ItemPrefixPatch.cs

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using System;
using Loot.Api.Ext;
using Mono.Cecil.Cil;
using MonoMod.Cil;
using Terraria;

namespace Loot.ILEditing
{
    internal class ItemPrefixPatch : ILEdit
    {
        public override void Apply(bool dedServ)
        {
            IL.Terraria.Item.Prefix += ItemOnPrefix;
        }

        private void ItemOnPrefix(ILContext il)
        {
            var cursor = new ILCursor(il);

            // Find !item.accessory check
            if (!cursor.TryGotoNext(MoveType.Before,
                i => i.MatchLdarg(0),
                i => i.MatchLdfld("Terraria.Item", "accessory")))
                return;

            // Append our check
            cursor.Emit(OpCodes.Ldarg_0); // emit item
            cursor.EmitDelegate<Func<Item, bool>>(IsArmor);

            // Clone cursor
            var retCursor = cursor.Clone();
            // Go after the vanilla false return statement
            // we may find either a return op or a br (branching) op for debug
            // in debug there is one ret op at the end of method, with br ops branching to it
            // so we can match either one and we will be in the right position.
            retCursor.TryGotoNext(MoveType.After, i => i.OpCode == OpCodes.Ret || i.OpCode == OpCodes.Br);

            // If our emitted delegate holds true, transfer control to our label
            cursor.Emit(OpCodes.Brtrue, retCursor.MarkLabel());
            // We essentially "goto" our new label is the item is armor, skipping the return or branching
        }

        private bool IsArmor(Item item) => item.IsArmor();
    }
}