Add the mod

src/main/java/net/shadowfacts/autoswap/mixin/MixinServerPlayerInteractionManager.java

@@ -0,0 +1,54 @@
+package net.shadowfacts.autoswap.mixin;
+
+import net.minecraft.entity.player.PlayerInventory;
+import net.minecraft.item.ItemStack;
+import net.minecraft.item.ItemUsageContext;
+import net.minecraft.server.network.ServerPlayerEntity;
+import net.minecraft.server.network.ServerPlayerInteractionManager;
+import net.minecraft.util.ActionResult;
+import net.minecraft.util.math.BlockPos;
+import net.shadowfacts.autoswap.AutoSwap;
+import org.spongepowered.asm.mixin.Mixin;
+import org.spongepowered.asm.mixin.Shadow;
+import org.spongepowered.asm.mixin.injection.At;
+import org.spongepowered.asm.mixin.injection.Inject;
+import org.spongepowered.asm.mixin.injection.Redirect;
+import org.spongepowered.asm.mixin.injection.callback.CallbackInfoReturnable;
+
+/**
+ * @author shadowfacts
+ */
+@Mixin(ServerPlayerInteractionManager.class)
+public abstract class MixinServerPlayerInteractionManager {
+
+	@Shadow
+	private ServerPlayerEntity player;
+
+	private ItemStack heldStackAtBeginningOfBreak = ItemStack.EMPTY;
+
+	@Inject(method = "tryBreakBlock", at = @At("HEAD"))
+	private void beginTryBreakBlock(BlockPos pos, CallbackInfoReturnable<Void> cb) {
+		PlayerInventory playerInv = player.inventory;
+		heldStackAtBeginningOfBreak = playerInv.getInvStack(playerInv.selectedSlot).copy();
+	}
+
+	@Inject(method = "tryBreakBlock", at = @At("RETURN"))
+	private void endTryBreakBlock(BlockPos pos, CallbackInfoReturnable<Void> cb) {
+		AutoSwap.INSTANCE.endBreakBlock(player.inventory, heldStackAtBeginningOfBreak);
+		heldStackAtBeginningOfBreak = ItemStack.EMPTY;
+	}
+
+	@Redirect(
+			method = "interactBlock",
+			at = @At(value = "INVOKE", target = "Lnet/minecraft/item/ItemStack;useOnBlock(Lnet/minecraft/item/ItemUsageContext;)Lnet/minecraft/util/ActionResult;")
+	)
+	private ActionResult useOnBlock(ItemStack self, ItemUsageContext context) {
+		ItemStack selfCopy = self.copy();
+		ActionResult result = self.useOnBlock(context);
+		if (result == ActionResult.SUCCESS) {
+			AutoSwap.INSTANCE.onUseOnItem(player, player.inventory, selfCopy);
+		}
+		return result;
+	}
+
+}

src/main/kotlin/net/shadowfacts/autoswap/AutoSwap.kt

@@ -1,7 +1,47 @@
 package net.shadowfacts.autoswap
 
+import net.fabricmc.api.ModInitializer
+import net.minecraft.client.network.packet.EntityEquipmentUpdateS2CPacket
+import net.minecraft.client.network.packet.ItemPickupAnimationS2CPacket
+import net.minecraft.entity.EquipmentSlot
+import net.minecraft.entity.player.PlayerEntity
+import net.minecraft.entity.player.PlayerInventory
+import net.minecraft.item.ItemStack
+import net.minecraft.server.network.ServerPlayerEntity
+import net.minecraft.util.math.BlockPos
+
 /**
  * @author shadowfacts
  */
-class AutoSwap {
+object AutoSwap: ModInitializer {
+
+	override fun onInitialize() {
+	}
+
+	fun endBreakBlock(playerInv: PlayerInventory, heldStackAtBeginningOfBreak: ItemStack) {
+		val currentlySelected = playerInv.mainHandStack
+		if (currentlySelected.isEmpty && !heldStackAtBeginningOfBreak.isEmpty) {
+			val index = playerInv.main.indexOfFirst {
+				ItemStack.areItemsEqualIgnoreDamage(heldStackAtBeginningOfBreak, it)
+			}
+			if (index >= 0) {
+				playerInv.swapSlotWithHotbar(index)
+			}
+		}
+	}
+
+	fun onUseOnItem(player: ServerPlayerEntity, playerInv: PlayerInventory, heldStackBeforeUse: ItemStack) {
+		val heldStack = playerInv.mainHandStack
+		if (!player.isCreative && heldStack.isEmpty) {
+			val index = playerInv.main.indexOfFirst {
+				ItemStack.areItemsEqualIgnoreDamage(heldStackBeforeUse, it)
+			}
+			if (index >= 0) {
+				playerInv.swapSlotWithHotbar(index)
+				val newStack = playerInv.mainHandStack
+				player.networkHandler.sendPacket(EntityEquipmentUpdateS2CPacket(player.entityId, EquipmentSlot.MAINHAND, newStack))
+			}
+		}
+	}
+
 }

src/main/resources/autoswap.mixins.json

@@ -0,0 +1,13 @@
+{
+	"required": true,
+	"package": "net.shadowfacts.autoswap.mixin",
+	"compatibilityLevel": "JAVA_8",
+	"mixins": [
+		"MixinServerPlayerInteractionManager"
+	],
+	"client": [
+	],
+	"injectors": {
+		"defaultRequire": 1
+	}
+}

src/main/resources/fabric.mod.json

@@ -24,7 +24,9 @@
 			}
 		]
 	},
-	"mixins": [],
+	"mixins": [
+		"autoswap.mixins.json"
+	],
 
 	"depends": {
 		"fabricloader": ">=0.4.0",