Add new experimental rule fun-keyword-spacing (#1362)

Lints and formats the spacing after the fun keyword.

This rule is required to create a rule which can rewrite the function signature automatically as is described in #1341

CHANGELOG.md

@@ -25,6 +25,7 @@ Please welcome [paul-dingemans](https://github.com/paul-dingemans) as an officia
 - Basic tests for CLI ([#540](https://github.com/pinterest/ktlint/issues/540))
 - Add experimental rule for unexpected spaces in a type reference before a function identifier (`function-type-reference-spacing`) ([#1341](https://github.com/pinterest/ktlint/issues/1341))
 - Add experimental rule for unnecessary parentheses in function call followed by lambda ([#1068](https://github.com/pinterest/ktlint/issues/1068))
+- Add rule to check spacing after fun keyword (`fun-keyword-spacing`) ([#1362](https://github.com/pinterest/ktlint/pull/1362))
 - Add experimental rules for unnecessary spacing between modifiers in and after the last modifier in a modifier list ([#1361](https://github.com/pinterest/ktlint/pull/1361))   
 
 ### Fixed

README.md

@@ -87,6 +87,7 @@ by passing the `--experimental` flag to `ktlint`.
 ### Spacing
 - `experimental:annotation-spacing`: Annotations should be separated by the annotated declaration by a single line break
 - `experimental:double-colon-spacing`: No spaces around `::`
+- `experimental:fun-keyword-spacing`: Consistent spacing after the fun keyword
 - `experimental:function-type-reference-spacing`: Consistent spacing in the type reference before a function
 - `experimental:modifier-list-spacing`: Consistent spacing between modifiers in and after the last modifier in a modifier list
 - `experimental:spacing-around-angle-brackets`: No spaces around angle brackets

ktlint-ruleset-experimental/src/main/kotlin/com/pinterest/ktlint/ruleset/experimental/ExperimentalRuleSetProvider.kt

@@ -22,6 +22,7 @@ public class ExperimentalRuleSetProvider : RuleSetProvider {
         SpacingAroundUnaryOperatorRule(),
         AnnotationSpacingRule(),
         UnnecessaryParenthesesBeforeTrailingLambdaRule(),
+        FunKeywordSpacingRule(),
         FunctionTypeReferenceSpacingRule(),
         ModifierListSpacingRule()
     )

ktlint-ruleset-experimental/src/main/kotlin/com/pinterest/ktlint/ruleset/experimental/FunKeywordSpacingRule.kt

@@ -0,0 +1,34 @@
+package com.pinterest.ktlint.ruleset.experimental
+
+import com.pinterest.ktlint.core.Rule
+import com.pinterest.ktlint.core.ast.ElementType
+import com.pinterest.ktlint.core.ast.ElementType.FUN_KEYWORD
+import com.pinterest.ktlint.core.ast.nextLeaf
+import org.jetbrains.kotlin.com.intellij.lang.ASTNode
+import org.jetbrains.kotlin.com.intellij.psi.impl.source.tree.LeafPsiElement
+
+/**
+ * Lints and formats the spacing after the fun keyword
+ */
+public class FunKeywordSpacingRule : Rule("fun-keyword-spacing") {
+    override fun visit(
+        node: ASTNode,
+        autoCorrect: Boolean,
+        emit: (offset: Int, errorMessage: String, canBeAutoCorrected: Boolean) -> Unit
+    ) {
+        node
+            .takeIf { it.elementType == FUN_KEYWORD }
+            ?.nextLeaf(includeEmpty = true)
+            ?.takeIf { it.elementType == ElementType.WHITE_SPACE && it.text != " " }
+            ?.let { whiteSpaceAfterFunKeyword ->
+                emit(
+                    whiteSpaceAfterFunKeyword.startOffset,
+                    "Single space expected after the fun keyword",
+                    true
+                )
+                if (autoCorrect) {
+                    (whiteSpaceAfterFunKeyword as LeafPsiElement).rawReplaceWithText(" ")
+                }
+            }
+    }
+}

ktlint-ruleset-experimental/src/test/kotlin/com/pinterest/ktlint/ruleset/experimental/FunKeywordSpacingRuleTest.kt

@@ -0,0 +1,42 @@
+package com.pinterest.ktlint.ruleset.experimental
+
+import com.pinterest.ktlint.core.LintError
+import com.pinterest.ktlint.test.format
+import com.pinterest.ktlint.test.lint
+import org.assertj.core.api.Assertions.assertThat
+import org.junit.jupiter.api.Test
+
+class FunKeywordSpacingRuleTest {
+    @Test
+    fun `Given a function signature with multiple spaces between the fun keyword and the function name then remove the redundant spaces`() {
+        val code =
+            """
+            fun  foo() = "some-result"
+            """.trimIndent()
+        val formattedCode =
+            """
+            fun foo() = "some-result"
+            """.trimIndent()
+        assertThat(FunKeywordSpacingRule().lint(code)).containsExactly(
+            LintError(1, 4, "fun-keyword-spacing", "Single space expected after the fun keyword")
+        )
+        assertThat(FunKeywordSpacingRule().format(code)).isEqualTo(formattedCode)
+    }
+
+    @Test
+    fun `Given a function signature with a newline between the fun keyword and the function name then remove the redundant newline`() {
+        val code =
+            """
+            fun
+            foo() = "some-result"
+            """.trimIndent()
+        val formattedCode =
+            """
+            fun foo() = "some-result"
+            """.trimIndent()
+        assertThat(FunKeywordSpacingRule().lint(code)).containsExactly(
+            LintError(1, 4, "fun-keyword-spacing", "Single space expected after the fun keyword")
+        )
+        assertThat(FunKeywordSpacingRule().format(code)).isEqualTo(formattedCode)
+    }
+}