Lesson 08 of 20

6 mins read

References & Borrowing (&T)

In Plain English

Borrowing lets you create references (&T) to data without taking ownership. Because the referencing function does not own the data, the original variable remains valid and no heap reallocation occurs.

The Mental Model

"Borrowing is like showing someone a photo on your phone: they can look at it (read reference), but you still keep the phone (retain ownership)."

Deep Dive: How It Works

The & Symbol: Indicates a reference that borrows access without moving ownership.

Immutability of References: By default, references (&T) are read-only.

Unlimited Readers: You can have any number of simultaneous immutable references to a value.

Syntax Blueprint

fn calculate_len(s: &String) -> usize { s.len() } // Borrowed, not moved

Passing &s borrows the string. The caller retains ownership after the function returns.

Core Rules to Remember

Borrowing: Access data via & reference without moving ownership.

No Heap Overhead: References are just pointers under the hood with zero runtime cost.

Referential Safety: Rust guarantees references always point to valid memory (no dangling pointers).

Common Beginner Traps & How to Fix Them

Mistake:Trying to modify data through an immutable reference.

Why it happens: &T references are strictly read-only.

How to fix: Use a mutable reference (&mut T) if mutation is required.

Pro Tip: Prefer passing &str instead of &String to functions to accept both string literals and heap strings.

Live Interactive Example

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Borrowing without Moving

Rust 1.78 / rustc

Output Console

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How it works: The message variable remains fully accessible after get_length because it was borrowed.

Your Turn: Micro Challenge

No pressure! Edit the starter code below and test your solution with instant feedback.

Micro Exercise

Borrow a string and print length

Write fn print_info(title: &String) that prints "Title: {title}".
In main(), let book = String::from("Rust Manifesto");
Call print_info(&book) and then print "Owner still has: Rust Manifesto".

Sandbox Output

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