[FIR] Add redundant single string expression template checker

compiler/fir/analysis-tests/testData/extendedCheckers/RedundantSingleExpressionStringTemplateChecker.kt

@@ -0,0 +1,11 @@
+val x = "Hello"
+
+val y = "$<!REDUNDANT_SINGLE_EXPRESSION_STRING_TEMPLATE!>x<!>"
+
+val z = "${y.hashCode()}"
+
+fun toString(x: String) = "IC$x"
+
+data class ProductGroup(val short_name: String, val parent: ProductGroup?) {
+    val name: String = if (parent == null) short_name else "${parent.name} $short_name"
+}

compiler/fir/analysis-tests/testData/extendedCheckers/RedundantSingleExpressionStringTemplateChecker.txt

@@ -0,0 +1,39 @@
+FILE: RedundantSingleExpressionStringTemplateChecker.kt
+    public final val x: R|kotlin/String| = String(Hello)
+        public get(): R|kotlin/String|
+    public final val y: R|kotlin/String| = R|/x|.R|kotlin/Any.toString|()
+        public get(): R|kotlin/String|
+    public final val z: R|kotlin/String| = R|/y|.R|kotlin/Any.hashCode|().R|kotlin/Any.toString|()
+        public get(): R|kotlin/String|
+    public final fun toString(x: R|kotlin/String|): R|kotlin/String| {
+        ^toString <strcat>(String(IC), R|<local>/x|.R|kotlin/Any.toString|())
+    }
+    public final data class ProductGroup : R|kotlin/Any| {
+        public constructor(short_name: R|kotlin/String|, parent: R|ProductGroup?|): R|ProductGroup| {
+            super<R|kotlin/Any|>()
+        }
+
+        public final val short_name: R|kotlin/String| = R|<local>/short_name|
+            public get(): R|kotlin/String|
+
+        public final val parent: R|ProductGroup?| = R|<local>/parent|
+            public get(): R|ProductGroup?|
+
+        public final val name: R|kotlin/String| = when () {
+            ==(R|<local>/parent|, Null(null)) ->  {
+                R|<local>/short_name|
+            }
+            else ->  {
+                <strcat>(R|<local>/parent|.R|/ProductGroup.name|.R|kotlin/Any.toString|(), String( ), R|<local>/short_name|.R|kotlin/Any.toString|())
+            }
+        }
+
+            public get(): R|kotlin/String|
+
+        public final fun component1(): R|kotlin/String|
+
+        public final fun component2(): R|ProductGroup?|
+
+        public final fun copy(short_name: R|kotlin/String| = this@R|/ProductGroup|.R|/ProductGroup.short_name|, parent: R|ProductGroup?| = this@R|/ProductGroup|.R|/ProductGroup.parent|): R|ProductGroup|
+
+    }

compiler/fir/analysis-tests/tests/org/jetbrains/kotlin/fir/ExtendedFirDiagnosticsTestGenerated.java

@@ -48,6 +48,11 @@ public class ExtendedFirDiagnosticsTestGenerated extends AbstractExtendedFirDiag
         runTest("compiler/fir/analysis-tests/testData/extendedCheckers/RedundantReturnUnitTypeChecker.kt");
     }
 
+    @TestMetadata("RedundantSingleExpressionStringTemplateChecker.kt")
+    public void testRedundantSingleExpressionStringTemplateChecker() throws Exception {
+        runTest("compiler/fir/analysis-tests/testData/extendedCheckers/RedundantSingleExpressionStringTemplateChecker.kt");
+    }
+
     @TestMetadata("RedundantVisibilityModifierChecker.kt")
     public void testRedundantVisibilityModifierChecker() throws Exception {
         runTest("compiler/fir/analysis-tests/testData/extendedCheckers/RedundantVisibilityModifierChecker.kt");

compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/checkers/expression/ExtendedExpressionCheckers.kt

@@ -7,10 +7,12 @@ package org.jetbrains.kotlin.fir.analysis.checkers.expression
 
 import org.jetbrains.kotlin.fir.analysis.checkers.extended.ArrayEqualityCanBeReplacedWithEquals
 import org.jetbrains.kotlin.fir.analysis.checkers.extended.CanBeReplacedWithOperatorAssignmentChecker
+import org.jetbrains.kotlin.fir.analysis.checkers.extended.RedundantSingleExpressionStringTemplateChecker
 
 object ExtendedExpressionCheckers : ExpressionCheckers() {
     override val expressionCheckers: List<FirBasicExpresionChecker> = listOf(
-        ArrayEqualityCanBeReplacedWithEquals
+        ArrayEqualityCanBeReplacedWithEquals,
+        RedundantSingleExpressionStringTemplateChecker
     )
     override val variableAssignmentCheckers: List<FirVariableAssignmentChecker> = listOf(
         CanBeReplacedWithOperatorAssignmentChecker

compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/checkers/extended/RedundantSingleExpressionStringTemplateChecker.kt

@@ -0,0 +1,39 @@
+/*
+ * Copyright 2010-2020 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * 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.fir.analysis.checkers.extended
+
+import com.intellij.psi.PsiElement
+import org.jetbrains.kotlin.fir.FirFakeSourceElementKind
+import org.jetbrains.kotlin.fir.analysis.checkers.context.CheckerContext
+import org.jetbrains.kotlin.fir.analysis.checkers.expression.FirBasicExpresionChecker
+import org.jetbrains.kotlin.fir.analysis.diagnostics.DiagnosticReporter
+import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.REDUNDANT_SINGLE_EXPRESSION_STRING_TEMPLATE
+import org.jetbrains.kotlin.fir.expressions.FirFunctionCall
+import org.jetbrains.kotlin.fir.expressions.FirStatement
+import org.jetbrains.kotlin.fir.psi
+import org.jetbrains.kotlin.fir.symbols.StandardClassIds
+import org.jetbrains.kotlin.fir.types.classId
+import org.jetbrains.kotlin.fir.types.coneType
+import org.jetbrains.kotlin.psi.KtStringTemplateExpression
+
+object RedundantSingleExpressionStringTemplateChecker : FirBasicExpresionChecker() {
+    override fun check(expression: FirStatement, context: CheckerContext, reporter: DiagnosticReporter) {
+        if (expression.source?.kind != FirFakeSourceElementKind.GeneratedToStringCallOnTemplateEntry) return
+        if (expression !is FirFunctionCall) return
+        if (
+            expression.explicitReceiver?.typeRef?.coneType?.classId == StandardClassIds.String
+            && expression.psi?.findStringParent()?.children?.size == 1 // there is no more children in original string template
+        ) {
+            reporter.report(expression.source, REDUNDANT_SINGLE_EXPRESSION_STRING_TEMPLATE)
+        }
+    }
+
+    private fun PsiElement.findStringParent(): KtStringTemplateExpression? {
+        if (this is KtStringTemplateExpression) return this
+        return if (this.parent != null) this.parent.findStringParent()
+        else null
+    }
+}

compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/diagnostics/FirErrors.kt

@@ -89,7 +89,10 @@ object FirErrors {
     val REDUNDANT_MODALITY_MODIFIER by warning0<FirSourceElement, PsiElement>()
     val REDUNDANT_RETURN_UNIT_TYPE by warning0<FirSourceElement, PsiTypeElement>()
     val REDUNDANT_EXPLICIT_TYPE by warning0<FirSourceElement, PsiElement>()
+    val REDUNDANT_SINGLE_EXPRESSION_STRING_TEMPLATE by warning0<FirSourceElement, PsiElement>()
     val CAN_BE_VAL by warning0<FirSourceElement, PsiElement>()
     val CAN_BE_REPLACED_WITH_OPERATOR_ASSIGNMENT by warning0<FirSourceElement, PsiElement>()
     val ARRAY_EQUALITY_OPERATOR_CAN_BE_REPLACED_WITH_EQUALS by warning0<FirSourceElement, PsiElement>()
 }
+
+