storage-alias: Check that prefix is not an underscore (#11704)

Besides that it also adds some UI tests.

substrate/frame/support/test/tests/storage_alias_ui.rs

@@ -0,0 +1,32 @@
+// This file is part of Substrate.
+
+// Copyright (C) 2020-2022 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.
+
+#[rustversion::attr(not(stable), ignore)]
+#[cfg(not(feature = "disable-ui-tests"))]
+#[test]
+fn storage_alias_ui() {
+	// Only run the ui tests when `RUN_UI_TESTS` is set.
+	if std::env::var("RUN_UI_TESTS").is_err() {
+		return
+	}
+
+	// As trybuild is using `cargo check`, we don't need the real WASM binaries.
+	std::env::set_var("SKIP_WASM_BUILD", "1");
+
+	let t = trybuild::TestCases::new();
+	t.compile_fail("tests/storage_alias_ui/*.rs");
+}

substrate/frame/support/test/tests/storage_alias_ui/checks_for_valid_storage_type.rs

@@ -0,0 +1,4 @@
+#[frame_support::storage_alias]
+type Ident = StorageValue<hello::World, u32>;
+
+fn main() {}

substrate/frame/support/test/tests/storage_alias_ui/checks_for_valid_storage_type.stderr

@@ -0,0 +1,5 @@
+error: If there are no generics, the prefix is only allowed to be an identifier.
+ --> tests/storage_alias_ui/checks_for_valid_storage_type.rs:2:27
+  |
+2 | type Ident = StorageValue<hello::World, u32>;
+  |                           ^^^^^^^^^^^^

substrate/frame/support/test/tests/storage_alias_ui/forbid_underscore_as_prefix.rs

@@ -0,0 +1,4 @@
+#[frame_support::storage_alias]
+type Ident = CustomStorage<Hello, u32>;
+
+fn main() {}

substrate/frame/support/test/tests/storage_alias_ui/forbid_underscore_as_prefix.stderr

@@ -0,0 +1,5 @@
+error: expected one of: `StorageValue`, `StorageMap`, `StorageDoubleMap`, `StorageNMap`
+ --> tests/storage_alias_ui/forbid_underscore_as_prefix.rs:2:14
+  |
+2 | type Ident = CustomStorage<Hello, u32>;
+  |              ^^^^^^^^^^^^^

substrate/frame/support/test/tests/storage_alias_ui/prefix_must_be_an_ident.rs

@@ -0,0 +1,4 @@
+#[frame_support::storage_alias]
+type NoUnderscore = StorageValue<_, u32>;
+
+fn main() {}

substrate/frame/support/test/tests/storage_alias_ui/prefix_must_be_an_ident.stderr

@@ -0,0 +1,5 @@
+error: `_` is not allowed as prefix by `storage_alias`.
+ --> tests/storage_alias_ui/prefix_must_be_an_ident.rs:2:34
+  |
+2 | type NoUnderscore = StorageValue<_, u32>;
+  |                                  ^