Implement RFC 3503: frontmatters

Supercedes #137193

crates/parser/src/lexed_str.rs

@@ -179,6 +179,15 @@ impl<'a> Converter<'a> {
                     COMMENT
                 }
 
+                rustc_lexer::TokenKind::Frontmatter  { has_invalid_preceding_whitespace, invalid_infostring } => {
+                    if *has_invalid_preceding_whitespace {
+                        err = "invalid preceding whitespace for frontmatter opening"
+                    } else if *invalid_infostring {
+                        err = "invalid infostring for frontmatter"
+                    }
+                    FRONTMATTER
+                }
+
                 rustc_lexer::TokenKind::Whitespace => WHITESPACE,
 
                 rustc_lexer::TokenKind::Ident if token_text == "_" => UNDERSCORE,

crates/syntax/rust.ungram

@@ -133,6 +133,7 @@ Meta =
 
 SourceFile =
   '#shebang'?
+  '#frontmatter'?
   Attr*
   Item*
 

crates/syntax/src/ast/generated/nodes.rs

@@ -1524,6 +1524,10 @@ impl ast::HasAttrs for SourceFile {}
 impl ast::HasDocComments for SourceFile {}
 impl ast::HasModuleItem for SourceFile {}
 impl SourceFile {
+    #[inline]
+    pub fn frontmatter_token(&self) -> Option<SyntaxToken> {
+        support::token(&self.syntax, T![frontmatter])
+    }
     #[inline]
     pub fn shebang_token(&self) -> Option<SyntaxToken> { support::token(&self.syntax, T![shebang]) }
 }

xtask/src/codegen/grammar.rs

@@ -670,6 +670,7 @@ fn generate_syntax_kinds(grammar: KindsSrc) -> String {
             [ident] => { $crate::SyntaxKind::IDENT };
             [string] => { $crate::SyntaxKind::STRING };
             [shebang] => { $crate::SyntaxKind::SHEBANG };
+            [frontmatter] => { $crate::SyntaxKind::FRONTMATTER };
         }
 
         impl ::core::marker::Copy for SyntaxKind {}