Implement RFC 1260 with feature_name imported_main.

Author: crlf0710

src/driver/jit.rs

@@ -103,7 +103,7 @@ pub(super) fn run_jit(tcx: TyCtxt<'_>, backend_config: BackendConfig) -> ! {
     });
 
     let (main_def_id, entry_ty) = tcx.entry_fn(LOCAL_CRATE).unwrap();
-    let instance = Instance::mono(tcx, main_def_id.to_def_id()).polymorphize(tcx);
+    let instance = Instance::mono(tcx, main_def_id).polymorphize(tcx);
 
     match entry_ty {
         EntryFnType::Main => {

src/main_shim.rs

@@ -13,7 +13,7 @@ pub(crate) fn maybe_create_entry_wrapper(
 ) {
     let (main_def_id, use_start_lang_item) = match tcx.entry_fn(LOCAL_CRATE) {
         Some((def_id, entry_ty)) => (
-            def_id.to_def_id(),
+            def_id,
             match entry_ty {
                 EntryFnType::Main => true,
                 EntryFnType::Start => false,