fix(parse): Feature gate frontmatter

compiler/rustc_ast_passes/src/feature_gate.rs

@@ -515,6 +515,7 @@ pub fn check_crate(krate: &ast::Crate, sess: &Session, features: &Features) {
     gate_all!(unsafe_binders, "unsafe binder types are experimental");
     gate_all!(contracts, "contracts are incomplete");
     gate_all!(contracts_internals, "contract internal machinery is for internal use only");
+    gate_all!(frontmatter, "frontmatter syntax is unstable");
 
     if !visitor.features.never_patterns() {
         if let Some(spans) = spans.get(&sym::never_patterns) {

compiler/rustc_parse/src/lexer/mod.rs

@@ -14,7 +14,7 @@ use rustc_session::lint::builtin::{
     TEXT_DIRECTION_CODEPOINT_IN_COMMENT,
 };
 use rustc_session::parse::ParseSess;
-use rustc_span::{BytePos, Pos, Span, Symbol};
+use rustc_span::{BytePos, Pos, Span, Symbol, sym};
 use tracing::debug;
 
 use crate::lexer::diagnostics::TokenTreeDiagInfo;
@@ -58,7 +58,11 @@ pub(crate) fn lex_token_trees<'psess, 'src>(
     // Skip frontmatter, if present.
     if let Some(frontmatter_len) = rustc_lexer::strip_frontmatter(src) {
         src = &src[frontmatter_len..];
+        let lo = start_pos;
         start_pos = start_pos + BytePos::from_usize(frontmatter_len);
+        let hi = start_pos;
+        let span = Span::with_root_ctxt(lo, hi);
+        psess.gated_spans.gate(sym::frontmatter, span);
     }
 
     let cursor = Cursor::new(src);

src/doc/unstable-book/src/language-features/frontmatter.md

@@ -18,6 +18,8 @@ For example, when used with Cargo:
 clap = "4"
 ---
 
+#![feature(frontmatter)]
+
 use clap::Parser;
 
 #[derive(Parser)]

tests/ui/feature-gates/feature-gate-frontmatter.rs

@@ -1,7 +1,7 @@
 ---
+//~^ frontmatter syntax is unstable [E0658]
 ---
 
-//@ check-pass
 
 pub fn main() {
 }

tests/ui/feature-gates/feature-gate-frontmatter.stderr

@@ -0,0 +1,16 @@
+error[E0658]: frontmatter syntax is unstable
+  --> $DIR/feature-gate-frontmatter.rs:1:1
+   |
+LL | / ---
+LL | |
+LL | | ---
+LL | |
+   | |_^
+   |
+   = note: see issue #136889 <https://github.com/rust-lang/rust/issues/136889> for more information
+   = help: add `#![feature(frontmatter)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0658`.