Apply a specific rule to exclude JUnit

This avoids excluding junit from the test classpath, even if explicitly added by a downstream user, which doesn't make sense, and allows people to add junit to the main classpath if they need to for some reason.

paperweight-userdev/src/main/kotlin/io/papermc/paperweight/userdev/PaperweightUser.kt

@@ -26,6 +26,7 @@ import io.papermc.paperweight.DownloadService
 import io.papermc.paperweight.PaperweightException
 import io.papermc.paperweight.tasks.*
 import io.papermc.paperweight.userdev.attribute.Obfuscation
+import io.papermc.paperweight.userdev.internal.JunitExclusionRule
 import io.papermc.paperweight.userdev.internal.setup.SetupHandler
 import io.papermc.paperweight.userdev.internal.setup.UserdevSetup
 import io.papermc.paperweight.userdev.internal.setup.util.*
@@ -162,6 +163,10 @@ abstract class PaperweightUser : Plugin<Project> {
                 }
             }
 
+            if (userdev.applyJunitExclusionRule.get()) {
+                applyJunitExclusionRule()
+            }
+
             // Print a friendly error message if the dev bundle is missing before we call anything else that will try and resolve it
             checkForDevBundle()
 
@@ -171,6 +176,12 @@ abstract class PaperweightUser : Plugin<Project> {
         }
     }
 
+    private fun Project.applyJunitExclusionRule() = dependencies {
+        components {
+            withModule<JunitExclusionRule>("com.googlecode.json-simple:json-simple")
+        }
+    }
+
     private fun Project.configureRepositories(userdevSetup: UserdevSetup) = repositories {
         maven(userdevSetup.paramMappings.url) {
             name = PARAM_MAPPINGS_REPO_NAME
@@ -235,7 +246,6 @@ abstract class PaperweightUser : Plugin<Project> {
         }
 
         val mojangMappedServerConfig = target.configurations.register(MOJANG_MAPPED_SERVER_CONFIG) {
-            exclude("junit", "junit") // json-simple exposes junit for some reason
             defaultDependencies {
                 val ctx = createContext(target)
                 userdevSetup.get().let { setup ->

paperweight-userdev/src/main/kotlin/io/papermc/paperweight/userdev/PaperweightUserExtension.kt

@@ -50,6 +50,13 @@ abstract class PaperweightUserExtension(
      */
     val injectPaperRepository: Property<Boolean> = objects.property<Boolean>().convention(true)
 
+    /**
+     * Whether to patch dependencies to exclude `junit:junit` from the transitive dependencies.
+     *
+     * True by default to avoid `junit:junit` appearing on the `compileClasspath`.
+     */
+    val applyJunitExclusionRule: Property<Boolean> = objects.property<Boolean>().convention(true)
+
     /**
      * The [ReobfArtifactConfiguration] is responsible for setting the input and output jars for `reobfJar`,
      * as well as changing the classifiers of other jars (i.e. `jar` or `shadowJar`).

paperweight-userdev/src/main/kotlin/io/papermc/paperweight/userdev/internal/JunitExclusionRule.kt

@@ -0,0 +1,43 @@
+/*
+ * paperweight is a Gradle plugin for the PaperMC project.
+ *
+ * Copyright (c) 2023 Kyle Wood (DenWav)
+ *                    Contributors
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation;
+ * version 2.1 only, no later versions.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301
+ * USA
+ */
+
+package io.papermc.paperweight.userdev.internal
+
+import org.gradle.api.artifacts.CacheableRule
+import org.gradle.api.artifacts.ComponentMetadataContext
+import org.gradle.api.artifacts.ComponentMetadataRule
+
+/**
+ * Excludes `junit:junit` from the transitive dependencies of all variants.
+ */
+@CacheableRule
+abstract class JunitExclusionRule : ComponentMetadataRule {
+    override fun execute(ctx: ComponentMetadataContext) {
+        ctx.details.allVariants {
+            withDependencies {
+                removeIf {
+                    it.group == "junit" && it.name == "junit"
+                }
+            }
+        }
+    }
+}