Exclude public API symbols in explicit mode from unused symbol inspection

#KT-41659 Fixed

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2000-2020 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Copyright 2010-2021 JetBrains s.r.o. and Kotlin Programming Language contributors.
  * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
  */
 
@@ -37,6 +37,7 @@ import org.jetbrains.kotlin.asJava.classes.KtLightClass
 import org.jetbrains.kotlin.asJava.toLightClass
 import org.jetbrains.kotlin.asJava.toLightMethods
 import org.jetbrains.kotlin.config.AnalysisFlags
+import org.jetbrains.kotlin.config.ExplicitApiMode
 import org.jetbrains.kotlin.descriptors.*
 import org.jetbrains.kotlin.idea.KotlinBundle
 import org.jetbrains.kotlin.idea.caches.project.implementingDescriptors
@@ -75,7 +76,9 @@ import org.jetbrains.kotlin.psi.psiUtil.*
 import org.jetbrains.kotlin.resolve.BindingContext
 import org.jetbrains.kotlin.resolve.DescriptorUtils
 import org.jetbrains.kotlin.resolve.bindingContextUtil.isUsedAsExpression
+import org.jetbrains.kotlin.resolve.checkers.explicitApiEnabled
 import org.jetbrains.kotlin.resolve.descriptorUtil.fqNameSafe
+import org.jetbrains.kotlin.resolve.descriptorUtil.isEffectivelyPublicApi
 import org.jetbrains.kotlin.resolve.isInlineClass
 import org.jetbrains.kotlin.resolve.isInlineClassType
 import org.jetbrains.kotlin.resolve.lazy.BodyResolveMode
@@ -266,6 +269,10 @@ class UnusedSymbolInspection : AbstractKotlinInspection() {
 
             // More expensive, resolve-based checks
             val descriptor = declaration.resolveToDescriptorIfAny() ?: return
+            if (declaration.languageVersionSettings.explicitApiEnabled
+                && (descriptor as? DeclarationDescriptorWithVisibility)?.isEffectivelyPublicApi == true) {
+                return
+            }
             if (descriptor is FunctionDescriptor && descriptor.isOperator) return
             val isCheapEnough = lazy(LazyThreadSafetyMode.NONE) {
                 isCheapEnoughToSearchUsages(declaration)

idea/testData/inspectionsLocal/unusedSymbol/unusedClassExplicitApi.kt

@@ -0,0 +1,5 @@
+// PROBLEM: none
+// COMPILER_ARGUMENTS: -Xexplicit-api=strict
+
+public class <caret>X {
+}

idea/testData/inspectionsLocal/unusedSymbol/unusedExtensionExplicitApi.kt

@@ -0,0 +1,4 @@
+// PROBLEM: none
+// COMPILER_ARGUMENTS: -Xexplicit-api=strict
+
+public fun String.<caret>baz() {}

idea/testData/inspectionsLocal/unusedSymbol/unusedFunctionExplicitApi.kt

@@ -0,0 +1,6 @@
+// PROBLEM: none
+// COMPILER_ARGUMENTS: -Xexplicit-api=strict
+
+public class X {
+    public fun <caret>foo() {}
+}

idea/tests/org/jetbrains/kotlin/idea/inspections/LocalInspectionTestGenerated.java

@@ -14529,11 +14529,26 @@ public class LocalInspectionTestGenerated extends AbstractLocalInspectionTest {
             runTest("idea/testData/inspectionsLocal/unusedSymbol/typeAlias.kt");
         }
 
+        @TestMetadata("unusedClassExplicitApi.kt")
+        public void testUnusedClassExplicitApi() throws Exception {
+            runTest("idea/testData/inspectionsLocal/unusedSymbol/unusedClassExplicitApi.kt");
+        }
+
         @TestMetadata("unusedEnumEntry.kt")
         public void testUnusedEnumEntry() throws Exception {
             runTest("idea/testData/inspectionsLocal/unusedSymbol/unusedEnumEntry.kt");
         }
 
+        @TestMetadata("unusedExtensionExplicitApi.kt")
+        public void testUnusedExtensionExplicitApi() throws Exception {
+            runTest("idea/testData/inspectionsLocal/unusedSymbol/unusedExtensionExplicitApi.kt");
+        }
+
+        @TestMetadata("unusedFunctionExplicitApi.kt")
+        public void testUnusedFunctionExplicitApi() throws Exception {
+            runTest("idea/testData/inspectionsLocal/unusedSymbol/unusedFunctionExplicitApi.kt");
+        }
+
         @TestMetadata("usedEnumFunction.kt")
         public void testUsedEnumFunction() throws Exception {
             runTest("idea/testData/inspectionsLocal/unusedSymbol/usedEnumFunction.kt");