Rust

fn print_name(name: &str) { println!("{name}"); }   // borrows; caller keeps ownership

let s = String::from("ada");
print_name(&s);                                       // Good: lend a reference
println!("{s}");                                      // still usable

// Bad: takes by value, moves it, then the caller can't use `s` anymore.
fn consume(name: String) { /* ... */ }
consume(s);
// println!("{s}");  // error[E0382]: borrow of moved value: `s`

// 1. "value moved here" (E0382): you used a value after moving it.
//    Fix: borrow instead of move, or .clone() only if you genuinely need two owners.
let v = vec![1, 2, 3];
let first = &v[0];           // Good: borrow
// let taken = v; let _ = first;  // Bad: moves v while `first` borrows it.

// 2. "cannot borrow as mutable more than once" (E0499): two &mut alive at once.
//    Fix: scope the first borrow so it ends before the second begins.
let mut data = vec![1, 2, 3];
{ let a = &mut data; a.push(4); }   // borrow ends here
let b = &mut data; b.push(5);       // Good: non-overlapping

// 3. "cannot borrow as mutable, already borrowed as immutable" (E0502).
//    Fix: don't hold a shared ref across a mutation; collect indices first, mutate after.

// 4. "does not live long enough" (E0597): a reference outlives the value it points to.
//    Fix: return an owned value, or restructure so the owner outlives the borrow.

use thiserror::Error;

// Library / domain layer: a typed enum callers can match on. #[from] gives free `?` conversion.
#[derive(Debug, Error)]
pub enum UserError {
    #[error("user {0} not found")]
    NotFound(i64),
    #[error("database error")]
    Db(#[from] sqlx::Error),   // any sqlx::Error becomes UserError::Db via `?`
}

// Application edge: anyhow when you just need context, not a match.
use anyhow::Context;
let config = std::fs::read_to_string(path)
    .with_context(|| format!("reading config at {path}"))?;   // adds a human breadcrumb

use axum::{http::StatusCode, response::{IntoResponse, Response}, Json};
use serde_json::json;

impl IntoResponse for UserError {
    fn into_response(self) -> Response {
        let status = match self {
            UserError::NotFound(_) => StatusCode::NOT_FOUND,            // 404
            UserError::Db(ref e) => {                                  // 500
                tracing::error!(error = %e, "unexpected db error");    // log here, not to the client
                StatusCode::INTERNAL_SERVER_ERROR
            }
        };
        (status, Json(json!({ "error": self.to_string() }))).into_response()
    }
}

use tokio::task::JoinSet;

let mut set = JoinSet::new();
for id in ids {                                  // Good: a JoinSet you can drain and cap
    set.spawn(async move { fetch(id).await });
}
let mut out = Vec::new();
while let Some(res) = set.join_next().await {
    out.push(res??);                             // join error, then task error
}

// Bad: std Mutex guard held across .await -> "future cannot be sent between threads safely".
let guard = state.lock().unwrap();
do_io().await;                  // guard is still alive here -> not Send
guard.update();

// Good: drop the lock before awaiting, or use tokio::sync::Mutex if the lock must span the await.
{
    let mut g = state.lock().unwrap();
    g.update();
}                               // guard dropped here
do_io().await;                  // nothing non-Send is held across the await

// Bad: a CPU-bound parse on the async worker thread starves every other task.
let parsed = heavy_parse(&blob);          // blocks the executor
// Good: move blocking/CPU work off the runtime.
let parsed = tokio::task::spawn_blocking(move || heavy_parse(&blob)).await?;

use axum::{extract::{Path, State}, routing::get, Router, Json};
use std::sync::Arc;

async fn get_user(
    State(app): State<Arc<AppState>>,            // shared state, not a global
    Path(id): Path<i64>,                         // {id} parsed and typed
) -> Result<Json<User>, UserError> {             // UserError: IntoResponse maps it
    let user = app.users.find(id).await?;        // `?` propagates the typed error
    Ok(Json(user))
}

let app = Router::new()
    .route("/users/{id}", get(get_user))         // 0.8 syntax: {id}, not :id
    .with_state(state);

my-service/
  Cargo.toml          # [dependencies], [profile.release], optional [workspace]
  src/
    main.rs           # entrypoint: parse config, build state, axum::serve — wiring only
    lib.rs            # pub mod error; pub mod app; pub mod users; — the testable surface
    error.rs          # the thiserror enum + IntoResponse
    users/
      mod.rs          # handlers + the domain model
      repo.rs         # sqlx queries
  tests/
    users_api.rs      # integration tests that spin up the Router

#[tokio::test]
async fn get_user_404_when_missing() {
    let app = build_router(test_state());            // the same Router main builds
    let res = app
        .oneshot(Request::get("/users/999").body(Body::empty()).unwrap())
        .await
        .unwrap();
    assert_eq!(res.status(), StatusCode::NOT_FOUND);
}