Rust: Reassign vs Mutate

Rust: Reassign vs Mutate

An implementation, memory, and performance comparison between reassignment and mutation in Rust

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Knowing when to use mutations versus reassignments is a common decision in many programming languages and in a memory-safe language like Rust, they are an essential question. As Rust is dominated by knowing how to borrow effectively, we can ease our way into its rules by looking at the Rust language through the lens of reassignment versus direct mutation.

Note: After this, read up on Ownership and how it relates to borrowing, slices, and memory management of your Rust variables. You probably already know some of this if you are using Rust, but it does not hurt to read through this section.

In a lot of cases, Rustaceans refer to the performance impact of using something like [Clone]( or [Copy]( traits in Rust, which allow duplication of a struct into a brand-new variable binding.

Below, we will look at the memory impact of using mutable object references, copied objects, and cloned objects alongside code examples (full source code here). Benchmarks at the end.

Simple Example: Bicycle — Clone vs Mutate

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Photo by Kiwihug on Unsplash

To begin, we have a Bicycle module, which is a considerably simple struct with a couple of methods: a constructor and a mutator for a Bicycle instance:

#[derive(Copy, Clone)]
    pub struct Bicycle {
        speed: u32,

    impl Bicycle {
        pub fn new(speed: u32) -> Bicycle {
            Bicycle { speed: speed }

        pub fn accelerate(&mut self) {
            println!("current speed: {}", self.speed);
            self.speed += 1;

Internally, the accelerate method mutates the speed field each time it is called.

Also, take note of the #[derive(Copy, Clone)] attribute which allows us to duplicate the struct easily.

Using this module, we can then create two different examples that demonstrate how a given bicycle instance can be modified.

Clone Bicycle

In this first example, the code clones the bicycle:

mod bicycle;

    const NUM_BIKES: u32 = 100_000;
    const TOTAL_EPOCHS: u64 = 5;

    use bicycle::Bicycle;

    fn main() {
            "=== clone_bikes - {} v{} ====",
        let mut bikes: Vec<Bicycle> = Vec::new();

        for i in 0..NUM_BIKES {

        println!("Lets reassign!");

        for _i in 0..TOTAL_EPOCHS {
            for j in 0..bikes.len() {
                let mut bike = bikes[j].clone();
                bikes[j] = bike;


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