docs(ref): Specify frontmatter

Author: epage

src/SUMMARY.md

@@ -6,6 +6,7 @@
 
 - [Lexical structure](lexical-structure.md)
     - [Input format](input-format.md)
+    - [Frontmatter](frontmatter.md)
     - [Keywords](keywords.md)
     - [Identifiers](identifiers.md)
     - [Comments](comments.md)

src/frontmatter.md

@@ -0,0 +1,53 @@
+r[frontmatter]
+# Frontmatter
+
+r[frontmatter.syntax]
+```grammar,lexer
+@root FRONTMATTER ->
+      FRONTMATTER_FENCE HORIZONTAL_WHITESPACE* INFOSTRING? HORIZONTAL_WHITESPACE* LF
+      (FRONTMATTER_LINE LF )*
+      FRONTMATTER_FENCE[^matched-fence] HORIZONTAL_WHITESPACE* LF
+
+FRONTMATTER_FENCE -> `---` `-`{..=255}
+
+INFOSTRING -> (XID_Start | `_`) ( XID_Continue | `-` | `.` )*
+
+FRONTMATTER_LINE -> (~INVALID_FRONTMATTER_LINE_START (~INVALID_FRONTMATTER_LINE_CONTINUE)*)?
+
+INVALID_FRONTMATTER_LINE_START -> (FRONTMATTER_FENCE[^escaped-fence] | LF)
+
+INVALID_FRONTMATTER_LINE_CONTINUE -> LF
+```
+
+[^matched-fence]: The closing fence must have the same number of `-` as the opening fence
+[^escaped-fence]: A `FRONTMATTER_FENCE` at the beginning of a `FRONTMATTER_LINE` is only invalid if it has the same or more `-` as the `FRONTMATTER_FENCE`
+
+r[frontmatter.intro]
+Frontmatter is an optional section for content intended for external tools without requiring these tools to have full knowledge of the Rust grammar.
+
+```rust
+#!/usr/bin/env cargo
+---
+[dependencies]
+fastrand = "2"
+---
+
+fn main() {
+    let num = fastrand::i32(..);
+    println!("{num}");
+}
+```
+
+r[frontmatter.document]
+Frontmatter may only be preceded by a [shebang] and whitespace.
+
+r[frontmatter.fence]
+The delimiters are referred to as a *fence*. The opening and closing fences must be at the start of a line. They must be a matching pair of three or more hyphens (`-`). A fence may be followed by horizontal whitespace.
+
+r[frontmatter.infostring]
+Following the opening fence may be an infostring for identifying the intention of the contained content. An infostring may be followed by horizontal whitespace.
+
+r[frontmatter.body]
+The body of the frontmatter may contain any content except for a line starting with as many or more hyphens (`-`) than in the fences.
+
+[shebang]: input-format.md#shebang-removal

src/input-format.md

@@ -59,6 +59,11 @@ This prevents an [inner attribute] at the start of a source file being removed.
 > [!NOTE]
 > The standard library [`include!`] macro applies byte order mark removal, CRLF normalization, and shebang removal to the file it reads. The [`include_str!`] and [`include_bytes!`] macros do not.
 
+r[input.frontmatter]
+## Frontmatter removal
+
+After some whitespace, [frontmatter] may next appear in the input.
+
 r[input.tokenization]
 ## Tokenization
 
@@ -69,4 +74,5 @@ The resulting sequence of characters is then converted into tokens as described
 [comments]: comments.md
 [Crates and source files]: crates-and-source-files.md
 [_shebang_]: https://en.wikipedia.org/wiki/Shebang_(Unix)
+[frontmatter]: frontmatter.md
 [whitespace]: whitespace.md

src/items/modules.md

@@ -152,7 +152,7 @@ r[items.mod.attributes]
 r[items.mod.attributes.intro]
 Modules, like all items, accept outer attributes. They also accept inner
 attributes: either after `{` for a module with a body, or at the beginning of the
-source file, after the optional BOM and shebang.
+source file, after the optional BOM, shebang, and frontmatter.
 
 r[items.mod.attributes.supported]
 The built-in attributes that have meaning on a module are [`cfg`],