Implement 'transactional' annotation for runtime functions. (#6763)

* Implement 'transactional' annotation for runtime functions.

* Allow function attributes for dispatchable calls in decl_module.

* decl_module docs: add transactional function example.

* decl_module docs: add function attributes notes.

* Fix license header.

substrate/frame/support/procedural/src/transactional.rs

@@ -0,0 +1,40 @@
+// This file is part of Substrate.
+
+// Copyright (C) 2020 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.
+
+use proc_macro::TokenStream;
+use quote::quote;
+use syn::{parse_macro_input, ItemFn};
+
+pub fn transactional(_attr: TokenStream, input: TokenStream) -> TokenStream {
+	let ItemFn { attrs, vis, sig, block } = parse_macro_input!(input as ItemFn);
+
+	let output = quote! {
+		#(#attrs)*
+        #vis #sig {
+			use frame_support::storage::{with_transaction, TransactionOutcome};
+			with_transaction(|| {
+				let r = #block;
+				if r.is_ok() {
+					TransactionOutcome::Commit(r)
+				} else {
+					TransactionOutcome::Rollback(r)
+				}
+			})
+        }
+    };
+    output.into()
+}