fix overflow in array length computation

compiler/rustc_hir_typeck/src/generator_interior/mod.rs

@@ -643,15 +643,15 @@ fn check_must_not_suspend_ty<'tcx>(
         }
         ty::Array(ty, len) => {
             let descr_pre = &format!("{}array{} of ", data.descr_pre, plural_suffix);
+            let target_usize = len.try_eval_target_usize(fcx.tcx, fcx.param_env).unwrap_or(0) as usize;
+            let plural_len = target_usize.saturating_add(1);
             check_must_not_suspend_ty(
                 fcx,
                 ty,
                 hir_id,
                 SuspendCheckData {
                     descr_pre,
-                    plural_len: len.try_eval_target_usize(fcx.tcx, fcx.param_env).unwrap_or(0)
+                    plural_len,
-                        as usize
-                        + 1,
                     ..data
                 },
             )