Improve the error message for missing fields

Author: ErrorCraft

src/main/java/net/errorcraft/codecium/mixin/mojang/serialization/codecs/FieldDecoderExtender.java

@@ -0,0 +1,30 @@
+package net.errorcraft.codecium.mixin.mojang.serialization.codecs;
+
+import com.llamalad7.mixinextras.sugar.Local;
+import com.mojang.serialization.MapLike;
+import com.mojang.serialization.codecs.FieldDecoder;
+import org.spongepowered.asm.mixin.Final;
+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.ModifyArg;
+
+import java.util.function.Supplier;
+
+@Mixin(value = FieldDecoder.class, remap = false)
+public class FieldDecoderExtender {
+    @Shadow
+    @Final
+    protected String name;
+
+    @ModifyArg(
+        method = "decode",
+        at = @At(
+            value = "INVOKE",
+            target = "Lcom/mojang/serialization/DataResult;error(Ljava/util/function/Supplier;)Lcom/mojang/serialization/DataResult;"
+        )
+    )
+    private <T> Supplier<String> useBetterErrorMessage(Supplier<String> message, @Local(argsOnly = true) final MapLike<T> input) {
+        return () -> "Key '" + this.name +  "' must be present in map: " + input;
+    }
+}

src/main/resources/codecium.mixins.json

@@ -10,6 +10,7 @@
     "mojang.serialization.JavaOpsExtender",
     "mojang.serialization.JsonOpsExtender",
     "mojang.serialization.codecs.EitherCodecExtender",
+    "mojang.serialization.codecs.FieldDecoderExtender",
     "mojang.serialization.codecs.OptionalFieldCodecExtender",
     "mojang.serialization.codecs.RecordCodecBuilderExtender$RecordMapCodecExtender"
   ],