fix: suppress trivial_numeric_casts in derived Encode/Decode

CHANGELOG.md

@@ -7,6 +7,10 @@ and this crate adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.h
 
 ## Unreleased
 
+### Fixed
+
+- Derived `Encode`/`Decode` for enums no longer fails under `#![deny(trivial_numeric_casts)]` ([#740](https://github.com/paritytech/parity-scale-codec/issues/740))
+
 ### [3.7.5] - 2025-05-20
 
 ### Fixed

derive/src/decode.rs

@@ -60,6 +60,7 @@ pub fn quote(
 				quote_spanned! { v.span() =>
 					#[allow(clippy::unnecessary_cast)]
 					#[allow(clippy::cast_possible_truncation)]
+					#[allow(trivial_numeric_casts)]
 					__codec_x_edqy if __codec_x_edqy == (#index) as ::core::primitive::u8 => {
 						// NOTE: This lambda is necessary to work around an upstream bug
 						// where each extra branch results in excessive stack usage:

derive/src/encode.rs

@@ -337,6 +337,7 @@ fn impl_encode(data: &Data, type_name: &Ident, crate_path: &syn::Path) -> TokenS
 						let encoding = quote_spanned! { f.span() =>
 							#type_name :: #name { #( ref #encoding_names, )* } => {
 								#[allow(clippy::unnecessary_cast)]
+								#[allow(trivial_numeric_casts)]
 								#dest.push_byte((#index) as ::core::primitive::u8);
 								#encode_fields
 							}
@@ -370,6 +371,7 @@ fn impl_encode(data: &Data, type_name: &Ident, crate_path: &syn::Path) -> TokenS
 						let encoding = quote_spanned! { f.span() =>
 							#type_name :: #name ( #( ref #encoding_names, )* ) => {
 								#[allow(clippy::unnecessary_cast)]
+								#[allow(trivial_numeric_casts)]
 								#dest.push_byte((#index) as ::core::primitive::u8);
 								#encode_fields
 							}
@@ -388,6 +390,7 @@ fn impl_encode(data: &Data, type_name: &Ident, crate_path: &syn::Path) -> TokenS
 							#type_name :: #name => {
 								#[allow(clippy::unnecessary_cast)]
 								#[allow(clippy::cast_possible_truncation)]
+								#[allow(trivial_numeric_casts)]
 								#dest.push_byte((#index) as ::core::primitive::u8);
 							}
 						};

derive/src/utils.rs

@@ -64,6 +64,7 @@ pub fn const_eval_check_variant_indexes(
 		const _: () = {
 			#[allow(clippy::unnecessary_cast)]
 			#[allow(clippy::cast_possible_truncation)]
+			#[allow(trivial_numeric_casts)]
 			const indices: [(usize, &'static str); #len] = [#( #recurse_indices ,)*];
 
 			const fn search_for_invalid_index(array: &[(usize, &'static str); #len]) -> (bool, usize) {

tests/clippy.rs

@@ -16,6 +16,8 @@
 //! This file is checked by clippy to make sure that the code generated by the derive macro
 //! doesn't spew out warnings/errors in users' code.
 
+#![deny(trivial_numeric_casts)]
+
 use parity_scale_codec_derive::{Decode, DecodeWithMemTracking, Encode};
 
 #[repr(u8)]
@@ -24,3 +26,9 @@ pub enum CLike {
 	Foo = 0,
 	Bar = 1,
 }
+
+#[derive(Decode, DecodeWithMemTracking, Encode)]
+pub enum UnitVariants {
+	One,
+	Two,
+}