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Rust Concepts: dyn Trait, Custom Iterators, Deref/Drop & Axum REST API (Part 5)

Wed, 27 May 2026 07:37:07 +0000

title: Rust Concepts: dyn Trait, Custom Iterators, Deref/Drop & Axum REST API (Part 5)
published: true
tags: rust, webdev, intermediate, systems
cover_image: https://images.unsplash.com/photo-1504639725590-34d0984388bd?w=1000

series: Core Rust Concepts

This is Part 5 of the Core Rust Concepts series.

Part 1 — Ownership, Borrowing, Lifetimes, Traits, Result/Option, Pattern Matching

Part 2 — Closures, Iterators, Generics, Enums, Smart Pointers, Async/Await

Part 3 — Macros, Modules, Testing, Unsafe Rust, FFI

Part 4 — Threads, Channels, Send/Sync, Mutex, Atomics, clap

Trait Objects and Dynamic Dispatch
The Deref Trait
The Drop Trait
Custom Iterators
Building a REST API with Axum

25. Trait Objects and Dynamic Dispatch

In Part 2 we used impl Trait — that's static dispatch: the compiler knows the exact type at compile time and monomorphizes it. Sometimes you don't know the type until runtime. That's where trait objects (dyn Trait) come in.

Static vs Dynamic dispatch

trait Greet {
    fn hello(&self) -> String;
}

struct English;
struct Spanish;

impl Greet for English {
    fn hello(&self) -> String { String::from("Hello!") }
}

impl Greet for Spanish {
    fn hello(&self) -> String { String::from("¡Hola!") }
}

// Static dispatch — type known at compile time, zero overhead
fn greet_static(g: &impl Greet) {
    println!("{}", g.hello());
}

// Dynamic dispatch — type known at runtime via vtable
fn greet_dynamic(g: &dyn Greet) {
    println!("{}", g.hello());
}

fn main() {
    let e = English;
    let s = Spanish;

    greet_static(&e);
    greet_dynamic(&s);

    // Only dyn Trait allows mixing types in a collection
    let greeters: Vec<Box<dyn Greet>> = vec![
        Box::new(English),
        Box::new(Spanish),
        Box::new(English),
    ];

    for g in &greeters {
        println!("{}", g.hello());
    }
}

When to use `dyn Trait` vs `impl Trait`

	`impl Trait`	`dyn Trait`
Dispatch	Static (compile time)	Dynamic (runtime vtable)
Performance	Zero overhead	Small indirection cost
Heterogeneous collections	❌	✅
Return from function	✅ (one concrete type)	✅ (any type at runtime)
Sized	✅	❌ (must be behind `&` or `Box`)

Object-safe traits

Not every trait can be made into a trait object. A trait is object-safe if:

It has no methods that return Self
It has no generic methods

// ✅ Object-safe
trait Drawable {
    fn draw(&self);
    fn area(&self) -> f64;
}

// ❌ NOT object-safe — returns Self
trait Clone2 {
    fn clone2(&self) -> Self; // can't use as dyn Clone2
}

Real-world pattern — plugin system

trait Plugin {
    fn name(&self) -> &str;
    fn run(&self, input: &str) -> String;
}

struct UppercasePlugin;
struct ReversePlugin;

impl Plugin for UppercasePlugin {
    fn name(&self) -> &str { "uppercase" }
    fn run(&self, input: &str) -> String { input.to_uppercase() }
}

impl Plugin for ReversePlugin {
    fn name(&self) -> &str { "reverse" }
    fn run(&self, input: &str) -> String { input.chars().rev().collect() }
}

struct Pipeline {
    plugins: Vec<Box<dyn Plugin>>,
}

impl Pipeline {
    fn new() -> Self { Self { plugins: vec![] } }

    fn add(mut self, p: Box<dyn Plugin>) -> Self {
        self.plugins.push(p);
        self
    }

    fn run(&self, input: &str) -> String {
        self.plugins.iter().fold(input.to_string(), |acc, p| p.run(&acc))
    }
}

fn main() {
    let pipeline = Pipeline::new()
        .add(Box::new(UppercasePlugin))
        .add(Box::new(ReversePlugin));

    println!("{}", pipeline.run("hello")); // "OLLEH"
}

26. The Deref Trait

Deref controls what happens when you use the * operator on a type. It's also what enables deref coercions — Rust automatically calls deref() to convert types so you don't have to do it manually.

use std::ops::Deref;

// A simple smart pointer
struct MyBox<T>(T);

impl<T> MyBox<T> {
    fn new(x: T) -> MyBox<T> { MyBox(x) }
}

impl<T> Deref for MyBox<T> {
    type Target = T;

    fn deref(&self) -> &Self::Target {
        &self.0 // return a reference to the inner value
    }
}

fn hello(name: &str) {
    println!("Hello, {name}!");
}

fn main() {
    let x = MyBox::new(String::from("Rustacean"));

    // These are all equivalent due to deref coercions:
    hello(&x);           // MyBox<String> → &String → &str  (auto!)
    hello(&(*x)[..]);    // manual — what Rust does behind the scenes
    hello(x.deref());    // explicit deref
}

Deref coercion chain

Rust automatically chains Deref calls as many times as needed:

MyBox<String>  →  String  →  str
    &MyBox<String>  →  &String  →  &str

fn main() {
    let boxed = Box::new(String::from("hello"));

    // Box<String> derefs to String, then to str
    println!("{}", boxed.to_uppercase()); // HELLO
    println!("{}", boxed.len());          // 5

    let s: &str = &boxed; // Box<String> → &String → &str — all automatic
    println!("{s}");
}

DerefMut

DerefMut does the same for mutable contexts:

use std::ops::{Deref, DerefMut};

struct Wrapper(Vec<i32>);

impl Deref for Wrapper {
    type Target = Vec<i32>;
    fn deref(&self) -> &Vec<i32> { &self.0 }
}

impl DerefMut for Wrapper {
    fn deref_mut(&mut self) -> &mut Vec<i32> { &mut self.0 }
}

fn main() {
    let mut w = Wrapper(vec![1, 2, 3]);
    w.push(4);                     // DerefMut lets you call Vec methods
    println!("{:?}", w.as_slice()); // Deref: [1, 2, 3, 4]
}

27. The Drop Trait

Drop lets you run custom cleanup code when a value goes out of scope — like a destructor in C++. Rust calls drop() automatically; you never call it yourself (use std::mem::drop() to drop early).

struct Resource {
    name: String,
}

impl Resource {
    fn new(name: &str) -> Self {
        println!("acquiring: {name}");
        Resource { name: name.to_string() }
    }
}

impl Drop for Resource {
    fn drop(&mut self) {
        println!("releasing: {}", self.name);
    }
}

fn main() {
    let _a = Resource::new("database connection");
    let _b = Resource::new("file handle");
    println!("resources in use...");
    // Drop order is LIFO: _b dropped first, then _a
}

Output:

acquiring: database connection
acquiring: file handle
resources in use...
releasing: file handle
releasing: database connection

Dropping early

fn main() {
    let conn = Resource::new("db");
    println!("doing work...");

    drop(conn); // explicit early drop — calls Drop::drop()
    println!("conn already released");

    // conn.name; ← compile error: value was dropped
}

Drop in practice — RAII pattern

use std::sync::{Arc, Mutex};

struct DbPool {
    connections: Arc<Mutex<Vec<String>>>,
}

struct DbConnection {
    id: String,
    pool: Arc<Mutex<Vec<String>>>,
}

impl Drop for DbConnection {
    fn drop(&mut self) {
        // Return the connection back to the pool automatically
        self.pool.lock().unwrap().push(self.id.clone());
        println!("connection '{}' returned to pool", self.id);
    }
}

impl DbPool {
    fn new() -> Self {
        let conns = vec!["conn-1".into(), "conn-2".into(), "conn-3".into()];
        Self { connections: Arc::new(Mutex::new(conns)) }
    }

    fn get(&self) -> Option<DbConnection> {
        let id = self.connections.lock().unwrap().pop()?;
        println!("checked out: {id}");
        Some(DbConnection { id, pool: Arc::clone(&self.connections) })
    }
}

fn main() {
    let pool = DbPool::new();
    {
        let _c1 = pool.get(); // "conn-3"
        let _c2 = pool.get(); // "conn-2"
    } // both returned to pool here automatically
    println!("pool size: {}", pool.connections.lock().unwrap().len()); // 3
}

This is the RAII (Resource Acquisition Is Initialization) pattern — the same one Rust's MutexGuard, file handles, and socket wrappers use internally.

28. Custom Iterators

You can make any type iterable by implementing the Iterator trait, which requires only one method: next().

struct Counter {
    count: u32,
    max: u32,
}

impl Counter {
    fn new(max: u32) -> Self { Self { count: 0, max } }
}

impl Iterator for Counter {
    type Item = u32;

    fn next(&mut self) -> Option<Self::Item> {
        if self.count < self.max {
            self.count += 1;
            Some(self.count)
        } else {
            None // signals the iterator is exhausted
        }
    }
}

fn main() {
    let counter = Counter::new(5);

    // You get ALL iterator methods for free!
    let sum: u32 = counter.sum();
    println!("sum: {sum}"); // 15

    // Chain with other iterators
    let result: Vec<u32> = Counter::new(5)
        .zip(Counter::new(5).skip(1))   // pair consecutive values
        .map(|(a, b)| a * b)            // multiply each pair
        .filter(|x| x % 3 == 0)        // keep multiples of 3
        .collect();

    println!("{:?}", result); // [6, 12]
}

A real-world custom iterator — paginated results

struct Paginator {
    current_page: u32,
    total_pages: u32,
    page_size: u32,
}

impl Paginator {
    fn new(total_items: u32, page_size: u32) -> Self {
        Self {
            current_page: 0,
            total_pages: (total_items + page_size - 1) / page_size,
            page_size,
        }
    }
}

#[derive(Debug)]
struct Page {
    number: u32,
    offset: u32,
    limit: u32,
}

impl Iterator for Paginator {
    type Item = Page;

    fn next(&mut self) -> Option<Page> {
        if self.current_page < self.total_pages {
            self.current_page += 1;
            Some(Page {
                number: self.current_page,
                offset: (self.current_page - 1) * self.page_size,
                limit: self.page_size,
            })
        } else {
            None
        }
    }
}

fn main() {
    let pages = Paginator::new(95, 20);

    for page in pages {
        println!("Page {}: OFFSET {} LIMIT {}", page.number, page.offset, page.limit);
    }
}

Output:

Page 1: OFFSET 0 LIMIT 20
Page 2: OFFSET 20 LIMIT 20
Page 3: OFFSET 40 LIMIT 20
Page 4: OFFSET 60 LIMIT 20
Page 5: OFFSET 80 LIMIT 20

IntoIterator — making structs work in for loops

struct Stack<T> {
    items: Vec<T>,
}

impl<T> Stack<T> {
    fn new() -> Self { Self { items: vec![] } }
    fn push(&mut self, item: T) { self.items.push(item); }
}

impl<T> IntoIterator for Stack<T> {
    type Item = T;
    type IntoIter = std::vec::IntoIter<T>;

    fn into_iter(self) -> Self::IntoIter {
        self.items.into_iter()
    }
}

fn main() {
    let mut stack = Stack::new();
    stack.push(1);
    stack.push(2);
    stack.push(3);

    for item in stack { // works because of IntoIterator
        println!("{item}");
    }
}

29. Building a REST API with Axum

axum is the most popular async web framework in Rust, built on top of tokio and hyper. It uses Rust's type system to make routing and request extraction ergonomic and safe.

Setup

# Cargo.toml
[dependencies]
axum      = "0.7"
tokio     = { version = "1", features = ["full"] }
serde     = { version = "1", features = ["derive"] }
serde_json = "1"
uuid      = { version = "1", features = ["v4"] }

Full CRUD API — Todo List

use axum::{
    extract::{Path, State},
    http::StatusCode,
    response::Json,
    routing::{delete, get, post, put},
    Router,
};
use serde::{Deserialize, Serialize};
use std::sync::{Arc, Mutex};
use uuid::Uuid;

// --- Models ---

#[derive(Debug, Clone, Serialize, Deserialize)]
struct Todo {
    id: String,
    title: String,
    completed: bool,
}

#[derive(Deserialize)]
struct CreateTodo {
    title: String,
}

#[derive(Deserialize)]
struct UpdateTodo {
    title: Option<String>,
    completed: Option<bool>,
}

// --- Shared state ---

type Db = Arc<Mutex<Vec<Todo>>>;

// --- Handlers ---

async fn list_todos(State(db): State<Db>) -> Json<Vec<Todo>> {
    let todos = db.lock().unwrap();
    Json(todos.clone())
}

async fn create_todo(
    State(db): State<Db>,
    Json(payload): Json<CreateTodo>,
) -> (StatusCode, Json<Todo>) {
    let todo = Todo {
        id: Uuid::new_v4().to_string(),
        title: payload.title,
        completed: false,
    };
    db.lock().unwrap().push(todo.clone());
    (StatusCode::CREATED, Json(todo))
}

async fn get_todo(
    State(db): State<Db>,
    Path(id): Path<String>,
) -> Result<Json<Todo>, StatusCode> {
    let todos = db.lock().unwrap();
    todos
        .iter()
        .find(|t| t.id == id)
        .cloned()
        .map(Json)
        .ok_or(StatusCode::NOT_FOUND)
}

async fn update_todo(
    State(db): State<Db>,
    Path(id): Path<String>,
    Json(payload): Json<UpdateTodo>,
) -> Result<Json<Todo>, StatusCode> {
    let mut todos = db.lock().unwrap();
    let todo = todos.iter_mut().find(|t| t.id == id).ok_or(StatusCode::NOT_FOUND)?;

    if let Some(title) = payload.title { todo.title = title; }
    if let Some(completed) = payload.completed { todo.completed = completed; }

    Ok(Json(todo.clone()))
}

async fn delete_todo(
    State(db): State<Db>,
    Path(id): Path<String>,
) -> StatusCode {
    let mut todos = db.lock().unwrap();
    let before = todos.len();
    todos.retain(|t| t.id != id);

    if todos.len() < before { StatusCode::NO_CONTENT }
    else { StatusCode::NOT_FOUND }
}

// --- Router ---

fn app(db: Db) -> Router {
    Router::new()
        .route("/todos",      get(list_todos).post(create_todo))
        .route("/todos/:id",  get(get_todo).put(update_todo).delete(delete_todo))
        .with_state(db)
}

// --- Main ---

#[tokio::main]
async fn main() {
    let db: Db = Arc::new(Mutex::new(vec![]));

    let listener = tokio::net::TcpListener::bind("0.0.0.0:3000").await.unwrap();
    println!("listening on http://localhost:3000");

    axum::serve(listener, app(db)).await.unwrap();
}

Test it with curl

# Create todos
curl -s -X POST http://localhost:3000/todos \
  -H "Content-Type: application/json" \
  -d '{"title": "Learn Rust"}' | jq

curl -s -X POST http://localhost:3000/todos \
  -H "Content-Type: application/json" \
  -d '{"title": "Build an API"}' | jq

# List all
curl -s http://localhost:3000/todos | jq

# Get one (replace ID)
curl -s http://localhost:3000/todos/<id> | jq

# Update
curl -s -X PUT http://localhost:3000/todos/<id> \
  -H "Content-Type: application/json" \
  -d '{"completed": true}' | jq

# Delete
curl -s -X DELETE http://localhost:3000/todos/<id>

Adding middleware — request logging

use axum::middleware::{self, Next};
use axum::extract::Request;
use axum::response::Response;

async fn log_request(req: Request, next: Next) -> Response {
    let method = req.method().clone();
    let uri    = req.uri().clone();
    let res    = next.run(req).await;
    println!("{} {} → {}", method, uri, res.status());
    res
}

fn app(db: Db) -> Router {
    Router::new()
        .route("/todos",     get(list_todos).post(create_todo))
        .route("/todos/:id", get(get_todo).put(update_todo).delete(delete_todo))
        .layer(middleware::from_fn(log_request)) // add logging to all routes
        .with_state(db)
}

Wrapping Up

Concept	What it gives you
dyn Trait	Runtime polymorphism, heterogeneous collections
Deref	Transparent smart pointer ergonomics, auto-coercions
Drop	RAII — deterministic cleanup, no GC needed
Custom iterators	Any type can plug into Rust's iterator ecosystem
Axum	Type-safe, async REST APIs on top of tokio

The full series

Part	Topics
Part 1	Ownership, Borrowing, Lifetimes, Traits, Result/Option, Pattern Matching
Part 2	Closures, Iterators, Generics, Enums, Smart Pointers, Async/Await
Part 3	Macros, Modules, Cargo, Testing, Unsafe, FFI
Part 4	Threads, Channels, Send/Sync, Mutex, Atomics, clap
Part 5	dyn Trait, Deref, Drop, Custom Iterators, Axum REST API

What's in Part 6?

Workspaces and multi-crate projects
serde — serialization deep dive
Benchmarking with criterion
Error handling patterns with thiserror and anyhow
Writing a real-world CLI + library crate

Found this useful? Drop a ❤️ and follow for Part 6!

Rust Concurrency: Threads, Channels, Mutex & Sync (Part 4)

Wed, 27 May 2026 07:36:28 +0000

title: Rust Concurrency: Threads, Channels, Mutex & Sync (Part 4)
published: true
tags: rust, concurrency, intermediate, systems
cover_image: https://images.unsplash.com/photo-1558494949-ef010cbdcc31?w=1000

series: Core Rust Concepts

This is Part 4 of the Core Rust Concepts series.

Part 1 — Ownership, Borrowing, Lifetimes, Traits, Result/Option, Pattern Matching

Part 2 — Closures, Iterators, Generics, Enums, Smart Pointers, Async/Await

Part 3 — Macros, Modules, Testing, Unsafe Rust, FFI

Threads
Message Passing with Channels
Send and Sync Traits
Mutex and RwLock
Atomics
Building a CLI Tool with clap

19. Threads

Rust's standard library provides OS threads via std::thread. The ownership and type system prevents data races at compile time — if your code compiles, it's free of data races. That's a guarantee no other systems language makes.

Spawning a thread

use std::thread;
use std::time::Duration;

fn main() {
    let handle = thread::spawn(|| {
        for i in 1..=5 {
            println!("thread: {i}");
            thread::sleep(Duration::from_millis(50));
        }
    });

    for i in 1..=3 {
        println!("main:   {i}");
        thread::sleep(Duration::from_millis(80));
    }

    handle.join().unwrap(); // wait for the thread to finish
}

Moving data into a thread

Use move to transfer ownership of data into the thread closure:

use std::thread;

fn main() {
    let data = vec![1, 2, 3, 4, 5];

    let handle = thread::spawn(move || {
        // data is moved into this thread
        let sum: i32 = data.iter().sum();
        println!("sum: {sum}");
    });

    // println!("{:?}", data); ← compile error: data was moved

    handle.join().unwrap();
}

Spawning many threads

use std::thread;

fn main() {
    let handles: Vec<_> = (0..8)
        .map(|i| {
            thread::spawn(move || {
                println!("worker {i} done");
                i * i
            })
        })
        .collect();

    let results: Vec<_> = handles
        .into_iter()
        .map(|h| h.join().unwrap())
        .collect();

    println!("squares: {:?}", results);
}

thread::spawn returns a JoinHandle<T> where T is the return type of the closure. Always .join() your handles — a panicking thread silently fails otherwise.

20. Message Passing with Channels

Rust's standard library includes multiple-producer, single-consumer (mpsc) channels. The philosophy: "do not communicate by sharing memory; share memory by communicating."

Basic channel

use std::sync::mpsc;
use std::thread;

fn main() {
    let (tx, rx) = mpsc::channel();

    thread::spawn(move || {
        let msgs = vec!["hello", "from", "the", "thread"];
        for msg in msgs {
            tx.send(msg).unwrap();
        }
    });

    for received in rx {
        println!("got: {received}");
    }
}

Multiple producers

use std::sync::mpsc;
use std::thread;

fn main() {
    let (tx, rx) = mpsc::channel::<String>();

    for id in 0..4 {
        let tx_clone = tx.clone(); // each thread gets its own sender
        thread::spawn(move || {
            tx_clone.send(format!("msg from worker {id}")).unwrap();
        });
    }

    drop(tx); // drop the original so rx knows when all senders are gone

    for msg in rx {
        println!("{msg}");
    }
}

Synchronous channel (bounded)

use std::sync::mpsc;
use std::thread;

fn main() {
    // Buffer of 2 — sender blocks when full
    let (tx, rx) = mpsc::sync_channel::<i32>(2);

    thread::spawn(move || {
        for i in 0..5 {
            println!("sending {i}");
            tx.send(i).unwrap(); // blocks at i=2 until receiver reads
        }
    });

    thread::sleep(std::time::Duration::from_millis(200));

    for val in rx {
        println!("received {val}");
    }
}

For more advanced channel patterns (broadcast, watch, oneshot), use the tokio::sync or crossbeam-channel crates.

21. Send and Sync Traits

These two marker traits are the foundation of Rust's fearless concurrency. You rarely implement them manually — the compiler derives them automatically.

Trait	Meaning
`Send`	Safe to transfer ownership to another thread
`Sync`	Safe to share a reference across threads (`&T` is `Send`)

use std::thread;
use std::rc::Rc;
use std::sync::Arc;

fn main() {
    // Rc<T> is NOT Send — can't move across threads
    // let rc = Rc::new(42);
    // thread::spawn(move || println!("{}", rc)); ← compile error!

    // Arc<T> IS Send — atomic reference counting
    let arc = Arc::new(42);
    let arc_clone = Arc::clone(&arc);

    thread::spawn(move || {
        println!("thread sees: {}", arc_clone);
    }).join().unwrap();

    println!("main sees: {}", arc);
}

Types and their Send/Sync status:

Type	Send	Sync
`i32`, `f64`, `bool`	✅	✅
`String`, `Vec<T>`	✅	✅
`Rc<T>`	❌	❌
`Arc<T>`	✅	✅
`RefCell<T>`	✅	❌
`Mutex<T>`	✅	✅
`*mut T` (raw pointer)	❌	❌

If you try to share a non-Sync type across threads, the compiler refuses to compile. No runtime surprises.

22. Mutex and RwLock

When multiple threads need to mutate shared data, use a Mutex (mutual exclusion lock). Wrap it in Arc to share ownership across threads.

Mutex

use std::sync::{Arc, Mutex};
use std::thread;

fn main() {
    let counter = Arc::new(Mutex::new(0));
    let mut handles = vec![];

    for _ in 0..10 {
        let counter = Arc::clone(&counter);
        let handle = thread::spawn(move || {
            let mut num = counter.lock().unwrap(); // blocks until lock acquired
            *num += 1;
        }); // lock is released here when `num` drops
        handles.push(handle);
    }

    for h in handles { h.join().unwrap(); }

    println!("result: {}", *counter.lock().unwrap()); // 10
}

RwLock — many readers, one writer

use std::sync::{Arc, RwLock};
use std::thread;

fn main() {
    let data = Arc::new(RwLock::new(vec![1, 2, 3]));

    // Spawn 4 reader threads
    let readers: Vec<_> = (0..4).map(|i| {
        let data = Arc::clone(&data);
        thread::spawn(move || {
            let r = data.read().unwrap(); // multiple readers allowed
            println!("reader {i}: {:?}", *r);
        })
    }).collect();

    // One writer thread
    let data_w = Arc::clone(&data);
    let writer = thread::spawn(move || {
        let mut w = data_w.write().unwrap(); // exclusive access
        w.push(4);
        println!("writer pushed 4");
    });

    for r in readers { r.join().unwrap(); }
    writer.join().unwrap();
}

Mutex vs RwLock:

	`Mutex<T>`	`RwLock<T>`
Readers	One at a time	Many simultaneously
Writers	One at a time	One at a time (exclusive)
Best for	Write-heavy workloads	Read-heavy workloads
Overhead	Lower	Higher

Avoiding deadlocks

use std::sync::{Arc, Mutex};

fn main() {
    let a = Arc::new(Mutex::new(1));
    let b = Arc::new(Mutex::new(2));

    // ✅ Always acquire locks in the same order
    {
        let _lock_a = a.lock().unwrap();
        let _lock_b = b.lock().unwrap();
        println!("safe");
    }

    // ❌ Acquiring in different orders across threads = deadlock risk
    // thread 1: lock a → lock b
    // thread 2: lock b → lock a  ← deadlock!
}

23. Atomics

For simple counters and flags shared between threads, std::sync::atomic types are faster than a Mutex — they use hardware atomic instructions with no locking.

use std::sync::Arc;
use std::sync::atomic::{AtomicUsize, Ordering};
use std::thread;

fn main() {
    let counter = Arc::new(AtomicUsize::new(0));
    let mut handles = vec![];

    for _ in 0..8 {
        let counter = Arc::clone(&counter);
        handles.push(thread::spawn(move || {
            for _ in 0..1000 {
                counter.fetch_add(1, Ordering::Relaxed);
            }
        }));
    }

    for h in handles { h.join().unwrap(); }
    println!("total: {}", counter.load(Ordering::SeqCst)); // 8000
}

Memory ordering cheat sheet

Ordering	Use case
`Relaxed`	Counters — no ordering guarantees needed
`Acquire`	Load that synchronizes with a `Release` store
`Release`	Store that publishes data to other threads
`AcqRel`	Read-modify-write (e.g. `fetch_add` with sync)
`SeqCst`	Strongest — total order across all threads

use std::sync::atomic::{AtomicBool, Ordering};
use std::sync::Arc;
use std::thread;

fn main() {
    let ready = Arc::new(AtomicBool::new(false));
    let ready_clone = Arc::clone(&ready);

    let worker = thread::spawn(move || {
        thread::sleep(std::time::Duration::from_millis(100));
        ready_clone.store(true, Ordering::Release);
        println!("worker: done");
    });

    // Spin-wait (not ideal in prod — use a Condvar or channel instead)
    while !ready.load(Ordering::Acquire) {
        thread::yield_now();
    }
    println!("main: worker finished");
    worker.join().unwrap();
}

Available atomic types: AtomicBool, AtomicI8/16/32/64, AtomicU8/16/32/64, AtomicUsize, AtomicIsize, AtomicPtr<T>.

24. Building a CLI Tool with clap

clap is the standard Rust library for building command-line interfaces. It gives you argument parsing, help text, and subcommands with minimal boilerplate.

# Cargo.toml
[dependencies]
clap = { version = "4", features = ["derive"] }

Basic CLI

use clap::Parser;

/// A simple file word counter
#[derive(Parser, Debug)]
#[command(name = "wordcount")]
#[command(about = "Count words in a string or file", long_about = None)]
struct Cli {
    /// The text to count words in
    #[arg(short, long)]
    text: Option<String>,

    /// Show character count too
    #[arg(short, long, default_value_t = false)]
    chars: bool,

    /// Verbosity level
    #[arg(short, long, action = clap::ArgAction::Count)]
    verbose: u8,
}

fn main() {
    let cli = Cli::parse();

    let input = cli.text.unwrap_or_else(|| String::from("hello world"));
    let words = input.split_whitespace().count();

    if cli.verbose > 0 {
        println!("Input: {:?}", input);
    }

    println!("Words: {words}");

    if cli.chars {
        println!("Chars: {}", input.chars().count());
    }
}

Running it:

$ cargo run -- --text "the quick brown fox" --chars -v
Input: "the quick brown fox"
Words: 4
Chars: 19

$ cargo run -- --help
Count words in a string or file

Usage: wordcount [OPTIONS]

Options:
  -t, --text <TEXT>   The text to count words in
  -c, --chars         Show character count too
  -v, --verbose...    Verbosity level
  -h, --help          Print help

Subcommands

use clap::{Parser, Subcommand};

#[derive(Parser)]
#[command(name = "mytool", about = "A multi-command CLI")]
struct Cli {
    #[command(subcommand)]
    command: Commands,
}

#[derive(Subcommand)]
enum Commands {
    /// Add two numbers
    Add { a: f64, b: f64 },
    /// Multiply two numbers
    Mul { a: f64, b: f64 },
    /// Greet someone
    Greet {
        name: String,
        #[arg(short, long, default_value = "Hello")]
        greeting: String,
    },
}

fn main() {
    let cli = Cli::parse();

    match cli.command {
        Commands::Add { a, b } => println!("{} + {} = {}", a, b, a + b),
        Commands::Mul { a, b } => println!("{} × {} = {}", a, b, a * b),
        Commands::Greet { name, greeting } => println!("{greeting}, {name}!"),
    }
}

Running subcommands:

$ cargo run -- add 3.5 2.5
3.5 + 2.5 = 6

$ cargo run -- greet Alice --greeting "Hey"
Hey, Alice!

$ cargo run -- mul 6 7
6 × 7 = 42

For a full production CLI, combine clap with anyhow for error handling, indicatif for progress bars, and colored or owo-colors for terminal color output.

Wrapping Up

Concept	What it gives you
Threads	OS-level parallelism, compile-time data race prevention
Channels	Safe message passing between threads (mpsc)
Send & Sync	Compiler-enforced thread-safety contracts
Mutex & RwLock	Shared mutable state without data races
Atomics	Lock-free counters and flags
clap	Production-grade CLI argument parsing

The full series so far

Part	Topics
Part 1	Ownership, Borrowing, Lifetimes, Traits, Result/Option, Pattern Matching
Part 2	Closures, Iterators, Generics, Enums, Smart Pointers, Async/Await
Part 3	Macros, Modules, Cargo, Testing, Unsafe, FFI
Part 4	Threads, Channels, Send/Sync, Mutex, Atomics, clap

What's in Part 5?

Trait objects and dynamic dispatch (dyn Trait)
The Deref and Drop traits
Custom iterators
Building a real REST API with axum

Found this helpful? Drop a ❤️ and follow for Part 5!

Coder Legion - Recent posts tagged intermediate

Rust Concepts: dyn Trait, Custom Iterators, Deref/Drop & Axum REST API (Part 5)

series: Core Rust Concepts

Table of Contents

25. Trait Objects and Dynamic Dispatch

Static vs Dynamic dispatch

When to use dyn Trait vs impl Trait

Object-safe traits

Real-world pattern — plugin system

26. The Deref Trait

Deref coercion chain

DerefMut

27. The Drop Trait

Dropping early

Drop in practice — RAII pattern

28. Custom Iterators

A real-world custom iterator — paginated results

IntoIterator — making structs work in for loops

29. Building a REST API with Axum

Setup

Full CRUD API — Todo List

Test it with curl

Adding middleware — request logging

Wrapping Up

The full series

What's in Part 6?

Rust Concurrency: Threads, Channels, Mutex & Sync (Part 4)

series: Core Rust Concepts

Table of Contents

19. Threads

Spawning a thread

Moving data into a thread

Spawning many threads

20. Message Passing with Channels

Basic channel

Multiple producers

Synchronous channel (bounded)

21. Send and Sync Traits

22. Mutex and RwLock

Mutex

RwLock — many readers, one writer

Avoiding deadlocks

23. Atomics

Memory ordering cheat sheet

24. Building a CLI Tool with clap

Basic CLI

Subcommands

Wrapping Up

The full series so far

What's in Part 5?

When to use `dyn Trait` vs `impl Trait`