IR: Don't use IrStringConcatenation for ordinary toString calls

We can only use IrStrinConcatentation to represent calls to Any?.toString
and toString calls on primitive types. Otherwise, x.toString() and "$x"
are observably different when x is a non-null type with null value
(e.g., an @NotNull value coming from Java).

compiler/fir/fir2ir/tests/org/jetbrains/kotlin/codegen/ir/FirBlackBoxCodegenTestGenerated.java

@@ -26325,6 +26325,11 @@ public class FirBlackBoxCodegenTestGenerated extends AbstractFirBlackBoxCodegenT
             runTest("compiler/testData/codegen/box/strings/interpolation.kt");
         }
 
+        @TestMetadata("javaToStringNPE.kt")
+        public void testJavaToStringNPE() throws Exception {
+            runTest("compiler/testData/codegen/box/strings/javaToStringNPE.kt");
+        }
+
         @TestMetadata("kt2592.kt")
         public void testKt2592() throws Exception {
             runTest("compiler/testData/codegen/box/strings/kt2592.kt");

compiler/ir/backend.common/src/org/jetbrains/kotlin/backend/common/lower/FlattenStringConcatenationLowering.kt

@@ -16,10 +16,7 @@ import org.jetbrains.kotlin.ir.expressions.IrCall
 import org.jetbrains.kotlin.ir.expressions.IrExpression
 import org.jetbrains.kotlin.ir.expressions.IrStringConcatenation
 import org.jetbrains.kotlin.ir.expressions.impl.IrStringConcatenationImpl
-import org.jetbrains.kotlin.ir.types.isAny
-import org.jetbrains.kotlin.ir.types.isNullableAny
-import org.jetbrains.kotlin.ir.types.isString
-import org.jetbrains.kotlin.ir.types.isStringClassType
+import org.jetbrains.kotlin.ir.types.*
 import org.jetbrains.kotlin.ir.util.fqNameWhenAvailable
 import org.jetbrains.kotlin.ir.visitors.IrElementTransformerVoid
 import org.jetbrains.kotlin.ir.visitors.IrElementVisitorVoid
@@ -126,9 +123,13 @@ class FlattenStringConcatenationLowering(val context: CommonBackendContext) : Fi
                 return function.isToString || function.isNullableToString
             }
 
-        /** @return true if the given expression is a [IrStringConcatenation], or an [IrCall] to [toString] or [String.plus]. */
+        /** @return true if the given expression is a call to [Any?.toString] or a call of [toString] on a primitive type. */
+        private val IrCall.isSpecialToStringCall: Boolean
+            get() = isToStringCall && dispatchReceiver?.type?.isPrimitiveType() != false
+
+        /** @return true if the given expression is a [IrStringConcatenation], or an [IrCall] to [String.plus]. */
         private fun isStringConcatenationExpression(expression: IrExpression): Boolean =
-            (expression is IrStringConcatenation) || (expression is IrCall) && (expression.isStringPlusCall || expression.isToStringCall)
+            (expression is IrStringConcatenation) || (expression is IrCall) && expression.isStringPlusCall
 
         /** Recursively collects string concatenation arguments from the given expression. */
         private fun collectStringConcatenationArguments(expression: IrExpression): List<IrExpression> {
@@ -141,7 +142,7 @@ class FlattenStringConcatenationLowering(val context: CommonBackendContext) : Fi
                 }
 
                 override fun visitCall(expression: IrCall) {
-                    if (isStringConcatenationExpression(expression)) {
+                    if (isStringConcatenationExpression(expression) || expression.isToStringCall) {
                         // Recursively collect from call arguments.
                         expression.acceptChildrenVoid(this)
                     } else {
@@ -172,7 +173,7 @@ class FlattenStringConcatenationLowering(val context: CommonBackendContext) : Fi
     override fun visitExpression(expression: IrExpression): IrExpression {
         // Only modify/flatten string concatenation expressions.
         val transformedExpression =
-            if (isStringConcatenationExpression(expression))
+            if (isStringConcatenationExpression(expression) || expression is IrCall && expression.isSpecialToStringCall)
                 expression.run {
                     IrStringConcatenationImpl(
                         startOffset,

compiler/testData/codegen/box/strings/javaToStringNPE.kt

@@ -0,0 +1,22 @@
+// TARGET_BACKEND: JVM
+// WITH_RUNTIME
+
+// FILE: J.java
+import org.jetbrains.annotations.NotNull;
+
+public class J {
+    @NotNull
+    public static String notNullStringIsNull() {
+        return null;
+    }
+}
+
+// FILE: test.kt
+fun box(): String {
+    try {
+        J.notNullStringIsNull().toString()
+    } catch (e: java.lang.NullPointerException) {
+        return "OK"
+    }
+    return "Fail"
+}

compiler/testData/codegen/bytecodeText/stringOperations/stringBuilderToString.kt

@@ -0,0 +1,12 @@
+val chars = listOf('O', 'K')
+
+fun box(): String {
+    val b = StringBuilder()
+    for (c in chars) {
+        b.append(c)
+    }
+    return b.toString()
+}
+
+// 0 INVOKESTATIC java/lang/String.valueOf
+// 1 INVOKEVIRTUAL java/lang/StringBuilder.toString

compiler/tests/org/jetbrains/kotlin/codegen/BlackBoxCodegenTestGenerated.java

@@ -27846,6 +27846,11 @@ public class BlackBoxCodegenTestGenerated extends AbstractBlackBoxCodegenTest {
             runTest("compiler/testData/codegen/box/strings/interpolation.kt");
         }
 
+        @TestMetadata("javaToStringNPE.kt")
+        public void testJavaToStringNPE() throws Exception {
+            runTest("compiler/testData/codegen/box/strings/javaToStringNPE.kt");
+        }
+
         @TestMetadata("kt2592.kt")
         public void testKt2592() throws Exception {
             runTest("compiler/testData/codegen/box/strings/kt2592.kt");

compiler/tests/org/jetbrains/kotlin/codegen/BytecodeTextTestGenerated.java

@@ -4245,6 +4245,11 @@ public class BytecodeTextTestGenerated extends AbstractBytecodeTextTest {
             runTest("compiler/testData/codegen/bytecodeText/stringOperations/singleConcat.kt");
         }
 
+        @TestMetadata("stringBuilderToString.kt")
+        public void testStringBuilderToString() throws Exception {
+            runTest("compiler/testData/codegen/bytecodeText/stringOperations/stringBuilderToString.kt");
+        }
+
         @TestMetadata("stringPlus.kt")
         public void testStringPlus() throws Exception {
             runTest("compiler/testData/codegen/bytecodeText/stringOperations/stringPlus.kt");

compiler/tests/org/jetbrains/kotlin/codegen/LightAnalysisModeTestGenerated.java

@@ -26663,6 +26663,11 @@ public class LightAnalysisModeTestGenerated extends AbstractLightAnalysisModeTes
             runTest("compiler/testData/codegen/box/strings/interpolation.kt");
         }
 
+        @TestMetadata("javaToStringNPE.kt")
+        public void testJavaToStringNPE() throws Exception {
+            runTest("compiler/testData/codegen/box/strings/javaToStringNPE.kt");
+        }
+
         @TestMetadata("kt2592.kt")
         public void testKt2592() throws Exception {
             runTest("compiler/testData/codegen/box/strings/kt2592.kt");

compiler/tests/org/jetbrains/kotlin/codegen/ir/IrBlackBoxCodegenTestGenerated.java

@@ -26325,6 +26325,11 @@ public class IrBlackBoxCodegenTestGenerated extends AbstractIrBlackBoxCodegenTes
             runTest("compiler/testData/codegen/box/strings/interpolation.kt");
         }
 
+        @TestMetadata("javaToStringNPE.kt")
+        public void testJavaToStringNPE() throws Exception {
+            runTest("compiler/testData/codegen/box/strings/javaToStringNPE.kt");
+        }
+
         @TestMetadata("kt2592.kt")
         public void testKt2592() throws Exception {
             runTest("compiler/testData/codegen/box/strings/kt2592.kt");

compiler/tests/org/jetbrains/kotlin/codegen/ir/IrBytecodeTextTestGenerated.java

@@ -4163,6 +4163,11 @@ public class IrBytecodeTextTestGenerated extends AbstractIrBytecodeTextTest {
             runTest("compiler/testData/codegen/bytecodeText/stringOperations/singleConcat.kt");
         }
 
+        @TestMetadata("stringBuilderToString.kt")
+        public void testStringBuilderToString() throws Exception {
+            runTest("compiler/testData/codegen/bytecodeText/stringOperations/stringBuilderToString.kt");
+        }
+
         @TestMetadata("stringPlus.kt")
         public void testStringPlus() throws Exception {
             runTest("compiler/testData/codegen/bytecodeText/stringOperations/stringPlus.kt");