block-on: Add example from Chapter 20, Asynchronous Programming.

block-on/.gitignore

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+/target/
+Cargo.lock

block-on/Cargo.toml

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+[package]
+name = "block-on"
+version = "0.1.0"
+authors = ["You <you@example.com>"]
+edition = "2018"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]
+waker-fn = "1.1"
+futures-lite = "1.11"
+crossbeam = "0.8"
+
+[dev-dependencies]
+async-std = "1.7"

block-on/src/lib.rs

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+use waker_fn::waker_fn;      // Cargo.toml: waker-fn = "1.1"
+use futures_lite::pin;       // Cargo.toml: futures-lite = "1.11"
+use crossbeam::sync::Parker; // Cargo.toml: crossbeam = "0.8"
+use std::future::Future;
+use std::task::{Context, Poll};
+
+pub fn block_on<F: Future>(future: F) -> F::Output {
+    let parker = Parker::new();
+    let unparker = parker.unparker().clone();
+    let waker = waker_fn(move || unparker.unpark());
+    let mut context = Context::from_waker(&waker);
+
+    pin!(future);
+
+    loop {
+        match future.as_mut().poll(&mut context) {
+            Poll::Ready(value) => return value,
+            Poll::Pending => parker.park(),
+        }
+    }
+}
+
+#[test]
+fn test() {
+    assert_eq!(block_on(std::future::ready(42)), 42);
+
+    use async_std::task::{spawn, sleep};
+    use futures_lite::FutureExt;
+    use std::time::Duration;
+
+    assert_eq!(
+        block_on({
+            let one_sec = async {
+                sleep(Duration::from_secs(1)).await;
+                43
+            };
+            let half_sec = async {
+                sleep(Duration::from_millis(500)).await;
+                44
+            };
+            spawn(one_sec.race(half_sec))
+        }),
+        44);
+}