Rollup merge of #140035 - fee1-dead-contrib:push-oszwkkvmpkks, r=jieyouxu,wesleywiser

Implement RFC 3503: frontmatters

Tracking issue: #136889

Supercedes #137193. This implements [RFC 3503](https://github.com/rust-lang/rfcs/blob/master/text/3503-frontmatter.md).

This might break rust-analyzer. Will look into how to fix that.

Suggestions welcome for how to improve diagnostics.

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) {