KT-13113: Add inspection to detect single-expression string template

(cherry picked from squashed commits 49a3ef3 and 4c7a42a)

idea/src/META-INF/plugin.xml

@@ -1273,6 +1273,11 @@
       <category>Kotlin</category>
     </intentionAction>
 
+    <intentionAction>
+      <className>org.jetbrains.kotlin.idea.intentions.RemoveSingleExpressionStringTemplateIntention</className>
+      <category>Kotlin</category>
+    </intentionAction>
+
     <localInspection implementationClass="org.jetbrains.kotlin.idea.intentions.ObjectLiteralToLambdaInspection"
                      displayName="Object literal can be converted to lambda"
                      groupName="Kotlin"
@@ -1644,6 +1649,14 @@
                      language="kotlin"
     />
 
+    <localInspection implementationClass="org.jetbrains.kotlin.idea.intentions.RemoveSingleExpressionStringTemplateInspection"
+                     displayName="Remove redundant string template"
+                     groupName="Kotlin"
+                     enabledByDefault="true"
+                     level="WARNING"
+                     language="kotlin"
+    />
+
     <referenceImporter implementation="org.jetbrains.kotlin.idea.quickfix.KotlinReferenceImporter"/>
 
     <fileType.fileViewProviderFactory filetype="KJSM" implementationClass="com.intellij.psi.ClassFileViewProviderFactory"/>

idea/src/org/jetbrains/kotlin/idea/intentions/RemoveSingleExpressionStringTemplateIntention.kt

@@ -0,0 +1,53 @@
+/*
+ * Copyright 2010-2016 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.
+ */
+
+package org.jetbrains.kotlin.idea.intentions
+
+import com.intellij.openapi.editor.Editor
+import org.jetbrains.kotlin.builtins.KotlinBuiltIns
+import org.jetbrains.kotlin.idea.caches.resolve.analyze
+import org.jetbrains.kotlin.idea.inspections.IntentionBasedInspection
+import org.jetbrains.kotlin.psi.KtExpression
+import org.jetbrains.kotlin.psi.KtPsiFactory
+import org.jetbrains.kotlin.psi.KtStringTemplateExpression
+import org.jetbrains.kotlin.psi.createExpressionByPattern
+import org.jetbrains.kotlin.resolve.calls.callUtil.getType
+
+private fun KtStringTemplateExpression.singleExpressionOrNull() =
+        children.singleOrNull()?.children?.firstOrNull() as? KtExpression
+
+class RemoveSingleExpressionStringTemplateInspection : IntentionBasedInspection<KtStringTemplateExpression>(
+        RemoveSingleExpressionStringTemplateIntention()
+) {
+    override val problemText = "Single-expression string template"
+}
+
+class RemoveSingleExpressionStringTemplateIntention : SelfTargetingOffsetIndependentIntention<KtStringTemplateExpression>(
+        KtStringTemplateExpression::class.java,
+        "Remove single-expression string template"
+) {
+    override fun isApplicableTo(element: KtStringTemplateExpression) =
+            element.singleExpressionOrNull() != null
+
+    override fun applyTo(element: KtStringTemplateExpression, editor: Editor?) {
+        val expression = element.singleExpressionOrNull() ?: return
+        val type = expression.getType(expression.analyze())
+        val newElement =
+                if (KotlinBuiltIns.isString(type)) expression
+                else KtPsiFactory(element).createExpressionByPattern("$0.$1()", expression, "toString")
+        element.replace(newElement)
+    }
+}