Borrowing in Rust: How It Works Under the Hood and Why It Matters

This content originally appeared on Level Up Coding - Medium and was authored by Juan Andrés Leiva

Rust Borrowing Explained: Ownership, Pointers, and Memory Safety

Introduction

Picture this: You’re debugging a C++ application that randomly crashes in production. After hours of investigation, you discover a use-after-free bug: a pointer accessing memory that was already deallocated. Sound familiar?

Rust’s borrowing system eliminates this entire class of bugs at compile time, without the runtime overhead of garbage collection. By the end of this article, you’ll understand how Rust achieves memory safety through its ownership model and why major companies are migrating critical systems to Rust.

Rust’s ownership model guarantees memory safety without garbage collection. A central part of this system is borrowing, meaning letting code use values without taking ownership. To really understand why this matters, let’s look at what’s happening in memory and how the Rust borrow checker enforces these rules.

What Is a Pointer in Rust?

At the hardware level, memory is just bytes. A pointer is a value that stores the address of some data in memory (stored in the heap).

The stack and heap are two different areas where your computer stores data while programs run. The stack is a small, fast memory region that works like a pile; data gets added to the top and removed from the top in strict order. The heap is a larger, more flexible memory area where programs can store data of any size anywhere there’s available space. However, accessing heap data is slower because the program has to follow a pointer (an address) to find where the data actually lives.

• If a variable lives on the stack, its address points to a fixed region tied to the function call.

• If it lives on the heap (like String or Vec), the pointer on the stack points to a dynamically allocated range within the heap.

Dangling pointers (pointing to freed memory) or multiple writers to the same data are classic causes of memory corruption in languages like C or C++.

Borrowing in Rust

In Rust, every value has exactly one owner. When you want to let other code access it without moving it, you borrow:

• &T → immutable reference (many readers, no writers)

•&mut T → mutable reference (one writer, no readers)

The Rust borrow checker tracks lifetimes and enforces these rules at compile time, making Rust’s ownership model both safe and efficient.

Rust Immutable Borrow Example

fn main() {
    let s = String::from("hello");
    print_length(&s); // borrow
    println!("Back in main: {}", s);
}

fn print_length(text: &String) {
    println!("Length: {}", text.len());
}

What’s happening in memory?

• s lives on the heap, but the stack holds a pointer to it.

• Passing &s creates another pointer, not a copy of the string, just a pointer to the same buffer.

• The compiler (via the borrow checker) ensures that while print_length is using &s, no one else is mutating s.

Rust Mutable Borrow Example

fn main() {
    let mut s = String::from("hello");
    change(&mut s);
    println!("After change: {}", s);
}

fn change(text: &mut String) {
    text.push_str(", world");
}

What’s happening in memory?

&mut s is just a pointer, but the compiler guarantees exclusive mutable access.
While change holds &mut s, no other borrows (even immutable) are allowed.
This prevents read/write data races and undefined behavior.

Why Rust Borrowing Matters for Modern Development

Rust’s borrowing system represents a fundamental shift in how we think about memory management.

Performance Without Sacrifice: Unlike garbage-collected languages, borrowing gives you zero-cost abstractions. Your references are just pointers with compile-time guarantees — no runtime overhead, no unpredictable pause times.

Fearless Concurrency: The same rules that prevent memory corruption also prevent data races. You can write concurrent code with confidence, knowing that if it compiles, it’s thread-safe.

Maintainable Systems: Borrowing rules encode important architectural decisions directly into your type system. When you see &mut self in a method signature, you immediately know it modifies the object. This self-documenting nature makes codebases easier to understand and maintain.

Industry Adoption: Companies aren’t migrating to Rust just for performance; they’re doing it to eliminate entire classes of bugs. Microsoft reports that 70% of their security vulnerabilities are memory safety issues. Rust eliminates these at compile time.

That’s why the Rust ownership model and borrow checker provide low-level control without classic memory-safety pitfalls.

✅ Borrowing looks strict, but it’s really a compile-time contract: if the code compiles, you can be sure that your pointers are always valid and race-free.