Define the DataClassFunctions rule

Author: ILIYANGERMANOV

src/main/kotlin/com/github/ivy/explicit/DataClassFunctionsRule.kt

@@ -0,0 +1,33 @@
+package com.github.ivy.explicit
+
+import io.gitlab.arturbosch.detekt.api.*
+import org.jetbrains.kotlin.psi.KtClass
+import org.jetbrains.kotlin.psi.KtNamedFunction
+
+class DataClassFunctionsRule(config: Config) : Rule(config) {
+    override val issue = Issue(
+        id = "DataClassFunctions",
+        severity = Severity.Maintainability,
+        description = "Data classes should not define behavior. " +
+                "Their purpose is to model data.",
+        debt = Debt.TWENTY_MINS,
+    )
+
+    override fun visitClass(klass: KtClass) {
+        super.visitClass(klass)
+        if (klass.isData()) {
+            klass.body?.declarations
+                ?.filterIsInstance<KtNamedFunction>()
+                ?.forEach { function ->
+                    report(
+                        CodeSmell(
+                            issue,
+                            Entity.from(function),
+                            message = "Data class '${klass.name}' should not contain functions. " +
+                                    "Found: function ${function.name}()."
+                        )
+                    )
+                }
+        }
+    }
+}

src/main/kotlin/com/github/ivy/explicit/MyRuleSetProvider.kt src/main/kotlin/com/github/ivy/explicit/IvyExplicitRuleSetProvider.kt

@@ -4,14 +4,14 @@ import io.gitlab.arturbosch.detekt.api.Config
 import io.gitlab.arturbosch.detekt.api.RuleSet
 import io.gitlab.arturbosch.detekt.api.RuleSetProvider
 
-class MyRuleSetProvider : RuleSetProvider {
-    override val ruleSetId: String = "MyRuleSet"
+class IvyExplicitRuleSetProvider : RuleSetProvider {
+    override val ruleSetId: String = "IvyExplicit"
 
     override fun instance(config: Config): RuleSet {
         return RuleSet(
             ruleSetId,
             listOf(
-                MyRule(config),
+                DataClassFunctionsRule(config),
             ),
         )
     }

src/main/kotlin/com/github/ivy/explicit/MyRule.kt

@@ -1 +1 @@
-com.github.ivy.explicit.MyRuleSetProvider
+com.github.ivy.explicit.IvyExplicitRuleSetProvider

src/main/resources/META-INF/services/io.gitlab.arturbosch.detekt.api.RuleSetProvider

@@ -0,0 +1,54 @@
+package com.github.ivy.explicit
+
+import io.gitlab.arturbosch.detekt.api.Config
+import io.gitlab.arturbosch.detekt.rules.KotlinCoreEnvironmentTest
+import io.gitlab.arturbosch.detekt.test.compileAndLintWithContext
+import io.kotest.matchers.collections.shouldHaveSize
+import io.kotest.matchers.string.shouldNotBeBlank
+import org.jetbrains.kotlin.cli.jvm.compiler.KotlinCoreEnvironment
+import org.junit.jupiter.api.Test
+
+@KotlinCoreEnvironmentTest
+internal class DataClassFunctionsRuleTest(private val env: KotlinCoreEnvironment) {
+
+    @Test
+    fun `reports data class having functions`() {
+        val code = """
+        data class A(
+            val x: Int
+        ) {
+            fun a() = 42
+        }
+        """
+        val findings = DataClassFunctionsRule(Config.empty).compileAndLintWithContext(env, code)
+        findings shouldHaveSize 1
+        val message = findings.first().message
+        message.shouldNotBeBlank()
+    }
+
+    @Test
+    fun `doesn't report data class without functions`() {
+        val code = """
+        data class A(
+            val x: Int
+        )
+        """
+        val findings = DataClassFunctionsRule(Config.empty).compileAndLintWithContext(env, code)
+        findings shouldHaveSize 0
+    }
+
+    @Test
+    fun `doesn't report data class with companion object`() {
+        val code = """
+        data class A(
+            val x: Int
+        ) {
+            companion object {
+                fun a() = 42
+            }
+        }
+        """
+        val findings = DataClassFunctionsRule(Config.empty).compileAndLintWithContext(env, code)
+        findings shouldHaveSize 0
+    }
+}