Add function expression body rule (#2151)

This rule rewrites function bodies only containining a `return` or `throw` expression to an expression body.

Closes #2150

CHANGELOG.md

@@ -6,7 +6,8 @@ This project adheres to [Semantic Versioning](https://semver.org/).
 
 ### Added
 
-* Add rule `class-signature`. This rule rewrites the class header to a consistent format. In code style `ktlint_official`, super types are always wrapped to a separate line. In other code styles, super types are only wrapped in classes having multiple super types. Especially for code style `ktlint_official` the class headers are rewritten in a more consistent format. See [examples in documentation](https://pinterest.github.io/ktlint/latest/rules/experimental/#class-signature). `class-signature` [#875](https://github.com/pinterest/ktlint/issues/1349), [#1349](https://github.com/pinterest/ktlint/issues/875)
+* Add experimental rule `class-signature`. This rule rewrites the class header to a consistent format. In code style `ktlint_official`, super types are always wrapped to a separate line. In other code styles, super types are only wrapped in classes having multiple super types. Especially for code style `ktlint_official` the class headers are rewritten in a more consistent format. See [examples in documentation](https://pinterest.github.io/ktlint/latest/rules/experimental/#class-signature). `class-signature` [#875](https://github.com/pinterest/ktlint/issues/1349), [#1349](https://github.com/pinterest/ktlint/issues/875)
+* Add experimental rule `function-expression-body`. This rule rewrites function bodies only contain a `return` or `throw` expression to an expression body. [#2150](https://github.com/pinterest/ktlint/issues/2150)
 
 ### Removed
 * As a part of public API stabilization, data classes are no longer used in the public API. As of that, functions like `copy()` or `componentN()` (used for destructuring declarations) are not available anymore. This is a binary incompatible change, breaking backwards compatibility. ([#2133](https://github.com/pinterest/ktlint/issues/2133))
@@ -35,7 +36,7 @@ This project adheres to [Semantic Versioning](https://semver.org/).
 * Update dependency com.google.jimfs:jimfs to v1.3.0 ([#2112](https://github.com/pinterest/ktlint/pull/2112))
 * As a part of public API stabilization, configure `binary-compatibility-validator` plugin for compile-time verification of binary compatibility with previous `ktlint` versions ([#2131](https://github.com/pinterest/ktlint/pull/2131))
 * Update dependency org.junit.jupiter:junit-jupiter to v5.10.0 ([#2148](https://github.com/pinterest/ktlint/pull/2148))
-* 
+
 ## [0.50.0] - 2023-06-29
 
 ### Deprecation of ktlint-enable and ktlint-disable directives

documentation/snapshot/docs/rules/experimental.md

@@ -434,6 +434,55 @@ This rule is only run when `ktlint_code_style` is set to `ktlint_official` or wh
 
 ## Function signature
 
+Rewrites a function body only containing a `return` or `throw` expression to an expression body.
+
+!!! note:
+    If the function body contains a comment, it is not rewritten to an expression body.
+
+=== "[:material-heart:](#) Ktlint"
+
+    ```kotlin
+    fun foo1() = "foo"
+    fun foo2(): String = "foo"
+    fun foo3(): Unit = throw IllegalArgumentException("some message")
+    fun foo4(): Foo = throw IllegalArgumentException("some message")
+    fun foo5() {
+        return "foo" // some comment
+    }
+    fun foo6(): String {
+        /* some comment */
+        return "foo"
+    }
+    fun foo7() {
+        throw IllegalArgumentException("some message")
+        /* some comment */
+    }
+    fun foo8(): Foo {
+        throw IllegalArgumentException("some message")
+        // some comment
+    }
+    ```
+=== "[:material-heart-off-outline:](#) Disallowed"
+
+    ```kotlin
+    fun foo1() {
+        return "foo"
+    }
+    fun foo2(): String {
+        return "foo"
+    }
+    fun foo3() {
+        throw IllegalArgumentException("some message")
+    }
+    fun foo4(): Foo {
+        throw IllegalArgumentException("some message")
+    }
+    ```
+
+Rule id: `function-expression-body` (`standard` rule set)
+
+## Function signature
+
 Rewrites the function signature to a single line when possible (e.g. when not exceeding the `max_line_length` property) or a multiline signature otherwise. In case of function with a body expression, the body expression is placed on the same line as the function signature when not exceeding the `max_line_length` property. Optionally the function signature can be forced to be written as a multiline signature in case the function has more than a specified number of parameters (`.editorconfig` property `ktlint_function_signature_wrapping_rule_always_with_minimum_parameters`)
 
 === "[:material-heart:](#) Ktlint"

ktlint-cli-reporter-format/src/main/kotlin/com/pinterest/ktlint/cli/reporter/format/FormatReporterProvider.kt

@@ -22,7 +22,8 @@ public class FormatReporterProvider : ReporterProviderV2<FormatReporter> {
 
     private fun String.emptyOrTrue() = this == "" || this == "true"
 
-    private fun getColor(color: String?): Color {
-        return Color.values().firstOrNull { it.name == color } ?: throw IllegalArgumentException("Invalid color parameter.")
-    }
+    private fun getColor(color: String?): Color =
+        Color.values().firstOrNull {
+            it.name == color
+        } ?: throw IllegalArgumentException("Invalid color parameter.")
 }

ktlint-cli-reporter-plain/src/main/kotlin/com/pinterest/ktlint/cli/reporter/plain/PlainReporterProvider.kt

@@ -20,7 +20,8 @@ public class PlainReporterProvider : ReporterProviderV2<PlainReporter> {
 
     private fun String.emptyOrTrue() = this == "" || this == "true"
 
-    private fun getColor(colorInput: String?): Color {
-        return Color.values().firstOrNull { it.name == colorInput } ?: throw IllegalArgumentException("Invalid color parameter.")
-    }
+    private fun getColor(colorInput: String?): Color =
+        Color.values().firstOrNull {
+            it.name == colorInput
+        } ?: throw IllegalArgumentException("Invalid color parameter.")
 }

ktlint-cli/src/main/kotlin/com/pinterest/ktlint/cli/internal/FileUtils.kt

@@ -134,15 +134,14 @@ internal fun FileSystem.fileSequence(
                     override fun preVisitDirectory(
                         dirPath: Path,
                         dirAttr: BasicFileAttributes,
-                    ): FileVisitResult {
-                        return if (Files.isHidden(dirPath)) {
+                    ): FileVisitResult =
+                        if (Files.isHidden(dirPath)) {
                             LOGGER.trace { "- Dir: $dirPath: Ignore" }
                             FileVisitResult.SKIP_SUBTREE
                         } else {
                             LOGGER.trace { "- Dir: $dirPath: Traverse" }
                             FileVisitResult.CONTINUE
                         }
-                    }
                 },
             )
         }

ktlint-cli/src/main/kotlin/com/pinterest/ktlint/cli/internal/KtlintCommandLine.kt

@@ -651,28 +651,18 @@ internal class KtlintCommandLine {
     ) {
         val pill =
             object : Future<T> {
-                override fun isDone(): Boolean {
-                    throw UnsupportedOperationException()
-                }
+                override fun isDone(): Boolean = throw UnsupportedOperationException()
 
                 override fun get(
                     timeout: Long,
                     unit: TimeUnit,
-                ): T {
-                    throw UnsupportedOperationException()
-                }
+                ): T = throw UnsupportedOperationException()
 
-                override fun get(): T {
-                    throw UnsupportedOperationException()
-                }
+                override fun get(): T = throw UnsupportedOperationException()
 
-                override fun cancel(mayInterruptIfRunning: Boolean): Boolean {
-                    throw UnsupportedOperationException()
-                }
+                override fun cancel(mayInterruptIfRunning: Boolean): Boolean = throw UnsupportedOperationException()
 
-                override fun isCancelled(): Boolean {
-                    throw UnsupportedOperationException()
-                }
+                override fun isCancelled(): Boolean = throw UnsupportedOperationException()
             }
         val q = ArrayBlockingQueue<Future<T>>(numberOfThreads)
         val producer =

ktlint-rule-engine-core/src/main/kotlin/com/pinterest/ktlint/rule/engine/core/api/RuleProvider.kt

@@ -28,9 +28,7 @@ public class RuleProvider private constructor(
     /**
      * Creates a new [Rule] instance.
      */
-    public fun createNewRuleInstance(): Rule {
-        return provider()
-    }
+    public fun createNewRuleInstance(): Rule = provider()
 
     /**
      * Lambda which creates a new instance of the [Rule]. Important: to ensure that a [Rule] can keep internal state and that processing of

ktlint-rule-engine-core/src/main/kotlin/com/pinterest/ktlint/rule/engine/core/api/editorconfig/EditorConfig.kt

@@ -200,13 +200,12 @@ public class EditorConfig(
         }
     }
 
-    private fun Set<EditorConfigProperty<*>>.defaultProperties(): Map<String, Property> {
-        return associate { editorConfigProperty ->
+    private fun Set<EditorConfigProperty<*>>.defaultProperties(): Map<String, Property> =
+        associate { editorConfigProperty ->
             editorConfigProperty
                 .writeDefaultValue()
                 .let { editorConfigProperty.name to editorConfigProperty.toPropertyWithValue(it) }
         }
-    }
 
     private fun <T> EditorConfigProperty<T>.writeDefaultValue() = propertyWriter(getDefaultValue())
 

ktlint-rule-engine/src/main/kotlin/com/pinterest/ktlint/rule/engine/internal/RuleExecutionContext.kt

@@ -204,12 +204,11 @@ internal class RuleExecutionContext private constructor(
             )
         }
 
-        private fun normalizeText(text: String): String {
-            return text
+        private fun normalizeText(text: String): String =
+            text
                 .replace("\r\n", "\n")
                 .replace("\r", "\n")
                 .replaceFirst(UTF8_BOM, "")
-        }
 
         private fun PsiElement.findErrorElement(): PsiErrorElement? {
             if (this is PsiErrorElement) {

ktlint-rule-engine/src/main/kotlin/com/pinterest/ktlint/rule/engine/internal/rulefilter/RunAfterRuleFilter.kt

@@ -150,8 +150,8 @@ internal class RunAfterRuleFilter : RuleFilter {
     private fun Set<RuleProvider>.canRunWith(loadedRuleProviders: Set<RuleProvider>): Set<RuleProvider> =
         canRunWithRuleIds(loadedRuleProviders.map { it.ruleId }.toSet())
 
-    private fun Set<RuleProvider>.canRunWithRuleIds(loadedRuleIds: Set<RuleId>): Set<RuleProvider> {
-        return this
+    private fun Set<RuleProvider>.canRunWithRuleIds(loadedRuleIds: Set<RuleId>): Set<RuleProvider> =
+        this
             .filter { it.canRunWith(loadedRuleIds) }
             .let { unblockedRuleProviders ->
                 if (unblockedRuleProviders.isEmpty()) {
@@ -165,7 +165,6 @@ internal class RunAfterRuleFilter : RuleFilter {
                         .plus(unblockedRuleProviders)
                 }
             }.toSet()
-    }
 
     private fun RuleProvider.canRunWith(loadedRuleIds: Set<RuleId>): Boolean =
         this

ktlint-rule-engine/src/main/kotlin/com/pinterest/ktlint/rule/engine/internal/rules/KtlintSuppressionRule.kt

@@ -456,8 +456,8 @@ public class KtlintSuppressionRule(
     private fun ASTNode.createFileAnnotation(
         suppressType: SuppressAnnotationType,
         suppressions: Set<String>,
-    ): ASTNode {
-        return suppressions
+    ): ASTNode =
+        suppressions
             .sorted()
             .joinToString()
             .let { sortedSuppressions -> "@file:${suppressType.annotationName}($sortedSuppressions)" }
@@ -468,7 +468,6 @@ public class KtlintSuppressionRule(
                     ?.firstChild
                     ?: throw IllegalStateException("Can not create annotation '$annotation'")
             }.node
-    }
 
     private fun ASTNode.createFileAnnotationList(annotation: ASTNode) {
         require(isRoot()) { "File annotation list can only be created for root node" }

ktlint-rule-engine/src/test/kotlin/com/pinterest/ktlint/rule/engine/internal/RuleProviderSorterTest.kt

@@ -468,10 +468,9 @@ class RuleProviderSorterTest {
             node: ASTNode,
             autoCorrect: Boolean,
             emit: (offset: Int, errorMessage: String, canBeAutoCorrected: Boolean) -> Unit,
-        ) {
+        ): Unit =
             throw UnsupportedOperationException(
                 "Rule should never be really invoked because that is not the aim of this unit test.",
             )
-        }
     }
 }

ktlint-rule-engine/src/test/kotlin/com/pinterest/ktlint/rule/engine/internal/rulefilter/InternalRuleProvidersFilterTest.kt

@@ -51,11 +51,10 @@ class InternalRuleProvidersFilterTest {
             node: ASTNode,
             autoCorrect: Boolean,
             emit: (offset: Int, errorMessage: String, canBeAutoCorrected: Boolean) -> Unit,
-        ) {
+        ): Unit =
             throw UnsupportedOperationException(
                 "Rule should never be really invoked because that is not the aim of this unit test.",
             )
-        }
     }
 
     private fun Set<RuleProvider>.toRuleId() = map { it.ruleId }

ktlint-rule-engine/src/test/kotlin/com/pinterest/ktlint/rule/engine/internal/rulefilter/RunAfterRuleFilterTest.kt

@@ -373,11 +373,10 @@ class RunAfterRuleFilterTest {
             node: ASTNode,
             autoCorrect: Boolean,
             emit: (offset: Int, errorMessage: String, canBeAutoCorrected: Boolean) -> Unit,
-        ) {
+        ): Unit =
             throw UnsupportedOperationException(
                 "Rule should never be really invoked because that is not the aim of this unit test.",
             )
-        }
     }
 
     private fun createRuleProviders(vararg rules: Rule) =

ktlint-ruleset-standard/api/ktlint-ruleset-standard.api

@@ -184,6 +184,16 @@ public final class com/pinterest/ktlint/ruleset/standard/rules/FunKeywordSpacing
 	public static final fun getFUN_KEYWORD_SPACING_RULE ()Lcom/pinterest/ktlint/rule/engine/core/api/RuleId;
 }
 
+public final class com/pinterest/ktlint/ruleset/standard/rules/FunctionExpressionBodyKt {
+	public static final fun getFUNCTION_EXPRESSION_BODY_RULE_ID ()Lcom/pinterest/ktlint/rule/engine/core/api/RuleId;
+}
+
+public final class com/pinterest/ktlint/ruleset/standard/rules/FunctionExpressionBodyRule : com/pinterest/ktlint/ruleset/standard/StandardRule, com/pinterest/ktlint/rule/engine/core/api/Rule$Experimental {
+	public fun <init> ()V
+	public fun beforeFirstNode (Lcom/pinterest/ktlint/rule/engine/core/api/editorconfig/EditorConfig;)V
+	public fun beforeVisitChildNodes (Lorg/jetbrains/kotlin/com/intellij/lang/ASTNode;ZLkotlin/jvm/functions/Function3;)V
+}
+
 public final class com/pinterest/ktlint/ruleset/standard/rules/FunctionLiteralKt {
 	public static final fun getFUNCTION_LITERAL_RULE_ID ()Lcom/pinterest/ktlint/rule/engine/core/api/RuleId;
 }

ktlint-ruleset-standard/src/main/kotlin/com/pinterest/ktlint/ruleset/standard/StandardRuleSetProvider.kt

@@ -21,6 +21,7 @@ import com.pinterest.ktlint.ruleset.standard.rules.EnumWrappingRule
 import com.pinterest.ktlint.ruleset.standard.rules.FilenameRule
 import com.pinterest.ktlint.ruleset.standard.rules.FinalNewlineRule
 import com.pinterest.ktlint.ruleset.standard.rules.FunKeywordSpacingRule
+import com.pinterest.ktlint.ruleset.standard.rules.FunctionExpressionBodyRule
 import com.pinterest.ktlint.ruleset.standard.rules.FunctionLiteralRule
 import com.pinterest.ktlint.ruleset.standard.rules.FunctionNamingRule
 import com.pinterest.ktlint.ruleset.standard.rules.FunctionReturnTypeSpacingRule
@@ -106,6 +107,7 @@ public class StandardRuleSetProvider : RuleSetProviderV3(RuleSetId.STANDARD) {
             RuleProvider { EnumWrappingRule() },
             RuleProvider { FilenameRule() },
             RuleProvider { FinalNewlineRule() },
+            RuleProvider { FunctionExpressionBodyRule() },
             RuleProvider { FunctionLiteralRule() },
             RuleProvider { FunctionNamingRule() },
             RuleProvider { FunctionReturnTypeSpacingRule() },

ktlint-ruleset-standard/src/main/kotlin/com/pinterest/ktlint/ruleset/standard/rules/ArgumentListWrappingRule.kt

@@ -228,14 +228,13 @@ public class ArgumentListWrappingRule :
             else -> throw UnsupportedOperationException()
         }
 
-    private fun ASTNode.textContainsIgnoringLambda(char: Char): Boolean {
-        return children().any { child ->
+    private fun ASTNode.textContainsIgnoringLambda(char: Char): Boolean =
+        children().any { child ->
             val elementType = child.elementType
             elementType == ElementType.WHITE_SPACE && child.textContains(char) ||
                 elementType == ElementType.COLLECTION_LITERAL_EXPRESSION && child.textContains(char) ||
                 elementType == ElementType.VALUE_ARGUMENT && child.children().any { it.textContainsIgnoringLambda(char) }
         }
-    }
 
     private fun ASTNode.hasTypeArgumentListInFront(): Boolean =
         treeParent.children()

ktlint-ruleset-standard/src/main/kotlin/com/pinterest/ktlint/ruleset/standard/rules/ChainWrappingRule.kt

@@ -143,10 +143,9 @@ public class ChainWrappingRule :
 
     private fun ASTNode.isInPrefixPosition() = treeParent?.treeParent?.elementType == PREFIX_EXPRESSION
 
-    private fun ASTNode.isElvisOperatorAndComment(): Boolean {
-        return elementType == ELVIS &&
+    private fun ASTNode.isElvisOperatorAndComment(): Boolean =
+        elementType == ELVIS &&
             leaves().takeWhile { it.isWhiteSpaceWithoutNewline() || it.isPartOfComment() }.any()
-    }
 }
 
 public val CHAIN_WRAPPING_RULE_ID: RuleId = ChainWrappingRule().ruleId