Implement RFC 3503: frontmatters

Supercedes rust-lang#137193

Author: fee1-dead

Diff for: compiler/rustc_ast_passes/src/feature_gate.rs

@@ -514,6 +514,7 @@ pub fn check_crate(krate: &ast::Crate, sess: &Session, features: &Features) {
     gate_all!(contracts_internals, "contract internal machinery is for internal use only");
     gate_all!(where_clause_attrs, "attributes in `where` clause are unstable");
     gate_all!(super_let, "`super let` is experimental");
+    gate_all!(frontmatter, "frontmatters are experimental");
 
     if !visitor.features.never_patterns() {
         if let Some(spans) = spans.get(&sym::never_patterns) {

Diff for: compiler/rustc_feature/src/unstable.rs

@@ -502,6 +502,8 @@ declare_features! (
     (incomplete, fn_delegation, "1.76.0", Some(118212)),
     /// Allows impls for the Freeze trait.
     (internal, freeze_impls, "1.78.0", Some(121675)),
+    /// Frontmatter `---` blocks for use by external tools.
+    (unstable, frontmatter, "CURRENT_RUSTC_VERSION", Some(136889)),
     /// Allows defining gen blocks and `gen fn`.
     (unstable, gen_blocks, "1.75.0", Some(117078)),
     /// Infer generic args for both consts and types.

Diff for: compiler/rustc_lexer/src/cursor.rs

@@ -8,17 +8,19 @@ pub struct Cursor<'a> {
     len_remaining: usize,
     /// Iterator over chars. Slightly faster than a &str.
     chars: Chars<'a>,
+    pub(crate) frontmatter_allowed: bool,
     #[cfg(debug_assertions)]
     prev: char,
 }
 
 pub(crate) const EOF_CHAR: char = '\0';
 
 impl<'a> Cursor<'a> {
-    pub fn new(input: &'a str) -> Cursor<'a> {
+    pub fn new(input: &'a str, frontmatter_allowed: bool) -> Cursor<'a> {
         Cursor {
             len_remaining: input.len(),
             chars: input.chars(),
+            frontmatter_allowed,
             #[cfg(debug_assertions)]
             prev: EOF_CHAR,
         }
@@ -95,6 +97,11 @@ impl<'a> Cursor<'a> {
         Some(c)
     }
 
+    /// Moves to a substring by a number of bytes.
+    pub(crate) fn bump_bytes(&mut self, n: usize) {
+        self.chars = self.as_str()[n..].chars();
+    }
+
     /// Eats symbols while predicate returns true or until the end of file is reached.
     pub(crate) fn eat_while(&mut self, mut predicate: impl FnMut(char) -> bool) {
         // It was tried making optimized version of this for eg. line comments, but

Diff for: compiler/rustc_lexer/src/lib.rs

@@ -57,17 +57,27 @@ impl Token {
 #[derive(Clone, Copy, Debug, PartialEq, Eq)]
 pub enum TokenKind {
     /// A line comment, e.g. `// comment`.
-    LineComment { doc_style: Option<DocStyle> },
+    LineComment {
+        doc_style: Option<DocStyle>,
+    },
 
     /// A block comment, e.g. `/* block comment */`.
     ///
     /// Block comments can be recursive, so a sequence like `/* /* */`
     /// will not be considered terminated and will result in a parsing error.
-    BlockComment { doc_style: Option<DocStyle>, terminated: bool },
+    BlockComment {
+        doc_style: Option<DocStyle>,
+        terminated: bool,
+    },
 
     /// Any whitespace character sequence.
     Whitespace,
 
+    Frontmatter {
+        has_invalid_preceding_whitespace: bool,
+        invalid_infostring: bool,
+    },
+
     /// An identifier or keyword, e.g. `ident` or `continue`.
     Ident,
 
@@ -109,10 +119,15 @@ pub enum TokenKind {
     /// this type will need to check for and reject that case.
     ///
     /// See [LiteralKind] for more details.
-    Literal { kind: LiteralKind, suffix_start: u32 },
+    Literal {
+        kind: LiteralKind,
+        suffix_start: u32,
+    },
 
     /// A lifetime, e.g. `'a`.
-    Lifetime { starts_with_number: bool },
+    Lifetime {
+        starts_with_number: bool,
+    },
 
     /// `;`
     Semi,
@@ -280,7 +295,7 @@ pub fn strip_shebang(input: &str) -> Option<usize> {
 #[inline]
 pub fn validate_raw_str(input: &str, prefix_len: u32) -> Result<(), RawStrError> {
     debug_assert!(!input.is_empty());
-    let mut cursor = Cursor::new(input);
+    let mut cursor = Cursor::new(input, false);
     // Move past the leading `r` or `br`.
     for _ in 0..prefix_len {
         cursor.bump().unwrap();
@@ -290,7 +305,7 @@ pub fn validate_raw_str(input: &str, prefix_len: u32) -> Result<(), RawStrError>
 
 /// Creates an iterator that produces tokens from the input string.
 pub fn tokenize(input: &str) -> impl Iterator<Item = Token> {
-    let mut cursor = Cursor::new(input);
+    let mut cursor = Cursor::new(input, false);
     std::iter::from_fn(move || {
         let token = cursor.advance_token();
         if token.kind != TokenKind::Eof { Some(token) } else { None }
@@ -361,7 +376,30 @@ impl Cursor<'_> {
             Some(c) => c,
             None => return Token::new(TokenKind::Eof, 0),
         };
+
         let token_kind = match first_char {
+            c if self.frontmatter_allowed && is_whitespace(c) => {
+                let mut last = first_char;
+                while is_whitespace(self.first()) {
+                    let Some(c) = self.bump() else {
+                        break;
+                    };
+                    last = c;
+                }
+                // invalid frontmatter opening as whitespace preceding it isn't newline.
+                // combine the whitespace and the frontmatter to a single token as we shall
+                // error later.
+                if last != '\n' && self.as_str().starts_with("---") {
+                    self.bump();
+                    self.frontmatter(true)
+                } else {
+                    Whitespace
+                }
+            }
+            '-' if self.frontmatter_allowed && self.as_str().starts_with("--") => {
+                // happy path
+                self.frontmatter(false)
+            }
             // Slash, comment or block comment.
             '/' => match self.first() {
                 '/' => self.line_comment(),
@@ -464,11 +502,82 @@ impl Cursor<'_> {
             c if !c.is_ascii() && c.is_emoji_char() => self.invalid_ident(),
             _ => Unknown,
         };
+        if self.frontmatter_allowed {
+            self.frontmatter_allowed = matches!(token_kind, Whitespace);
+        }
         let res = Token::new(token_kind, self.pos_within_token());
         self.reset_pos_within_token();
         res
     }
 
+    /// Given that one `-` was eaten, eat the rest of the frontmatter.
+    fn frontmatter(&mut self, has_invalid_preceding_whitespace: bool) -> TokenKind {
+        debug_assert_eq!('-', self.prev());
+
+        let pos = self.pos_within_token();
+        self.eat_while(|c| c == '-');
+
+        // one `-` is eaten by the caller.
+        let length_opening = self.pos_within_token() - pos + 1;
+
+        // must be ensured by the caller
+        debug_assert!(length_opening >= 3);
+
+        self.eat_identifier();
+        self.eat_while(|ch| ch != '\n' && is_whitespace(ch));
+        let invalid_infostring = self.first() != '\n';
+
+        let mut s = self.as_str();
+        let mut found = false;
+        while let Some(closing) = s.find(&"-".repeat(length_opening as usize)) {
+            let preceding_chars_start = s[..closing].rfind("\n").map_or(0, |i| i + 1);
+            if s[preceding_chars_start..closing].chars().all(is_whitespace) {
+                // candidate found
+                self.bump_bytes(closing);
+                // in case like
+                // ---cargo
+                // --- blahblah
+                // or
+                // ---cargo
+                // ----
+                // combine those stuff into this frontmatter token such that it gets detected later.
+                self.eat_until(b'\n');
+                found = true;
+                break;
+            } else {
+                s = &s[closing + length_opening as usize..];
+            }
+        }
+
+        if !found {
+            // recovery strategy: a closing statement might have precending whitespace/newline
+            // but not have enough dashes to properly close. In this case, we eat until there,
+            // and report a mismatch in the parser.
+            let mut potential_closing = None;
+            let mut rest = self.as_str();
+            while let Some(closing) = rest.find("---") {
+                let preceding_chars_start = rest[..closing].rfind("\n").map_or(0, |i| i + 1);
+                if rest[preceding_chars_start..closing].chars().all(is_whitespace) {
+                    // candidate found
+                    potential_closing = Some(closing);
+                    break;
+                } else {
+                    rest = &rest[closing + 3..];
+                }
+            }
+            if let Some(potential_closing) = potential_closing {
+                // bump to the potential closing, and eat everything on that line.
+                self.bump_bytes(potential_closing);
+                self.eat_until(b'\n');
+            } else {
+                // eat everything. this will get reported as an unclosed frontmatter.
+                self.eat_while(|_| true);
+            }
+        }
+
+        Frontmatter { has_invalid_preceding_whitespace, invalid_infostring }
+    }
+
     fn line_comment(&mut self) -> TokenKind {
         debug_assert!(self.prev() == '/' && self.first() == '/');
         self.bump();

Diff for: compiler/rustc_lexer/src/tests.rs

@@ -4,7 +4,7 @@ use super::*;
 
 fn check_raw_str(s: &str, expected: Result<u8, RawStrError>) {
     let s = &format!("r{}", s);
-    let mut cursor = Cursor::new(s);
+    let mut cursor = Cursor::new(s, false);
     cursor.bump();
     let res = cursor.raw_double_quoted_string(0);
     assert_eq!(res, expected);

Diff for: compiler/rustc_parse/messages.ftl

@@ -297,6 +297,19 @@ parse_forgot_paren = perhaps you forgot parentheses?
 parse_found_expr_would_be_stmt = expected expression, found `{$token}`
     .label = expected expression
 
+parse_frontmatter_extra_characters_after_close = extra characters after frontmatter close are not allowed
+parse_frontmatter_invalid_close_preceding_whitespace = invalid preceding whitespace for frontmatter close
+    .note = frontmatter close should not be preceded by whitespace
+parse_frontmatter_invalid_infostring = invalid infostring for frontmatter
+    .note = frontmatter infostrings must be a single identifier immediately following the opening
+parse_frontmatter_invalid_opening_preceding_whitespace = invalid preceding whitespace for frontmatter opening
+    .note = frontmatter opening should not be preceded by whitespace
+parse_frontmatter_length_mismatch = frontmatter close does not match the opening
+    .label_opening = the opening here has {$len_opening} dashes...
+    .label_close = ...while the close has {$len_close} dashes
+parse_frontmatter_unclosed = unclosed frontmatter
+    .note = frontmatter opening here was not closed
+
 parse_function_body_equals_expr = function body cannot be `= expression;`
     .suggestion = surround the expression with `{"{"}` and `{"}"}` instead of `=` and `;`
 

Diff for: compiler/rustc_parse/src/errors.rs

@@ -730,6 +730,61 @@ pub(crate) struct FoundExprWouldBeStmt {
     pub suggestion: ExprParenthesesNeeded,
 }
 
+#[derive(Diagnostic)]
+#[diag(parse_frontmatter_extra_characters_after_close)]
+pub(crate) struct FrontmatterExtraCharactersAfterClose {
+    #[primary_span]
+    pub span: Span,
+}
+
+#[derive(Diagnostic)]
+#[diag(parse_frontmatter_invalid_infostring)]
+#[note]
+pub(crate) struct FrontmatterInvalidInfostring {
+    #[primary_span]
+    pub span: Span,
+}
+
+#[derive(Diagnostic)]
+#[diag(parse_frontmatter_invalid_opening_preceding_whitespace)]
+pub(crate) struct FrontmatterInvalidOpeningPrecedingWhitespace {
+    #[primary_span]
+    pub span: Span,
+    #[note]
+    pub note_span: Span,
+}
+
+#[derive(Diagnostic)]
+#[diag(parse_frontmatter_unclosed)]
+pub(crate) struct FrontmatterUnclosed {
+    #[primary_span]
+    pub span: Span,
+    #[note]
+    pub note_span: Span,
+}
+
+#[derive(Diagnostic)]
+#[diag(parse_frontmatter_invalid_close_preceding_whitespace)]
+pub(crate) struct FrontmatterInvalidClosingPrecedingWhitespace {
+    #[primary_span]
+    pub span: Span,
+    #[note]
+    pub note_span: Span,
+}
+
+#[derive(Diagnostic)]
+#[diag(parse_frontmatter_length_mismatch)]
+pub(crate) struct FrontmatterLengthMismatch {
+    #[primary_span]
+    pub span: Span,
+    #[label(parse_label_opening)]
+    pub opening: Span,
+    #[label(parse_label_close)]
+    pub close: Span,
+    pub len_opening: usize,
+    pub len_close: usize,
+}
+
 #[derive(Diagnostic)]
 #[diag(parse_leading_plus_not_supported)]
 pub(crate) struct LeadingPlusNotSupported {