Constant Functions in Rust: Harness the Power of Compile-time Evaluation

Drashti Shah
2 min readApr 11, 2023

Rust is a systems programming language that prioritizes safety, performance, and concurrency. One of the features that Rust offers to achieve better performance is the concept of constant functions.

What are Constant Functions?

Constant functions in Rust are functions that can be evaluated at compile-time. The compiler calculates the result of the function during compilation and replaces the function call with the computed result. This can lead to improved performance, as the function doesn’t need to be called at runtime.

Declaring a Constant Function

To declare a constant function in Rust, use the const keyword before the fn keyword. Here's an example of a simple constant function that returns the square of a given integer:

const fn square(x: i32) -> i32 {
x * x

Benefits of Constant Functions

  1. Performance improvement: Since constant functions are evaluated at compile-time, there’s no need to call them at runtime. This can lead to faster execution of your code.
  2. Code readability: Constant functions can make your code more expressive and easier to read, as the value of a constant function is fixed and known at compile-time.

Limitations of Constant Functions

  1. Restricted functionality: Constant functions can only call other constant functions. They can’t have mutable local variables or perform I/O operations, among other restrictions.
  2. Limited input types: Constant functions can only take parameters of certain types, such as integers, floats, and bools.

When to Use Constant Functions

  1. Simple calculations: Constant functions are suitable for simple calculations that don’t require complex logic or external data.
  2. Fixed values: If you have values that are fixed and known at compile-time, consider using constant functions to calculate those values.
  3. Performance-critical code: If you’re working on performance-critical code, consider using constant functions to reduce runtime overhead.


Constant functions in Rust offer a powerful way to improve the performance of your code by leveraging compile-time evaluation. By understanding their benefits and limitations, you can make informed decisions about when to use constant functions in your Rust projects. Keep in mind that constant functions are just one of the many features Rust provides to help you write efficient and safe code.



Drashti Shah

ESG & climate data scientist by day. Aspiring computational biologist by night.