Add inspection for wrong mixin class type. Closes #2549

Author: Earthcomputer

src/main/kotlin/platform/mixin/inspection/MixinClassTypeInspection.kt

@@ -0,0 +1,118 @@
+/*
+ * Minecraft Development for IntelliJ
+ *
+ * https://mcdev.io/
+ *
+ * Copyright (C) 2025 minecraft-dev
+ *
+ * This program 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 3.0 only.
+ *
+ * This program 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 General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+package com.demonwav.mcdev.platform.mixin.inspection
+
+import com.demonwav.mcdev.platform.mixin.util.hasAccess
+import com.demonwav.mcdev.platform.mixin.util.isAccessorMixin
+import com.demonwav.mcdev.platform.mixin.util.isMixin
+import com.demonwav.mcdev.platform.mixin.util.mixinTargets
+import com.intellij.codeInspection.LocalQuickFix
+import com.intellij.codeInspection.LocalQuickFixOnPsiElement
+import com.intellij.codeInspection.ProblemsHolder
+import com.intellij.openapi.project.Project
+import com.intellij.psi.JavaElementVisitor
+import com.intellij.psi.JavaPsiFacade
+import com.intellij.psi.JavaTokenType
+import com.intellij.psi.PsiClass
+import com.intellij.psi.PsiElement
+import com.intellij.psi.PsiFile
+import com.intellij.psi.PsiKeyword
+import com.intellij.psi.tree.TokenSet
+import org.objectweb.asm.Opcodes
+
+class MixinClassTypeInspection : MixinInspection() {
+    override fun getStaticDescription() = "Reports when a Mixin uses the wrong class type"
+
+    override fun buildVisitor(holder: ProblemsHolder) = object : JavaElementVisitor() {
+        override fun visitClass(mixinClass: PsiClass) {
+            if (!mixinClass.isMixin) {
+                return
+            }
+            if (mixinClass.isAccessorMixin) {
+                return
+            }
+
+            val classKeywordElement = mixinClass.children.firstOrNull {
+                (it as? PsiKeyword)?.tokenType in CLASS_KEYWORD_SET
+            }
+            val problemElement = classKeywordElement ?: mixinClass.nameIdentifier ?: mixinClass
+
+            val needsToBeClass = mixinClass.mixinTargets.any { !it.hasAccess(Opcodes.ACC_INTERFACE) }
+            val needsToBeInterface = mixinClass.mixinTargets.any { it.hasAccess(Opcodes.ACC_INTERFACE) }
+
+            val fixes = mutableListOf<LocalQuickFix>()
+            if (classKeywordElement != null) {
+                if (needsToBeClass && !needsToBeInterface) {
+                    fixes += ChangeClassTypeFix(classKeywordElement, PsiKeyword.CLASS)
+                } else if (needsToBeInterface && !needsToBeClass) {
+                    fixes += ChangeClassTypeFix(classKeywordElement, PsiKeyword.INTERFACE)
+                }
+            }
+
+            if (mixinClass.isEnum) {
+                holder.registerProblem(problemElement, "Mixins cannot be enums", *fixes.toTypedArray())
+                return
+            }
+
+            if (mixinClass.isAnnotationType) {
+                holder.registerProblem(problemElement, "Mixins cannot be annotation types", *fixes.toTypedArray())
+                return
+            }
+
+            if (mixinClass.isRecord) {
+                holder.registerProblem(problemElement, "Mixins cannot be records", *fixes.toTypedArray())
+                return
+            }
+
+            if (mixinClass.isInterface) {
+                if (needsToBeClass) {
+                    holder.registerProblem(problemElement, "Interface Mixin targets a class", *fixes.toTypedArray())
+                }
+            } else {
+                if (needsToBeInterface) {
+                    holder.registerProblem(problemElement, "Class Mixin targets an interface", *fixes.toTypedArray())
+                }
+            }
+        }
+    }
+
+    companion object {
+        private val CLASS_KEYWORD_SET = TokenSet.create(
+            JavaTokenType.CLASS_KEYWORD,
+            JavaTokenType.INTERFACE_KEYWORD,
+            JavaTokenType.ENUM_KEYWORD,
+            JavaTokenType.RECORD_KEYWORD,
+        )
+    }
+
+    private class ChangeClassTypeFix(
+        classKeywordElement: PsiElement,
+        private val newKeyword: String
+    ) : LocalQuickFixOnPsiElement(classKeywordElement) {
+        override fun getFamilyName() = "Change class type to $newKeyword"
+        override fun getText() = "Change class type to $newKeyword"
+
+        override fun invoke(project: Project, file: PsiFile, startElement: PsiElement, endElement: PsiElement) {
+            val factory = JavaPsiFacade.getElementFactory(project)
+            startElement.replace(factory.createKeyword(newKeyword))
+        }
+    }
+}

src/main/kotlin/platform/mixin/inspection/MixinSuperClassInspection.kt

@@ -46,6 +46,11 @@ class MixinSuperClassInspection : MixinInspection() {
                 return
             }
 
+            if (psiClass.isEnum || psiClass.isAnnotationType || psiClass.isRecord) {
+                // these will be reported by MixinClassTypeInspection
+                return
+            }
+
             val superClass = psiClass.superClass ?: return
             if (superClass.qualifiedName == CommonClassNames.JAVA_LANG_OBJECT) {
                 return
@@ -87,7 +92,7 @@ class MixinSuperClassInspection : MixinInspection() {
         }
 
         private fun reportSuperClass(psiClass: PsiClass, description: String) {
-            holder.registerProblem(psiClass.extendsList?.referenceElements?.firstOrNull() ?: psiClass, description)
+            holder.registerProblem(psiClass.extendsList?.referenceElements?.firstOrNull() ?: psiClass.nameIdentifier ?: psiClass, description)
         }
     }
 }

src/main/kotlin/platform/mixin/util/Mixin.kt

@@ -116,7 +116,7 @@ val PsiClass.bytecode: ClassNode?
  * 1. The class given is a Mixin.
  * 2. The class given is an interface.
  * 3. All member methods are decorated with either `@Accessor` or `@Invoker`.
- * 4. All Mixin targets are classes.
+ * 4. None of the Mixin targets are interfaces.
  *
  * @receiver The class to check
  * @return True if the above checks are satisfied.

src/main/resources/META-INF/plugin.xml

@@ -1102,6 +1102,14 @@
                          level="ERROR"
                          hasStaticDescription="true"
                          implementationClass="com.demonwav.mcdev.platform.mixin.inspection.MixinInnerClassInspection"/>
+        <localInspection displayName="Mixin is wrong class type"
+                         shortName="MixinWrongClassType"
+                         groupName="Mixin"
+                         language="JAVA"
+                         enabledByDefault="true"
+                         level="ERROR"
+                         hasStaticDescription="true"
+                         implementationClass="com.demonwav.mcdev.platform.mixin.inspection.MixinClassTypeInspection"/>
         <localInspection displayName="Reference to @Mixin class"
                          shortName="ReferenceToMixin"
                          groupName="Mixin"