Allow pallet error enum variants to contain fields

frame/support/procedural/src/pallet/parse/error.rs

@@ -18,7 +18,7 @@
 use super::helper;
 use frame_support_procedural_tools::get_doc_literals;
 use quote::ToTokens;
-use syn::spanned::Spanned;
+use syn::{spanned::Spanned, Fields};
 
 /// List of additional token to be used for parsing.
 mod keyword {
@@ -70,12 +70,18 @@ impl ErrorDef {
 			.variants
 			.iter()
 			.map(|variant| {
-				if !matches!(variant.fields, syn::Fields::Unit) {
-					let msg = "Invalid pallet::error, unexpected fields, must be `Unit`";
-					return Err(syn::Error::new(variant.fields.span(), msg))
+				match &variant.fields {
+					Fields::Unit => {},
+					Fields::Named(f) if f.named.len() < 2 => {},
+					Fields::Unnamed(u) if u.unnamed.len() < 2 => {},
+					_ => {
+						let msg = "Invalid pallet::error, unexpected fields, must be `Unit` or \
+							contain only 1 field";
+						return Err(syn::Error::new(variant.fields.span(), msg))
+					},
 				}
 				if variant.discriminant.is_some() {
-					let msg = "Invalid pallet::error, unexpected discriminant, discriminant \
+					let msg = "Invalid pallet::error, unexpected discriminant, discriminants \
 						are not supported";
 					let span = variant.discriminant.as_ref().unwrap().0.span();
 					return Err(syn::Error::new(span, msg))

frame/support/src/lib.rs

@@ -1614,10 +1614,16 @@ pub mod pallet_prelude {
 /// pub enum Error<T> {
 /// 	/// $some_optional_doc
 /// 	$SomeFieldLessVariant,
+/// 	/// $some_more_optional_doc
+/// 	$SomeSingleFieldVariant(FieldType),
 /// 	...
 /// }
 /// ```
-/// I.e. a regular rust enum named `Error`, with generic `T` and fieldless variants.
+/// I.e. a regular rust enum named `Error`, with generic `T` and fieldless or single-field
+/// variants.
+/// Any field in the enum variants must implement `scale_info::TypeInfo` in order to be
+/// properly used in the metadata, and its encoded size should be as small as possible,
+/// preferably 1 byte in size.
 /// The generic `T` mustn't bound anything and where clause is not allowed. But bounds and
 /// where clause shouldn't be needed for any usecase.
 ///

frame/support/test/tests/pallet_ui/error_no_fieldless.rs → frame/support/test/tests/pallet_ui/error_more_than_1_field.rs

@@ -17,6 +17,7 @@ mod pallet {
 
 	#[pallet::error]
 	pub enum Error<T> {
+		Tuple(u8, u8),
 		U8(u8),
 	}
 }

frame/support/test/tests/pallet_ui/error_more_than_1_field.stderr

@@ -0,0 +1,5 @@
+error: Invalid pallet::error, unexpected fields, must be `Unit` or contain only 1 field
+  --> $DIR/error_more_than_1_field.rs:20:8
+   |
+20 |         Tuple(u8, u8),
+   |              ^^^^^^^^