Ownership and Borrowing

Ownership is Rust's most distinctive feature and the key to its memory safety guarantees. Understanding ownership, moves, references, and borrowing is essential for writing Rust programs.

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The Ownership Rules

Rust enforces three ownership rules at compile time:

1. Each value has exactly one owner (a variable)
2. There can only be one owner at a time
3. When the owner goes out of scope, the value is dropped (freed)

{
    let s = String::from("hello"); // s owns the String
    // s is valid here
} // s goes out of scope — the String is dropped (freed)

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Moves

When you assign a heap-allocated value to another variable, ownership moves:

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println!("{s1}"); // ERROR: s1 is no longer valid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

This prevents double free errors — only one variable can free the memory.

Copy Types

Types that are small and live on the stack implement the Copy trait. Assignment copies instead of moves:

let x = 5;
let y = x; // x is COPIED, not moved