fix(parse): Limit frontmatter fences to 255 dashes

compiler/rustc_parse/messages.ftl

@@ -347,6 +347,7 @@ parse_frontmatter_invalid_opening_preceding_whitespace = invalid preceding white
 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_too_many_dashes = too many `-` symbols: frontmatter openings may be delimited by up to 255 `-` symbols, but found {$len_opening}
 parse_frontmatter_unclosed = unclosed frontmatter
     .note = frontmatter opening here was not closed
 

compiler/rustc_parse/src/errors.rs

@@ -822,6 +822,12 @@ pub(crate) struct FrontmatterLengthMismatch {
     pub len_close: usize,
 }
 
+#[derive(Diagnostic)]
+#[diag(parse_frontmatter_too_many_dashes)]
+pub(crate) struct FrontmatterTooManyDashes {
+    pub len_opening: usize,
+}
+
 #[derive(Diagnostic)]
 #[diag(parse_leading_plus_not_supported)]
 pub(crate) struct LeadingPlusNotSupported {

compiler/rustc_parse/src/lexer/mod.rs

@@ -665,6 +665,11 @@ impl<'psess, 'src> Lexer<'psess, 'src> {
             });
         }
 
+        // Only up to 255 `-`s are allowed in code fences
+        if u8::try_from(len_opening).is_err() {
+            self.dcx().emit_err(errors::FrontmatterTooManyDashes { len_opening });
+        }
+
         if !rest.trim_matches(is_horizontal_whitespace).is_empty() {
             let span = self.mk_sp(last_line_start_pos, self.pos);
             self.dcx().emit_err(errors::FrontmatterExtraCharactersAfterClose { span });

tests/ui/frontmatter/fence-too-many-dashes.rs

@@ -0,0 +1,11 @@
+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
+//~? ERROR: too many `-` symbols: frontmatter openings may be delimited by up to 255 `-` symbols
+// ignore-tidy-linelength
+[dependencies]
+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
+
+#![feature(frontmatter)]
+
+// check that we limit fence lengths
+
+fn main() {}

tests/ui/frontmatter/fence-too-many-dashes.stderr

@@ -0,0 +1,4 @@
+error: too many `-` symbols: frontmatter openings may be delimited by up to 255 `-` symbols, but found 256
+
+error: aborting due to 1 previous error
+