Comments

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.
  1. fn main() {
  2.     // This is an example of a line comment
  3.     // Notice how there are two slashes at the beginning of the line
  4.     // And that nothing written inside these will be read by the compiler
  6.     // println!("Hello, world!");
  8.     // Run it. See? Now try deleting the two slashes, and run it again.
  10.     /* 
  11.      * This is another type of comment, the block comment. In general,
  12.      * the line comment is the recommended comment style however the
  13.      * block comment is extremely useful for temporarily disabling
  14.      * a large chunk of code. /* Block comments can be /* nested, */ */
  15.      * so it takes only a few keystrokes to comment out all the lines
  16.      * in this main() function. /*/*/* Try it yourself! */*/*/
  17.      */
  19.     /*
  20.     Note, the previous column of `*` was entirely for style. There's
  21.     no actual need for it.
  22.     */
  24.     // Observe how block comments allow easy expression manipulation
  25.     // which line comments do not. Deleting the comment delimiters
  26.     // will change the result:
  27.     let x = 5 + /* 90 + */ 5;
  28.     println!("Is `x` 10 or 100? x = {}", x);
  29. }

See also:

Library documentation