Fix MaxEncodedLen derive macro for enum with skipped variant (#622)

* fix enum skip variant max encoded len

* trigger CI

* trigger CI

This reverts commit 40f632b90663e49ef79b1d19d054c672d46b22d5.

* fmt

* fix warnings

derive/src/max_encoded_len.rs

@@ -119,12 +119,15 @@ fn data_length_expr(data: &Data, crate_path: &syn::Path) -> proc_macro2::TokenSt
 			//
 			// Each variant expression's sum is computed the way an equivalent struct's would be.
 
-			let expansion = data.variants.iter().map(|variant| {
-				let variant_expression = fields_length_expr(&variant.fields, crate_path);
-				quote! {
-					.max(#variant_expression)
-				}
-			});
+			let expansion =
+				data.variants.iter().filter(|variant| !should_skip(&variant.attrs)).map(
+					|variant| {
+						let variant_expression = fields_length_expr(&variant.fields, crate_path);
+						quote! {
+							.max(#variant_expression)
+						}
+					},
+				);
 
 			quote! {
 				0_usize #( #expansion )* .saturating_add(1)

tests/max_encoded_len.rs

@@ -246,3 +246,41 @@ fn skip_type_params() {
 
 	assert_eq!(SomeData::<u32, SomeStruct>::max_encoded_len(), 4);
 }
+
+#[test]
+fn skip_enum_struct_test() {
+	#[derive(Default)]
+	struct NoCodecType;
+
+	struct NoCodecNoDefaultType;
+
+	#[derive(Encode, Decode, MaxEncodedLen)]
+	enum Enum<T = NoCodecType, S = NoCodecNoDefaultType> {
+		#[codec(skip)]
+		A(S),
+		B {
+			#[codec(skip)]
+			_b1: T,
+			b2: u32,
+		},
+		C(#[codec(skip)] T, u32),
+	}
+
+	#[derive(Encode, Decode, MaxEncodedLen)]
+	struct StructNamed<T = NoCodecType> {
+		#[codec(skip)]
+		a: T,
+		b: u32,
+	}
+
+	#[derive(Encode, Decode, MaxEncodedLen)]
+	struct StructUnnamed<T = NoCodecType>(#[codec(skip)] T, u32);
+
+	assert_eq!(Enum::<NoCodecType, NoCodecNoDefaultType>::max_encoded_len(), 5);
+	assert_eq!(StructNamed::<NoCodecType>::max_encoded_len(), 4);
+	assert_eq!(StructUnnamed::<NoCodecType>::max_encoded_len(), 4);
+
+	// Use the fields to avoid unused warnings.
+	let _ = Enum::<NoCodecType, NoCodecNoDefaultType>::A(NoCodecNoDefaultType);
+	let _ = StructNamed::<NoCodecType> { a: NoCodecType, b: 0 }.a;
+}