Rust Comments

A comment is a small note that a programmer may leave in the code to offer explanations to other programmers or even himself later on.

Such notes are meant for people and not the computer. So the compiler will ignore them during build process.

Rust supports a few different varieties:

• Regular comments which are ignored by the compiler:
  • // Line comments which go to the end of the line.
  • /* Block comments which go to the closing delimiter. */
• Doc comments which are parsed into HTML library documentation:
  • /// Generate library docs for the following item.
  • //! Generate library docs for the enclosing item.

Here is an example:

fn main() {
    // This is an example of a line comment
    // There are two slashes at the beginning of the line
    // And nothing written inside these will be read by the compiler

    // println!("Hello, world!");

    // Run it. See? Now try deleting the two slashes, and run it again.

    /*
     * This is another type of comment, a block comment. In general,
     * line comments are the recommended comment style. But
     * block comments are extremely useful for temporarily disabling
     * chunks of code. /* Block comments can be /* nested, */ */
     * so it takes only a few keystrokes to comment out everything
     * in this main() function. /*/*/* Try it yourself! */*/*/
     */

    /*
    Note: The previous column of * was entirely for style. There's
    no actual need for it.
    */

    // You can manipulate expressions more easily with block comments
    // than with line comments. Try deleting the comment delimiters
    // to change the result:
    let x = 5 + /* 90 + */ 5;
    println!("Is x 10 or 100? x = {}", x);
}

More Comment Examples

Here are simple programs that show comments:

Example 1

fn main() {
    // I’m feeling lucky today
    let lucky_number = 7;
}

Example 2

fn main() {
    let lucky_number = 7; // I’m feeling lucky today
}

Example 3

//! A doc comment that applies to the implicit anonymous module of this crate

pub mod outer_module {

    //!  - Inner line doc
    //!! - Still an inner line doc (but with a bang at the beginning)

    /*!  - Inner block doc */
    /*!! - Still an inner block doc (but with a bang at the beginning) */

    //   - Only a comment
    ///  - Outer line doc (exactly 3 slashes)
    //// - Only a comment

    /*   - Only a comment */
    /**  - Outer block doc (exactly) 2 asterisks */
    /*** - Only a comment */

    pub mod inner_module {}

    pub mod nested_comments {
        /* In Rust /* we can /* nest comments */ */ */

        // All three types of block comments can contain or be nested inside
        // any other type:

        /*   /* */  /** */  /*! */  */
        /*!  /* */  /** */  /*! */  */
        /**  /* */  /** */  /*! */  */
        pub mod dummy_item {}
    }

    pub mod degenerate_cases {
        // empty inner line doc
        //!

        // empty inner block doc
        /*!*/

        // empty line comment
        //

        // empty outer line doc
        ///

        // empty block comment
        /**/

        pub mod dummy_item {}

        // empty 2-asterisk block isn't a doc block, it is a block comment
        /***/

    }

    /* The next one isn't allowed because outer doc comments
       require an item that will receive the doc */

    /// Where is my item?
}