guide: add rt guide

Author: dswij

_stable/init/runtime.md

@@ -0,0 +1,112 @@
+---
+title:  Runtime
+layout: guide
+permalink: /guides/1/init/runtime/
+---
+
+With hyper v1.0 removing `tokio` as runtime dependency, a new runtime trait `hyper::rt` is introduced. If you still want to use `tokio`, a `tokio` implementation for `hyper::rt` is provided by `hyper-util` crate.
+
+## Building your own `hyper::rt` implementations with `Tokio`
+
+Let's build a simple `hyper::rt` implementations with the help of tokio. First, make sure you have `tokio` as a dependency in your `Cargo.toml`:
+
+```toml
+[dependencies]
+tokio = { version = "1", features = ["full"] }
+```
+
+Now, let's try to write a simple executor `hyper::rt::Executor`, we'll call it `TokioExecutor`:
+
+```rust
+# extern crate hyper;
+/// Future executor that utilises `tokio` threads.
+#[non_exhaustive]
+#[derive(Default, Debug, Clone)]
+pub struct TokioExecutor {}
+```
+
+the trait `hyper::rt::Executor` expects an `execute` method that asks the runtime to execute futures. `tokio` allows this easily with `tokio::spawn`
+
+```rust
+# extern crate hyper;
+# extern crate tokio;
+
+use std::future::Future;
+
+use hyper::rt::Executor;
+
+# #[non_exhaustive]
+# #[derive(Default, Debug, Clone)]
+# pub struct TokioExecutor {}
+
+impl<Fut> Executor<Fut> for TokioExecutor
+where
+    Fut: Future + Send + 'static,
+    Fut::Output: Send + 'static,
+{
+    fn execute(&self, fut: Fut) {
+        tokio::spawn(fut);
+    }
+}
+```
+
+We now have a working `hyper::rt::Executor` with Tokio, and is ready to be supplied to anything that requires `Executor`. For example, with the auto connection from hyper-util:
+
+```rust
+# extern crate hyper;
+# extern crate hyper_util;
+# extern crate tokio;
+
+use std::future::Future;
+
+use hyper_util::server::conn::auto;
+use hyper::rt::Executor;
+
+# #[non_exhaustive]
+# #[derive(Default, Debug, Clone)]
+# pub struct TokioExecutor {}
+
+# impl<Fut> Executor<Fut> for TokioExecutor
+# where
+#     Fut: Future + Send + 'static,
+#     Fut::Output: Send + 'static,
+# {
+#     fn execute(&self, fut: Fut) {
+#         tokio::spawn(fut);
+#     }
+# }
+
+impl TokioExecutor {
+    pub fn new() -> Self {
+        Self {}
+    }
+}
+
+auto::Builder::new(TokioExecutor::new());
+```
+
+## Using `hyper::rt` implementations with tokio in hyper-util
+
+The crate `hyper-util` provides implementations for `hyper::rt` traits with Tokio. To use, we'll need to have `hyper-util` as a dependency.
+
+```toml
+[dependencies]
+hyper-util = { version = "0.1", features = ["full"] }
+```
+
+Then you'll simply need to import and use it as in the example above
+
+```rust
+# extern crate hyper;
+# extern crate hyper_util;
+
+use hyper::rt::Executor;
+use hyper_util::rt::TokioExecutor;
+use hyper_util::server::conn::auto;
+
+auto::Builder::new(TokioExecutor::new());
+```
+
+There are more implementations in the hyper_util crate. Check out the docs on [`hyper_util::rt`][] for more details.
+
+[`hyper_util::rt`]: https://docs.rs/hyper-util/latest/hyper_util/rt/index.html