Show inlay hints for implicit parameters and receivers of lambdas

#KT-20533 Fixed

idea/src/org/jetbrains/kotlin/idea/parameterInfo/KotlinInlayParameterHintsProvider.kt

@@ -84,7 +84,8 @@ enum class HintType(desc: String, enabled: Boolean) {
     },
 
     LAMBDA_RETURN_EXPRESSION("Show lambda return expression hints", true) {
-        override fun isApplicable(elem: PsiElement) = elem is KtExpression
+        override fun isApplicable(elem: PsiElement) =
+            elem is KtExpression && elem !is KtLambdaExpression && elem !is KtFunctionLiteral
 
         override fun provideHints(elem: PsiElement): List<InlayInfo> {
             if (elem is KtExpression) {
@@ -92,8 +93,18 @@ enum class HintType(desc: String, enabled: Boolean) {
             }
             return emptyList()
         }
-    }
-    ;
+    },
+
+    LAMBDA_IMPLICIT_PARAMETER_RECEIVER("Show hints for implicit receivers and parameters of lambdas", true) {
+        override fun isApplicable(elem: PsiElement) = elem is KtLambdaExpression
+
+        override fun provideHints(elem: PsiElement): List<InlayInfo> {
+            (elem as? KtLambdaExpression)?.let {
+                return provideLambdaImplicitHints(elem)
+            }
+            return emptyList()
+        }
+    };
 
     companion object {
 

idea/src/org/jetbrains/kotlin/idea/parameterInfo/LambdaImpicitHints.kt

@@ -0,0 +1,61 @@
+/*
+ * Copyright 2000-2018 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license
+ * that can be found in the license/LICENSE.txt file.
+ */
+
+package org.jetbrains.kotlin.idea.parameterInfo
+
+import com.intellij.codeInsight.hints.InlayInfo
+import com.intellij.lang.ASTNode
+import com.intellij.psi.PsiComment
+import com.intellij.psi.TokenType
+import org.jetbrains.kotlin.idea.caches.resolve.analyze
+import org.jetbrains.kotlin.psi.KtLambdaExpression
+import org.jetbrains.kotlin.psi.psiUtil.siblings
+import org.jetbrains.kotlin.resolve.BindingContext
+import org.jetbrains.kotlin.resolve.lazy.BodyResolveMode
+import org.jetbrains.kotlin.types.typeUtil.isUnit
+
+fun provideLambdaImplicitHints(lambda: KtLambdaExpression): List<InlayInfo> {
+    val lbrace = lambda.leftCurlyBrace
+    if (!lbrace.isFollowedByNewLine()) {
+        return emptyList()
+    }
+    val bindingContext = lambda.analyze(BodyResolveMode.PARTIAL)
+    val functionDescriptor = bindingContext[BindingContext.FUNCTION, lambda.functionLiteral] ?: return emptyList()
+
+    val implicitReceiver = functionDescriptor.extensionReceiverParameter
+    if (implicitReceiver != null) {
+        val text = buildString {
+            append(TYPE_INFO_PREFIX)
+            append("this: ")
+            append(inlayHintsTypeRenderer.renderType(implicitReceiver.type))
+        }
+        return listOf(InlayInfo(text, lbrace.textRange.endOffset))
+    }
+
+    val singleParameter = functionDescriptor.valueParameters.singleOrNull()
+    if (singleParameter != null && bindingContext[BindingContext.AUTO_CREATED_IT, singleParameter] == true) {
+        val type = singleParameter.type
+        if (type.isUnit()) return emptyList()
+        val text = buildString {
+            append(TYPE_INFO_PREFIX)
+            append("it: ")
+            append(inlayHintsTypeRenderer.renderType(type))
+        }
+        return listOf(InlayInfo(text, lbrace.textRange.endOffset))
+    }
+    return emptyList()
+}
+
+private fun ASTNode.isFollowedByNewLine(): Boolean {
+    for (sibling in siblings()) {
+        if (sibling.elementType != TokenType.WHITE_SPACE && sibling.psi !is PsiComment) {
+            return false
+        }
+        if (sibling.elementType == TokenType.WHITE_SPACE && sibling.textContains('\n')) {
+            return true
+        }
+    }
+    return false
+}

idea/src/org/jetbrains/kotlin/idea/parameterInfo/TypeHints.kt

@@ -1,17 +1,6 @@
 /*
- * Copyright 2010-2017 JetBrains s.r.o.
- *
- * Licensed under the Apache License, Version 2.0 (the "License");
- * you may not use this file except in compliance with the License.
- * You may obtain a copy of the License at
- *
- * http://www.apache.org/licenses/LICENSE-2.0
- *
- * Unless required by applicable law or agreed to in writing, software
- * distributed under the License is distributed on an "AS IS" BASIS,
- * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
- * See the License for the specific language governing permissions and
- * limitations under the License.
+ * Copyright 2000-2018 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license
+ * that can be found in the license/LICENSE.txt file.
  */
 
 package org.jetbrains.kotlin.idea.parameterInfo
@@ -35,7 +24,7 @@ import org.jetbrains.kotlin.types.typeUtil.immediateSupertypes
 
 //hack to separate type presentation from param info presentation
 const val TYPE_INFO_PREFIX = "@TYPE@"
-private val typeRenderer = DescriptorRenderer.COMPACT_WITH_SHORT_TYPES.withOptions {
+val inlayHintsTypeRenderer = DescriptorRenderer.COMPACT_WITH_SHORT_TYPES.withOptions {
     textFormat = RenderingFormat.PLAIN
     renderUnabbreviatedType = false
 }
@@ -76,7 +65,7 @@ fun provideTypeHint(element: KtCallableDeclaration, offset: Int): List<InlayInfo
             append(":")
             if (settings.SPACE_AFTER_TYPE_COLON)
                 append(" ")
-            append(typeRenderer.renderType(type))
+            append(inlayHintsTypeRenderer.renderType(type))
         }
         listOf(InlayInfo(declString, offset))
     }

idea/tests/org/jetbrains/kotlin/idea/parameterInfo/LambdaImplicitHintsTest.kt

@@ -0,0 +1,45 @@
+/*
+ * Copyright 2000-2018 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license
+ * that can be found in the license/LICENSE.txt file.
+ */
+
+package org.jetbrains.kotlin.idea.parameterInfo
+
+import org.jetbrains.kotlin.idea.test.KotlinLightCodeInsightFixtureTestCase
+import org.jetbrains.kotlin.idea.test.KotlinLightProjectDescriptor
+import org.jetbrains.kotlin.idea.test.KotlinWithJdkAndRuntimeLightProjectDescriptor
+
+class LambdaImplicitHintsTest : KotlinLightCodeInsightFixtureTestCase() {
+    override fun getProjectDescriptor(): KotlinLightProjectDescriptor = KotlinWithJdkAndRuntimeLightProjectDescriptor.INSTANCE
+
+    fun check(text: String) {
+        myFixture.configureByText("A.kt", text)
+        myFixture.testInlays()
+    }
+
+    fun testSimpleIt() {
+        check(
+            """
+            val x = listOf("").filter {<hint text="it: String" />
+                it.startsWith("")
+            }"""
+        )
+    }
+
+    fun testSimpleThis() {
+        check(
+            """
+            val x = buildString {<hint text="this: StringBuilder" />
+                append("foo")
+            }"""
+        )
+    }
+
+    fun testSingleLine() {
+        check(
+            """
+            val x = listOf("").filter { it.startsWith("") }
+            """
+        )
+    }
+}