Repository: cqfn/diKTat (kotlin)

Issue:
EMPTY_BLOCK_STRUCTURE_ERROR shouldn't trigger on empty lambdas
## Describe the bug
Sometimes empty lambda can be used as a function that does nothing:
```kotlin
fun foo(bar: () -> Unit = {})
```
or empty function in implementation of functional interface:
```kotlin
A({})  // instead of A(Disposable {}), where Disposable is functional interface from Java
```
Diktat shouldn't issue a warning in these cases, because they are valid and can't be done differentlty. Also, kotlin copiler warns that `Unit` is redundant in `{ Unit }`.

## Environment information
* diktat version: 0.4.2

Pull request description:
### What's done:
  * Fixed bug

This pull request closes #796

Patch:
diff --git a/diktat-rules/src/main/kotlin/org/cqfn/diktat/ruleset/rules/chapter3/EmptyBlock.kt b/diktat-rules/src/main/kotlin/org/cqfn/diktat/ruleset/rules/chapter3/EmptyBlock.kt
index f0405440c5..8a1dd69960 100644
--- a/diktat-rules/src/main/kotlin/org/cqfn/diktat/ruleset/rules/chapter3/EmptyBlock.kt
+++ b/diktat-rules/src/main/kotlin/org/cqfn/diktat/ruleset/rules/chapter3/EmptyBlock.kt
@@ -9,13 +9,21 @@ import org.cqfn.diktat.ruleset.utils.*
 
 import com.pinterest.ktlint.core.ast.ElementType
 import com.pinterest.ktlint.core.ast.ElementType.CALL_EXPRESSION
+import com.pinterest.ktlint.core.ast.ElementType.DOT_QUALIFIED_EXPRESSION
+import com.pinterest.ktlint.core.ast.ElementType.FILE
 import com.pinterest.ktlint.core.ast.ElementType.FUNCTION_LITERAL
 import com.pinterest.ktlint.core.ast.ElementType.IDENTIFIER
+import com.pinterest.ktlint.core.ast.ElementType.LAMBDA_EXPRESSION
+import com.pinterest.ktlint.core.ast.ElementType.LPAR
 import com.pinterest.ktlint.core.ast.ElementType.RBRACE
+import com.pinterest.ktlint.core.ast.ElementType.VALUE_ARGUMENT
+import com.pinterest.ktlint.core.ast.ElementType.VALUE_ARGUMENT_LIST
+import com.pinterest.ktlint.core.ast.ElementType.VALUE_PARAMETER
 import com.pinterest.ktlint.core.ast.ElementType.WHITE_SPACE
 import org.jetbrains.kotlin.com.intellij.lang.ASTNode
 import org.jetbrains.kotlin.com.intellij.psi.impl.source.tree.LeafPsiElement
 import org.jetbrains.kotlin.com.intellij.psi.impl.source.tree.PsiWhiteSpaceImpl
+import org.jetbrains.kotlin.psi.psiUtil.parents
 
 /**
  * Rule that checks if empty code blocks (`{  }`) are used and checks their formatting.
@@ -41,7 +49,7 @@ class EmptyBlock(configRules: List<RulesConfig>) : DiktatRule(
 
     @Suppress("UnsafeCallOnNullableType", "TOO_LONG_FUNCTION")
     private fun checkEmptyBlock(node: ASTNode, configuration: EmptyBlockStyleConfiguration) {
-        if (node.treeParent.isOverridden() || isAnonymousSamClass(node)) {
+        if (node.treeParent.isOverridden() || isAnonymousSamClass(node) || isLambdaUsedAsFunction(node)) {
             return
         }
         if (node.isBlockEmpty()) {
@@ -82,11 +90,11 @@ class EmptyBlock(configRules: List<RulesConfig>) : DiktatRule(
         }
     }
 
-    @Suppress("UnsafeCallOnNullableType", "WRONG_WHITESPACE")
-    private fun isAnonymousSamClass(node: ASTNode) : Boolean =
+    @Suppress("UnsafeCallOnNullableType")
+    private fun isAnonymousSamClass(node: ASTNode): Boolean =
             if (node.elementType == FUNCTION_LITERAL && node.hasParent(CALL_EXPRESSION)) {
-                // We are checking identifier because it is not class in AST
-                // , SAM conversions are indistinguishable from lambdas.
+                // We are checking identifier because it is not class in AST,
+                // SAM conversions are indistinguishable from lambdas.
                 // So we just verify that identifier is in PascalCase
                 val valueArgument = node.findParentNodeWithSpecificType(CALL_EXPRESSION)!!
                 valueArgument.findLeafWithSpecificType(IDENTIFIER)?.text?.isPascalCase() ?: false
@@ -94,6 +102,25 @@ class EmptyBlock(configRules: List<RulesConfig>) : DiktatRule(
                 false
             }
 
+    @Suppress("UnsafeCallOnNullableType")
+    private fun isLambdaUsedAsFunction(node: ASTNode): Boolean {
+        val parents = node.parents()
+        return when {
+            parents.any { it.elementType == CALL_EXPRESSION } -> {
+                val callExpression = parents.find { it.elementType == CALL_EXPRESSION }!!
+                // excepting cases like list.map { }. In this case call expression will not have value argument list
+                // And in this case: Parser.parse({}, some, thing) it will have value argument list
+                callExpression.hasChildOfType(VALUE_ARGUMENT_LIST)
+            }
+            parents.any { it.elementType == LAMBDA_EXPRESSION } -> {
+                val lambdaExpression = parents.find { it.elementType == LAMBDA_EXPRESSION }!!
+                // cases like A({}). Here Lambda expression is used as a value parameter.
+                lambdaExpression.treeParent.elementType == VALUE_PARAMETER
+            }
+            else -> false
+        }
+    }
+
     /**
      * [RuleConfiguration] for empty blocks formatting
      */
diff --git a/diktat-rules/src/main/kotlin/org/cqfn/diktat/ruleset/utils/KotlinParser.kt b/diktat-rules/src/main/kotlin/org/cqfn/diktat/ruleset/utils/KotlinParser.kt
index 3a994703f7..abeb23661b 100644
--- a/diktat-rules/src/main/kotlin/org/cqfn/diktat/ruleset/utils/KotlinParser.kt
+++ b/diktat-rules/src/main/kotlin/org/cqfn/diktat/ruleset/utils/KotlinParser.kt
@@ -10,7 +10,6 @@ import org.jetbrains.kotlin.cli.jvm.compiler.EnvironmentConfigFiles
 import org.jetbrains.kotlin.cli.jvm.compiler.KotlinCoreEnvironment
 import org.jetbrains.kotlin.com.intellij.lang.ASTNode
 import org.jetbrains.kotlin.com.intellij.mock.MockProject
-import org.jetbrains.kotlin.com.intellij.openapi.Disposable
 import org.jetbrains.kotlin.com.intellij.openapi.project.Project
 import org.jetbrains.kotlin.com.intellij.openapi.util.UserDataHolderBase
 import org.jetbrains.kotlin.com.intellij.pom.PomModel
@@ -49,8 +48,7 @@ class KotlinParser {
                 return null
             }
         }  // I don't really understand what's going on here, but thanks to this, you can use this node in the future
-        val project = KotlinCoreEnvironment.createForProduction(
-            Disposable { },
+        val project = KotlinCoreEnvironment.createForProduction({},
             compilerConfiguration,
             EnvironmentConfigFiles.JVM_CONFIG_FILES
         ).project  // create project
diff --git a/info/guide/guide-chapter-3.md b/info/guide/guide-chapter-3.md
index e764e325af..69d855e7c5 100644
--- a/info/guide/guide-chapter-3.md
+++ b/info/guide/guide-chapter-3.md
@@ -305,7 +305,7 @@ class A
 Avoid empty blocks, and ensure braces start on a new line. An empty code block can be closed immediately on the same line and the next line. However, a newline is recommended between opening and closing braces `{}` (see the examples below.)
 
 Generally, empty code blocks are prohibited; using them is considered a bad practice (especially for catch block).
-They are only appropriate for overridden functions when the base class's functionality is not needed in the class-inheritor.
+They are appropriate for overridden functions, when the base class's functionality is not needed in the class-inheritor, for lambdas used as a function and for empty function in implementation of functional interface. 
 ```kotlin
 override fun foo() {    
 }
@@ -318,6 +318,8 @@ fun doNothing() {}
 
 fun doNothingElse() {
 }
+
+fun foo(bar: () -> Unit = {})
 ```
 
 **Invalid examples:**


Test patch:
diff --git a/diktat-rules/src/test/kotlin/org/cqfn/diktat/ruleset/chapter3/EmptyBlockWarnTest.kt b/diktat-rules/src/test/kotlin/org/cqfn/diktat/ruleset/chapter3/EmptyBlockWarnTest.kt
index 66e82eb25e..c2a87615f0 100644
--- a/diktat-rules/src/test/kotlin/org/cqfn/diktat/ruleset/chapter3/EmptyBlockWarnTest.kt
+++ b/diktat-rules/src/test/kotlin/org/cqfn/diktat/ruleset/chapter3/EmptyBlockWarnTest.kt
@@ -8,6 +8,8 @@ import org.cqfn.diktat.util.LintTestBase
 
 import com.pinterest.ktlint.core.LintError
 import generated.WarningNames
+import org.jetbrains.kotlin.cli.jvm.compiler.EnvironmentConfigFiles
+import org.jetbrains.kotlin.cli.jvm.compiler.KotlinCoreEnvironment
 import org.junit.jupiter.api.Tag
 import org.junit.jupiter.api.Test
 
@@ -204,4 +206,38 @@ class EmptyBlockWarnTest : LintTestBase(::EmptyBlock) {
             rulesConfigList = rulesConfigListEmptyBlockExist
         )
     }
+
+    @Test
+    @Tag(WarningNames.EMPTY_BLOCK_STRUCTURE_ERROR)
+    fun `should not trigger on empty lambdas as a functions`() {
+        lintMethod(
+            """
+                |fun foo(bar: () -> Unit = {})
+                |
+                |class Some {
+                |   fun bar() {
+                |       A({})
+                |   }
+                |}
+            """.trimMargin(),
+            rulesConfigList = rulesConfigListEmptyBlockExist
+        )
+    }
+
+    @Test
+    @Tag(WarningNames.EMPTY_BLOCK_STRUCTURE_ERROR)
+    fun `should not trigger on empty lambdas as a functions #2`() {
+        lintMethod(
+            """
+                |fun some() {
+                |    val project = KotlinCoreEnvironment.createForProduction(
+                |       { },
+                |       compilerConfiguration,
+                |       EnvironmentConfigFiles.JVM_CONFIG_FILES
+                |    ).project
+                |}
+            """.trimMargin(),
+            rulesConfigList = rulesConfigListEmptyBlockExist
+        )
+    }
 }