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/procedural/src/lib.rs

@@ -1411,3 +1411,30 @@ pub fn validate_unsigned(_: TokenStream, _: TokenStream) -> TokenStream {
 pub fn origin(_: TokenStream, _: TokenStream) -> TokenStream {
 	pallet_macro_stub()
 }
+
+/// The `#[pallet::composable_enum]` attribute allows you to define an enum that gets composed as an
+/// aggregate enum by `construct_runtime`. This is similar in principle with `#[pallet::event]` and
+/// `#[pallet::error]`.
+///
+/// The attribute currently only supports enum definitions, and identifiers that are named
+/// `FreezeReason`, `HoldReason`, `LockId` or `SlashReason`. Arbitrary identifiers for the enum are
+/// not supported. The aggregate enum generated by `construct_runtime` will have the name of
+/// `RuntimeFreezeReason`, `RuntimeHoldReason`, `RuntimeLockId` and `RuntimeSlashReason`
+/// respectively.
+///
+/// NOTE: The aggregate enum generated by `construct_runtime` generates a conversion function from
+/// the pallet enum to the aggregate enum, and automatically derives the following traits:
+///
+/// ```ignore
+/// Copy, Clone, Eq, PartialEq, Ord, PartialOrd, Encode, Decode, MaxEncodedLen, TypeInfo,
+/// RuntimeDebug
+/// ```
+///
+/// For ease of usage, when no `#[derive]` attributes are found for the enum under
+/// `#[pallet::composable_enum]`, the aforementioned traits are automatically derived for it. The
+/// inverse is also true: if there are any `#[derive]` attributes found for the enum, then no traits
+/// will automatically be derived for it.
+#[proc_macro_attribute]
+pub fn composable_enum(_: TokenStream, _: TokenStream) -> TokenStream {
+	pallet_macro_stub()
+}