doc: demonstrate use of all non-optional upstream RLP fields

Author: ARR4N

core/types/rlp_backwards_compat.libevm_test.go

@@ -198,15 +198,20 @@ type cChainBodyExtras struct {
 var _ BodyHooks = (*cChainBodyExtras)(nil)
 
 func (e *cChainBodyExtras) RLPFieldsForEncoding(b *Body) ([]any, []any) {
-	return []any{
-		b.Transactions,
-		b.Uncles,
-		e.Version,
-		e.ExtData,
-	}, nil
+	// The Avalanche C-Chain uses all of the geth required fields (but none of
+	// the optional ones) so there's no need to explicitly list them. This
+	// pattern might not be ideal for readability but is used here for
+	// demonstrative purposes.
+	//
+	// All new fields will always be tagged as optional for backwards
+	// compatibility so this is safe to do, but only for the required fields.
+	req, _ /*drop all optional*/ := NOOPBodyHooks{}.RLPFieldsForEncoding(b)
+	return append(req, e.Version, e.ExtData), nil
 }
 
 func (e *cChainBodyExtras) RLPFieldPointersForDecoding(b *Body) ([]any, []any) {
+	// An alternative to the pattern used above is to explicitly list all
+	// fields for better introspection.
 	return []any{
 		&b.Transactions,
 		&b.Uncles,