r[comments]
# Comments

r[comments.syntax]
```grammar,lexer
@root LINE_COMMENT ->
      `//` (~[`/` `!` LF] | `//`) ~LF*
    | `//`

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

@root INNER_LINE_DOC ->
    `//!` ~[LF CR]*

INNER_BLOCK_DOC ->
    `/*!` ( BLOCK_COMMENT_OR_DOC | ~[`*/` CR] )* `*/`

@root OUTER_LINE_DOC ->
    `///` (~`/` ~[LF CR]*)?

OUTER_BLOCK_DOC ->
    `/**`
      ( ~`*` | BLOCK_COMMENT_OR_DOC )
      ( BLOCK_COMMENT_OR_DOC | ~[`*/` CR] )*
    `*/`

@root 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

```rust
//! 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 {}
}
```

[`doc` attributes]: ../rustdoc/the-doc-attribute.html
