Rust Ownership Borrowing

Rust Ownership and Borrowing

Understanding Rust's ownership and borrowing system — from owners and references through lifetimes, borrowing rules, and common patterns.

When to Use

• Learning Rust's ownership model
• Writing safe Rust without fighting the borrow checker
• Managing lifetimes in complex data structures
• Implementing safe abstractions

Ownership Patterns

// Ownership transfer
fn take_ownership(s: String) -> usize { s.len() }

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

// Mutable borrow
fn append(s: &mut String, text: &str) { s.push_str(text); }

// Lifetime annotations
struct Config<'a> {
    name: &'a str,
    version: &'a str,
}

impl<'a> Config<'a> {
    fn new(name: &'a str, version: &'a str) -> Self {
        Config { name, version }
    }
}

// Lifetime elision rules
fn first_word(s: &str) -> &str {
    s.split_whitespace().next().unwrap_or(s)
}

Verification Checklist

• Each value has exactly one owner
• References never outlive borrowed data
• Mutable references are exclusive (no aliasing)
• Lifetime annotations only where needed (elision rules)
• Interior mutability (RefCell, Mutex) for shared mutation
• Clone vs Copy semantics understood