Do not report "unused symbol" on experimental annotations

#KT-25609 Fixed

idea/src/org/jetbrains/kotlin/idea/inspections/UnusedSymbolInspection.kt

@@ -36,6 +36,7 @@ import org.jetbrains.kotlin.asJava.LightClassUtil
 import org.jetbrains.kotlin.asJava.classes.KtLightClass
 import org.jetbrains.kotlin.asJava.toLightClass
 import org.jetbrains.kotlin.asJava.toLightMethods
+import org.jetbrains.kotlin.config.AnalysisFlag
 import org.jetbrains.kotlin.descriptors.*
 import org.jetbrains.kotlin.fileClasses.JvmFileClassUtil
 import org.jetbrains.kotlin.idea.caches.project.implementingDescriptors
@@ -47,6 +48,7 @@ import org.jetbrains.kotlin.idea.core.toDescriptor
 import org.jetbrains.kotlin.idea.findUsages.KotlinFindUsagesHandlerFactory
 import org.jetbrains.kotlin.idea.findUsages.handlers.KotlinFindClassUsagesHandler
 import org.jetbrains.kotlin.idea.imports.importableFqName
+import org.jetbrains.kotlin.idea.project.languageVersionSettings
 import org.jetbrains.kotlin.idea.quickfix.RemoveUnusedFunctionParameterFix
 import org.jetbrains.kotlin.idea.references.mainReference
 import org.jetbrains.kotlin.idea.references.resolveMainReferenceToDescriptors
@@ -65,6 +67,7 @@ import org.jetbrains.kotlin.psi.*
 import org.jetbrains.kotlin.psi.psiUtil.*
 import org.jetbrains.kotlin.resolve.BindingContext
 import org.jetbrains.kotlin.resolve.DescriptorUtils
+import org.jetbrains.kotlin.resolve.descriptorUtil.fqNameSafe
 import org.jetbrains.kotlin.util.findCallableMemberBySignature
 import java.awt.GridBagConstraints
 import java.awt.GridBagLayout
@@ -163,6 +166,14 @@ class UnusedSymbolInspection : AbstractKotlinInspection() {
             if (declaration is KtNamedFunction && declaration.isSerializationImplicitlyUsedMethod()) return
             // properties can be referred by component1/component2, which is too expensive to search, don't mark them as unused
             if (declaration is KtParameter && declaration.dataClassComponentFunction() != null) return
+            // experimental annotations
+            if (descriptor is ClassDescriptor && descriptor.kind == ClassKind.ANNOTATION_CLASS) {
+                val fqName = descriptor.fqNameSafe.asString()
+                val languageVersionSettings = declaration.languageVersionSettings
+                if (fqName in languageVersionSettings.getFlag(AnalysisFlag.experimental) ||
+                    fqName in languageVersionSettings.getFlag(AnalysisFlag.useExperimental)
+                ) return
+            }
 
             // Main checks: finding reference usages && text usages
             if (hasNonTrivialUsages(declaration, descriptor)) return

idea/testData/quickfix/experimental/experimentalUnused.kt

@@ -0,0 +1,10 @@
+// "Safe delete 'Marker'" "false"
+// COMPILER_ARGUMENTS: -Xuse-experimental=kotlin.Experimental -Xuse-experimental=test.Marker
+// WITH_RUNTIME
+// ACTION: Rename file to Marker.kt
+// TOOL: org.jetbrains.kotlin.idea.inspections.UnusedSymbolInspection
+
+package test
+
+@Experimental
+annotation class <caret>Marker

idea/testData/quickfix/experimental/experimentalUnused2.kt

@@ -0,0 +1,10 @@
+// "Safe delete 'Marker'" "false"
+// COMPILER_ARGUMENTS: -Xuse-experimental=kotlin.Experimental -Xexperimental=test.Marker
+// WITH_RUNTIME
+// ACTION: Rename file to Marker.kt
+// TOOL: org.jetbrains.kotlin.idea.inspections.UnusedSymbolInspection
+
+package test
+
+@Experimental
+annotation class <caret>Marker

idea/tests/org/jetbrains/kotlin/idea/quickfix/QuickFixTestGenerated.java

@@ -6204,6 +6204,16 @@ public class QuickFixTestGenerated extends AbstractQuickFixTest {
             runTest("idea/testData/quickfix/experimental/doNotSwitchOn.kt");
         }
 
+        @TestMetadata("experimentalUnused.kt")
+        public void testExperimentalUnused() throws Exception {
+            runTest("idea/testData/quickfix/experimental/experimentalUnused.kt");
+        }
+
+        @TestMetadata("experimentalUnused2.kt")
+        public void testExperimentalUnused2() throws Exception {
+            runTest("idea/testData/quickfix/experimental/experimentalUnused2.kt");
+        }
+
         @TestMetadata("functionInLocalClass.kt")
         public void testFunctionInLocalClass() throws Exception {
             runTest("idea/testData/quickfix/experimental/functionInLocalClass.kt");