Nested class, interface or enum shouldn't be publicly visible

detekt-rules/src/main/kotlin/io/gitlab/arturbosch/detekt/rules/style/NestedClassesVisibility.kt

@@ -0,0 +1,47 @@
+package io.gitlab.arturbosch.detekt.rules.style
+
+import io.gitlab.arturbosch.detekt.api.CodeSmell
+import io.gitlab.arturbosch.detekt.api.Config
+import io.gitlab.arturbosch.detekt.api.DetektVisitor
+import io.gitlab.arturbosch.detekt.api.Entity
+import io.gitlab.arturbosch.detekt.api.Issue
+import io.gitlab.arturbosch.detekt.api.Rule
+import io.gitlab.arturbosch.detekt.api.Severity
+import org.jetbrains.kotlin.lexer.KtTokens
+import org.jetbrains.kotlin.psi.KtClass
+import org.jetbrains.kotlin.psi.psiUtil.visibilityModifierType
+
+class NestedClassesVisibility(config: Config = Config.empty) : Rule(config) {
+	override val issue: Issue = Issue("NestedClassesVisibility", severity = Severity.Security, description = "Nested types are often used for implementing private functionality. Therefore, they shouldn't be externally visible.")
+
+	private val visitor = VisibilityVisitor(this)
+
+	override fun visitClass(klass: KtClass) {
+		super.visitClass(klass)
+		if (isInternal(klass)) {
+			klass.declarations.forEach {
+				it.accept(visitor)
+			}
+		}
+	}
+
+	private fun isInternal(klass: KtClass): Boolean {
+		return klass.visibilityModifierType() == KtTokens.INTERNAL_KEYWORD
+	}
+
+	private fun isPrivate(klass: KtClass): Boolean {
+		return klass.visibilityModifierType() == KtTokens.PRIVATE_KEYWORD
+	}
+
+	private fun handleClass(klass: KtClass) {
+		report(CodeSmell(issue, Entity.from(klass)))
+	}
+
+	private class VisibilityVisitor(val rule: NestedClassesVisibility) : DetektVisitor() {
+		override fun visitClass(klass: KtClass) {
+			if (!rule.isInternal(klass) && !rule.isPrivate(klass)) {
+				rule.handleClass(klass)
+			}
+		}
+	}
+}

detekt-rules/src/test/kotlin/io/gitlab/arturbosch/detekt/rules/KT.kt

@@ -46,7 +46,8 @@ enum class Case(val file: String) {
 	TooManyFunctions("/cases/TooManyFunctions.kt"),
 	TooManyFunctionsTopLevel("/cases/TooManyFunctionsTopLevel.kt"),
 	UseDataClass("/cases/UseDataClass.kt"),
-	UnconditionalJumpStatementInLoop("/cases/UnconditionalJumpStatementInLoop.kt");
+	UnconditionalJumpStatementInLoop("/cases/UnconditionalJumpStatementInLoop.kt"),
+	NestedClassesVisibility("/cases/NestedClassesVisibility.kt");
 
 	fun path(): Path = Paths.get(resource(file))
 }

detekt-rules/src/test/kotlin/io/gitlab/arturbosch/detekt/rules/style/NestedClassesVisibilitySpec.kt

@@ -0,0 +1,18 @@
+package io.gitlab.arturbosch.detekt.rules.style
+
+import io.gitlab.arturbosch.detekt.rules.Case
+import io.gitlab.arturbosch.detekt.test.lint
+import org.assertj.core.api.Assertions
+import org.jetbrains.spek.api.dsl.given
+import org.jetbrains.spek.api.dsl.it
+import org.jetbrains.spek.subject.SubjectSpek
+
+class NestedClassesVisibilitySpec : SubjectSpek<NestedClassesVisibility>({
+	subject { NestedClassesVisibility() }
+
+	given("several nested classes") {
+		it("check interfaces,enums,classes") {
+			Assertions.assertThat(subject.lint(Case.NestedClassesVisibility.path())).hasSize(3)
+		}
+	}
+})

detekt-rules/src/test/resources/cases/NestedClassesVisibility.kt

@@ -0,0 +1,16 @@
+package cases
+
+internal class ParentType {
+	public class NestedType { // report
+	}
+
+	enum class NestedEnum { One, Two } // report
+
+	private enum class NestedPrivateEnum { Three } // valid
+
+	interface Test {} // report
+
+	private interface PrivateTest {} // valid
+
+	internal interface InternalTest {} // valid
+}