Comments in Rust

Good code is well-documented, and comments are an essential part of that documentation. In this section, we’ll explore how comments work in Rust and compare them to JavaScript.

Regular Comments

Single-Line Comments

Both Rust and JavaScript use // for single-line comments:

// This is a single-line comment in Rust
let x = 5; // This is an end-of-line comment

// This is a single-line comment in JavaScript
let x = 5; // This is an end-of-line comment

Multi-Line Comments

Both languages also support multi-line comments, though with slightly different syntax:

Rust:

/* This is a multi-line comment in Rust.
   It can span multiple lines.
   Rust allows nested /* multi-line */ comments. */

JavaScript:

/* This is a multi-line comment in JavaScript.
   It can span multiple lines.
   JavaScript doesn't allow nested /* multi-line */ comments. */

The key difference is that Rust allows nesting multi-line comments, while JavaScript doesn’t.

Documentation Comments

This is where Rust really shines compared to JavaScript. Rust has built-in documentation comments that can be processed by rustdoc to generate HTML documentation.

Rust Documentation Comments

Rust has two types of documentation comments:

/// - Used to document the item that follows
//! - Used to document the containing item (like a module)

Example of /// documentation comments:

/// Calculates the sum of two numbers.
///
/// # Examples
///
/// ```
/// let result = add(2, 3);
/// assert_eq!(result, 5);
/// ```
fn add(a: i32, b: i32) -> i32 {
    a + b
}

Example of //! documentation comments:

//! # My Math Library
//!
//! A collection of mathematical functions.

/// Calculates the sum of two numbers.
pub fn add(a: i32, b: i32) -> i32 {
    a + b
}

JavaScript Documentation Comments

JavaScript doesn’t have built-in documentation comments, but it does have JSDoc, a documentation standard:

/**
 * Calculates the sum of two numbers.
 *
 * @param {number} a - The first number
 * @param {number} b - The second number
 * @returns {number} The sum of a and b
 *
 * @example
 * // returns 5
 * add(2, 3);
 */
function add(a, b) {
  return a + b;
}

Markdown Support

Rust’s documentation comments support Markdown, which allows for rich formatting:

/// # Add Function
///
/// Calculates the sum of two numbers.
///
/// ## Examples
///
/// ```
/// let result = add(2, 3);
/// assert_eq!(result, 5);
/// ```
///
/// ## Panics
///
/// This function does not panic.
///
/// ## Safety
///
/// This function is safe to use.
fn add(a: i32, b: i32) -> i32 {
    a + b
}

JSDoc also supports HTML formatting and some Markdown-like syntax:

/**
 * # Add Function
 *
 * Calculates the sum of two numbers.
 *
 * ## Examples
 *
 * ```js
 * // returns 5
 * add(2, 3);
 * ```
 *
 * @param {number} a - The first number
 * @param {number} b - The second number
 * @returns {number} The sum of a and b
 */
function add(a, b) {
  return a + b;
}

Generating Documentation

Rust Documentation Generation

Rust’s documentation can be generated and viewed locally using Cargo:

# Generate documentation for your project and its dependencies
cargo doc --open

This generates HTML documentation and opens it in your browser.

JavaScript Documentation Generation

JavaScript requires external tools like JSDoc, TypeDoc, or ESDoc:

# Example with JSDoc
npm install --save-dev jsdoc
npx jsdoc yourfile.js -d docs

Doc Tests

One of the coolest features of Rust’s documentation system is that code examples in documentation are actually run as tests:

/// Returns the square of a number.
///
/// # Examples
///
/// ```
/// let result = square(4);
/// assert_eq!(result, 16);
/// ```
fn square(x: i32) -> i32 {
    x * x
}

When you run cargo test, the example in the comment is compiled and run as a test. This ensures that your documentation examples stay up-to-date with your code.

JavaScript doesn’t have this feature built-in, though some tools can extract and run examples from JSDoc comments.

Common Documentation Conventions

Rust Documentation Sections

Rust has some conventional section headers used in documentation comments:

# Examples - Code examples
# Panics - Situations where the function might panic
# Errors - Error types that might be returned
# Safety - Safety considerations for unsafe functions
# Implementation Details - Notes about the implementation

JavaScript Documentation Tags

JSDoc uses tags for similar purposes:

@example - Code examples
@throws - Exceptions that might be thrown
@param - Parameter descriptions
@returns - Return value description
@see - Related functions or resources

Comments as a Last Resort

Both Rust and JavaScript communities encourage writing self-documenting code:

“Good code is its own best documentation.” — Steve McConnell

Comments should explain “why” something is done a certain way, not “what” is being done (the code should make that clear).

Comment Best Practices

Write self-documenting code first - Use descriptive variable and function names
Explain why, not what - The code shows what it’s doing; comments should explain why
Keep comments updated - Outdated comments are worse than no comments
Use documentation comments for public APIs - Help users understand how to use your code
Comment complex algorithms - Even if the code is clear, the underlying algorithm might not be

Next Steps

Now that you understand how to comment your Rust code, let’s move on to Control Flow to learn how to use conditionals and loops in Rust.