Mark @RUSTC_BUILTIN search path usage as unstable

compiler/rustc_interface/src/tests.rs

@@ -321,6 +321,7 @@ fn test_search_paths_tracking_hash_different_order() {
             &opts.target_triple,
             &early_dcx,
             search_path,
+            false,
         ));
     };
 

compiler/rustc_session/src/config.rs

@@ -2548,7 +2548,13 @@ pub fn build_session_options(early_dcx: &mut EarlyDiagCtxt, matches: &getopts::M
 
     let mut search_paths = vec![];
     for s in &matches.opt_strs("L") {
-        search_paths.push(SearchPath::from_cli_opt(&sysroot, &target_triple, early_dcx, s));
+        search_paths.push(SearchPath::from_cli_opt(
+            &sysroot,
+            &target_triple,
+            early_dcx,
+            s,
+            unstable_opts.unstable_options,
+        ));
     }
 
     let working_dir = std::env::current_dir().unwrap_or_else(|e| {

compiler/rustc_session/src/search_paths.rs

@@ -52,6 +52,7 @@ impl SearchPath {
         triple: &TargetTriple,
         early_dcx: &EarlyDiagCtxt,
         path: &str,
+        is_unstable_enabled: bool,
     ) -> Self {
         let (kind, path) = if let Some(stripped) = path.strip_prefix("native=") {
             (PathKind::Native, stripped)
@@ -68,6 +69,14 @@ impl SearchPath {
         };
         let dir = match path.strip_prefix("@RUSTC_BUILTIN") {
             Some(stripped) => {
+                if !is_unstable_enabled {
+                    #[allow(rustc::untranslatable_diagnostic)] // FIXME: make this translatable
+                    early_dcx.early_fatal(
+                        "the `-Z unstable-options` flag must also be passed to \
+                         enable the use of `@RUSTC_BUILTIN`",
+                    );
+                }
+
                 make_target_lib_path(sysroot, triple.triple()).join("builtin").join(stripped)
             }
             None => PathBuf::from(path),

src/librustdoc/config.rs

@@ -635,7 +635,16 @@ impl Options {
         let libs = matches
             .opt_strs("L")
             .iter()
-            .map(|s| SearchPath::from_cli_opt(&sysroot, &target, early_dcx, s))
+            .map(|s| {
+                SearchPath::from_cli_opt(
+                    &sysroot,
+                    &target,
+                    early_dcx,
+                    s,
+                    #[allow(rustc::bad_opt_access)] // we have no `Session` here
+                    unstable_opts.unstable_options,
+                )
+            })
             .collect();
 
         let show_coverage = matches.opt_present("show-coverage");