Repository: analysis-dev/diktat (kotlin)

Issue:
Add warning to suggest to use inline classes when a class has single property
https://kotlinlang.org/docs/reference/inline-classes.html

However, only since 1.3. Maybe needs some more considerations.

Pull request description:
### What's done:
  * Logic made
  * Added warn tests

This pull request closes #698 

## Actions checklist
* [X] Implemented Rule, added Warnings
* [X] Added tests on checks
* [ ] Added tests on fixers
* [X] Updated diktat-analysis.yml
* [X] Updated available-rules.md

## Fixme
Since it is still experimental feature it shouldn't be fixed

Patch:
diff --git a/diktat-analysis.yml b/diktat-analysis.yml
index bb2e9fa348..a8a7cf4f91 100644
--- a/diktat-analysis.yml
+++ b/diktat-analysis.yml
@@ -430,4 +430,7 @@
   enabled: true
 # If there exists negated version of function you should prefer it instead of !functionCall
 - name: INVERSE_FUNCTION_PREFERRED
+  enabled: true
+# Checks if class can be converted to inline class
+- name: INLINE_CLASS_CAN_BE_USED
   enabled: true
\ No newline at end of file
diff --git a/diktat-rules/src/main/kotlin/generated/WarningNames.kt b/diktat-rules/src/main/kotlin/generated/WarningNames.kt
index bb6e65e535..b7dd05f033 100644
--- a/diktat-rules/src/main/kotlin/generated/WarningNames.kt
+++ b/diktat-rules/src/main/kotlin/generated/WarningNames.kt
@@ -239,4 +239,6 @@ public object WarningNames {
     public const val OBJECT_IS_PREFERRED: String = "OBJECT_IS_PREFERRED"
 
     public const val INVERSE_FUNCTION_PREFERRED: String = "INVERSE_FUNCTION_PREFERRED"
+
+    public const val INLINE_CLASS_CAN_BE_USED: String = "INLINE_CLASS_CAN_BE_USED"
 }
diff --git a/diktat-rules/src/main/kotlin/org/cqfn/diktat/ruleset/constants/Warnings.kt b/diktat-rules/src/main/kotlin/org/cqfn/diktat/ruleset/constants/Warnings.kt
index 478c71fdaa..50746275e5 100644
--- a/diktat-rules/src/main/kotlin/org/cqfn/diktat/ruleset/constants/Warnings.kt
+++ b/diktat-rules/src/main/kotlin/org/cqfn/diktat/ruleset/constants/Warnings.kt
@@ -152,6 +152,7 @@ enum class Warnings(
     AVOID_USING_UTILITY_CLASS(false, "6.4.1", "avoid using utility classes/objects, use extensions functions"),
     OBJECT_IS_PREFERRED(true, "6.4.2", "it is better to use object for stateless classes"),
     INVERSE_FUNCTION_PREFERRED(true, "5.1.4", "it is better to use inverse function"),
+    INLINE_CLASS_CAN_BE_USED(true, "6.1.12", "inline class can be used"),
     ;
 
     /**
diff --git a/diktat-rules/src/main/kotlin/org/cqfn/diktat/ruleset/rules/DiktatRuleSetProvider.kt b/diktat-rules/src/main/kotlin/org/cqfn/diktat/ruleset/rules/DiktatRuleSetProvider.kt
index 8f7e7dee7e..9cf80aa44e 100644
--- a/diktat-rules/src/main/kotlin/org/cqfn/diktat/ruleset/rules/DiktatRuleSetProvider.kt
+++ b/diktat-rules/src/main/kotlin/org/cqfn/diktat/ruleset/rules/DiktatRuleSetProvider.kt
@@ -63,6 +63,7 @@ import org.cqfn.diktat.ruleset.rules.chapter6.UselessSupertype
 import org.cqfn.diktat.ruleset.rules.chapter6.classes.AbstractClassesRule
 import org.cqfn.diktat.ruleset.rules.chapter6.classes.CompactInitialization
 import org.cqfn.diktat.ruleset.rules.chapter6.classes.DataClassesRule
+import org.cqfn.diktat.ruleset.rules.chapter6.classes.InlineClassesRule
 import org.cqfn.diktat.ruleset.rules.chapter6.classes.SingleConstructorRule
 import org.cqfn.diktat.ruleset.rules.chapter6.classes.SingleInitRule
 import org.cqfn.diktat.ruleset.rules.chapter6.classes.StatelessClassesRule
@@ -153,6 +154,7 @@ class DiktatRuleSetProvider(private var diktatConfigFile: String = DIKTAT_ANALYS
             ::CustomGetterSetterRule,
             ::CompactInitialization,
             // other rules
+            ::InlineClassesRule,
             ::CheckInverseMethodRule,
             ::StatelessClassesRule,
             ::ImplicitBackingPropertyRule,
diff --git a/diktat-rules/src/main/kotlin/org/cqfn/diktat/ruleset/rules/chapter6/classes/InlineClassesRule.kt b/diktat-rules/src/main/kotlin/org/cqfn/diktat/ruleset/rules/chapter6/classes/InlineClassesRule.kt
new file mode 100644
index 0000000000..e600621a5d
--- /dev/null
+++ b/diktat-rules/src/main/kotlin/org/cqfn/diktat/ruleset/rules/chapter6/classes/InlineClassesRule.kt
@@ -0,0 +1,77 @@
+package org.cqfn.diktat.ruleset.rules.chapter6.classes
+
+import org.cqfn.diktat.common.config.rules.RulesConfig
+import org.cqfn.diktat.ruleset.constants.EmitType
+import org.cqfn.diktat.ruleset.constants.Warnings.INLINE_CLASS_CAN_BE_USED
+import org.cqfn.diktat.ruleset.utils.getFirstChildWithType
+import org.cqfn.diktat.ruleset.utils.hasChildOfType
+
+import com.pinterest.ktlint.core.Rule
+import com.pinterest.ktlint.core.ast.ElementType.CLASS
+import com.pinterest.ktlint.core.ast.ElementType.CONSTRUCTOR_CALLEE
+import com.pinterest.ktlint.core.ast.ElementType.FINAL_KEYWORD
+import com.pinterest.ktlint.core.ast.ElementType.INTERNAL_KEYWORD
+import com.pinterest.ktlint.core.ast.ElementType.MODIFIER_LIST
+import com.pinterest.ktlint.core.ast.ElementType.PRIVATE_KEYWORD
+import com.pinterest.ktlint.core.ast.ElementType.PROTECTED_KEYWORD
+import com.pinterest.ktlint.core.ast.ElementType.PUBLIC_KEYWORD
+import com.pinterest.ktlint.core.ast.ElementType.SUPER_TYPE_LIST
+import com.pinterest.ktlint.core.ast.ElementType.VAR_KEYWORD
+import com.pinterest.ktlint.core.ast.children
+import org.jetbrains.kotlin.com.intellij.lang.ASTNode
+import org.jetbrains.kotlin.psi.KtClass
+import org.jetbrains.kotlin.psi.psiUtil.visibilityModifierType
+
+/**
+ * This rule checks if inline class can be used.
+ */
+class InlineClassesRule(private val configRule: List<RulesConfig>) : Rule("inline-classes") {
+    private var isFixMode: Boolean = false
+    private lateinit var emitWarn: EmitType
+
+    override fun visit(
+        node: ASTNode,
+        autoCorrect: Boolean,
+        emit: EmitType
+    ) {
+        emitWarn = emit
+        isFixMode = autoCorrect
+
+        if (node.elementType == CLASS) {
+            handleClasses(node.psi as KtClass)
+        }
+    }
+
+    private fun handleClasses(classPsi: KtClass) {
+        // Fixme: In Kotlin 1.4.30 inline classes may be used with internal constructors. When it will be released need to check it
+        if (hasValidProperties(classPsi) &&
+                !isExtendingClass(classPsi.node) &&
+                classPsi.node.getFirstChildWithType(MODIFIER_LIST)?.getChildren(null)?.all { it.elementType in goodModifiers } != false) {
+            INLINE_CLASS_CAN_BE_USED.warnAndFix(configRule, emitWarn, isFixMode, "class ${classPsi.name}", classPsi.node.startOffset, classPsi.node) {
+                // Fixme: since it's an experimental feature we shouldn't do fixer
+            }
+        }
+    }
+
+    private fun hasValidProperties(classPsi: KtClass): Boolean {
+        if (classPsi.getProperties().size == 1 && !classPsi.hasExplicitPrimaryConstructor()) {
+            return !classPsi.getProperties().single().isVar
+        } else if (classPsi.getProperties().isEmpty() && classPsi.hasExplicitPrimaryConstructor()) {
+            return classPsi.primaryConstructorParameters.size == 1 &&
+                    !classPsi.primaryConstructorParameters.first().node.hasChildOfType(VAR_KEYWORD) &&
+                    classPsi.primaryConstructor?.visibilityModifierType()?.value?.let { it == "public" } ?: true
+        }
+        return false
+    }
+
+    private fun isExtendingClass(node: ASTNode): Boolean =
+            node
+                .getFirstChildWithType(SUPER_TYPE_LIST)
+                ?.children()
+                ?.any { it.hasChildOfType(CONSTRUCTOR_CALLEE) }
+                ?: false
+
+    companion object {
+        val goodModifiers = listOf(PUBLIC_KEYWORD, PRIVATE_KEYWORD, FINAL_KEYWORD, PROTECTED_KEYWORD, INTERNAL_KEYWORD)
+    }
+}
diff --git a/diktat-rules/src/main/resources/diktat-analysis-huawei.yml b/diktat-rules/src/main/resources/diktat-analysis-huawei.yml
index ad2a5e1dcf..8e560744ca 100644
--- a/diktat-rules/src/main/resources/diktat-analysis-huawei.yml
+++ b/diktat-rules/src/main/resources/diktat-analysis-huawei.yml
@@ -430,4 +430,7 @@
   enabled: true
 # If there exists negated version of function you should prefer it instead of !functionCall
 - name: INVERSE_FUNCTION_PREFERRED
+  enabled: true
+# Checks if class can be converted to inline class
+- name: INLINE_CLASS_CAN_BE_USED
   enabled: true
\ No newline at end of file
diff --git a/diktat-rules/src/main/resources/diktat-analysis.yml b/diktat-rules/src/main/resources/diktat-analysis.yml
index 9af524733e..f083c75b57 100644
--- a/diktat-rules/src/main/resources/diktat-analysis.yml
+++ b/diktat-rules/src/main/resources/diktat-analysis.yml
@@ -427,4 +427,7 @@
   enabled: true
 # If there exists negated version of function you should prefer it instead of !functionCall
 - name: INVERSE_FUNCTION_PREFERRED
+  enabled: true
+# Checks if class can be converted to inline class
+- name: INLINE_CLASS_CAN_BE_USED
   enabled: true
\ No newline at end of file
diff --git a/info/available-rules.md b/info/available-rules.md
index e04df4cb4a..f8fe984053 100644
--- a/info/available-rules.md
+++ b/info/available-rules.md
@@ -110,6 +110,7 @@
 |    6  |    6.1.10  |  TRIVIAL_ACCESSORS_ARE_NOT_RECOMMENDED   |  Check: if there are any trivial getters or setters <br> Fix: Delete trivial getter or setter                                                                                  |  yes  |  no  |  - |
 |    6  |     6.1.8  |  CUSTOM_GETTERS_SETTERS                  |  Check: Inspection that checks that no custom getters and setters are used for properties                                                                                      |  no  |  no  |  - |
 |    6  |    6.1.11  |  COMPACT_OBJECT_INITIALIZATION           |  Checks if class instantiation can be wrapped in `apply` for better readability  |    |  no  |  |
+|    6  |    6.1.12  |  INLINE_CLASS_CAN_BE_USED                |  Check: warns if class can be transferred to the inline class.                                                                                                                 | no | no | yes |
 |    6  |     6.2.2  |  EXTENSION_FUNCTION_SAME_SIGNATURE       |  Checks if extension function has the same signature as another extension function and their classes are related                                                               |  no  |  no  |  + |
 |    6  |     6.4.1  |  AVOID_USING_UTILITY_CLASS               | Checks if there is class/object that can be replace with extension function                                                                                                  | no  | no | - |
 |    6  |     6.4.2  |  OBJECT_IS_PREFERRED                     | Checks: if class is stateless it is preferred to use `object`                                                                                                                | yes | no | + |
\ No newline at end of file
diff --git a/info/guide/guide-TOC.md b/info/guide/guide-TOC.md
index 89233b325f..b63fecf1c5 100644
--- a/info/guide/guide-TOC.md
+++ b/info/guide/guide-TOC.md
@@ -105,6 +105,7 @@ I [Preface](#c0)
     * [6.1.9 Never use the name of a variable in the custom getter or setter (possible_bug)](#r6.1.9) 
     * [6.1.10 No trivial getters and setters are allowed in the code](#r6.1.10)
     * [6.1.11 Use 'apply' for grouping object initialization](#r6.1.11) 
+    * [6.1.12 Prefer Inline classes when a class has a single property](#r6.1.12)
 * [6.2 Classes](#c6.2)   
     * [6.2.1 Use extension functions for making logic of classes less coupled](#r6.2.1)
     * [6.2.2 No extension functions with the same name and signature if they extend base and inheritor classes (possible_bug)](#r6.2.2) 
diff --git a/info/guide/guide-chapter-6.md b/info/guide/guide-chapter-6.md
index ba7aff7765..db0969b4dd 100644
--- a/info/guide/guide-chapter-6.md
+++ b/info/guide/guide-chapter-6.md
@@ -357,6 +357,23 @@ fun main() {
 }
 ```
 
+### <a name="r6.1.12"></a> 6.1.12 Prefer Inline classes when a class has a single property
+If a class has only one immutable property, then it can be converted to the inline class.
+
+Sometimes it is necessary for business logic to create a wrapper around some type. However, it introduces runtime overhead due to additional heap allocations. Moreover, if the wrapped type is primitive, the performance hit is terrible, because primitive types are usually heavily optimized by the runtime, while their wrappers don't get any special treatment.
+
+**Invalid example**:
+```kotlin
+class Password {
+    val value: String
+}
+```
+
+**Valid example**:
+```kotlin
+inline class Password(val value: String)
+```
+
 <!-- =============================================================================== -->
 ### <a name="c6.2"></a>6.2 Extension functions
 This section describes the rules of using extension functions in your code.


Test patch:
diff --git a/diktat-rules/src/test/kotlin/org/cqfn/diktat/ruleset/chapter6/InlineClassesWarnTest.kt b/diktat-rules/src/test/kotlin/org/cqfn/diktat/ruleset/chapter6/InlineClassesWarnTest.kt
new file mode 100644
index 0000000000..2a3cf65760
--- /dev/null
+++ b/diktat-rules/src/test/kotlin/org/cqfn/diktat/ruleset/chapter6/InlineClassesWarnTest.kt
@@ -0,0 +1,163 @@
+package org.cqfn.diktat.ruleset.chapter6
+
+import org.cqfn.diktat.ruleset.constants.Warnings.INLINE_CLASS_CAN_BE_USED
+import org.cqfn.diktat.ruleset.rules.DIKTAT_RULE_SET_ID
+import org.cqfn.diktat.ruleset.rules.chapter6.classes.InlineClassesRule
+import org.cqfn.diktat.util.LintTestBase
+
+import com.pinterest.ktlint.core.LintError
+import generated.WarningNames
+import org.junit.jupiter.api.Tag
+import org.junit.jupiter.api.Test
+
+class InlineClassesWarnTest : LintTestBase(::InlineClassesRule) {
+    private val ruleId = "$DIKTAT_RULE_SET_ID:inline-classes"
+
+    @Test
+    @Tag(WarningNames.INLINE_CLASS_CAN_BE_USED)
+    fun `should not trigger on inline class`() {
+        lintMethod(
+            """
+                |inline class Name(val s: String) {}
+            """.trimMargin()
+        )
+    }
+
+    @Test
+    @Tag(WarningNames.INLINE_CLASS_CAN_BE_USED)
+    fun `should trigger on regular class`() {
+        lintMethod(
+            """
+                |class Some {
+                |   val config = Config()
+                |}
+            """.trimMargin(),
+            LintError(1, 1, ruleId, "${INLINE_CLASS_CAN_BE_USED.warnText()} class Some", true)
+        )
+    }
+
+    @Test
+    @Tag(WarningNames.INLINE_CLASS_CAN_BE_USED)
+    fun `should trigger on class with appropriate modifiers`() {
+        lintMethod(
+            """
+                |final class Some {
+                |   val config = Config()
+                |}
+            """.trimMargin(),
+            LintError(1, 1, ruleId, "${INLINE_CLASS_CAN_BE_USED.warnText()} class Some", true)
+        )
+    }
+
+    @Test
+    @Tag(WarningNames.INLINE_CLASS_CAN_BE_USED)
+    fun `should not trigger on class with inappropriate modifiers`() {
+        lintMethod(
+            """
+                |abstract class Some {
+                |   val config = Config()
+                |}
+            """.trimMargin()
+        )
+    }
+
+    @Test
+    @Tag(WarningNames.INLINE_CLASS_CAN_BE_USED)
+    fun `should trigger on class with val prop in constructor`() {
+        lintMethod(
+            """
+                |class Some(val anything: Int) {
+                |
+                |}
+            """.trimMargin(),
+            LintError(1, 1, ruleId, "${INLINE_CLASS_CAN_BE_USED.warnText()} class Some", true)
+        )
+    }
+
+    @Test
+    @Tag(WarningNames.INLINE_CLASS_CAN_BE_USED)
+    fun `should not trigger on class with var prop #1`() {
+        lintMethod(
+            """
+                |class Some(var anything: Int) {
+                |
+                |}
+            """.trimMargin()
+        )
+    }
+
+    @Test
+    @Tag(WarningNames.INLINE_CLASS_CAN_BE_USED)
+    fun `should not trigger on class with var prop #2`() {
+        lintMethod(
+            """
+                |class Some {
+                |   var some = 3
+                |}
+            """.trimMargin()
+        )
+    }
+
+    @Test
+    @Tag(WarningNames.INLINE_CLASS_CAN_BE_USED)
+    fun `should not trigger on class that extends class`() {
+        lintMethod(
+            """
+                |class Some : Any() {
+                |   val some = 3
+                |}
+            """.trimMargin()
+        )
+    }
+
+    @Test
+    @Tag(WarningNames.INLINE_CLASS_CAN_BE_USED)
+    fun `should trigger on class that extends interface`() {
+        lintMethod(
+            """
+                |class Some : Any {
+                |   val some = 3
+                |}
+            """.trimMargin(),
+            LintError(1, 1, ruleId, "${INLINE_CLASS_CAN_BE_USED.warnText()} class Some", true)
+        )
+    }
+
+    @Test
+    @Tag(WarningNames.INLINE_CLASS_CAN_BE_USED)
+    fun `should not trigger on class with internal constructor`() {
+        lintMethod(
+            """
+                |class LocalCommandExecutor internal constructor(private val command: String) {
+                |   
+                |}
+            """.trimMargin()
+        )
+    }
+
+    @Test
+    @Tag(WarningNames.INLINE_CLASS_CAN_BE_USED)
+    fun `should trigger on class with public constructor`() {
+        lintMethod(
+            """
+                |class LocalCommandExecutor public constructor(private val command: String) {
+                |   
+                |}
+            """.trimMargin(),
+            LintError(1, 1, ruleId, "${INLINE_CLASS_CAN_BE_USED.warnText()} class LocalCommandExecutor", true)
+        )
+    }
+
+    @Test
+    @Tag(WarningNames.INLINE_CLASS_CAN_BE_USED)
+    fun `should trigger on class with annotation before the constructor`() {
+        lintMethod(
+            """
+                |class LocalCommandExecutor @Inject constructor(private val command: String) {
+                |   
+                |}
+            """.trimMargin(),
+            LintError(1, 1, ruleId, "${INLINE_CLASS_CAN_BE_USED.warnText()} class LocalCommandExecutor", true)
+        )
+    }
+}
diff --git a/diktat-rules/src/test/kotlin/org/cqfn/diktat/ruleset/utils/RulesConfigYamlTest.kt b/diktat-rules/src/test/kotlin/org/cqfn/diktat/ruleset/utils/RulesConfigYamlTest.kt
index b7c4776a01..da18bc44ea 100644
--- a/diktat-rules/src/test/kotlin/org/cqfn/diktat/ruleset/utils/RulesConfigYamlTest.kt
+++ b/diktat-rules/src/test/kotlin/org/cqfn/diktat/ruleset/utils/RulesConfigYamlTest.kt
@@ -17,11 +17,10 @@ import kotlinx.serialization.encodeToString
  * Special test that checks that developer has not forgotten to add his warning to a diktat-analysis.yml
  * This file is needed to be in tact with latest changes in Warnings.kt
  */
-class RulesConfigYamlTest {
-    private val pathMap = mapOf("diktat-analysis.yml" to "diKTat/diktat-rules/src/main/resources/diktat-analysis.yml",
-        "diktat-analysis-huawei.yml" to "diKTat/diktat-rules/src/main/resources/diktat-analysis-huawei.yml",
-        "parent/diktat-analysis.yml" to "diKTat/diktat-analysis.yml")
-
+inline class RulesConfigYamlTest(private val pathMap: Map<String, String> =
+        mapOf("diktat-analysis.yml" to "diKTat/diktat-rules/src/main/resources/diktat-analysis.yml",
+            "diktat-analysis-huawei.yml" to "diKTat/diktat-rules/src/main/resources/diktat-analysis-huawei.yml",
+            "parent/diktat-analysis.yml" to "diKTat/diktat-analysis.yml")) {
     @Test
     fun `read rules config yml`() {
         compareRulesAndConfig("diktat-analysis.yml")