log::PaintExt;
 /// To launch an instance of `Rocket`, it _must_ progress through all three
 /// phases. To progress into the ignite or launch phases, a tokio `async`
 /// runtime is required. The [`#[main]`](crate::main) attribute initializes a
-/// Rocket-specific tokio runtime and runs attributed async code inside of it:
+/// Rocket-specific tokio runtime and runs the attributed `async fn` inside of
+/// it:
 ///
 /// ```rust,no_run
 /// #[rocket::main]