Add way to add new launch arguments, because this can change without regenerating the run configs.

src/main/java/net/fabricmc/loom/LoomGradleExtension.java

@@ -55,6 +55,7 @@ import net.fabricmc.loom.api.decompilers.LoomDecompiler;
 import net.fabricmc.loom.configuration.LoomDependencyManager;
 import net.fabricmc.loom.configuration.ide.RunConfig;
 import net.fabricmc.loom.configuration.ide.RunConfigSettings;
+import net.fabricmc.loom.configuration.launch.LaunchProviderSettings;
 import net.fabricmc.loom.configuration.processors.JarProcessor;
 import net.fabricmc.loom.configuration.processors.JarProcessorManager;
 import net.fabricmc.loom.configuration.providers.MinecraftProvider;
@@ -112,6 +113,7 @@ public class LoomGradleExtension {
 	public final List<Consumer<RunConfig>> settingsPostEdit = new ArrayList<>();
 
 	private NamedDomainObjectContainer<RunConfigSettings> runConfigs;
+	private NamedDomainObjectContainer<LaunchProviderSettings> launchConfigs;
 
 	/**
 	 * Loom will generate a new genSources task (with a new name, based off of {@link LoomDecompiler#name()})
@@ -137,6 +139,7 @@ public class LoomGradleExtension {
 	}
 
 	public Mercury getOrCreateSrcMercuryCache(int id, Supplier<Mercury> factory) {
+		if (id == -1) return factory.get();
 		return srcMercuryCache[id] != null ? srcMercuryCache[id] : (srcMercuryCache[id] = factory.get());
 	}
 
@@ -216,6 +219,8 @@ public class LoomGradleExtension {
 		this.forge = new LazyBool(() -> Boolean.parseBoolean(Objects.toString(project.findProperty(FORGE_PROPERTY))));
 		this.runConfigs = project.container(RunConfigSettings.class,
 				baseName -> new RunConfigSettings(project, baseName));
+		this.launchConfigs = project.container(LaunchProviderSettings.class,
+				baseName -> new LaunchProviderSettings(project, baseName));
 	}
 
 	/**
@@ -495,8 +500,18 @@ public class LoomGradleExtension {
 		action.execute(runConfigs);
 	}
 
+	@ApiStatus.Experimental
+	public void launches(Action<NamedDomainObjectContainer<LaunchProviderSettings>> action) {
+		action.execute(launchConfigs);
+	}
+
 	@ApiStatus.Experimental
 	public NamedDomainObjectContainer<RunConfigSettings> getRunConfigs() {
 		return runConfigs;
 	}
+
+	@ApiStatus.Experimental
+	public NamedDomainObjectContainer<LaunchProviderSettings> getLaunchConfigs() {
+		return launchConfigs;
+	}
 }

src/main/java/net/fabricmc/loom/configuration/launch/LaunchProviderSettings.java

@@ -0,0 +1,54 @@
+package net.fabricmc.loom.configuration.launch;
+
+import java.util.AbstractMap;
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.Collection;
+import java.util.List;
+import java.util.Map;
+
+import org.gradle.api.Named;
+import org.gradle.api.Project;
+
+public class LaunchProviderSettings implements Named {
+	private final String name;
+	private List<Map.Entry<String, String>> properties = new ArrayList<>();
+	private List<String> arguments = new ArrayList<>();
+
+	public LaunchProviderSettings(Project project, String name) {
+		this.name = name;
+	}
+
+	@Override
+	public String getName() {
+		return name;
+	}
+
+	public void arg(String argument) {
+		this.arguments.add(argument);
+	}
+
+	public void arg(String... arguments) {
+		this.arguments.addAll(Arrays.asList(arguments));
+	}
+
+	public void arg(Collection<String> arguments) {
+		this.arguments.addAll(arguments);
+	}
+
+	public void property(String key, String value) {
+		this.properties.add(new AbstractMap.SimpleEntry<>(key, value));
+	}
+
+	public void properties(Map<String, String> arguments) {
+		this.properties.addAll(arguments.entrySet());
+	}
+
+	public List<Map.Entry<String, String>> getProperties() {
+		return properties;
+	}
+
+	public List<String> getArguments() {
+		return arguments;
+	}
+}

src/main/java/net/fabricmc/loom/configuration/providers/LaunchProvider.java

@@ -45,6 +45,7 @@ import org.gradle.api.plugins.JavaPlugin;
 
 import net.fabricmc.loom.configuration.DependencyProvider;
 import net.fabricmc.loom.configuration.RemappedConfigurationEntry;
+import net.fabricmc.loom.configuration.launch.LaunchProviderSettings;
 import net.fabricmc.loom.util.Constants;
 
 public class LaunchProvider extends DependencyProvider {
@@ -116,6 +117,16 @@ public class LaunchProvider extends DependencyProvider {
 			}
 		}
 
+		for (LaunchProviderSettings settings : getExtension().getLaunchConfigs()) {
+			for (String argument : settings.getArguments()) {
+				launchConfig.argument(settings.getName(), argument);
+			}
+
+			for (Map.Entry<String, String> property : settings.getProperties()) {
+				launchConfig.property(settings.getName(), property.getKey(), property.getValue());
+			}
+		}
+
 		//Enable ansi by default for idea and vscode
 		if (new File(getProject().getRootDir(), ".vscode").exists()
 				|| new File(getProject().getRootDir(), ".idea").exists()