^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

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.

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.

# #![allow(unused_variables)]
#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 {}
}
#}