add EarlyBinder::no_bound_vars

compiler/rustc_codegen_cranelift/src/main_shim.rs

@@ -46,7 +46,7 @@ pub(crate) fn maybe_create_entry_wrapper(
         is_main_fn: bool,
         sigpipe: u8,
     ) {
-        let main_ret_ty = tcx.fn_sig(rust_main_def_id).subst_identity().output();
+        let main_ret_ty = tcx.fn_sig(rust_main_def_id).no_bound_vars().unwrap().output();
         // Given that `main()` has no arguments,
         // then its return type cannot have
         // late-bound regions, since late-bound

compiler/rustc_codegen_ssa/src/base.rs

@@ -436,7 +436,7 @@ pub fn maybe_create_entry_wrapper<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
             cx.type_func(&[], cx.type_int())
         };
 
-        let main_ret_ty = cx.tcx().fn_sig(rust_main_def_id).subst_identity().output();
+        let main_ret_ty = cx.tcx().fn_sig(rust_main_def_id).no_bound_vars().unwrap().output();
         // Given that `main()` has no arguments,
         // then its return type cannot have
         // late-bound regions, since late-bound

compiler/rustc_middle/src/ty/subst.rs

@@ -758,6 +758,11 @@ impl<'tcx, T: TypeFoldable<'tcx>> ty::EarlyBinder<T> {
     pub fn subst_identity(self) -> T {
         self.0
     }
+
+    /// Returns the inner value, but only if it contains no bound vars.
+    pub fn no_bound_vars(self) -> Option<T> {
+        if !self.0.needs_subst() { Some(self.0) } else { None }
+    }
 }
 
 ///////////////////////////////////////////////////////////////////////////

compiler/rustc_monomorphize/src/collector.rs

@@ -1296,7 +1296,7 @@ impl<'v> RootCollector<'_, 'v> {
         };
 
         let start_def_id = self.tcx.require_lang_item(LangItem::Start, None);
-        let main_ret_ty = self.tcx.fn_sig(main_def_id).subst_identity().output();
+        let main_ret_ty = self.tcx.fn_sig(main_def_id).no_bound_vars().unwrap().output();
 
         // Given that `main()` has no arguments,
         // then its return type cannot have

src/tools/miri/src/eval.rs

@@ -357,7 +357,7 @@ pub fn create_ecx<'mir, 'tcx: 'mir>(
     match entry_type {
         EntryFnType::Main { .. } => {
             let start_id = tcx.lang_items().start_fn().unwrap();
-            let main_ret_ty = tcx.fn_sig(entry_id).subst_identity().output();
+            let main_ret_ty = tcx.fn_sig(entry_id).no_bound_vars().unwrap().output();
             let main_ret_ty = main_ret_ty.no_bound_vars().unwrap();
             let start_instance = ty::Instance::resolve(
                 tcx,