Escape each element of the IntelliJ module name

src/main/java/net/neoforged/moddevgradle/internal/IntelliJIntegration.java

@@ -256,21 +256,39 @@ private static String getIdeaOutName(final SourceSet sourceSet) {
      * Convert a project and source set to an IntelliJ module name.
      * Do not use {@link ModuleRef} as it does not correctly handle projects with a space in their name!
      */
-    private static String getIntellijModuleName(Project project, SourceSet sourceSet) {
+    static String getIntellijModuleName(Project project, SourceSet sourceSet) {
+        // IntelliJ escapes each element of the Gradle path on its own and joins them with '.',
+        // so an element that itself contains a '.' (a subproject named "1.21.1-neoforge", say)
+        // becomes "1_21_1-neoforge". Joining the raw path with '.' instead produces a module
+        // name that does not exist, and the generated run configuration ends up without a module.
+        // See GradleProjectResolverUtil#getHolderModuleName and #escapeModuleNameElement:
+        // https://github.com/JetBrains/intellij-community/blob/711ef1fbda55f43c3943d379f3f16dccfa6760eb/plugins/gradle/src/org/jetbrains/plugins/gradle/service/project/GradleProjectResolverUtil.java
         var moduleName = new StringBuilder();
-        // The `replace` call here is our bug fix compared to ModuleRef!
-        // The actual IDEA logic is more complicated, but this should cover the majority of use cases.
-        // See https://github.com/JetBrains/intellij-community/blob/a32fd0c588a6da11fd6d5d2fb0362308da3206f3/plugins/gradle/src/org/jetbrains/plugins/gradle/service/project/GradleProjectResolverUtil.java#L205
-        // which calls https://github.com/JetBrains/intellij-community/blob/a32fd0c588a6da11fd6d5d2fb0362308da3206f3/platform/util-rt/src/com/intellij/util/PathUtilRt.java#L120
-        moduleName.append(project.getRootProject().getName().replace(" ", "_"));
+        moduleName.append(escapeModuleNameElement(project.getRootProject().getName()));
         if (project != project.getRootProject()) {
-            moduleName.append(project.getPath().replaceAll(":", "."));
+            for (var element : project.getPath().split(":")) {
+                if (!element.isEmpty()) {
+                    moduleName.append(".").append(escapeModuleNameElement(element));
+                }
+            }
         }
         moduleName.append(".");
-        moduleName.append(sourceSet.getName());
+        moduleName.append(escapeModuleNameElement(sourceSet.getName()));
         return moduleName.toString();
     }
 
+    /**
+     * Escapes a single element of an IntelliJ module name, mirroring
+     * {@code GradleProjectResolverUtil#escapeModuleNameElement}.
+     */
+    static String escapeModuleNameElement(String element) {
+        return element
+                .replace(" ", "_")
+                .replace("/", "_")
+                .replace("\\", "_")
+                .replace(".", "_");
+    }
+
     private static Map<String, Object> getExtraIntelijRunProperties(RunModel run) {
         var extraProperties = new HashMap<String, Object>();
         if (!run.getIdeFolderName().get().isEmpty()) {

src/test/java/net/neoforged/moddevgradle/internal/IntelliJIntegrationTest.java

@@ -0,0 +1,56 @@
+package net.neoforged.moddevgradle.internal;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import net.neoforged.moddevgradle.internal.utils.ExtensionUtils;
+import org.gradle.api.Project;
+import org.gradle.api.tasks.SourceSet;
+import org.gradle.testfixtures.ProjectBuilder;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.CsvSource;
+
+class IntelliJIntegrationTest {
+    @ParameterizedTest
+    @CsvSource(textBlock = """
+            plain|plain
+            with space|with_space
+            1.21.1-neoforge|1_21_1-neoforge
+            with/slash|with_slash
+            """, delimiter = '|')
+    void testEscapeModuleNameElement(String element, String escaped) {
+        assertEquals(escaped, IntelliJIntegration.escapeModuleNameElement(element));
+    }
+
+    @Test
+    void testModuleNameOfRootProject() {
+        var root = ProjectBuilder.builder().withName("xaeronav").build();
+        assertEquals("xaeronav.main", getMainModuleName(root));
+    }
+
+    /**
+     * IntelliJ escapes each element of the path separately, so a '.' in a subproject name becomes
+     * '_'. Joining the raw path would produce "xaeronav.1.21.1-neoforge.main", which is not a
+     * module that exists, and the run configuration would be written without a module.
+     */
+    @Test
+    void testModuleNameOfSubprojectContainingDots() {
+        var root = ProjectBuilder.builder().withName("xaeronav").build();
+        var subproject = ProjectBuilder.builder().withName("1.21.1-neoforge").withParent(root).build();
+        assertEquals("xaeronav.1_21_1-neoforge.main", getMainModuleName(subproject));
+    }
+
+    @Test
+    void testModuleNameOfNestedSubproject() {
+        var root = ProjectBuilder.builder().withName("root").build();
+        var parent = ProjectBuilder.builder().withName("versions").withParent(root).build();
+        var subproject = ProjectBuilder.builder().withName("1.21.1").withParent(parent).build();
+        assertEquals("root.versions.1_21_1.main", getMainModuleName(subproject));
+    }
+
+    private static String getMainModuleName(Project project) {
+        project.getPluginManager().apply("java");
+        SourceSet sourceSet = ExtensionUtils.getSourceSets(project).getByName(SourceSet.MAIN_SOURCE_SET_NAME);
+        return IntelliJIntegration.getIntellijModuleName(project, sourceSet);
+    }
+}