Make mixin entries from the mods.toml optional.

loader/build.gradle

@@ -1,5 +1,5 @@
 plugins {
-    id 'net.neoforged.jarcompatibilitychecker' version '0.1.3'
+    id 'net.neoforged.jarcompatibilitychecker' version '0.1.13'
 }
 
 checkJarCompatibility {

loader/src/main/java/net/neoforged/fml/loading/LoadingModList.java

@@ -77,7 +77,13 @@ public void addMixinConfigs() {
                 .map(ModFileInfo::getFile)
                 .forEach(file -> {
                     final String modId = file.getModInfos().get(0).getModId();
-                    file.getMixinConfigs().forEach(cfg -> DeferredMixinConfigRegistration.addMixinConfig(cfg, modId));
+                    file.getMixinConfigs()
+                            .stream()
+                            .filter(potential -> potential.requiredModIds().isEmpty() ||
+                                    potential.requiredModIds().stream().allMatch(
+                                            id -> this.getModFileById(id) != null
+                                    ))
+                            .forEach(cfg -> DeferredMixinConfigRegistration.addMixinConfig(cfg.name(), modId));
                 });
     }
 

loader/src/main/java/net/neoforged/fml/loading/moddiscovery/ModFile.java

@@ -56,7 +56,7 @@ public class ModFile implements IModFile {
     private ModFileScanData fileModFileScanData;
     private volatile CompletableFuture<ModFileScanData> futureScanResult;
     private List<CoreModFile> coreMods;
-    private List<String> mixinConfigs;
+    private List<ModFileParser.PotentialMixinConfig> mixinConfigs;
     private List<Path> accessTransformers;
 
     public static final Attributes.Name TYPE = new Attributes.Name("FMLModType");
@@ -135,7 +135,7 @@ public List<CoreModFile> getCoreMods() {
         return coreMods;
     }
 
-    public List<String> getMixinConfigs() {
+    public List<ModFileParser.PotentialMixinConfig> getMixinConfigs() {
         return mixinConfigs;
     }
 

loader/src/main/java/net/neoforged/fml/loading/moddiscovery/ModFileParser.java

@@ -16,12 +16,14 @@
 import java.lang.reflect.Type;
 import java.nio.file.Files;
 import java.nio.file.Path;
+import java.util.ArrayList;
 import java.util.Collections;
 import java.util.List;
 import java.util.Map;
 import java.util.Optional;
 import net.neoforged.fml.loading.LogMarkers;
 import net.neoforged.fml.loading.moddiscovery.readers.JarModsDotTomlModFileReader;
+import net.neoforged.neoforgespi.language.IConfigurable;
 import net.neoforged.neoforgespi.language.IModFileInfo;
 import net.neoforged.neoforgespi.locating.IModFile;
 import net.neoforged.neoforgespi.locating.InvalidModFileException;
@@ -85,17 +87,28 @@ protected static List<CoreModFile> getCoreMods(final ModFile modFile) {
                 .toList();
     }
 
-    protected static List<String> getMixinConfigs(IModFileInfo modFileInfo) {
+    /**
+     * Represents a potential mixin configuration.
+     *
+     * @param name The name of the mixin configuration.
+     * @param requiredModIds The mod ids that are required for this mixin configuration to be loaded. If empty, will be loaded regardless.
+     */
+    public record PotentialMixinConfig(String name, List<String> requiredModIds) {}
+
+    protected static List<PotentialMixinConfig> getMixinConfigs(IModFileInfo modFileInfo) {
         try {
             var config = modFileInfo.getConfig();
             var mixinsEntries = config.getConfigList("mixins");
-            return mixinsEntries
-                    .stream()
-                    .map(entry -> entry
-                            .<String>getConfigElement("config")
-                            .orElseThrow(
-                                    () -> new InvalidModFileException("Missing \"config\" in [[mixins]] entry", modFileInfo)))
-                    .toList();
+
+            var potentialMixins = new ArrayList<PotentialMixinConfig>();
+            for (IConfigurable mixinsEntry : mixinsEntries) {
+                var name = mixinsEntry.<String>getConfigElement("config")
+                        .orElseThrow(() -> new InvalidModFileException("Missing \"config\" in [[mixins]] entry", modFileInfo));
+                var requiredModIds = mixinsEntry.<List<String>>getConfigElement("requiredMods").orElse(List.of());
+                potentialMixins.add(new PotentialMixinConfig(name, requiredModIds));
+            }
+
+            return potentialMixins;
         } catch (Exception exception) {
             LOGGER.error("Failed to load mixin configs from mod file", exception);
             return List.of();