Constants

In Rust, constants are values bound to a name that remain unchanged throughout the execution of your program. Unlike immutable and mutable variables, constants are inherently immutable, declared with the keyword const, and require explicit type annotations.

For example, the following constant represents the maximum number of users:

const MAX_USERS: u32 = 1000;

In this snippet, MAX_USERS is a constant with a value of 1000. The type annotation u32 indicates that this constant is an unsigned 32-bit integer.

Differences Between Constants and Variables

There are several key differences between constants and variables in Rust:

1. Immutability Enforcement:
   Constants are inherently immutable. Unlike variables, you cannot use the mut keyword with constants.

2. Scope Flexibility:
   Constants can be declared in any scope, including the global scope, making them ideal for values accessed by multiple parts of your application (for example, configuration settings or device addresses).

3. Compile-Time Evaluation:
   Constants must be assigned constant expressions, meaning their values are determined at compile time rather than runtime. This ensures that the value remains fixed, which is essential for certain computations.

Below is an example that demonstrates compile-time computation using constants:

const SECONDS_IN_A_MINUTE: u32 = 60;
const MINUTES_IN_AN_HOUR: u32 = 60;
const HOURS_IN_A_DAY: u32 = 24;
const SECONDS_IN_A_DAY: u32 = SECONDS_IN_A_MINUTE * MINUTES_IN_AN_HOUR * HOURS_IN_A_DAY;

const SERVER_ADDRESS: &str = "192.168.1.1";

fn main() {
    println!("Server address: {}", SERVER_ADDRESS);
}

In this code, SECONDS_IN_A_DAY is computed at compile time using other constant values, ensuring accuracy and efficiency.

Naming Conventions

Rust follows a clear naming convention for constants: use all uppercase letters with underscores to separate words. This practice makes constants easily identifiable compared to variables. For example:

const PI: f64 = 3.141592653589793;
const MAX_SCORE: u32 = 100;
const DEFAULT_TIMEOUT: u64 = 30;

Using descriptive names like PI, MAX_SCORE, and DEFAULT_TIMEOUT helps eliminate ambiguous "magic numbers" and improves overall code readability.

Benefits of Using Constants

By leveraging constants in Rust, you can write more robust, maintainable, and readable code. Their use enhances clarity, reduces errors, and simplifies future modifications.