Rust チートシート

let x = 5;            // immutable by default
let mut y = 10;        // mutable with 'mut'
y += 1;                // OK, y is mutable

const MAX: u32 = 100;  // compile-time constant
static GREETING: &str = "Hi"; // global static

let z: i32 = -5;       // explicit type annotation

let x = 5;
let x = x * 2;         // shadow previous x, now 10
let x = "text";        // can even change type!
println!("{}", x);     // "text"

// shadowing vs mut: shadowing creates a NEW variable
// with the same name, allowing type changes

// Line comment
/* Block comment */
/// Doc comment (renders in rustdoc)
//! Module-level doc comment

let name = "Alice";
println!("Hello, {}!", name);        // format string
println!("{0} {1} {0}", "a", "b");   // positional args
println!("{:>5}", 42);               // right-align, width 5
println!("{:.2}", 3.14159);          // 2 decimal places: 3.14
println!("{:#b}", 0b1010);           // binary: 0b1010
eprintln!("Error to stderr");        // error output

// Scalar types
let a: i32 = 42;          // signed 32-bit integer
let b: u64 = 100;         // unsigned 64-bit
let c: f64 = 3.14;        // 64-bit float (default)
let d: bool = true;
let e: char = '🦀';        // 4-byte Unicode scalar

// isize/usize: pointer-sized (32 or 64 bit depending on arch)
let len: usize = vec![1,2,3].len();

// Compound types
let pair: (i32, &str) = (1, "hello");
let arr: [i32; 3] = [0; 3];   // [0, 0, 0]

// 'as' keyword for primitive casts (may truncate)
let x: i32 = 42;
let y: f64 = x as f64;        // 42.0
let z: u8 = 300u16 as u8;     // truncated: 44

// From/Into traits for safe conversion
let s = String::from("hi");
let owned: String = "hi".into();

// Type aliases
type Kilometers = i32;
let distance: Kilometers = 5;

// &str: immutable string slice (borrowed)
let s1: &str = "literal";           // stored in binary
let s2: &str = &s3[..2];            // slice of a String

// String: owned, growable, heap-allocated
let mut s3: String = String::from("hello");
s3.push_str(", world");             // append
s3.push('!');                       // append char
s3.insert(0, '>');                  // insert at index
println!("{}", s3);                 // >hello, world!

// Convert: &str -> String
let owned = "literal".to_string();
let owned2 = String::from("literal");

let s = String::from("Hello, World");

// inspection
println!("len: {}", s.len());              // 12
println!("is_empty: {}", s.is_empty());    // false
println!("contains 'World': {}", s.contains("World")); // true
println!("starts_with 'Hello': {}", s.starts_with("Hello")); // true

// transformation
let upper = s.to_uppercase();              // HELLO, WORLD
let replaced = s.replace("o", "0");        // Hell0, W0rld
let trimmed = "  hi  ".trim().to_string(); // hi

// splitting
for word in s.split(", ") {
    println!("{}", word); // Hello / World
}
let parts: Vec<&str> = s.split_whitespace().collect();

// format! macro creates a new String
let name = "Alice";
let msg = format!("Hi {}, you have {} messages", name, 5);

// concatenation
let s1 = String::from("Hello");
let s2 = String::from(" World");
let s3 = s1 + &s2;        // s1 moved, s2 borrowed
// s1 is now invalid!

let s4 = format!("{}{}", s2, s3); // neither moved

// concat! macro for literals
let s5 = concat!("foo", "bar"); // "foobar" at compile time

let s = "héllo";  // é is 2 bytes in UTF-8

// iterate over chars (Unicode scalar values)
for c in s.chars() {
    print!("{} ", c);  // h é l l o
}

// iterate over bytes
for b in s.bytes() {
    print!("{} ", b);  // 104 195 169 108 108 111
}

// get char at index (O(n) — must walk UTF-8)
let third = s.chars().nth(2); // Some('l')

// string slices must be on char boundaries
let slice = &s[0..1]; // "h" — OK
// let bad = &s[0..2]; // PANIC if mid-é!

// String/str -> number
let n: i32 = "42".parse().unwrap();
let n2 = "42".parse::<i32>().unwrap(); // turbofish syntax
let f: f64 = "3.14".parse().unwrap();

// number -> String
let s = 42.to_string();
let s2 = format!("{}", 42);

// String -> &str (free, just dereference)
let owned = String::from("hi");
let borrowed: &str = &owned;

// collect chars into String
let upper: String = "hello".chars().map(|c| c.to_uppercase().next().unwrap()).collect();

// Fixed-size array (stack allocated)
let arr: [i32; 3] = [1, 2, 3];
let zeros = [0; 5];           // [0, 0, 0, 0, 0]
println!("first: {}", arr[0]);
println!("len: {}", arr.len());

// Slice: a view into an array/vector
let slice: &[i32] = &arr[1..3];  // [2, 3]
let full: &[i32] = &arr;          // whole array
let first = &arr[..1];            // [1]

// iterating
for n in &arr {
    println!("{}", n);
}

// growable heap array
let mut v: Vec<i32> = Vec::new();
let v2 = vec![1, 2, 3];          // macro shorthand

v.push(4);                        // append
v.pop();                          // remove last -> Option
v.insert(0, 0);                   // insert at index (O(n))
v.remove(0);                      // remove at index (O(n))
v.extend([5, 6]);                 // append multiple

// access
println!("{}", v[0]);             // panics if out of bounds
println!("{:?}", v.get(0));       // Some(&4) — safe

// iterate by value, ref, or mut ref
for n in &v { print!("{}", n); }
for n in &mut v { *n *= 2; }      // double each
let owned: Vec<i32> = v.into_iter().collect();

use std::collections::HashMap;
use std::collections::BTreeMap;

// HashMap: O(1) average lookup, unordered
let mut scores: HashMap<String, i32> = HashMap::new();
scores.insert(String::from("Alice"), 10);
scores.entry("Bob".into()).or_insert(0); // insert if absent
let alice = scores.get("Alice"); // Some(&10)

// iterate (unordered)
for (name, score) in &scores {
    println!("{}: {}", name, score);
}

// BTreeMap: O(log n) lookup, sorted by key
let mut bt: BTreeMap<String, i32> = BTreeMap::new();
bt.insert("zebra".into(), 1);
bt.insert("apple".into(), 2);
// iterates in sorted order: apple, zebra

use std::collections::HashSet;

let mut a: HashSet<i32> = [1, 2, 3].into_iter().collect();
let b: HashSet<i32> = [3, 4, 5].into_iter().collect();

a.insert(4);
a.remove(&1);
println!("contains 2: {}", a.contains(&2));

// set operations
let union: HashSet<_> = a.union(&b).copied().collect();
let inter: HashSet<_> = a.intersection(&b).copied().collect();
let diff: HashSet<_> = a.difference(&b).copied().collect();
let sym: HashSet<_> = a.symmetric_difference(&b).copied().collect();

// tuples can hold different types
let tup: (i32, f64, &str) = (42, 3.14, "hi");

// access by index
println!("{} {} {}", tup.0, tup.1, tup.2);

// destructuring
let (n, pi, s) = tup;
println!("{} {} {}", n, pi, s);

// nested
let ((a, b), c) = ((1, 2), 3);

// unit tuple (empty)
let unit: () = ();

// function returning multiple values
fn divmod(a: i32, b: i32) -> (i32, i32) {
    (a / b, a % b)
}
let (q, r) = divmod(17, 5); // (3, 2)

let score = 85;

// if is an expression — returns a value
let grade = if score >= 90 {
    "A"
} else if score >= 80 {
    "B"
} else if score >= 70 {
    "C"
} else {
    "F"
};
println!("Grade: {}", grade);

// arms must return same type
// let bad = if true { 1 } else { "two" }; // ERROR!

// loop: infinite, use break to exit
let mut count = 0;
let result = loop {
    count += 1;
    if count == 10 {
        break count * 2; // break with value
    }
};
println!("{}", result); // 20

// while: condition-checked loop
let mut n = 5;
while n > 0 {
    println!("{}", n);
    n -= 1;
}

// for: iterate over anything with IntoIterator
for i in 0..5 { print!("{}", i); }      // 01234
for i in (1..=3).rev() { print!("{}", i); } // 321
for ch in "hello".chars() { print!("{}", ch); }

let coin = 25;
match coin {
    25 => println!("quarter"),
    10 => println!("dime"),
    5 => println!("nickel"),
    1 => println!("penny"),
    _ => println!("unknown"),  // catch-all (required)
}

// matching with bindings
let x = 3;
match x {
    1 | 2 => println!("one or two"),   // or-pattern
    3..=9 => println!("single digit"),  // range
    n if n % 2 == 0 => println!("even: {}", n), // guard
    _ => println!("other"),
}

// match is exhaustive — all cases must be covered

// if let: shorthand for matching one pattern
let maybe: Option<i32> = Some(5);

// verbose: match
match maybe {
    Some(x) => println!("got {}", x),
    None => println!("nothing"),
}

// concise: if let
if let Some(x) = maybe {
    println!("got {}", x);
} else {
    println!("nothing");
}

// while let: loop while pattern matches
let mut iter = vec![1, 2, 3].into_iter();
while let Some(n) = iter.next() {
    println!("{}", n);
}

// continue: skip to next iteration
for i in 0..10 {
    if i % 2 == 0 { continue; }
    println!("{}", i); // prints odd numbers
}

// break: exit loop
for i in 0..10 {
    if i == 5 { break; }
    println!("{}", i); // prints 0..4
}

// labeled loops (for nested break/continue)
'outer: for i in 0..3 {
    for j in 0..3 {
        if i == 1 && j == 1 {
            break 'outer; // breaks the outer loop
        }
        println!("{} {}", i, j);
    }
}

// basic function with return type
fn add(a: i32, b: i32) -> i32 {
    a + b  // no semicolon = expression = return value
}

// statements (with semicolon) return ()
fn greet(name: &str) {
    println!("Hi, {}", name);
    // implicit return ()
}

// explicit return
fn abs(x: i32) -> i32 {
    if x < 0 {
        return -x;  // early return needs 'return'
    }
    x  // tail expression
}

// diverging function (never returns)
fn forever() -> ! {
    loop {}
}

// immutable borrow
fn len(s: &String) -> usize { s.len() }

// mutable borrow
fn push(v: &mut Vec<i32>) { v.push(42); }

// take ownership
fn consume(s: String) { println!("{}", s); }

// multiple return via tuple
fn swap(a: i32, b: i32) -> (i32, i32) { (b, a) }

// variadic-ish via slices
fn sum(nums: &[i32]) -> i32 {
    nums.iter().sum()
}
println!("{}", sum(&[1, 2, 3, 4])); // 10

// closure syntax: |params| body
let add = |a, b| a + b;
println!("{}", add(1, 2)); // 3

// type annotations (rarely needed)
let square = |x: i32| -> i32 { x * x };

// capturing environment
let multiplier = 3;
let multiply = |x| x * multiplier; // borrows multiplier
println!("{}", multiply(5)); // 15

// move closure: takes ownership of captured vars
let name = String::from("Alice");
let greet = move || println!("Hi {}", name);
// name is now moved into greet
greet();

let nums = vec![1, 2, 3, 4, 5];

// map: transform each element
let doubled: Vec<i32> = nums.iter().map(|x| x * 2).collect();

// filter: keep elements matching predicate
let evens: Vec<&i32> = nums.iter().filter(|&&x| x % 2 == 0).collect();

// fold/reduce: accumulate
let sum: i32 = nums.iter().sum();              // 15
let product: i32 = nums.iter().product();       // 120
let combined = nums.iter().fold(0, |acc, x| acc + x);

// chain multiple operations (lazy!)
let result: Vec<i32> = nums.iter()
    .filter(|&&x| x > 1)
    .map(|&x| x * x)
    .collect(); // [4, 9, 16, 25]

// function pointer type
type MathFn = fn(i32, i32) -> i32;

fn add(a: i32, b: i32) -> i32 { a + b }
fn mul(a: i32, b: i32) -> i32 { a * b }

fn apply(f: MathFn, a: i32, b: i32) -> i32 {
    f(a, b)
}
println!("{}", apply(add, 3, 4)); // 7
println!("{}", apply(mul, 3, 4)); // 12

// generic over Fn trait (accepts closures too)
fn apply_fn<F: Fn(i32, i32) -> i32>(f: F, a: i32, b: i32) -> i32 {
    f(a, b)
}
let closure = |a, b| a - b;
println!("{}", apply_fn(closure, 10, 3)); // 7

// Rule 1: Each value has ONE owner
let s1 = String::from("hello");
let s2 = s1;  // s1's ownership MOVED to s2
// println!("{}", s1); // ERROR: s1 is invalid after move

// Rule 2: When owner goes out of scope, value is dropped
{
    let s = String::from("temp");
    // s is valid here
} // s is automatically dropped (memory freed)

// Rule 3: Copy types (i32, bool, char, etc.) are copied, not moved
let a = 5;
let b = a;  // a is copied, both valid
println!("{} {}", a, b); // OK

// &T: immutable borrow (read-only, multiple allowed)
fn calc_len(s: &String) -> usize {
    s.len()
    // s goes out of scope but is NOT dropped (we don't own it)
}
let s = String::from("hello");
let len = calc_len(&s);  // borrow s, don't move it
println!("'{}' has length {}", s, len); // s still valid

// &mut T: mutable borrow (exclusive, only ONE at a time)
fn push_world(s: &mut String) {
    s.push_str(", world");
}
let mut s2 = String::from("hello");
push_world(&mut s2);
println!("{}", s2); // hello, world

// string slice: &str
let s = String::from("hello world");
let hello: &str = &s[0..5];   // "hello"
let world: &str = &s[6..];    // "world"
let full: &str = &s[..];      // "hello world"

// array slice: &[T]
let arr = [1, 2, 3, 4, 5];
let mid: &[i32] = &arr[1..4]; // [2, 3, 4]

// function accepting slices (idiomatic)
fn first_word(s: &str) -> &str {
    let bytes = s.as_bytes();
    for (i, &byte) in bytes.iter().enumerate() {
        if byte == b' ' {
            return &s[0..i];
        }
    }
    &s[..]
}

// explicit lifetime annotation
fn longest<'a>(x: &'a str, y: &'a str) -> &'a str {
    if x.len() > y.len() { x } else { y }
}
// 'a means: the returned reference lives as long as
// the SHORTEST of x and y's lifetimes

let s1 = String::from("long string");
let s2 = String::from("hi");
let result = longest(s1.as_str(), s2.as_str());
println!("Longest: {}", result);

// struct holding references needs lifetime
struct Excerpt<'a> {
    part: &'a str,
}
let novel = String::from("call me Ishmael. years ago...");
let first_sentence = novel.split('.').next().unwrap();
let ex = Excerpt { part: first_sentence };

use std::rc::Rc;
use std::sync::Arc;
use std::cell::RefCell;

// Box<T>: heap allocation, single owner
let b = Box::new(5); // 5 lives on heap
println!("{}", b);   // dereferenced automatically

// Rc<T>: reference counting, multiple owners (single-threaded)
let a = Rc::new(String::from("shared"));
let b = Rc::clone(&a); // increments ref count
println!("count: {}", Rc::strong_count(&a)); // 2

// Arc<T>: atomic Rc, thread-safe
let arc = Arc::new(vec![1, 2, 3]);

// RefCell<T>: interior mutability (runtime borrow check)
let cell = RefCell::new(5);
*cell.borrow_mut() += 1; // mutable borrow checked at runtime

// named-field struct
struct User {
    name: String,
    age: u32,
    active: bool,
}

let u = User {
    name: String::from("Alice"),
    age: 30,
    active: true,
};

// field init shorthand
let name = String::from("Bob");
let u2 = User { name, age: 25, active: true };

// update syntax (copy remaining fields from another)
let u3 = User { age: 40, ..u2 };

// tuple struct
struct Color(u8, u8, u8);
let red = Color(255, 0, 0);

// unit struct (no fields, useful for traits)
struct AlwaysEqual;

struct Rectangle {
    width: f64,
    height: f64,
}

impl Rectangle {
    // associated function (constructor, no &self)
    fn new(w: f64, h: f64) -> Self {
        Rectangle { width: w, height: h }
    }

    // method (borrows self)
    fn area(&self) -> f64 {
        self.width * self.height
    }

    // mutable method
    fn scale(&mut self, factor: f64) {
        self.width *= factor;
        self.height *= factor;
    }

    // consuming method (takes ownership)
    fn into_square(self) -> Rectangle {
        let side = (self.width + self.height) / 2.0;
        Rectangle { width: side, height: side }
    }
}

let mut r = Rectangle::new(10.0, 5.0);
println!("Area: {}", r.area()); // 50
r.scale(2.0);
let sq = r.into_square(); // r consumed

// enum with data (algebraic data type)
enum Message {
    Quit,                          // no data
    Move { x: i32, y: i32 },       // named fields
    Write(String),                 // tuple variant
    ChangeColor(i32, i32, i32),    // tuple variant
}

// pattern matching with destructuring
fn process(msg: Message) {
    match msg {
        Message::Quit => println!("Quit"),
        Message::Move { x, y } => println!("Move to ({}, {})", x, y),
        Message::Write(text) => println!("Write: {}", text),
        Message::ChangeColor(r, g, b) => println!("RGB({}, {}, {})", r, g, b),
    }
}

process(Message::Move { x: 10, y: 20 });
process(Message::Write(String::from("hello")));

// enums can have methods too
impl Message {
    fn is_quit(&self) -> bool {
        matches!(self, Message::Quit)
    }
}

// Option<T>: Some(value) or None (replaces null)
fn find_user(id: i32) -> Option<String> {
    if id == 1 { Some(String::from("Alice")) }
    else { None }
}

let user = find_user(1);
match user {
    Some(name) => println!("Found: {}", name),
    None => println!("Not found"),
}

// convenient methods
let name = find_user(1).unwrap_or("Anonymous".into());
let upper = find_user(1).map(|n| n.to_uppercase());
let len = find_user(1).and_then(|n| Some(n.len()));

// Result<T, E>: Ok(value) or Err(error)
fn parse_num(s: &str) -> Result<i32, std::num::ParseIntError> {
    s.parse()
}

match parse_num("42") {
    Ok(n) => println!("Parsed: {}", n),
    Err(e) => println!("Error: {}", e),
}

// define a trait (interface)
trait Summary {
    fn summarize(&self) -> String;

    // default method
    fn author(&self) -> String {
        String::from("Unknown")
    }
}

struct Article { title: String, content: String }

impl Summary for Article {
    fn summarize(&self) -> String {
        format!("{}: {}", self.title, self.content)
    }
    // author() uses default implementation
}

let a = Article {
    title: "Rust".into(),
    content: "Great".into(),
};
println!("{}", a.summarize());
println!("Author: {}", a.author()); // Unknown

// generic with trait bound
fn print_summary<T: Summary>(item: &T) {
    println!("{}", item.summarize());
}

// multiple bounds with +, or where clause
fn display<T: Summary + std::fmt::Display>(item: &T) {}
// or: fn display<T>(item: &T) where T: Summary + std::fmt::Display {}

// derive common traits automatically
#[derive(Debug, Clone, PartialEq, Eq, Hash)]
struct Point {
    x: i32,
    y: i32,
}

let p1 = Point { x: 1, y: 2 };
let p2 = p1.clone();          // Clone
println!("{:?}", p1);          // Debug
println!("{}", p1 == p2);      // PartialEq -> true

// trait object: dynamic dispatch
trait Animal {
    fn sound(&self) -> String;
}

struct Dog;
struct Cat;

impl Animal for Dog { fn sound(&self) -> String { "Woof".into() } }
impl Animal for Cat { fn sound(&self) -> String { "Meow".into() } }

// Vec of trait objects (dynamic dispatch via vtable)
let animals: Vec<Box<dyn Animal>> = vec![
    Box::new(Dog),
    Box::new(Cat),
];
for a in &animals {
    println!("{}", a.sound());
}

use std::fs;
use std::io;
use std::num::ParseIntError;

// ? propagates errors: if Err, return early; if Ok, unwrap
fn read_config(path: &str) -> Result<i32, io::Error> {
    let content = fs::read_to_string(path)?; // ? on io::Result
    Ok(content.len() as i32)
}

// ? converts error types via From
fn parse_and_read(path: &str) -> Result<i32, Box<dyn std::error::Error>> {
    let content = fs::read_to_string(path)?;     // io::Error -> Box
    let n: i32 = content.trim().parse()?;         // ParseIntError -> Box
    Ok(n)
}

// ? on Option too
fn first_char(s: &str) -> Option<char> {
    s.lines().next()?.chars().next()
}

// panic: unrecoverable error, crashes the program
fn divide(a: i32, b: i32) -> i32 {
    if b == 0 {
        panic!("Division by zero!"); // unwinds stack
    }
    a / b
}

// Result: recoverable error, caller decides
fn safe_divide(a: i32, b: i32) -> Result<i32, String> {
    if b == 0 {
        Err(String::from("division by zero"))
    } else {
        Ok(a / b)
    }
}

// unwrap/expect: panic on Err (use sparingly)
let n: i32 = "42".parse().unwrap();       // panics on error
let n2: i32 = "42".parse().expect("valid number"); // panic with msg

// when to panic vs Result:
// panic: bugs, invariant violations, impossible states
// Result: expected failures (file not found, parse error)

use std::fmt;
use std::error::Error;

#[derive(Debug)]
enum AppError {
    Io(std::io::Error),
    Parse(std::num::ParseIntError),
    NotFound(String),
}

// implement Display (required by Error)
impl fmt::Display for AppError {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        match self {
            AppError::Io(e) => write!(f, "IO error: {}", e),
            AppError::Parse(e) => write!(f, "Parse error: {}", e),
            AppError::NotFound(name) => write!(f, "Not found: {}", name),
        }
    }
}

// implement Error
impl Error for AppError {
    fn source(&self) -> Option<&(dyn Error + 'static)> {
        match self {
            AppError::Io(e) => Some(e),
            AppError::Parse(e) => Some(e),
            _ => None,
        }
    }
}

// From impls for ? to work automatically
impl From<std::io::Error> for AppError {
    fn from(e: std::io::Error) -> Self { AppError::Io(e) }
}

// match on Result
let result: Result<i32, &str> = Ok(42);
match result {
    Ok(n) => println!("Got: {}", n),
    Err(e) => println!("Error: {}", e),
}

// combinators
let r1: Result<i32, &str> = Ok(5);
let doubled = r1.map(|n| n * 2);           // Ok(10)
let mapped_err = r1.map_err(|e| format!("{}", e));

// and_then: chain fallible operations
let r2 = r1.and_then(|n| if n > 0 { Ok(n * 2) } else { Err("negative") });

// unwrap_or family
let val = r1.unwrap_or(0);        // 5 or 0
let val2 = r1.unwrap_or_default(); // 5 or T::default()
let val3 = r1.unwrap_or_else(|_| 0); // lazy default

// check variants
println!("{}", r1.is_ok());  // true
println!("{}", r1.is_err()); // false

use std::fs;
use std::num::ParseIntError;

// Without From: manual conversion needed
fn manual(path: &str) -> Result<i32, String> {
    let content = fs::read_to_string(path)
        .map_err(|e| e.to_string())?;  // manual convert
    let n: i32 = content.trim().parse()
        .map_err(|e| e.to_string())?;  // manual convert
    Ok(n)
}

// With Box<dyn Error>: auto-converts via From
fn boxed(path: &str) -> Result<i32, Box<dyn std::error::Error>> {
    let content = fs::read_to_string(path)?;  // auto-converts
    let n: i32 = content.trim().parse()?;      // auto-converts
    Ok(n)
}

// anyhow::Result (from anyhow crate) is ergonomic for apps
// fn anyhow_fn() -> anyhow::Result<i32> {
//     let n: i32 = something()?; // any error auto-converted
//     Ok(n)
// }

// mod.rs or mod declaration in lib.rs/main.rs
// File: src/lib.rs
mod network {
    pub mod connection {
        pub fn connect() -> bool { true }
        fn disconnect() {} // private
    }
}

// use: bring paths into scope
use network::connection::connect;

// calling with full path
fn main() {
    network::connection::connect();
    connect(); // after 'use'
}

// re-export with pub use
pub use network::connection::connect as open_connection;

// nested file: src/network/connection.rs
// mod network; in lib.rs loads src/network/mod.rs
// which can have: pub mod connection;

mod my_module {
    // private by default
    fn internal_helper() {}

    // public: accessible from outside
    pub fn public_api() {
        internal_helper(); // can call private within module
    }

    // pub(crate): visible within this crate only
    pub(crate) fn crate_wide() {}

    // pub(super): visible to parent module
    pub(super) fn parent_visible() {}

    // struct fields are private even if struct is pub
    pub struct Config {
        pub name: String,    // public field
        secret: String,      // private field
    }

    // enum variants inherit the enum's visibility
    pub enum Status {
        Active,  // public because Status is public
        Inactive,
    }
}

// basic use
use std::collections::HashMap;
use std::fs::read_to_string;

// grouped use
use std::io::{self, Read, Write, BufRead};
// equivalent to:
// use std::io;
// use std::io::Read;
// use std::io::Write;
// use std::io::BufRead;

// aliasing with 'as'
use std::collections::HashMap as Map;

// glob import (use sparingly)
use std::prelude::v1::*;

// bringing trait methods into scope
use std::io::Read; // now .read() method is available
let mut f = std::fs::File::open("x")?;
let mut buf = String::new();
f.read_to_string(&mut buf)?; // Read trait method

# Cargo.toml
[package]
name = "my_app"
version = "0.1.0"
edition = "2021"

[dependencies]
serde = { version = "1.0", features = ["derive"] }
tokio = { version = "1", features = ["full"] }
rand = "0.8"

# in Rust code
use serde::{Serialize, Deserialize};
use rand::Rng;

#[derive(Serialize, Deserialize)]
struct User { name: String, age: u32 }

fn main() {
    let mut rng = rand::thread_rng();
    let n: i32 = rng.gen_range(1..=100);
    println!("{}", n);
}

# CLI commands:
# cargo new my_app      # create new project
# cargo build           # compile
# cargo run             # compile + run
# cargo test            # run tests
# cargo add serde       # add dependency
# cargo update          # update dependencies

// Unit tests in same file (convention: tests module)
pub fn add(a: i32, b: i32) -> i32 { a + b }

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn test_add() {
        assert_eq!(add(2, 3), 5);
        assert_ne!(add(2, 3), 6);
    }

    #[test]
    fn test_with_message() {
        let result = add(1, 1);
        assert_eq!(result, 2, "Expected 2, got {}", result);
    }

    #[test]
    #[should_panic(expected = "division by zero")]
    fn test_panic() {
        panic!("division by zero");
    }

    #[test]
    fn test_result() -> Result<(), String> {
        if add(2, 2) == 4 { Ok(()) }
        else { Err(String::from("math is broken")) }
    }
}

// Integration tests: tests/integration_test.rs
// Run: cargo test

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

// spawn a thread
let handle = thread::spawn(|| {
    for i in 0..5 {
        println!("spawned thread: {}", i);
        thread::sleep(Duration::from_millis(10));
    }
});

// main thread continues
for i in 0..3 {
    println!("main thread: {}", i);
}

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

// move closure: transfer ownership to thread
let data = vec![1, 2, 3];
let h = thread::spawn(move || {
    println!("got data: {:?}", data); // data moved here
});
h.join().unwrap();

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

// mpsc: multiple producer, single consumer
let (tx, rx) = mpsc::channel();

// clone transmitter for multiple producers
let tx2 = tx.clone();

thread::spawn(move || {
    let vals = vec!["a", "b", "c"];
    for v in vals {
        tx.send(v).unwrap();
    }
});

thread::spawn(move || {
    tx2.send("from tx2").unwrap();
});

// receiver is an iterator
for received in rx {
    println!("Got: {}", received);
}

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

// Arc: thread-safe reference counting
// Mutex: mutual exclusion lock
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 || {
        // lock() returns MutexGuard, auto-unlocks when dropped
        let mut num = counter.lock().unwrap();
        *num += 1;
    }); // lock released here
    handles.push(handle);
}

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

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

// RwLock: multiple readers OR one writer
use std::sync::RwLock;
let data = RwLock::new(5);
let r1 = data.read().unwrap();  // shared read
let r2 = data.read().unwrap();
// let w = data.write().unwrap(); // would block until r1, r2 dropped

use std::fs;
use std::io::{Read, Write, BufReader, BufWriter};
use std::fs::File;

// read entire file
let content = fs::read_to_string("file.txt")?;
fs::write("output.txt", "Hello")?;

// buffered read (efficient for large files)
let file = File::open("file.txt")?;
let mut reader = BufReader::new(file);
let mut buf = String::new();
reader.read_to_string(&mut buf)?;

// line by line
use std::io::BufRead;
let file = File::open("file.txt")?;
for line in BufReader::new(file).lines() {
    println!("{}", line?);
}

// buffered write
let file = File::create("out.txt")?;
let mut writer = BufWriter::new(file);
writeln!(writer, "Line 1")?;
writeln!(writer, "Line 2")?;
writer.flush()?; // ensure written to disk

use tokio::{fs, task, time};
use std::time::Duration;

// async fn returns a Future
async fn fetch_data() -> String {
    time::sleep(Duration::from_secs(1)).await;
    String::from("data")
}

// spawn concurrent tasks
#[tokio::main]
async fn main() {
    // run tasks concurrently
    let (a, b, c) = tokio::join!(
        fetch_data(),
        fetch_data(),
        fetch_data()
    );
    println!("{} {} {}", a, b, c);

    // spawn a background task
    let handle = tokio::spawn(async {
        let data = fs::read_to_string("file.txt").await.unwrap();
        println!("Read {} bytes", data.len());
    });
    handle.await.unwrap();

    // select: first to complete wins
    tokio::select! {
        val = fetch_data() => println!("Got: {}", val),
        _ = time::sleep(Duration::from_millis(500)) => {
            println!("Timeout!");
        }
    }
}

// Lifetimes tell the compiler how long references live
fn longest<'a>(x: &'a str, y: &'a str) -> &'a str {
    if x.len() > y.len() { x } else { y }
}
// 'a means: the returned reference lives as long as
// the SHORTEST of x and y

let s1 = String::from("long string");
let result;
{
    let s2 = String::from("hi");
    result = longest(s1.as_str(), s2.as_str());
    // result valid here
    println!("{}", result);
}
// result NOT valid here — s2 is dropped

// The compiler applies 3 elision rules automatically:
// 1. Each input reference gets its own lifetime
// 2. If one input lifetime, output gets that lifetime
// 3. If &self/&mut self, output gets self's lifetime

// These DON'T need explicit annotations (rule 2):
fn first_word(s: &str) -> &str {
    let bytes = s.as_bytes();
    for (i, &byte) in bytes.iter().enumerate() {
        if byte == b' ' { return &s[0..i]; }
    }
    &s[..]
}

// This NEEDS annotation (multiple inputs, rule doesn't apply):
// fn longest<'a>(x: &'a str, y: &'a str) -> &'a str

// 'static means the reference lives for the entire program
let s: &'static str = "I live forever";
// All string literals are &'static str

// Static variables (global, program-wide)
static COUNTER: AtomicUsize = AtomicUsize::new(0);
COUNTER.fetch_add(1, Ordering::SeqCst);

// 'static in bounds: T: 'static means T contains no
// non-static references (owned data always satisfies this)
fn spawn_thread<T: Send + 'static>(t: T) {
    std::thread::spawn(move || {
        drop(t);
    });
}

// Structs holding references need lifetime annotations
struct Parser<'a> {
    text: &'a str,
    pos: usize,
}

impl<'a> Parser<'a> {
    fn new(text: &'a str) -> Self {
        Parser { text, pos: 0 }
    }
    fn peek(&self) -> Option<char> {
        self.text[self.pos..].chars().next()
    }
    fn advance(&mut self) {
        self.pos += 1;
    }
}

// The struct cannot outlive the text it borrows
let text = String::from("hello");
let mut p = Parser::new(&text);
p.advance();

// Multiple lifetime parameters
fn parse<'src, 'ctx>(src: &'src str, ctx: &'ctx Context) -> &'src str {
    // returns something tied to src, not ctx
    src
}

// 'static: 'a is always true (static outlives everything)
fn static_ref<'a>(_: &'a str) -> &'static str {
    "constant"  // 'static coerces to 'a
}

// Variance: &'long can be used where &'short is expected
// (covariance) — longer lifetime is a subtype
fn use_ref<'short>(r: &'short str) {}
let long: &'static str = "hi";
use_ref(long);  // OK: 'static coerces to any 'short

// Box moves data to the heap (single owner)
let b = Box::new(5);
println!("{}", b);  // derefs automatically

// Recursive types NEED Box (size unknown at compile time)
enum List {
    Cons(i32, Box<List>),
    Nil,
}
let list = List::Cons(1, Box::new(List::Cons(2, Box::new(List::Nil))));

// Trait objects (dynamic dispatch)
let shapes: Vec<Box<dyn Draw>> = vec![
    Box::new(Circle { radius: 1.0 }),
    Box::new(Square { side: 2.0 }),
];
for s in &shapes { s.draw(); }

// Box has zero runtime overhead when dereferencing

use std::rc::Rc;

// Rc allows multiple owners via reference counting
let a = Rc::new(String::from("shared"));
let b = Rc::clone(&a);  // increments count, doesn't copy
let c = Rc::clone(&a);

println!("count = {}", Rc::strong_count(&a));  // 3
// Data dropped when count reaches 0

// Shared graph/tree structures
struct Node {
    children: Vec<Rc<Node>>,
    value: i32,
}
let leaf = Rc::new(Node { children: vec![], value: 1 });
let branch = Rc::new(Node {
    children: vec![Rc::clone(&leaf)],
    value: 2,
});

use std::sync::Arc;
use std::thread;

// Arc: thread-safe version of Rc (atomic ref counting)
let data = Arc::new(vec![1, 2, 3, 4, 5]);

let handles: Vec<_> = (0..3).map(|i| {
    let data = Arc::clone(&data);  // atomic increment
    thread::spawn(move || {
        println!("Thread {} sees: {:?}", i, *data);
    })
}).collect();

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

// Arc is slightly slower than Rc due to atomic operations
// Only use Arc when actually sharing across threads

use std::cell::RefCell;

// RefCell moves borrow checking to RUNTIME
let cell = RefCell::new(vec![1, 2, 3]);

// Multiple immutable borrows OR one mutable borrow
{
    let mut borrowed = cell.borrow_mut();
    borrowed.push(4);
}  // borrow released here

{
    let r1 = cell.borrow();     // OK
    let r2 = cell.borrow();     // OK — multiple immutable
    println!("{:?} {:?}", r1, r2);
}

// cell.borrow_mut() while r1 alive → PANIC at runtime
// Combine with Rc: Rc<RefCell<T>> for mutable shared graphs

use std::rc::{Rc, Weak, RefCell};

// Weak references don't contribute to the strong count
// — prevents memory leaks in cycles
struct Node {
    parent: RefCell<Weak<Node>>,        // weak: child → parent
    children: RefCell<Vec<Rc<Node>>>,   // strong: parent → children
}

let leaf = Rc::new(Node {
    parent: RefCell::new(Weak::new()),
    children: RefCell::new(vec![]),
});
let branch = Rc::new(Node {
    parent: RefCell::new(Weak::new()),
    children: RefCell::new(vec![Rc::clone(&leaf)]),
});
*leaf.parent.borrow_mut() = Rc::downgrade(&branch);

// Upgrade returns Option — parent may already be dropped
if let Some(p) = leaf.parent.borrow().upgrade() {
    println!("parent exists");
}

trait Animal {
    fn name(&self) -> &str;
    fn sound(&self) -> String;
}

struct Dog { name: String }
struct Cat { name: String }

impl Animal for Dog {
    fn name(&self) -> &str { &self.name }
    fn sound(&self) -> String { "Woof".into() }
}
impl Animal for Cat {
    fn name(&self) -> &str { &self.name }
    fn sound(&self) -> String { "Meow".into() }
}

// dyn Trait = erased type, runtime dispatch via vtable
let animals: Vec<Box<dyn Animal>> = vec![
    Box::new(Dog { name: "Rex".into() }),
    Box::new(Cat { name: "Whiskers".into() }),
];
for a in &animals {
    println!("{} says {}", a.name(), a.sound());
}

// Object-safe traits CAN be used as dyn Trait:
trait Draw {
    fn draw(&self);  // OK: &self, no generics
}

// NOT object-safe:
trait Bad {
    fn create() -> Self;        // returns Self — needs known type
    fn process<T>(&self, x: T); // generic method — vtable can't cover all T
    const SIZE: usize;          // associated const (sometimes OK)
}

// Workaround for Self returns — use a factory or Box<Self>
trait GoodFactory {
    fn new_boxed() -> Box<dyn GoodFactory>;
}

// Sized bound makes trait non-object-safe
// trait Foo: Sized {}  // NOT object-safe

// Generics: monomorphization, static dispatch, zero overhead
fn max_generic<T: PartialOrd>(a: T, b: T) -> T {
    if a > b { a } else { b }
}
// Each type instantiation creates a separate function:
// max_generic::<i32>, max_generic::<f64>, etc.

// Trait objects: dynamic dispatch, single function
fn max_dyn(a: &dyn PartialOrd, b: &dyn PartialOrd) -> bool {
    // can't return — don't know size at compile time
    a.partial_cmp(b) == Some(std::cmp::Ordering::Less)
}

// Rule of thumb:
// - Few types, performance-critical → generics (static dispatch)
// - Many/unknown types, flexibility needed → dyn (dynamic dispatch)
// - Heterogeneous collections → must use dyn

use std::any::Any;

// Any enables runtime type checking & downcasting
let x: Box<dyn Any> = Box::new(42i32);

// downcast_ref returns Option<&T>
if let Some(n) = x.downcast_ref::<i32>() {
    println!("It's an i32: {}", n);
}

// downcast returns Option<T> (for Box)
let boxed: Box<dyn Any> = Box::new("hello");
if let Ok(s) = boxed.downcast::<&str>() {
    println!("Recovered: {}", s);
}

// Useful for: plugin systems, error types, heterogeneous storage
// Avoid overusing — prefer enums when the type set is known

trait Shape {
    fn area(&self) -> f64;
    // Default method — can be overridden
    fn describe(&self) -> String {
        format!("Shape with area {:.2}", self.area())
    }
}

// Supertrait: trait that requires another trait
trait Named: Shape {
    fn name(&self) -> &str;
}

struct Circle { radius: f64 }
impl Shape for Circle {
    fn area(&self) -> f64 { std::f64::consts::PI * self.radius.powi(2) }
}
impl Named for Circle {
    fn name(&self) -> &str { "Circle" }
}

let c = Circle { radius: 2.0 };
println!("{}", c.describe());  // uses default method

// macro_rules! defines pattern-matching macros
macro_rules! vec_of {
    ($($x:expr),*) => {{
        let mut v = Vec::new();
        $( v.push($x); )*
        v
    }};
}

let nums = vec_of!(1, 2, 3, 4);

// Recursive macro: build a HashMap
macro_rules! hashmap {
    ($($k:expr => $v:expr),*) => {{
        let mut m = std::collections::HashMap::new();
        $( m.insert($k, $v); )*
        m
    }};
}
let config = hashmap!("host" => "localhost", "port" => 8080);

// Common fragment specifiers:
macro_rules! build_fn {
    // $name:ident — identifier (function/variable name)
    // $body:block — a block { ... }
    // $ty:ty — a type
    // $expr:expr — an expression
    // $stmt:stmt — a statement
    // $lit:literal — a literal (string, number)
    ($name:ident, $ret:ty, $body:block) => {
        fn $name() -> $ret $body
    };
}

build_fn!(get_answer, i32, { 42 });
println!("{}", get_answer());

// :pat — pattern, :path — module path
// :meta — attribute meta-item, :vis — visibility

// Formatting & printing
println!("x = {}, y = {:?}", 1, "two");
eprintln!("Error: {}", "oops");     // stderr
format!("{}-{}", "a", "b");          // returns String

// Debug helpers
dbg!(2 + 2);                          // prints [src.rs:1] 2 + 2 = 4
assert!(1 + 1 == 2);                  // panic if false
assert_eq!(2 + 2, 4);                 // panic if not equal
assert_ne!(1, 2);                     // panic if equal

// Code generation
todo!("not implemented yet");         // unimplemented!()
unimplemented!();
panic!("fatal: {}", "reason");

// Environment & file info
println!("{}:{}", file!(), line!());  // src.rs:1
println!("{}", env!("CARGO_PKG_NAME"));

// Procedural macros are Rust functions that transform code
// Three kinds (must be in a separate crate with proc-macro=true):

// 1. Derive macros — #[derive(MyTrait)]
#[derive(Debug, Clone, MyTrait)]
struct Point { x: f64, y: f64 }

// 2. Attribute macros — #[my_attr]
#[my_attr]
fn function() {}

// 3. Function-like macros — my_macro!(...)
sqlx::query!("SELECT * FROM users");

// Cargo.toml for a proc-macro crate:
// [lib]
// proc-macro = true
//
// [dependencies]
// syn = "2.0"   # parse Rust code
// quote = "1.0" # generate code
// proc-macro2 = "1.0"

// Hygiene: macro-introduced identifiers don't leak
macro_rules! using_temp {
    ($e:expr) => {
        let temp = $e;  // this 'temp' is distinct from caller's
        println!("{}", temp);
    };
}
let temp = 10;
using_temp!(temp + 5);  // no conflict — hygienic

// Conditional compilation macro
macro_rules! cfg_debug {
    ($($e:tt)*) => {
        #[cfg(debug_assertions)]
        { $($e)* }
    };
}
cfg_debug! {
    println!("Debug mode on");
}

// stringify! converts tokens to a string literal
let s = stringify!(a + b * c);  // "a + b * c"

// Raw pointers: *const T (immutable) and *mut T (mutable)
let x = 42;
let r1: *const i32 = &x;       // coerce from reference
let r2: *mut i32 = x as *mut i32;  // cast

// Can be null, can alias, no borrow checking
let null: *const i32 = std::ptr::null();

// Creating raw pointers is safe, but DEREFERENCING is unsafe
unsafe {
    println!("r1 = {}", *r1);
}

// Convert between types (transmute-like)
let bytes: [u8; 4] = [0x78, 0x56, 0x34, 0x12];
let ptr = bytes.as_ptr() as *const u32;
unsafe { println!("0x{:x}", *ptr); }  // little-endian int

// unsafe block: a localized unsafe region
let ptr: *const i32 = &42;
let val = unsafe { *ptr };

// unsafe fn: the ENTIRE function body is unsafe
unsafe fn dangerous(ptr: *const u8) -> u8 {
    *ptr
}
// Callers must use unsafe block:
let b = 5u8;
unsafe { dangerous(&b) };

// Splitting borrows safely (compiler is conservative)
let mut v = vec![1, 2, 3, 4];
let len = v.len();
unsafe {
    let first = v.get_unchecked(0);   // no bounds check
    let last = v.get_unchecked(len - 1);
    println!("{} {}", first, last);
}

// extern "C" declares foreign functions
extern "C" {
    fn abs(x: i32) -> i32;
}

fn main() {
    let x = -5;
    let positive = unsafe { abs(x) };
    println!("{}", positive);  // 5
}

// Exporting Rust functions to C
#[no_mangle]  // prevent name mangling
pub extern "C" fn add(a: i64, b: i64) -> i64 {
    a + b
}

// C-compatible struct
#[repr(C)]
struct Point { x: f64, y: f64 }

// Some traits are unsafe to implement — the compiler can't verify invariants
use std::marker::Send;

// Send/Sync are auto-implemented, but sometimes you must manually impl
struct RawPointer<T>(*mut T);

// SAFETY: We guarantee the pointer is only used on one thread
unsafe impl<T> Send for RawPointer<T> where T: Send {}

// Splitting a slice into disjoint mutable parts
let mut v = vec![1, 2, 3, 4, 5, 6];
let (left, right) = v.split_at_mut(3);
// left = [1,2,3], right = [4,5,6] — disjoint, but compiler
// couldn't prove this without unsafe internally

// Global mutable state
static mut COUNTER: u64 = 0;
unsafe { COUNTER += 1; }  // unsafe: data races possible

// Unions: multiple fields share the same memory (like C unions)
#[repr(C)]
union IntOrFloat {
    i: i32,
    f: f32,
}

let mut u = IntOrFloat { i: 42 };
unsafe { println!("as int: {}", u.i); }
u.f = 3.14;
unsafe { println!("as float: {}", u.f); }
// Reading the WRONG field is undefined behavior!

// Inline assembly (nightly / asm!)
#[cfg(feature = "asm")]
unsafe fn halt() {
    std::arch::asm!("hlt", options(nostack));
}

// Transmute: reinterpret bits as another type (same size)
let bits: u32 = 0x40490FDB;  // ~3.14159 in IEEE 754
let pi: f32 = unsafe { std::mem::transmute(bits) };

// The Iterator trait: one required method
trait Iterator {
    type Item;
    fn next(&mut self) -> Option<Self::Item>;
    // ... many provided methods (map, filter, etc.)
}

// Creating iterators
let v = vec![1, 2, 3];
let iter = v.iter();        // borrows: &i32
let into_iter = v.into_iter(); // owns: i32 (consumes v)
let mut_iter = v.iter_mut();   // mutably borrows: &mut i32

// Range is an iterator
for i in 1..=5 { print!("{} ", i); }  // 1 2 3 4 5

// String iteration
for c in "héllo".chars() { print!("{} ", c); }  // h é l l o
for b in "hi".bytes() { print!("{} ", b); }      // 104 105

let nums = vec![1, 2, 3, 4, 5, 6];

// map: transform each element
let doubled: Vec<_> = nums.iter().map(|x| x * 2).collect();

// filter: keep elements matching predicate
let evens: Vec<_> = nums.iter().filter(|&&x| x % 2 == 0).collect();

// take / skip: limit or skip elements
let first3: Vec<_> = nums.iter().take(3).collect();     // [1,2,3]
let after2: Vec<_> = nums.iter().skip(2).collect();     // [3,4,5,6]

// flat_map: map then flatten one level
let words: Vec<_> = ["a b", "c"].iter()
    .flat_map(|s| s.split(' '))
    .collect();  // ["a","b","c"]

// enumerate: add index
for (i, v) in nums.iter().enumerate() {
    println!("{}: {}", i, v);
}

let nums = vec![1, 2, 3, 4, 5];

// collect: gather into a collection
let v: Vec<i32> = nums.iter().copied().collect();
let s: std::collections::HashSet<i32> = nums.iter().copied().collect();

// sum, product, count
let total: i32 = nums.iter().sum();        // 15
let prod: i32 = nums.iter().product();     // 120
let count = nums.iter().count();           // 5

// reduce / fold: accumulate into a single value
let max = nums.iter().copied().reduce(i32::max);  // Some(5)
let sum = nums.iter().fold(0, |acc, &x| acc + x); // 15

// find / any / all: short-circuiting
let first_even = nums.iter().find(|&&x| x % 2 == 0);  // Some(2)
let has_neg = nums.iter().any(|&x| *x < 0);  // false
let all_pos = nums.iter().all(|&x| *x > 0);  // true

// min / max
nums.iter().copied().min();  // Some(1)
nums.iter().copied().max();  // Some(5)

// Build a custom iterator for a counter
struct Counter {
    current: usize,
    max: usize,
}

impl Counter {
    fn new(max: usize) -> Self {
        Counter { current: 0, max }
    }
}

impl Iterator for Counter {
    type Item = usize;
    fn next(&mut self) -> Option<Self::Item> {
        if self.current < self.max {
            let val = self.current;
            self.current += 1;
            Some(val)
        } else {
            None
        }
    }
}

// Now all iterator methods work!
let sum: usize = Counter::new(5).sum();  // 0+1+2+3+4 = 10
let doubled: Vec<_> = Counter::new(3).map(|x| x * 10).collect();

use std::iter;

// Infinite iterators (use with take!)
let naturals = (1..).take(10);  // 1..10
let zeros = iter::repeat(0).take(5);  // [0,0,0,0,0]
let alternating = iter::repeat_with(|| rand::random::<u8>());

// cycle: repeat a finite iterator infinitely
let pattern = [1, 2, 3].iter().cycle().take(7);
// [1,2,3,1,2,3,1]

// chain: concatenate two iterators
let combined = [1, 2].iter().chain([3, 4].iter());
// [1,2,3,4]

// zip: pair elements from two iterators
let pairs: Vec<_> = [1, 2, 3].iter().zip(['a', 'b', 'c']).collect();
// [(1,'a'), (2,'b'), (3,'c')]

// peekable: look ahead without consuming
let mut iter = [1, 2, 3].iter().peekable();
if let Some(&&first) = iter.peek() { println!("{}", first); }

// Result combinators chain operations without match
fn parse_and_double(s: &str) -> Result<i32, std::num::ParseIntError> {
    s.parse::<i32>().map(|n| n * 2)
}

// and_then: chain fallible operations
fn validate(n: i32) -> Result<i32, String> {
    if n > 0 { Ok(n) } else { Err("must be positive".into()) }
}
let result = "5".parse::<i32>().and_then(validate);  // Ok(5)

// map_err: transform the error type
let r = "abc".parse::<i32>()
    .map_err(|e| format!("Parse failed: {}", e));

// ok_or / ok_or_else: convert Option to Result
let opt: Option<i32> = None;
let val = opt.ok_or("missing value");  // Err("missing value")

// unwrap_or / unwrap_or_else / unwrap_or_default
let x: i32 = "abc".parse().unwrap_or(0);

#[derive(Debug)]
enum AppError {
    Io(std::io::Error),
    Parse(std::num::ParseIntError),
    NotFound(String),
    Unauthorized,
}

// Implement From for automatic conversion with ?
impl From<std::io::Error> for AppError {
    fn from(e: std::io::Error) -> Self { AppError::Io(e) }
}
impl From<std::num::ParseIntError> for AppError {
    fn from(e: std::num::ParseIntError) -> Self { AppError::Parse(e) }
}

impl std::fmt::Display for AppError {
    fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
        match self {
            AppError::Io(e) => write!(f, "IO error: {}", e),
            AppError::Parse(e) => write!(f, "Parse error: {}", e),
            AppError::NotFound(s) => write!(f, "Not found: {}", s),
            AppError::Unauthorized => write!(f, "Unauthorized"),
        }
    }
}

// Now ? works for io::Error and ParseIntError automatically
fn read_config() -> Result<i32, AppError> {
    let s = std::fs::read_to_string("config.txt")?;  // io → AppError
    let n: i32 = s.trim().parse()?;                   // parse → AppError
    Ok(n)
}

use std::error::Error;

// std::error::Error is the trait for all errors
// Requires: Debug + Display
fn do_something() -> Result<(), Box<dyn Error>> {
    let f = std::fs::read_to_string("file.txt")?;  // io::Error
    let n: i32 = f.parse()?;                         // ParseIntError
    println!("{}", n);
    Ok(())
}

// Box<dyn Error> is the quick-and-dirty error type
// — accepts any error via ?, but loses specific type info

// downcast to recover specific error type
match do_something() {
    Err(e) => {
        if let Some(io_err) = e.downcast_ref::<std::io::Error>() {
            println!("IO: {}", io_err);
        }
    }
    _ => {}
}

use thiserror::Error;

// thiserror auto-generates Display and From impls
#[derive(Debug, Error)]
enum DataError {
    #[error("IO error: {0}")]
    Io(#[from] std::io::Error),

    #[error("parse failed: {0}")]
    Parse(#[from] std::num::ParseIntError),

    #[error("item {id} not found")]
    NotFound { id: u32 },

    #[error("invalid state: {msg}")]
    Invalid { msg: String },
}

// #[from] auto-implements From, so ? just works:
fn load() -> Result<i32, DataError> {
    let s = std::fs::read_to_string("data.txt")?;  // auto-converts
    let n: i32 = s.trim().parse()?;
    Ok(n)
}

use anyhow::{Context, Result, anyhow};

// anyhow::Result = Result<T, anyhow::Error>
fn load_config() -> Result<String> {
    let content = std::fs::read_to_string("config.toml")
        .context("Failed to read config.toml")?;  // add context
    Ok(content)
}

fn main() -> Result<()> {
    let config = load_config()?;
    if config.is_empty() {
        // bail! / anyhow! create errors with format! syntax
        return Err(anyhow!("config is empty"));
    }
    println!("{}", config);
    Ok(())
}

// Error chain: "Failed to read config.toml: No such file..."
// anyhow preserves the full chain and backtrace

[package]
name = "myapp"
version = "0.1.0"
edition = "2021"
authors = ["You <[email protected]>"]
license = "MIT"
description = "A sample app"

[dependencies]
serde = { version = "1.0", features = ["derive"] }
tokio = { version = "1", optional = true }
rand = "0.8"

[dev-dependencies]
criterion = "0.5"  # only for tests/benches

[features]
default = ["tokio"]
async-mode = ["dep:tokio"]

[[bin]]
name = "myapp"
path = "src/main.rs"

[profile.release]
opt-level = 3
lto = true
strip = true

# Cargo.toml
[dependencies]
# Version requirements: ^1.2 (compatible), =1.2.3 (exact), >=1.0,<2.0 (range)
serde = "1.0"                    # ^1.0 (default: compatible)
serde = { version = "1.0", features = ["derive"] }
serde = { version = "1.0", default-features = false }  # disable defaults

# Optional dependencies (enabled by a feature)
tokio = { version = "1", optional = true }

[features]
# "async" feature enables the optional tokio dep
async = ["dep:tokio"]
# Features can enable other features
full = ["async", "serde/derive"]

# In code:
# #[cfg(feature = "async")]
# fn run_async() { ... }

# Root Cargo.toml — workspace manifest
[workspace]
members = [
    "core",
    "cli",
    "server",
    "utils",
]
resolver = "2"

# Shared dependencies across all members
[workspace.dependencies]
serde = "1.0"
tokio = "1"

# In member crates (e.g., cli/Cargo.toml):
# [dependencies]
# serde = { workspace = true }
# core = { path = "../core" }  # local path dependency

# Cargo.toml
[profile.dev]
opt-level = 0        # no optimization (fast compile)
debug = true         # include debug symbols
overflow-checks = true

[profile.release]
opt-level = 3        # max optimization
lto = "fat"          # link-time optimization across crates
codegen-units = 1    # single codegen unit (better opt, slower compile)
strip = true         # strip debug symbols from binary
panic = "abort"      # smaller binary, no unwinding

[profile.release.package."*"]
opt-level = 2  # optimize dependencies less than your code

# Custom profile
[profile.bench]
inherits = "release"
debug = true  # keep symbols for profiling

# Usage: cargo build --release --profile bench

# Before publishing:
# 1. Login (one-time)
# $ cargo login <token>  (token from crates.io)

# 2. Check the package
# $ cargo package        # creates .crate file, checks metadata
# $ cargo publish --dry-run

# 3. Publish
# $ cargo publish        # uploads to crates.io (irreversible!)

# Required fields in Cargo.toml for publishing:
# name, version, description, license, repository

# Documentation:
# $ cargo doc            # generate docs for your crate + deps
# $ cargo doc --open     # generate and open in browser
# $ cargo doc --no-deps  # only your crate

# Doc tests run automatically:
/// Adds two numbers.
/// 
/// # Examples
/// ```
/// let result = mycrate::add(2, 3);
/// assert_eq!(result, 5);
/// ```
pub fn add(a: i32, b: i32) -> i32 { a + b }

trait Draw {
    fn draw(&self);
}

struct Circle { radius: f64 }
struct Square { side: f64 }

impl Draw for Circle {
    fn draw(&self) { println!("Circle r={}", self.radius); }
}
impl Draw for Square {
    fn draw(&self) { println!("Square s={}", self.side); }
}

// Trait object: type-erased, dynamic dispatch
let shapes: Vec<Box<dyn Draw>> = vec![
    Box::new(Circle { radius: 1.0 }),
    Box::new(Square { side: 2.0 }),
];
for s in &shapes { s.draw(); }

// Function taking trait object
fn render(shape: &dyn Draw) { shape.draw(); }

// Object-safe trait (can be made into dyn)
trait Animal {
    fn sound(&self) -> String;
    fn name(&self) -> &str;
}

// NOT object-safe: returns Self
trait Clone {
    fn clone(&self) -> Self;  // Self is unknown for dyn
}

// NOT object-safe: takes Self by value
trait Add {
    fn add(&self, other: Self) -> Self;
}

// NOT object-safe: generic method
trait From {
    fn from<T>(t: T) -> Self;
}

// Fix: use where clauses or separate traits
trait AnimalSafe {
    fn sound(&self) -> String;
    fn box_clone(&self) -> Box<dyn AnimalSafe>;
}

// Static dispatch (monomorphization)
fn max<T: Ord>(a: T, b: T) -> T {
    if a > b { a } else { b }
}
// Compiler generates max_i32, max_f64, etc.

// Dynamic dispatch (vtable lookup)
fn max_dyn(a: &dyn Ord, b: &dyn Ord) -> bool {
    // a > b  // Cannot use operators on dyn
    false
}

// Trait bound (static)
fn process<T: Display>(item: &T) {
    println!("{}", item);
}

// impl Trait (static, syntactic sugar)
fn process2(item: &impl Display) {
    println!("{}", item);
}

// dyn Trait (dynamic)
fn process3(item: &dyn Display) {
    println!("{}", item);
}

trait Parser {
    fn parse(&self, input: &str) -> &str;
}

// Trait object with lifetime
fn make_parser() -> Box<dyn Parser> {
    Box::new(MyParser)
}

// Trait object holding references
struct Runner<'a> {
    parsers: Vec<Box<dyn Parser + 'a>>,
}

impl<'a> Runner<'a> {
    fn add(&mut self, p: Box<dyn Parser + 'a>) {
        self.parsers.push(p);
    }
}

// dyn Trait defaults to 'static when no lifetime given
fn static_parser() -> Box<dyn Parser> {
    Box::new(MyParser)
}

use std::any::Any;

// Any enables runtime type identification
let x: Box<dyn Any> = Box::new(42_i32);

// Downcast to concrete type
if let Some(n) = x.downcast_ref::<i32>() {
    println!("Got i32: {}", n);
}

// Store heterogeneous values
let mut bag: Vec<Box<dyn Any>> = vec![
    Box::new(42_i32),
    Box::new("hello".to_string()),
    Box::new(3.14_f64),
];

for item in &bag {
    if let Some(s) = item.downcast_ref::<String>() {
        println!("String: {}", s);
    } else if let Some(n) = item.downcast_ref::<i32>() {
        println!("i32: {}", n);
    }
}

macro_rules! vec_of {
    ( $( $x:expr ),* ) => {
        {
            let mut v = Vec::new();
            $(
                v.push($x);
            )*
            v
        }
    };
}

let nums = vec_of!(1, 2, 3, 4);
let strs = vec_of!("a", "b", "c");

// Multiple patterns
macro_rules! greet {
    () => { println!("Hello!") };
    ($name:expr) => { println!("Hello, {}!", $name) };
    ($name:expr, $greeting:expr) => {
        println!("{}, {}!", $greeting, $name)
    };
}
greet!();
greet!("Alice");
greet!("Bob", "Hi");

macro_rules! items {
    // $x:expr - expression (1 + 2, foo())
    // $x:stmt - statement (let x = 5;)
    // $x:ty - type (Vec<i32>, &str)
    // $x:pat - pattern (Some(x), (a, b))
    // $x:path - path (std::vec::Vec, Module::Type)
    // $x:ident - identifier (foo, Bar)
    // $x:literal - literal (42, "hello")
    // $x:tt - token tree (anything)
    // $x:block - block ({ ... })
    // $x:item - item (fn, struct)

    ($e:expr, $t:ty, $i:ident) => {
        let $i: $t = $e;
    };
}

items!(42, i32, my_num);
items!("hi", &str, greeting);

macro_rules! sum {
    // Zero or more: $(...)*
    ( $( $x:expr ),* ) => {
        {
            let mut total = 0;
            $(
                total += $x;
            )*
            total
        }
    };

    // One or more: $(...)+
    ( first $(, $x:expr )+ ) => {
        println!("First and more");
    };

    // With separator and count
    ( $( $x:expr ),+ $(; $sep:expr )? ) => {
        println!("List with optional separator");
    };
}

// Nested repetition
macro_rules! matrix {
    ( $( [ $( $x:expr ),* ] ),* ) => {
        vec![ vec![ $( $x ),* ],* ]
    };
}

// Export at crate root
#[macro_export]
macro_rules! log {
    ($($arg:tt)*) => {
        // Hygienic: this T does not collide with caller's T
        let _ts: std::time::Instant = std::time::Instant::now();
        eprintln!("[{}] {}", "LOG", format!($($arg)*));
    };
}

// Use $crate to refer to crate items (works after re-export)
#[macro_export]
macro_rules! make_error {
    ($msg:expr) => {
        $crate::Error::new($msg)
    };
}

// Internal helper rule (convention: @ prefix)
macro_rules! count {
    (@count $x:expr, $($rest:expr),*) => { 1 + count!(@count $($rest),*) };
    (@count $x:expr) => { 1 };
    () => { 0 };
}

// 1. Builder DSL
macro_rules! html {
    ($tag:ident { $($body:tt)* }) => {
        format!("<{}>{}</{}>", stringify!($tag), html_inner!($($body)*), stringify!($tag))
    };
}

// 2. Test assertion
macro_rules! assert_approx {
    ($a:expr, $b:expr, $eps:expr) => {
        assert!(($a - $b).abs() < $eps, "{} != {} within {}", $a, $b, $eps);
    };
}

// 3. Configuration
macro_rules! config {
    ( $( $key:ident : $val:expr ),* ) => {
        {
            let mut m = std::collections::HashMap::new();
            $(
                m.insert(stringify!($key).to_string(), $val.to_string());
            )*
            m
        }
    };
}

let cfg = config! { host: "localhost", port: 8080 };

# Root Cargo.toml
[workspace]
members = ["crates/*", "cli", "server"]
resolver = "2"

[workspace.package]
version = "0.1.0"
edition = "2021"
authors = ["Team <[email protected]>"]
license = "MIT"

[workspace.dependencies]
serde = { version = "1.0", features = ["derive"] }
tokio = { version = "1", features = ["full"] }
anyhow = "1.0"

# Member crate: crates/mylib/Cargo.toml
[package]
name = "mylib"
version.workspace = true
edition.workspace = true

[dependencies]
serde.workspace = true
tokio.workspace = true

# Cargo.toml
[profile.dev]
opt-level = 0        # No optimization (fast compile)
debug = true         # Include debug symbols
overflow-checks = true

[profile.release]
opt-level = 3        # Max optimization
debug = false
lto = "fat"          # Link-time optimization
codegen-units = 1    # Single unit (better opt, slower compile)
panic = "abort"      # Smaller binary, no unwinding
strip = true         # Strip symbols

[profile.dev.package."*"]
opt-level = 2        # Optimize dependencies in dev

# Custom profile
[profile.bench]
inherits = "release"
debug = true

# Usage: cargo build --release, cargo build --profile=bench

# Cargo.toml
[features]
default = ["json"]
json = ["serde_json"]
yaml = ["serde_yaml"]
async-runtime = ["tokio"]

[dependencies]
serde = { version = "1.0", optional = true }
serde_json = { version = "1.0", optional = true }
serde_yaml = { version = "0.9", optional = true }
tokio = { version = "1", optional = true, features = ["full"] }

# Code: conditional compilation
#[cfg(feature = "json")]
pub fn parse_json(s: &str) -> Result<Value, Error> {
    serde_json::from_str(s)
}

#[cfg(not(feature = "json"))]
pub fn parse_json(_: &str) -> Result<Value, Error> {
    Err(Error::FeatureNotEnabled)
}

// build.rs: runs before compilation
use std::env;
use std::fs;
use std::path::Path;

fn main() {
    // Tell Cargo to rerun if env changes
    println!("cargo:rerun-if-env-changed=DATABASE_URL");

    let out_dir = env::var("OUT_DIR").unwrap();
    let dest = Path::new(&out_dir).join("config.rs");

    let db_url = env::var("DATABASE_URL")
        .unwrap_or_else(|_| "sqlite://default.db".to_string());

    // Generate Rust code at build time
    fs::write(&dest, format!(
        "pub const DATABASE_URL: &str = \"{}\";",
        db_url
    )).unwrap();

    // Link a C library
    println!("cargo:rustc-link-lib=static=mylib");
    println!("cargo:rustc-link-search=native=/usr/local/lib");
}

// In code: include generated file
include!(concat!(env!("OUT_DIR"), "/config.rs"));

# Publish to crates.io
cargo login <token>     # One-time authentication
cargo publish           # Publish current crate

# Before publishing:
cargo publish --dry-run # Verify package contents
cargo package           # Inspect the .crate file

# Documentation
cargo doc               # Generate docs
cargo doc --open        # Generate and open
cargo doc --no-deps     # Only this crate

# In code: doc comments
/// Adds two numbers.
///
/// # Examples
/// ```
/// let result = mycrate::add(2, 3);
/// assert_eq!(result, 5);
/// ```
pub fn add(a: i32, b: i32) -> i32 { a + b }

# README and metadata in Cargo.toml
[package]
description = "A short description"
repository = "https://github.com/user/repo"
readme = "README.md"
keywords = ["parser", "cli"]
categories = ["command-line-utilities"]

// Three types of procedural macros:
// 1. Function-like: my_macro!(...)
// 2. Derive: #[derive(MyMacro)]
// 3. Attribute: #[my_macro]

// Cargo.toml for a proc-macro crate
// [lib]
// proc-macro = true

// [dependencies]
// syn = { version = "2", features = ["full"] }
// quote = "1"
// proc-macro2 = "1"

use proc_macro::TokenStream;

#[proc_macro]
pub fn make_answer(_item: TokenStream) -> TokenStream {
    "fn answer() -> i32 { 42 }".parse().unwrap()
}

// Usage: make_answer!();
// Generates: fn answer() -> i32 { 42 }

use proc_macro::TokenStream;
use quote::quote;
use syn::{parse_macro_input, DeriveInput};

#[proc_macro_derive(HelloMacro)]
pub fn hello_macro_derive(input: TokenStream) -> TokenStream {
    let ast = parse_macro_input!(input as DeriveInput);
    let name = &ast.ident;

    let expanded = quote! {
        impl HelloMacro for #name {
            fn hello() {
                println!("Hello from {}!", stringify!(#name));
            }
        }
    };

    expanded.into()
}

// Usage:
// #[derive(HelloMacro)]
// struct Pancakes;
// Pancakes::hello();  // "Hello from Pancakes!"

// Helper attributes
#[proc_macro_derive(HelloMacro, attributes(hello))]
pub fn hello_with_attr(input: TokenStream) -> TokenStream { /* ... */ }

use proc_macro::TokenStream;
use quote::quote;
use syn::{parse_macro_input, ItemFn};

#[proc_macro_attribute]
pub fn log_calls(attr: TokenStream, item: TokenStream) -> TokenStream {
    let attr_args = syn::parse_macro_input!(attr as syn::AttributeArgs);
    let input_fn = parse_macro_input!(item as ItemFn);

    let fn_name = &input_fn.sig.ident;
    let fn_block = &input_fn.block;

    let expanded = quote! {
        fn #fn_name() {
            println!("Calling {}", stringify!(#fn_name));
            let __result = (|| #fn_block)();
            println!("Finished {}", stringify!(#fn_name));
            __result
        }
    };

    expanded.into()
}

// Usage:
// #[log_calls]
// fn my_function() -> i32 { 42 }

use proc_macro::TokenStream;
use quote::quote;
use syn::{parse::Parse, parse::ParseStream, parse_macro_input};

// Custom syntax: sql!(SELECT * FROM users WHERE id = $1)
struct SqlQuery {
    query: String,
}

impl Parse for SqlQuery {
    fn parse(input: ParseStream) -> syn::Result<Self> {
        let query = input.to_string();
        Ok(SqlQuery { query })
    }
}

#[proc_macro]
pub fn sql(input: TokenStream) -> TokenStream {
    let SqlQuery { query } = parse_macro_input!(input as SqlQuery);

    let expanded = quote! {
        {
            static QUERY: &str = #query;
            // Compile-time SQL validation could go here
            QUERY
        }
    };

    expanded.into()
}

// Cargo.toml
// [dev-dependencies]
// trybuild = "1"

// tests/ui/my_macro.rs - test file
// #[derive(MyMacro)]
// struct Foo;
// fn main() { Foo::hello(); }

// tests/ui/my_macro.stderr - expected error
// error: ...

// Test runner
#[test]
fn ui() {
    let t = trybuild::TestCases::new();
    t.pass("tests/ui/pass_*.rs");
    t.compile_fail("tests/ui/fail_*.rs");
}

// Debugging with eprintln
#[proc_macro_derive(Debug)]
pub fn debug_derive(input: TokenStream) -> TokenStream {
    eprintln!("Input tokens: {}", input);
    let ast = syn::parse2(input.clone().into()).unwrap();
    eprintln!("Parsed AST: {:#?}", ast);
    TokenStream::new()
}

macro_rules! vec_of {
    ($($x:expr),*) => {{
        let mut v = Vec::new();
        $(v.push($x);)*
        v
    }};
}
let nums = vec_of!(1, 2, 3);

// In a separate crate with proc-macro = true
use proc_macro::TokenStream;
#[proc_macro_derive(HelloMacro)]
pub fn hello_macro_derive(input: TokenStream) -> TokenStream {
    // Generate impl HelloMacro for the type
    // Returns new TokenStream
}

println!("Hello, {}!", "world");
format!("x = {}", 42);
vec![1, 2, 3];
assert!(1 + 1 == 2);
assert_eq!(2 + 2, 4);
dbg!(some_variable);  // Debug print
todo!("Not implemented");
unreachable!();

macro_rules! swap {
    ($a:expr, $b:expr) => {
        let temp = $a;
        $a = $b;
        $b = temp;
    };
}
// temp is hygienic: does not conflict with outer temp
let mut temp = 1;
let mut x = 2;
swap!(temp, x);  // Works correctly

macro_rules! sum {
    ($($x:expr),*) => {
        0 $(+ $x)*
    };
}
let total = sum!(1, 2, 3, 4);  // 10
// $(...)* zero or more, $(...)+ one or more
// $(...),? optional trailing comma

async fn fetch_data() -> String {
    // Simulate async work
    String::from("data")
}
async fn process() {
    let data = fetch_data().await;
    println!("{}", data);
}

#[tokio::main]
async fn main() {
    let task1 = tokio::spawn(async { work1().await });
    let task2 = tokio::spawn(async { work2().await });
    let (r1, r2) = tokio::join!(task1, task2);
}

use tokio::sync::mpsc;
#[tokio::main]
async fn main() {
    let (tx, mut rx) = mpsc::channel(32);
    tokio::spawn(async move {
        tx.send("hello").await.unwrap();
    });
    while let Some(msg) = rx.recv().await {
        println!("{}", msg);
    }
}

tokio::select! {
    result = task1 => {
        println!("Task1 done: {:?}", result);
    }
    result = task2 => {
        println!("Task2 done: {:?}", result);
    }
    _ = tokio::time::sleep(Duration::from_secs(5)) => {
        println!("Timeout");
    }
}

use tokio_stream::{self as stream, StreamExt};
let mut stream = stream::iter(vec![1, 2, 3]);
while let Some(item) = stream.next().await {
    println!("{}", item);
}
// Map, filter like iterators but async
stream.map(|x| x * 2).filter(|x| *x > 2).for_each(|x| async move {
    println!("{}", x);
}).await;

[package]
name = "myapp"
version = "0.1.0"
edition = "2021"

[dependencies]
serde = { version = "1.0", features = ["derive"] }
tokio = { version = "1", features = ["full"] }

[dev-dependencies]
pretty_assertions = "1"

cargo new myapp        # Create new project
cargo build            # Compile
cargo build --release  # Optimized build
cargo run              # Build and run
cargo test             # Run tests
cargo check            # Fast type-check
cargo fmt              # Format code
cargo clippy           # Lint
cargo doc --open       # Generate docs
cargo add serde        # Add dependency

# Root Cargo.toml
[workspace]
members = ["crate-a", "crate-b"]

# Shared dependencies
[workspace.dependencies]
serde = "1.0"

# In crate-a/Cargo.toml
[dependencies]
serde = { workspace = true }

# Cargo.toml
[features]
default = ["csv"]
csv = ["dep:csv-parse"]
json = ["dep:serde_json"]

# Conditional compilation
#[cfg(feature = "csv")]
pub fn parse_csv() { /* ... */ }

# Login (one time)
cargo login <token>

# Check before publishing
cargo publish --dry-run

# Publish to crates.io
cargo publish

# Version bumping
cargo bump patch  # 0.1.0 -> 0.1.1
cargo bump minor  # 0.1.0 -> 0.2.0
cargo bump major  # 0.1.0 -> 1.0.0

pub fn add(a: i32, b: i32) -> i32 { a + b }

#[cfg(test)]
mod tests {
    use super::*;
    #[test]
    fn test_add() {
        assert_eq!(add(2, 3), 5);
    }
    #[test]
    #[should_panic]
    fn test_panic() {
        panic!("expected");
    }
}

// tests/integration_test.rs
use myapp::add;

#[test]
fn test_add_integration() {
    assert_eq!(add(2, 3), 5);
}
// Run: cargo test --test integration_test
// Each file in tests/ is a separate crate

#[test]
fn it_works() { /* ... */ }

// Custom test harness
#[test]
#[ignore = "slow test"]
fn slow_test() { /* ... */ }

// Run only ignored tests
cargo test -- --ignored

// Filter by name
cargo test test_add

// Show output
cargo test -- --nocapture

assert!(true);                          // Boolean
assert_eq!(2 + 2, 4);                   // Equality
assert_ne!(3, 4);                       // Inequality
assert!(x > 0, "x must be positive");   // Custom message
assert_eq!(a, b, "got {}, expected {}", a, b);
// Debug output on failure
assert_eq!(vec![1, 2], vec![1, 2]);

// Cargo.toml: proptest = "1"
use proptest::prelude::*;

proptest! {
    #[test]
    fn test_add_commutative(a in -1000..1000, b in -1000..1000) {
        prop_assert_eq!(add(a, b), add(b, a));
    }
    #[test]
    fn test_string_len(s in ".{0,100}") {
        prop_assert!(s.len() <= 100);
    }
}