#[error("Database error")]
Database(#[from] sqlx::Error),

#[error("IO error")]
Io(#[from] std::io::Error),

}


### Propagation with ?
```rust
fn read_config(path: &str) -> Result<Config, AppError> {
    let content = std::fs::read_to_string(path)?;  // Auto-converts io::Error
    let config: Config = serde_json::from_str(&content)
        .map_err(|e| AppError::NotFound(format!("Invalid config: {e}")))?;
    Ok(config)
}

Option Combinators

fn find_user(id: &str) -> Option<User> {
    users.iter()
        .find(|u| u.id == id)
        .filter(|u| u.is_active)
        .map(|u| u.clone())
}

// Chain with unwrap_or, unwrap_or_else, unwrap_or_default
let name = user.name.as_deref().unwrap_or("Anonymous");

Ownership and Borrowing

Prefer Borrowing

// Good: Borrows data, doesn't take ownership
fn process(data: &[u8]) -> Result<Output, Error> { /* ... */ }

// Good: Mutable borrow when needed
fn update(config: &mut Config) { /* ... */ }

// Take ownership only when necessary (storing, sending to thread)
fn spawn_worker(data: Vec<u8>) { /* ... */ }

Builder Pattern

pub struct ServerBuilder {
    addr: String,
    port: u16,
    max_connections: Option<usize>,
}

impl ServerBuilder {
    pub fn new(addr: impl Into<String>) -> Self {
        Self {
            addr: addr.into(),
            port: 8080,
            max_connections: None,
        }
    }

    pub fn port(mut self, port: u16) -> Self {
        self.port = port;
        self
    }

    pub fn max_connections(mut self, max: usize) -> Self {
        self.max_connections = Some(max);
        self
    }

    pub fn build(self) -> Server {
        Server { /* ... */ }
    }
}

// Usage: Server::builder("localhost").port(3000).build()

Traits

Define Behavior, Not Data

pub trait Repository {
    type Error;

    fn find_by_id(&self, id: &str) -> Result<Option<User>, Self::Error>;
    fn save(&self, user: &User) -> Result<(), Self::Error>;
    fn delete(&self, id: &str) -> Result<bool, Self::Error>;
}

Blanket Implementations

// Implement for all types that satisfy bounds
impl<T: Display + Debug> Loggable for T {
    fn log(&self) {
        println!("[LOG] {self}");
    }
}

Async Rust

Tokio Patterns

use tokio::task::JoinSet;

async fn fetch_all(urls: Vec<String>) -> Vec<Result<Response, Error>> {
    let mut set = JoinSet::new();

    for url in urls {
        set.spawn(async move {
            reqwest::get(&url).await?.text().await
        });
    }

    let mut results = Vec::new();
    while let Some(result) = set.join_next().await {
        results.push(result.unwrap());
    }
    results
}

Graceful Shutdown

use tokio::signal;

async fn run_server() -> Result<(), Error> {
    let server = start_server().await?;

    signal::ctrl_c().await?;
    println!("Shutting down...");

    server.shutdown().await;
    Ok(())
}

Tooling

Tool	Purpose	Command
cargo check	Type checking	`cargo check`
cargo clippy	Linting	`cargo clippy -- -D warnings`
cargo fmt	Formatting	`cargo fmt -- --check`
cargo test	Testing	`cargo test`
cargo audit	Security	`cargo audit`

Checklist

clippy passes with no warnings
Error types defined with thiserror
? operator for error propagation
Prefer borrowing over ownership transfer
Clone only when necessary
Send + Sync bounds for async types
No unwrap() in library code (use ? or expect)
Builder pattern for types with many options

rust-patterns

Rust Patterns

Error Handling

Custom Error Types with thiserror