Comments - The Rust Reference

# Comments

> Lexer
> LINE_COMMENT :
> `//` (~[`/` `!`] | `//`) ~`\n`*
> | `//`

> BLOCK_COMMENT :
> `/*` (~[`*` `!`] | `**` | BlockCommentOrDoc) (BlockCommentOrDoc | ~`*/`)* `*/`
> | `/**/`
> | `/***/`

> INNER_LINE_DOC :
> `//!` ~[`\n` IsolatedCR]*

> INNER_BLOCK_DOC :
> `/*!` ( BlockCommentOrDoc | ~[`*/` IsolatedCR] )* `*/`

> OUTER_LINE_DOC :
> `///` (~`/` ~[`\n` IsolatedCR]*)?

> OUTER_BLOCK_DOC :
> `/**` (~`*` | BlockCommentOrDoc ) (BlockCommentOrDoc | ~[`*/` IsolatedCR])* `*/`

> BlockCommentOrDoc :
> BLOCK_COMMENT
> | OUTER_BLOCK_DOC
> | INNER_BLOCK_DOC

> IsolatedCR :
> A `\r` not followed by a `\n`

## Non-doc comments

Comments in Rust code follow the general C++ style of line (`//`) and block (`/* ... */`) comment forms. Nested block comments are supported.

Non-doc comments are interpreted as a form of whitespace.

## Doc comments

Line doc comments beginning with exactly three slashes (`///`), and block doc comments (`/** ... */`), both inner doc comments, are interpreted as a special syntax for `doc` attributes. That is, they are equivalent to writing `#[doc="..."]` around the body of the comment, i.e., `/// Foo` turns into `#[doc="Foo"]` and `/** Bar */` turns into `#[doc="Bar"]`.

Line comments beginning with `//!` and block comments `/*! ... */` are doc comments that apply to the parent of the comment, rather than the item that follows. That is, they are equivalent to writing `#![doc="..."]` around the body of the comment. `//!` comments are usually used to document modules that occupy a source file.

Isolated CRs (`\r`), i.e. not followed by LF (`\n`), are not allowed in doc comments.

## Examples

    #![allow(unused)]
    fn main() {
    //! 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?
      mod boo {}
    }
    }