Move all examples to their own crates (#201)

This makes it much clearer which dependencies each example has.

examples/tokio-postgres/Cargo.toml

@@ -0,0 +1,14 @@
+[package]
+name = "example-tokio-postgres"
+version = "0.1.0"
+edition = "2018"
+publish = false
+
+[dependencies]
+axum = { path = "../.." }
+tokio = { version = "1.0", features = ["full"] }
+tracing = "0.1"
+tracing-subscriber = "0.2"
+bb8 = "0.7.1"
+bb8-postgres = "0.7.0"
+tokio-postgres = "0.7.2"

examples/tokio-postgres/src/main.rs

@@ -0,0 +1,109 @@
+//! Run with
+//!
+//! ```not_rust
+//! cargo run -p example-tokio-postgres
+//! ```
+
+use axum::{
+    async_trait,
+    extract::{Extension, FromRequest, RequestParts},
+    handler::get,
+    http::StatusCode,
+    route,
+    routing::RoutingDsl,
+    AddExtensionLayer,
+};
+use bb8::{Pool, PooledConnection};
+use bb8_postgres::PostgresConnectionManager;
+use std::net::SocketAddr;
+use tokio_postgres::NoTls;
+
+#[tokio::main]
+async fn main() {
+    // Set the RUST_LOG, if it hasn't been explicitly defined
+    if std::env::var("RUST_LOG").is_err() {
+        std::env::set_var("RUST_LOG", "example_tokio_postgres=debug")
+    }
+    tracing_subscriber::fmt::init();
+
+    // setup connection pool
+    let manager =
+        PostgresConnectionManager::new_from_stringlike("host=localhost user=postgres", NoTls)
+            .unwrap();
+    let pool = Pool::builder().build(manager).await.unwrap();
+
+    // build our application with some routes
+    let app = route(
+        "/",
+        get(using_connection_pool_extractor).post(using_connection_extractor),
+    )
+    .layer(AddExtensionLayer::new(pool));
+
+    // run it with hyper
+    let addr = SocketAddr::from(([127, 0, 0, 1], 3000));
+    tracing::debug!("listening on {}", addr);
+    axum::Server::bind(&addr)
+        .serve(app.into_make_service())
+        .await
+        .unwrap();
+}
+
+type ConnectionPool = Pool<PostgresConnectionManager<NoTls>>;
+
+// we can exact the connection pool with `Extension`
+async fn using_connection_pool_extractor(
+    Extension(pool): Extension<ConnectionPool>,
+) -> Result<String, (StatusCode, String)> {
+    let conn = pool.get().await.map_err(internal_error)?;
+
+    let row = conn
+        .query_one("select 1 + 1", &[])
+        .await
+        .map_err(internal_error)?;
+    let two: i32 = row.try_get(0).map_err(internal_error)?;
+
+    Ok(two.to_string())
+}
+
+// we can also write a custom extractor that grabs a connection from the pool
+// which setup is appropriate depends on your application
+struct DatabaseConnection(PooledConnection<'static, PostgresConnectionManager<NoTls>>);
+
+#[async_trait]
+impl<B> FromRequest<B> for DatabaseConnection
+where
+    B: Send,
+{
+    type Rejection = (StatusCode, String);
+
+    async fn from_request(req: &mut RequestParts<B>) -> Result<Self, Self::Rejection> {
+        let Extension(pool) = Extension::<ConnectionPool>::from_request(req)
+            .await
+            .map_err(internal_error)?;
+
+        let conn = pool.get_owned().await.map_err(internal_error)?;
+
+        Ok(Self(conn))
+    }
+}
+
+async fn using_connection_extractor(
+    DatabaseConnection(conn): DatabaseConnection,
+) -> Result<String, (StatusCode, String)> {
+    let row = conn
+        .query_one("select 1 + 1", &[])
+        .await
+        .map_err(internal_error)?;
+    let two: i32 = row.try_get(0).map_err(internal_error)?;
+
+    Ok(two.to_string())
+}
+
+/// Utility function for mapping any error into a `500 Internal Server Error`
+/// response.
+fn internal_error<E>(err: E) -> (StatusCode, String)
+where
+    E: std::error::Error,
+{
+    (StatusCode::INTERNAL_SERVER_ERROR, err.to_string())
+}