Any program requires comments, and 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.
    // There are two slashes at the beginning of the line.
    // And nothing written after 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);

See also:

Library documentation