[FIR] Explicit API mode: No REDUNDANT_VISIBILITY_MODIFIER for public

- In explicit API mode, the `public` visibility modifier is not
  redundant unless a declaration is hidden by a container. The
  `REDUNDANT_VISIBILITY_MODIFIER` diagnostic is now not reported in such
  cases.

^KTIJ-24485 fixed

analysis/low-level-api-fir/tests/org/jetbrains/kotlin/analysis/low/level/api/fir/diagnostic/compiler/based/DiagnosisCompilerFirTestdataTestGenerated.java

@@ -2741,6 +2741,12 @@ public class DiagnosisCompilerFirTestdataTestGenerated extends AbstractDiagnosis
                 runTest("compiler/fir/analysis-tests/testData/resolve/extendedCheckers/RedundantVisibilityModifierCheckerForInnerComponents.kt");
             }
 
+            @Test
+            @TestMetadata("RedundantVisibilityModifierCheckerWithExplicitApiMode.kt")
+            public void testRedundantVisibilityModifierCheckerWithExplicitApiMode() throws Exception {
+                runTest("compiler/fir/analysis-tests/testData/resolve/extendedCheckers/RedundantVisibilityModifierCheckerWithExplicitApiMode.kt");
+            }
+
             @Nested
             @TestMetadata("compiler/fir/analysis-tests/testData/resolve/extendedCheckers/canBeReplacedWithOperatorAssignment")
             @TestDataPath("$PROJECT_ROOT")

compiler/fir/analysis-tests/legacy-fir-tests/tests-gen/org/jetbrains/kotlin/fir/LazyBodyIsNotTouchedTilContractsPhaseTestGenerated.java

@@ -2394,6 +2394,11 @@ public class LazyBodyIsNotTouchedTilContractsPhaseTestGenerated extends Abstract
             runTest("compiler/fir/analysis-tests/testData/resolve/extendedCheckers/RedundantVisibilityModifierCheckerForInnerComponents.kt");
         }
 
+        @TestMetadata("RedundantVisibilityModifierCheckerWithExplicitApiMode.kt")
+        public void testRedundantVisibilityModifierCheckerWithExplicitApiMode() throws Exception {
+            runTest("compiler/fir/analysis-tests/testData/resolve/extendedCheckers/RedundantVisibilityModifierCheckerWithExplicitApiMode.kt");
+        }
+
         @TestMetadata("compiler/fir/analysis-tests/testData/resolve/extendedCheckers/canBeReplacedWithOperatorAssignment")
         @TestDataPath("$PROJECT_ROOT")
         @RunWith(JUnit3RunnerWithInners.class)

compiler/fir/analysis-tests/testData/resolve/extendedCheckers/RedundantVisibilityModifierCheckerWithExplicitApiMode.fir.txt

@@ -0,0 +1,190 @@
+FILE: RedundantVisibilityModifierCheckerWithExplicitApiMode.kt
+    public final fun f1(): R|kotlin/Unit| {
+        local final class LocalClass : R|kotlin/Any| {
+            public constructor(): R|LocalClass| {
+                super<R|kotlin/Any|>()
+            }
+
+            public final var foo: R|kotlin/Int| = Int(0)
+                public get(): R|kotlin/Int|
+                public set(value: R|kotlin/Int|): R|kotlin/Unit|
+
+        }
+
+        R|/LocalClass.LocalClass|().R|<local>/foo| = Int(1)
+    }
+    public final fun f2(): R|kotlin/Unit| {
+        R|/f1|()
+    }
+    public final inline fun h1(): R|kotlin/Unit| {
+        R|/f1|()
+    }
+    public final inline fun h2(): R|kotlin/Unit| {
+        R|/f1|()
+    }
+    public final class C1 : R|kotlin/Any| {
+        public constructor(string: R|kotlin/String|): R|C1| {
+            super<R|kotlin/Any|>()
+        }
+
+        public final val foo: R|kotlin/Int| = Int(0)
+            public get(): R|kotlin/Int|
+
+        public final val z: R|kotlin/Any| = object : R|kotlin/Any| {
+            private constructor(): R|<anonymous>| {
+                super<R|kotlin/Any|>()
+            }
+
+            public final fun foo(): R|kotlin/Int| {
+                ^foo Int(13)
+            }
+
+        }
+
+            public get(): R|kotlin/Any|
+
+        public final fun x(): R|kotlin/Unit| {
+        }
+
+    }
+    public final class C2 : R|kotlin/Any| {
+        public constructor(string: R|kotlin/String|): R|C2| {
+            super<R|kotlin/Any|>()
+        }
+
+        public final val foo: R|kotlin/Int| = Int(0)
+            public get(): R|kotlin/Int|
+
+        public final val z: R|kotlin/Any| = object : R|kotlin/Any| {
+            private constructor(): R|<anonymous>| {
+                super<R|kotlin/Any|>()
+            }
+
+            public final fun foo(): R|kotlin/Int| {
+                ^foo Int(13)
+            }
+
+        }
+
+            public get(): R|kotlin/Any|
+
+        public final fun x(): R|kotlin/Unit| {
+        }
+
+    }
+    public open class D1 : R|kotlin/Any| {
+        public constructor(): R|D1| {
+            super<R|kotlin/Any|>()
+        }
+
+        public open fun willRemainPublic(): R|kotlin/Unit| {
+        }
+
+        protected open fun willBecomePublic(): R|kotlin/Unit| {
+        }
+
+    }
+    public open class D2 : R|kotlin/Any| {
+        public constructor(): R|D2| {
+            super<R|kotlin/Any|>()
+        }
+
+        public open fun willRemainPublic(): R|kotlin/Unit| {
+        }
+
+        protected open fun willBecomePublic(): R|kotlin/Unit| {
+        }
+
+    }
+    public abstract interface I1 : R|kotlin/Any| {
+        public abstract fun bar(): R|kotlin/Unit|
+
+    }
+    public abstract interface I2 : R|kotlin/Any| {
+        public abstract fun bar(): R|kotlin/Unit|
+
+    }
+    public final var baz1: R|kotlin/Int| = Int(0)
+        public get(): R|kotlin/Int|
+        public set(value: R|kotlin/Int|): R|kotlin/Unit|
+    public final var baz2: R|kotlin/Int| = Int(0)
+        public get(): R|kotlin/Int|
+        public set(value: R|kotlin/Int|): R|kotlin/Unit|
+    public final class J1 : R|kotlin/Any| {
+        public constructor(): R|J1| {
+            super<R|kotlin/Any|>()
+        }
+
+        protected final val baz: R|kotlin/Int| = Int(0)
+            protected get(): R|kotlin/Int| {
+                ^ this@R|/J1|.F|/J1.baz|.R|kotlin/Int.times|(Int(2))
+            }
+
+        public final var baf: R|kotlin/Int| = Int(0)
+            public get(): R|kotlin/Int| {
+                ^ Int(1)
+            }
+            public set(value: R|kotlin/Int|): R|kotlin/Unit| {
+                this@R|/J1|.F|/J1.baf| = R|<local>/value|
+            }
+
+        public final var buf: R|kotlin/Int| = Int(0)
+            private get(): R|kotlin/Int| {
+                ^ Int(42)
+            }
+            protected set(value: R|kotlin/Int|): R|kotlin/Unit| {
+                this@R|/J1|.F|/J1.buf| = R|<local>/value|
+            }
+
+        public final var bar: R|kotlin/Int| = Int(0)
+            public get(): R|kotlin/Int| {
+                ^ Double(3.1415926535)
+            }
+            public set(value: R|kotlin/Int|): R|kotlin/Unit| {
+            }
+
+    }
+    public final class J2 : R|kotlin/Any| {
+        public constructor(): R|J2| {
+            super<R|kotlin/Any|>()
+        }
+
+        protected final val baz: R|kotlin/Int| = Int(0)
+            protected get(): R|kotlin/Int| {
+                ^ this@R|/J2|.F|/J2.baz|.R|kotlin/Int.times|(Int(2))
+            }
+
+        public final var baf: R|kotlin/Int| = Int(0)
+            public get(): R|kotlin/Int| {
+                ^ Int(1)
+            }
+            public set(value: R|kotlin/Int|): R|kotlin/Unit| {
+                this@R|/J2|.F|/J2.baf| = R|<local>/value|
+            }
+
+        public final var buf: R|kotlin/Int| = Int(0)
+            private get(): R|kotlin/Int| {
+                ^ Int(42)
+            }
+            protected set(value: R|kotlin/Int|): R|kotlin/Unit| {
+                this@R|/J2|.F|/J2.buf| = R|<local>/value|
+            }
+
+        public final var bar: R|kotlin/Int| = Int(0)
+            public get(): R|kotlin/Int| {
+                ^ Double(3.1415926535)
+            }
+            public set(value: R|kotlin/Int|): R|kotlin/Unit| {
+            }
+
+    }
+    private final class Hidden : R|kotlin/Any| {
+        public constructor(): R|Hidden| {
+            super<R|kotlin/Any|>()
+        }
+
+        public final fun f(): R|kotlin/Int| {
+            ^f Int(5)
+        }
+
+    }

compiler/fir/analysis-tests/testData/resolve/extendedCheckers/RedundantVisibilityModifierCheckerWithExplicitApiMode.kt

@@ -0,0 +1,116 @@
+// EXPLICIT_API_MODE: STRICT
+
+fun f1() {
+    class LocalClass {
+        public var foo = 0
+    }
+    LocalClass().foo = 1
+}
+
+public fun f2() {
+    f1()
+}
+
+<!NO_EXPLICIT_VISIBILITY_IN_API_MODE!><!NOTHING_TO_INLINE!>inline<!> fun h1<!>() {
+    f1()
+}
+
+public <!NOTHING_TO_INLINE!>inline<!> fun h2() {
+    f1()
+}
+
+<!NO_EXPLICIT_VISIBILITY_IN_API_MODE!>class C1<!> {
+    <!NO_EXPLICIT_VISIBILITY_IN_API_MODE!>constructor<!>(string: String) { }
+
+    <!NO_EXPLICIT_VISIBILITY_IN_API_MODE!>val foo<!>: Int = 0
+
+    <!NO_EXPLICIT_VISIBILITY_IN_API_MODE!>val z<!>: Any = object {
+        fun foo() = 13
+    }
+
+    <!NO_EXPLICIT_VISIBILITY_IN_API_MODE!>fun x<!>() { }
+}
+
+public class C2 {
+    public constructor(string: String) { }
+
+    public val foo: Int = 0
+
+    public val <!NO_EXPLICIT_RETURN_TYPE_IN_API_MODE!>z<!> = object {
+        public fun foo() = 13
+    }
+
+    public fun x() { }
+}
+
+<!NO_EXPLICIT_VISIBILITY_IN_API_MODE!>open class D1<!> {
+    <!NO_EXPLICIT_VISIBILITY_IN_API_MODE!>open fun willRemainPublic<!>() {
+    }
+
+    protected open fun willBecomePublic() {
+    }
+}
+
+public open class D2 {
+    public open fun willRemainPublic() {
+    }
+
+    protected open fun willBecomePublic() {
+    }
+}
+
+<!NO_EXPLICIT_VISIBILITY_IN_API_MODE!>interface I1<!> {
+    <!NO_EXPLICIT_VISIBILITY_IN_API_MODE!>fun bar<!>()
+}
+
+public interface I2 {
+    public fun bar()
+}
+
+<!NO_EXPLICIT_VISIBILITY_IN_API_MODE!>var baz1<!>: Int = 0
+
+public var baz2: Int = 0
+
+class J1 {
+    protected val baz: Int = 0
+        <!REDUNDANT_VISIBILITY_MODIFIER!>protected<!> get() = field * 2
+    <!NO_EXPLICIT_VISIBILITY_IN_API_MODE!>var baf<!>: Int = 0
+        get() = 1
+        set(value) {
+            field = value
+        }
+
+    var buf: Int = 0
+        <!GETTER_VISIBILITY_DIFFERS_FROM_PROPERTY_VISIBILITY!>private<!> get() = 42
+        protected set(value) {
+            field = value
+        }
+
+    <!NO_EXPLICIT_VISIBILITY_IN_API_MODE!>var bar<!>: Int = <!PROPERTY_INITIALIZER_NO_BACKING_FIELD!>0<!>
+        get() = <!RETURN_TYPE_MISMATCH!>3.1415926535<!>
+        set(value) {}
+}
+
+public class J2 {
+    protected val baz: Int = 0
+        <!REDUNDANT_VISIBILITY_MODIFIER!>protected<!> get() = field * 2
+    public var baf: Int = 0
+        public get() = 1
+        public set(value) {
+            field = value
+        }
+
+    public var buf: Int = 0
+        <!GETTER_VISIBILITY_DIFFERS_FROM_PROPERTY_VISIBILITY!>private<!> get() = 42
+        protected set(value) {
+            field = value
+        }
+
+    public var bar: Int = <!PROPERTY_INITIALIZER_NO_BACKING_FIELD!>0<!>
+        get() = <!RETURN_TYPE_MISMATCH!>3.1415926535<!>
+        set(value) {}
+}
+
+private class Hidden {
+    <!REDUNDANT_VISIBILITY_MODIFIER!>public<!> fun f(): Int = 5
+}

compiler/fir/analysis-tests/tests-gen/org/jetbrains/kotlin/test/runners/FirDiagnosticTestGenerated.java

@@ -2741,6 +2741,12 @@ public class FirDiagnosticTestGenerated extends AbstractFirDiagnosticTest {
                 runTest("compiler/fir/analysis-tests/testData/resolve/extendedCheckers/RedundantVisibilityModifierCheckerForInnerComponents.kt");
             }
 
+            @Test
+            @TestMetadata("RedundantVisibilityModifierCheckerWithExplicitApiMode.kt")
+            public void testRedundantVisibilityModifierCheckerWithExplicitApiMode() throws Exception {
+                runTest("compiler/fir/analysis-tests/testData/resolve/extendedCheckers/RedundantVisibilityModifierCheckerWithExplicitApiMode.kt");
+            }
+
             @Nested
             @TestMetadata("compiler/fir/analysis-tests/testData/resolve/extendedCheckers/canBeReplacedWithOperatorAssignment")
             @TestDataPath("$PROJECT_ROOT")

compiler/fir/analysis-tests/tests-gen/org/jetbrains/kotlin/test/runners/FirDiagnosticsWithLightTreeTestGenerated.java

@@ -2741,6 +2741,12 @@ public class FirDiagnosticsWithLightTreeTestGenerated extends AbstractFirDiagnos
                 runTest("compiler/fir/analysis-tests/testData/resolve/extendedCheckers/RedundantVisibilityModifierCheckerForInnerComponents.kt");
             }
 
+            @Test
+            @TestMetadata("RedundantVisibilityModifierCheckerWithExplicitApiMode.kt")
+            public void testRedundantVisibilityModifierCheckerWithExplicitApiMode() throws Exception {
+                runTest("compiler/fir/analysis-tests/testData/resolve/extendedCheckers/RedundantVisibilityModifierCheckerWithExplicitApiMode.kt");
+            }
+
             @Nested
             @TestMetadata("compiler/fir/analysis-tests/testData/resolve/extendedCheckers/canBeReplacedWithOperatorAssignment")
             @TestDataPath("$PROJECT_ROOT")

compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/checkers/extended/RedundantVisibilityModifierSyntaxChecker.kt

@@ -7,6 +7,8 @@ package org.jetbrains.kotlin.fir.analysis.checkers.extended
 
 import org.jetbrains.kotlin.KtFakeSourceElementKind
 import org.jetbrains.kotlin.KtSourceElement
+import org.jetbrains.kotlin.config.AnalysisFlags
+import org.jetbrains.kotlin.config.ExplicitApiMode
 import org.jetbrains.kotlin.descriptors.Visibilities
 import org.jetbrains.kotlin.descriptors.Visibility
 import org.jetbrains.kotlin.diagnostics.DiagnosticReporter
@@ -121,11 +123,23 @@ object RedundantVisibilityModifierSyntaxChecker : FirDeclarationSyntaxChecker<Fi
 
         val explicitVisibility = element.source?.explicitVisibility
         val isHidden = explicitVisibility.isEffectivelyHiddenBy(containingMemberDeclaration)
-
+        if (isHidden) {
-        if (explicitVisibility != implicitVisibility && !isHidden) {
+            reportElement(element, context, reporter)
             return
         }
 
+        // In explicit API mode, `public` is explicitly required.
+        val explicitApiMode = context.languageVersionSettings.getFlag(AnalysisFlags.explicitApiMode)
+        if (explicitApiMode != ExplicitApiMode.DISABLED && explicitVisibility == Visibilities.Public) {
+            return
+        }
+
+        if (explicitVisibility == implicitVisibility) {
+            reportElement(element, context, reporter)
+        }
+    }
+
+    private fun reportElement(element: FirDeclaration, context: CheckerContext, reporter: DiagnosticReporter) {
         reporter.reportOn(element.source, FirErrors.REDUNDANT_VISIBILITY_MODIFIER, context)
     }