Drop AST on a separate thread and prefetch hir_crate

compiler/rustc_interface/src/passes.rs

@@ -900,6 +900,12 @@ fn run_required_analyses(tcx: TyCtxt<'_>) {
     // is not defined. So we need to cfg it out.
     #[cfg(all(not(doc), debug_assertions))]
     rustc_passes::hir_id_validator::check_crate(tcx);
+
+    // Prefetch this to prevent multiple threads from blocking on it later.
+    // This is needed since the `hir_id_validator::check_crate` call above is not guaranteed
+    // to use `hir_crate`.
+    tcx.ensure_done().hir_crate(());
+
     let sess = tcx.sess;
     sess.time("misc_checking_1", || {
         parallel!(