Add example showing how to return anyhow::Errors (#1398)

examples/anyhow-error-response/Cargo.toml

@@ -0,0 +1,10 @@
+[package]
+name = "example-anyhow-error-response"
+version = "0.1.0"
+edition = "2021"
+publish = false
+
+[dependencies]
+anyhow = "1.0"
+axum = { path = "../../axum" }
+tokio = { version = "1.0", features = ["full"] }

examples/anyhow-error-response/src/main.rs

@@ -0,0 +1,59 @@
+//! Run with
+//!
+//! ```not_rust
+//! cd examples && cargo run -p example-anyhow-error-response
+//! ```
+
+use axum::{
+    http::StatusCode,
+    response::{IntoResponse, Response},
+    routing::get,
+    Router,
+};
+use std::net::SocketAddr;
+
+#[tokio::main]
+async fn main() {
+    let app = Router::new().route("/", get(handler));
+
+    let addr = SocketAddr::from(([127, 0, 0, 1], 3000));
+    println!("listening on {}", addr);
+    axum::Server::bind(&addr)
+        .serve(app.into_make_service())
+        .await
+        .unwrap();
+}
+
+async fn handler() -> Result<(), AppError> {
+    try_thing()?;
+    Ok(())
+}
+
+fn try_thing() -> Result<(), anyhow::Error> {
+    anyhow::bail!("it failed!")
+}
+
+// Make our own error that wraps `anyhow::Error`.
+struct AppError(anyhow::Error);
+
+// Tell axum how to convert `AppError` into a response.
+impl IntoResponse for AppError {
+    fn into_response(self) -> Response {
+        (
+            StatusCode::INTERNAL_SERVER_ERROR,
+            format!("Something went wrong: {}", self.0),
+        )
+            .into_response()
+    }
+}
+
+// This enables using `?` on functions that return `Result<_, anyhow::Error>` to turn them into
+// `Result<_, AppError>`. That way you don't need to do that manually.
+impl<E> From<E> for AppError
+where
+    E: Into<anyhow::Error>,
+{
+    fn from(err: E) -> Self {
+        Self(err.into())
+    }
+}