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.rs

@@ -476,6 +476,11 @@ pub mod pallet {
 		}
 	}
 
+	#[pallet::composite_enum]
+	pub enum HoldReason {
+		Staking,
+	}
+
 	#[derive(codec::Encode, sp_runtime::RuntimeDebug)]
 	#[cfg_attr(feature = "std", derive(codec::Decode))]
 	pub enum InherentError {
@@ -570,6 +575,16 @@ pub mod pallet2 {
 	{
 		fn build(&self) {}
 	}
+
+	#[pallet::composite_enum]
+	pub enum HoldReason {
+		Governance,
+	}
+
+	#[pallet::composite_enum]
+	pub enum SlashReason {
+		Equivocation,
+	}
 }
 
 /// Test that the supertrait check works when we pass some parameter to the `frame_system::Config`.
@@ -974,6 +989,17 @@ fn validate_unsigned_expand() {
 	assert_eq!(validity, ValidTransaction::default());
 }
 
+#[test]
+fn composite_expand() {
+	let hold_reason: RuntimeHoldReason = pallet::HoldReason::Staking.into();
+	let hold_reason2: RuntimeHoldReason = pallet2::HoldReason::Governance.into();
+	let slash_reason: RuntimeSlashReason = pallet2::SlashReason::Equivocation.into();
+
+	assert_eq!(hold_reason, RuntimeHoldReason::Example(pallet::HoldReason::Staking));
+	assert_eq!(hold_reason2, RuntimeHoldReason::Example2(pallet2::HoldReason::Governance));
+	assert_eq!(slash_reason, RuntimeSlashReason::Example2(pallet2::SlashReason::Equivocation));
+}
+
 #[test]
 fn pallet_expand_deposit_event() {
 	TestExternalities::default().execute_with(|| {