---
id: f4c04f44-c7c3-4fa0-a4d6-e78995257b9c
title: Rust Comments
---

# Basics

In Rust, the idiomatic comment style starts a comment with two slashes,
and the comment continues until the end of the line. For comments that
extend beyond a single line, you’ll need to include // on each line,
like this:

``` rust
// So we’re doing something complicated here, long enough that we need
// multiple lines of comments to do it! Whew! Hopefully, this comment will
// explain what’s going on.
```

Comments can also be placed at the end of lines containing code:

``` rust
fn main() {
    println!("tralala") // This prints something
}
```

But if you're a sane person you'll comment above the code you're
annotating

``` rust
// This prints something
fn main() {
    println!("tralala")
}
```

# Documentation comments

## */*

This is like phpdoc, but for Rust. Documentation comments use three
slashes `///` and support Markdown notation:

``` rust
/// Adds one to the number given.
///
/// # Examples
///
/// ```
/// let arg = 5;
/// let answer = my_crate::add_one(arg);
///
/// assert_eq!(6, answer);
/// ```
pub fn add_one(x: i32) -> i32 {
    x + 1
}
```

Run `cargo doc` to generate HTML documentation from the comments.
`cargo doc --open` will open documentation in the browser.

### Commonly used sections

1.  Panics

    The scenarios in which the function being documented could panic.
    Callers of the function who don’t want their programs to panic
    should make sure they don’t call the function in these situations.

2.  Errors

    If the function returns a `Result`, describing the kinds of errors
    that might occur and what conditions might cause those errors to be
    returned can be helpful to callers so they can write code to handle
    the different kinds of errors in different ways.

3.  Safety

    If the function is `unsafe` to call, there should be a section
    explaining why the function is unsafe and covering the invariants
    that the function expects callers to uphold.

## //!

This is used for general comments:

``` rust
//! # My Crate
//!
//! `my_crate` is a collection of utilities to make performing certain
//! calculations more convenient.

/// Adds one to the number given.
// --snip--
///
/// # Examples
///
/// ```
/// let arg = 5;
/// let answer = my_crate::add_one(arg);
///
/// assert_eq!(6, answer);
/// ```
pub fn add_one(x: i32) -> i32 {
    x + 1
}
```

# Changes

-   [Linking To Items By Name](20201119170237-linking_to_items_by_name)
-   [Search Aliases](20201119170710-search_aliases)