Implement #[pallet::composite_enum] (#13722)

* Implement #[pallet::hold_reason]

* Appease clippy

* cargo fmt

* Update test expectations

* Update test expectations

* Support composite_enum attribute instead

* Update test expectations

* Change hold_reason to composite_enum

* Add UI test for unsupported identifier when using composite_enum

* Fix comment

* Add documentation for pallet::composable_enum

* More docs

* cargo fmt

substrate/frame/support/test/tests/pallet_ui/composite_enum_unsupported_identifier.rs

@@ -0,0 +1,13 @@
+#[frame_support::pallet]
+mod pallet {
+	#[pallet::config]
+	pub trait Config: frame_system::Config {}
+
+	#[pallet::pallet]
+	pub struct Pallet<T>(core::marker::PhantomData<T>);
+
+	#[pallet::composite_enum]
+	pub enum HoldReasons {}
+}
+
+fn main() {}

substrate/frame/support/test/tests/pallet_ui/composite_enum_unsupported_identifier.stderr

@@ -0,0 +1,5 @@
+error: expected one of: `FreezeReason`, `HoldReason`, `LockId`, `SlashReason`
+  --> tests/pallet_ui/composite_enum_unsupported_identifier.rs:10:11
+   |
+10 |     pub enum HoldReasons {}
+   |              ^^^^^^^^^^^

substrate/frame/support/test/tests/pallet_ui/event_wrong_item.stderr

@@ -1,4 +1,4 @@
-error: Invalid pallet::event, expected item enum
+error: Invalid pallet::event, expected enum item
   --> $DIR/event_wrong_item.rs:19:2
    |
 19 |     pub struct Foo;

substrate/frame/support/test/tests/pallet_ui/hold_reason_non_enum.rs

@@ -0,0 +1,14 @@
+#[frame_support::pallet]
+mod pallet {
+	#[pallet::config]
+	pub trait Config: frame_system::Config {}
+
+	#[pallet::pallet]
+	pub struct Pallet<T>(core::marker::PhantomData<T>);
+
+	#[pallet::composite_enum]
+	pub struct HoldReason;
+}
+
+fn main() {
+}

substrate/frame/support/test/tests/pallet_ui/hold_reason_non_enum.stderr

@@ -0,0 +1,5 @@
+error: Invalid pallet::composite_enum, expected enum item
+  --> tests/pallet_ui/hold_reason_non_enum.rs:10:2
+   |
+10 |     pub struct HoldReason;
+   |     ^^^

substrate/frame/support/test/tests/pallet_ui/hold_reason_not_pub.rs

@@ -0,0 +1,14 @@
+#[frame_support::pallet]
+mod pallet {
+	#[pallet::config]
+	pub trait Config: frame_system::Config {}
+
+	#[pallet::pallet]
+	pub struct Pallet<T>(core::marker::PhantomData<T>);
+
+	#[pallet::composite_enum]
+    enum HoldReason {}
+}
+
+fn main() {
+}

substrate/frame/support/test/tests/pallet_ui/hold_reason_not_pub.stderr

@@ -0,0 +1,5 @@
+error: Invalid pallet::composite_enum, `HoldReason` must be public
+  --> tests/pallet_ui/hold_reason_not_pub.rs:10:5
+   |
+10 |     enum HoldReason {}
+   |     ^^^^

substrate/frame/support/test/tests/pallet_ui/lock_id_duplicate.rs

@@ -0,0 +1,17 @@
+#[frame_support::pallet]
+mod pallet {
+	#[pallet::config]
+	pub trait Config: frame_system::Config {}
+
+	#[pallet::pallet]
+	pub struct Pallet<T>(core::marker::PhantomData<T>);
+
+	#[pallet::composite_enum]
+    pub enum LockId {}
+
+	#[pallet::composite_enum]
+    pub enum LockId {}
+}
+
+fn main() {
+}

substrate/frame/support/test/tests/pallet_ui/lock_id_duplicate.stderr

@@ -0,0 +1,5 @@
+error: Invalid duplicated `LockId` definition
+  --> tests/pallet_ui/lock_id_duplicate.rs:13:14
+   |
+13 |     pub enum LockId {}
+   |              ^^^^^^