Better error for missing index in CRV2

substrate/frame/support/procedural/src/runtime/parse/mod.rs

@@ -153,7 +153,9 @@ impl Def {
 
 		for item in items.iter_mut() {
 			let mut pallet_item = None;
-			let mut pallet_index = 0;
+			let mut pallet_index = None;
+
+			let mut is_item_runtime_struct = false;
 
 			let mut disable_call = false;
 			let mut disable_unsigned = false;
@@ -165,12 +167,13 @@ impl Def {
 					RuntimeAttr::Runtime(span) if runtime_struct.is_none() => {
 						let p = runtime_struct::RuntimeStructDef::try_from(span, item)?;
 						runtime_struct = Some(p);
+						is_item_runtime_struct = true;
 					},
 					RuntimeAttr::Derive(_, types) if runtime_types.is_none() => {
 						runtime_types = Some(types);
 					},
 					RuntimeAttr::PalletIndex(span, index) => {
-						pallet_index = index;
+						pallet_index = Some(index);
 						pallet_item = if let syn::Item::Type(item) = item {
 							Some(item.clone())
 						} else {
@@ -187,6 +190,11 @@ impl Def {
 				}
 			}
 
+			if !is_item_runtime_struct && pallet_index.is_none() {
+				let msg = "Missing pallet index for pallet declaration. Please add `#[runtime::pallet_index(...)]`";
+				return Err(syn::Error::new(item.span(), &msg))
+			}
+
 			if let Some(pallet_item) = pallet_item {
 				match *pallet_item.ty.clone() {
 					syn::Type::Path(ref path) => {
@@ -209,7 +217,7 @@ impl Def {
 						let pallet = Pallet::try_from(
 							item.span(),
 							&pallet_item,
-							pallet_index,
+							pallet_index.expect("Pallet item is present only for valid pallet index").into(),
 							disable_call,
 							disable_unsigned,
 							&bounds,

substrate/frame/support/test/tests/runtime_ui/missing_pallet_index.rs

@@ -0,0 +1,27 @@
+// This file is part of Substrate.
+
+// Copyright (C) Parity Technologies (UK) Ltd.
+// SPDX-License-Identifier: Apache-2.0
+
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// 	http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#[frame_support::runtime]
+mod runtime {
+    #[runtime::runtime]
+    #[runtime::derive(RuntimeCall)]
+    pub struct Runtime;
+
+    pub type System = frame_system;
+}
+
+fn main() {}

substrate/frame/support/test/tests/runtime_ui/missing_pallet_index.stderr

@@ -0,0 +1,5 @@
+error: Missing pallet index for pallet declaration. Please add `#[runtime::pallet_index(...)]`
+  --> tests/runtime_ui/missing_pallet_index.rs:24:5
+   |
+24 |     pub type System = frame_system;
+   |     ^^^