Added grammar of comments

src/comments.md

@@ -1,20 +1,125 @@
 # Comments
 
+> **<sup>Lexer</sup>**  
+> LINE_COMMENT :  
+> &nbsp;&nbsp; &nbsp;&nbsp; `//` (~[`/` `!`] | `//`) ~`\n`<sup>\*</sup>  
+> &nbsp;&nbsp; | `//`
+>  
+> BLOCK_COMMENT :  
+> &nbsp;&nbsp; &nbsp;&nbsp; `/*` (~[`*` `!`] | `**` | _BlockCommentOrDoc_)
+>      (_BlockCommentOrDoc_ | ~`*/`)<sup>\*</sup> `*/`  
+> &nbsp;&nbsp; | `/**/`  
+> &nbsp;&nbsp; | `/***/`  
+>  
+> OUTER_LINE_DOC :  
+> &nbsp;&nbsp; `//!` ~[`\n` _IsolatedCR_]<sup>\*</sup>  
+>  
+> OUTER_BLOCK_DOC :  
+> &nbsp;&nbsp; `/*!` ( _BlockCommentOrDoc_ | ~[`*/` _IsolatedCR_] )<sup>\*</sup> `*/`  
+>  
+> INNER_LINE_DOC :  
+> &nbsp;&nbsp; `///` (~`/` ~[`\n` _IsolatedCR_]<sup>\*</sup>)<sup>?</sup>  
+>  
+> INNER_BLOCK_DOC :  
+> &nbsp;&nbsp; `/**` (~`*` | _BlockCommentOrDoc_ )
+>              (_BlockCommentOrDoc_ | ~[`*/` _IsolatedCR_])<sup>\*</sup> `*/`  
+>  
+> _BlockCommentOrDoc_ :  
+> &nbsp;&nbsp; &nbsp;&nbsp; BLOCK_COMMENT  
+> &nbsp;&nbsp; | OUTER_BLOCK_DOC  
+> &nbsp;&nbsp; | INNER_BLOCK_DOC  
+>  
+> _IsolatedCR_ :  
+> &nbsp;&nbsp; _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.
 
-Line comments beginning with exactly _three_ slashes (`///`), and block
-comments (`/** ... */`), are interpreted as a special syntax for `doc`
-[attributes]. That is, they are equivalent to writing
+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"]`.
+`#[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.
 
-Non-doc comments are interpreted as a form of whitespace.
+Isolated CRs (`\r`), i.e. not followed by LF (`\n`), are not allowed in doc
+comments.
+
+## 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 {}
+}
+```
 
 [attributes]: attributes.html