RenderDifferentItemInHandMixin.java

@ModifyVariable(method = "renderArmWithItem(Lnet/minecraft/client/player/AbstractClientPlayer;FFLnet/minecraft/world/InteractionHand;FLnet/minecraft/world/item/ItemStack;FLcom/mojang/blaze3d/vertex/PoseStack;Lnet/minecraft/client/renderer/MultiBufferSource;I)V",
                    at = @At("HEAD"), index = 6, ordinal = 0, argsOnly = true)
private ItemStack overwriteItemStack(ItemStack stack, AbstractClientPlayer player, float f, float g, InteractionHand hand) {
  // Your code here. You can check the hand to see what hand we're currently rendering, and the ItemStack returned from
  // this function is what will be used in rendering (you can return the provided stack to just render the item that was
  // going to be rendered anyways).
}