Rust Core Knowledge

Full Reference: See advanced.md for builder pattern, advanced traits, async patterns (join!/select!/spawn), custom errors with thiserror, and production configuration.

Deep Knowledge: Use mcp__documentation__fetch_docs with technology: rust for comprehensive documentation.

Ownership System

Rust's memory safety is guaranteed through its ownership system without a garbage collector.

The Three Ownership Rules

1. Each value in Rust has an owner
2. There can only be one owner at a time
3. When the owner goes out of scope, the value will be dropped

Move Semantics (Heap Data)

fn main() {
    let s1 = String::from("hello");
    let s2 = s1;  // s1 is MOVED to s2

    // println!("{s1}");  // ERROR: s1 is no longer valid
    println!("{s2}");     // OK: s2 owns the data
}

Copy Semantics (Stack Data)

fn main() {
    let x = 5;
    let y = x;  // x is COPIED (not moved)

    println!("x = {x}, y = {y}");  // Both valid
}

Types implementing Copy trait:

• All integer types (u32, i64, etc.)
• Boolean (bool)
• Floating-point types (f64)
• Character (char)
• Tuples of Copy types

Clone for Deep Copying

fn main() {
    let s1 = String::from("hello");
    let s2 = s1.clone();  // Explicit deep copy

    println!("s1 = {s1}, s2 = {s2}");  // Both valid
}

Borrowing and References

Immutable References

fn main() {
    let s1 = String::from("hello");
    let len = calculate_length(&s1);  // Borrow s1
    println!("Length of '{s1}' is {len}");  // s1 still valid
}

fn calculate_length(s: &String) -> usize {
    s.len()
}  // s goes out of scope but doesn't drop data (just a reference)

Mutable References

fn main() {
    let mut s = String::from("hello");
    change(&mut s);
    println!("{s}");  // Prints "hello, world"
}

fn change(s: &mut String) {
    s.push_str(", world");
}

Borrowing Rules

1. You can have either:
   • One mutable reference, OR
   • Any number of immutable references
2. References must always be valid (no dangling references)

fn main() {
    let mut s = String::from("hello");

    let r1 = &s;      // OK: first immutable borrow
    let r2 = &s;      // OK: second immutable borrow
    println!("{r1} and {r2}");
    // r1 and r2 no longer used after this point

    let r3 = &mut s;  // OK: mutable borrow (after immutables are done)
    println!("{r3}");
}

Lifetimes

Lifetimes ensure references are valid for as long as they're used.

Lifetime Annotations

fn longest<'a>(x: &'a str, y: &'a str) -> &'a str {
    if x.len() > y.len() { x } else { y }
}

fn main() {
    let string1 = String::from("long string");
    let result;
    {
        let string2 = String::from("xyz");
        result = longest(string1.as_str(), string2.as_str());
        println!("Longest: {result}");  // Must use result here
    }
    // Can't use result here - string2 dropped
}

Lifetime Elision Rules

The compiler applies these rules automatically:

1. Each input reference gets its own lifetime
2. If exactly one input lifetime, output gets that lifetime
3. If &self or &mut self, output gets self's lifetime

Static Lifetime

let s: &'static str = "I live forever";

Error Handling

Result Type

use std::fs::File;
use std::io::{self, Read};

fn read_username_from_file() -> Result<String, io::Error> {
    let mut file = File::open("hello.txt")?;  // ? propagates errors
    let mut username = String::new();
    file.read_to_string(&mut username)?;
    Ok(username)
}

// Even shorter with fs::read_to_string
fn read_username_shortest() -> Result<String, io::Error> {
    std::fs::read_to_string("hello.txt")
}

Option Type

fn find_user(id: u32) -> Option<User> {
    users.iter().find(|u| u.id == id).cloned()
}

// Using Option
match find_user(42) {
    Some(user) => println!("Found: {}", user.name),
    None => println!("User not found"),
}

// Combinators
let name = find_user(42)
    .map(|u| u.name)
    .unwrap_or_else(|| "Anonymous".to_string());

Async/Await

Basic Async Function

async fn fetch_data(url: &str) -> Result<String, reqwest::Error> {
    let response = reqwest::get(url).await?;
    let body = response.text().await?;
    Ok(body)
}

#[tokio::main]
async fn main() {
    match fetch_data("https://api.example.com").await {
        Ok(data) => println!("Got: {data}"),
        Err(e) => eprintln!("Error: {e}"),
    }
}

Structs and Implementations

Struct Definition

#[derive(Debug, Clone)]
pub struct User {
    pub id: u64,
    pub name: String,
    pub email: String,
    active: bool,  // private field
}

impl User {
    // Associated function (constructor)
    pub fn new(id: u64, name: String, email: String) -> Self {
        Self {
            id,
            name,
            email,
            active: true,
        }
    }

    // Method with immutable borrow
    pub fn is_active(&self) -> bool {
        self.active
    }

    // Method with mutable borrow
    pub fn deactivate(&mut self) {
        self.active = false;
    }
}

Traits

Trait Definition and Implementation

pub trait Summary {
    fn summarize(&self) -> String;

    // Default implementation
    fn summarize_author(&self) -> String {
        String::from("Unknown author")
    }
}

impl Summary for Article {
    fn summarize(&self) -> String {
        format!("{}, by {}", self.headline, self.author)
    }
}

Common Derive Traits

#[derive(Debug, Clone, PartialEq, Eq, Hash, Default, Serialize, Deserialize)]
pub struct Config {
    pub host: String,
    pub port: u16,
}

Cargo Essentials

Project Structure

my_project/
├── Cargo.toml
├── Cargo.lock
├── src/
│   ├── main.rs      # Binary entry point
│   ├── lib.rs       # Library root
│   └── module/      # Module directory
│       └── mod.rs
├── tests/           # Integration tests
├── benches/         # Benchmarks
└── examples/        # Example programs

Cargo.toml

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

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

[dev-dependencies]
tokio-test = "0.4"

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

Common Commands

cargo new my_project      # Create new project
cargo build               # Build debug
cargo build --release     # Build optimized
cargo run                 # Build and run
cargo test                # Run tests
cargo doc --open          # Generate docs
cargo clippy              # Lint code
cargo fmt                 # Format code
cargo check               # Fast type checking

When NOT to Use This Skill

Anti-Patterns

Quick Troubleshooting

Reference Documentation

• Ownership
• Async Patterns
• Error Handling

Deep Knowledge: Use mcp__documentation__fetch_docs with technology: rust for comprehensive documentation.