Any program requires comments and indeed 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.

fn main() {
    // This is an example of a line comment
    // Notice how there are two slashes at the beginning of the line
    // And that 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, the block comment. In general,
     * the line comment is the recommended comment style however the
     * block comment is extremely useful for temporarily disabling
     * a large chunk of code. /* Block comments can be /* nested, */ */
     * so it takes only a few keystrokes to comment out all the lines
     * in this main() function. /*/*/* Try it yourself! */*/*/
     */

    /*
    Note, the previous column of `*` was entirely for style. There's
    no actual need for it.
    */

    // Observe how block comments allow easy expression manipulation
    // which line comments do not. Deleting the comment delimiters
    // will change the result:
    let x = 5 + /* 90 + */ 5;
    println!("Is `x` 10 or 100? x = {}", x);
}

Library documentation