For now, this reference is a best-effort document. We strive for validity and completeness, but are not yet there. In the future, the docs and lang teams will work together to figure out how best to do this. Until then, this is a best-effort attempt. If you find something wrong or missing, file an issue or send in a pull request.

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_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 {}
}
#}