Memory Management in Rust

4 min readJan 31, 2025

Memory in computing systems is a finite resource, and engineers must manage it effectively to build robust and sustainable systems. Different programming languages adopt various approaches to memory management. Languages like C leave full control to the programmer, allowing flexibility but risking memory leaks due to human error. On the other hand, Java uses Garbage Collection, an automated cleaning operation that eliminates manual errors but can impose a performance overhead.

In contrast, Rust introduces an innovative solution called Ownership and Borrowing, which ensures efficient memory usage without compromising performance or safety. In this article, I will explain how Rust manages memory uniquely.

Importance of Memory Management

Before diving into Rust, let’s recall the role of memory in computing and what it means to manage it efficiently.

Modern computers are processor-centric, requiring data to be loaded into CPU registers for processing. Accessing data directly from permanent storage like SSDs is too slow, so computer architects designed the memory hierarchy to bridge the gap. DRAM is a critical part of this system, providing faster access to frequently used data. However, memory is not infinite — most PCs today have around 16 GB of DRAM to handle thousands of processes simultaneously.

This is where memory management becomes crucial. A process must use its allocated memory efficiently to avoid waste. To ensure this, modern programming languages provide various memory management techniques, as discussed earlier.

Rust’s Approach

Rust uses a system based on ownership, borrowing, and lifetimes, which is enforced at compile time. This ensures memory safety without needing a garbage collector, meaning Rust’s memory management has minimal impact on runtime performance.

Ownership

Ownership is Rust’s core memory management system that dictates how memory is allocated and deallocated at compile time, preventing common issues like dangling pointers, double frees, and memory leaks.

There are three key rules of ownership:

Each value in Rust has a single owner at a time.
When the owner goes out of scope, the value is dropped.
You can transfer ownership, but there is no implicit copying of heap data.

fn main() {
    let s1 = String::from("hello"); // `s1` owns the string
    let s2 = s1; // Ownership moves to `s2`, `s1` is now invalid

    println!("{}", s2); // This works
    // println!("{}", s1); // Error! `s1` is invalid after the move
}

These rules apply differently to types with fixed sizes that are allocated on the stack. Such types implement the Copy trait, which allows them to be implicitly copied instead of moving ownership. This makes operations on them more efficient and avoids ownership transfer complexities.

fn main() {
    let x = 42;
    let y = x; // `x` is still valid because integers implement `Copy`
    println!("x: {}, y: {}", x, y);
}

Borrowing

Borrowing is Rust’s way of allowing multiple parts of a program to access a value without taking ownership. This prevents unnecessary memory copies while ensuring memory safety at compile time.

To borrow a value, references (&T for immutable and &mut T for mutable) are used. Rust enforces two borrowing rules:

Immutable Borrowing (&T): Multiple immutable references can exist at the same time, but they cannot modify the value.
Mutable Borrowing (&mut T): Only one mutable reference can exist at a time, ensuring no simultaneous reads and writes, which prevents data races.

fn main() {
    let s1 = String::from("Rust");

    let ir1 = &s1; // Immutable borrow
    let ir2 = &s1; // Another immutable borrow

    println!("{}, {}", ir1, ir2); // Works fine
    
    let mut s2 = String::from("Memory");
    let mr1= &mut s2;
    mr1.push_str(" management is easy!");
    //let mr2= &mut s2; Won't compile!
    println!("{}", mr1); 
}

Lifetimes

Lifetimes in Rust ensure that references remain valid for as long as they are needed, preventing dangling pointers at compile time. Unlike other high-level languages where memory safety is handled at runtime, Rust enforces lifetimes statically.

Lifetimes are particularly useful when dealing with function parameters and struct references, where the compiler needs to know how long a reference should be valid. While it may seem complex at first, it ultimately helps developers write safe and efficient code without runtime overhead.

fn main() {
    let s1 = String::from("Rust");
    let s2 = String::from("Awesome");

    let result = longest(&s1, &s2); 
    println!("{}", result);
}

fn longest(s1: &str, s2: &str) -> &str {
    if s1.len() > s2.len() {
        s1
    } else {
        s2
    }
}

Let’s examine the code above. At first glance, it appears straightforward. We have a function called longest that takes two string references and returns a reference to the longer one. However, when we try to run it, we encounter an error:

error[E0106]: missing lifetime specifier

help: this function's return type contains a borrowed value, 
but the signature does not say whether it is borrowed from `s1` or `s2`

consider introducing a named lifetime parameter

Lifetimes in Rust are denoted using 'a and have a syntax similar to generics. They explicitly define how long references remain valid. Let’s fix our code:

fn main() {
    let s1 = String::from("Rust");
    let s2 = String::from("Awesome");

    let result = longest(&s1, &s2); // Both s1 & s2 live long enough
    println!("{}", result);
}

fn longest<'a>(s1: &'a str, s2: &'a str) -> &'a str {
    if s1.len() > s2.len() {
        s1
    } else {
        s2
    }
}

This code works as expected!

Conclusion

Rust takes a unique approach to memory management compared to other popular programming languages. This article aimed to provide a quick and simple introduction to spark curiosity in engineers interested in Rust. There’s much more to explore, such as smart pointers and advanced memory management techniques. Feel free to check the resources below to dive deeper!