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    // println!("{s1}");  // ERROR: s1 is no longer valid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

Scenario	Use Instead
Actix/Axum framework specifics	Framework-specific skills
Tauri desktop apps	tauri skill
WebAssembly compilation	WASM-specific skill
Diesel ORM	ORM-specific skill
Rocket web framework	Framework-specific skill

Anti-Patterns

Anti-Pattern	Why It's Bad	Correct Approach
.clone() everywhere	Unnecessary allocations	Use references or Arc
.unwrap() in production	Panics on error	Use proper error handling
Large match statements	Hard to maintain	Use methods or polymorphism
Ignoring Clippy warnings	Misses best practices	Fix all warnings
Not using ? operator	Verbose error handling	Use ? for propagation
Mutex for everything	Lock contention	Use channels or RwLock
Blocking in async	Blocks executor	Use spawn_blocking
Manual memory management	Defeats ownership	Trust the compiler

Quick Troubleshooting

Issue	Cause	Solution
"cannot borrow as mutable"	Multiple borrows	Drop immutable refs first
"move occurs because X has type Y"	Ownership moved	Clone or use references
"lifetime may not live long enough"	Lifetime mismatch	Add lifetime annotations
"trait X is not implemented"	Missing trait impl	Implement trait or derive
"cannot return reference to local"	Dangling reference	Return owned value
"cyclic dependency detected"	Circular deps	Refactor dependency tree
Slow compilation	Too many dependencies	Use cargo workspaces
Memory leak with Arc	Circular references	Use Weak references

Reference Documentation

• Ownership
• Async Patterns
• Error Handling

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