r[comments]

Comments

r[comments.syntax]

@root COMMENT ->
      LINE_COMMENT
    | INNER_LINE_DOC
    | OUTER_LINE_DOC
    | INNER_BLOCK_DOC
    | OUTER_BLOCK_DOC
    | BLOCK_COMMENT

LINE_COMMENT ->
      `//` (~[`/` `!` LF] | `//`) ~LF*
    | `//` EOF
    | `//` _immediately followed by LF_

BLOCK_COMMENT ->
      `/**/`
    | `/***/`
    | `/*`
        ^
        ( ~[`*` `!`] | `**` | BLOCK_COMMENT_OR_DOC )
        ( BLOCK_COMMENT_OR_DOC | ~`*/` )*
      `*/`

INNER_LINE_DOC ->
    `//!` ^ LINE_DOC_COMMENT_CONTENT (LF | EOF)

LINE_DOC_COMMENT_CONTENT -> (!CR ~LF)*

INNER_BLOCK_DOC ->
    `/*!` ^ ( BLOCK_COMMENT_OR_DOC | BLOCK_CHAR )* `*/`

OUTER_LINE_DOC ->
    `///` ^ LINE_DOC_COMMENT_CONTENT (LF | EOF)

OUTER_BLOCK_DOC ->
    `/**` ![`*` `/`]
      ^
      ( ~`*` | BLOCK_COMMENT_OR_DOC )
      ( BLOCK_COMMENT_OR_DOC | BLOCK_CHAR )*
    `*/`

BLOCK_CHAR -> (!(`*/` | CR) CHAR)

BLOCK_COMMENT_OR_DOC ->
      BLOCK_COMMENT
    | OUTER_BLOCK_DOC
    | INNER_BLOCK_DOC

r[comments.normal]

Non-doc comments

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

r[comments.normal.tokenization] Non-doc comments are interpreted as a form of whitespace.

r[comments.doc]

Doc comments

r[comments.doc.syntax] Line doc comments beginning with exactly three slashes (///), and block doc comments (/** ... */), both outer doc comments, are interpreted as a special syntax for doc attributes.

r[comments.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 "]. They must therefore appear before something that accepts an outer attribute.

r[comments.doc.inner-syntax] Line comments beginning with //! and block comments /*! ... */ are doc comments that apply to the parent of the comment, rather than the item that follows.

r[comments.doc.inner-attributes] 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.

r[comments.doc.bare-crs] The character U+000D (CR) is not allowed in doc comments.

[!NOTE] It is conventional for doc comments to contain Markdown, as expected by rustdoc. However, the comment syntax does not respect any internal Markdown. /** `glob = "*/*.rs";` */ terminates the comment at the first */, and the remaining code would cause a syntax error. This slightly limits the content of block doc comments compared to line doc comments.

[!NOTE] The sequence U+000D (CR) immediately followed by U+000A (LF) would have been previously transformed into a single U+000A (LF).

Examples

//! 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 {}
}