[JS IR] Don't concatenate string template segments with '' when possible

If concatenating with an empty string is a sure no-op, just don't do it.

compiler/ir/backend.js/src/org/jetbrains/kotlin/ir/backend/js/transformers/irToJs/IrElementToJsExpressionTransformer.kt

@@ -13,6 +13,7 @@ import org.jetbrains.kotlin.ir.backend.js.utils.emptyScope
 import org.jetbrains.kotlin.ir.backend.js.utils.getJsNameOrKotlinName
 import org.jetbrains.kotlin.ir.declarations.*
 import org.jetbrains.kotlin.ir.expressions.*
+import org.jetbrains.kotlin.ir.types.isString
 import org.jetbrains.kotlin.ir.types.isUnit
 import org.jetbrains.kotlin.ir.util.isEffectivelyExternal
 import org.jetbrains.kotlin.ir.util.isEnumClass
@@ -68,7 +69,15 @@ class IrElementToJsExpressionTransformer : BaseIrElementToJsNodeTransformer<JsEx
 
     override fun visitStringConcatenation(expression: IrStringConcatenation, context: JsGenerationContext): JsExpression {
         // TODO revisit
-        return expression.arguments.fold<IrExpression, JsExpression>(JsStringLiteral("")) { jsExpr, irExpr ->
+
+        val firstArgument = expression.arguments.firstOrNull()
+        val (head, tail) = if (firstArgument?.type?.isString() == true) {
+            Pair(firstArgument.accept(this, context), expression.arguments.asSequence().drop(1))
+        } else {
+            Pair(JsStringLiteral(""), expression.arguments.asSequence())
+        }
+
+        return tail.fold(head) { jsExpr, irExpr ->
             JsBinaryOperation(
                 JsBinaryOperator.ADD,
                 jsExpr,

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

@@ -1,9 +1,33 @@
+// CHECK_STRING_LITERAL_COUNT: function=foo count=1
+fun foo(x: Int) = "foo $x"
+
+// CHECK_STRING_LITERAL_COUNT: function=bar count=2 IGNORED_BACKENDS=JS
+fun bar(x: Int) = "$x bar"
+
+// CHECK_STRING_LITERAL_COUNT: function=baz count=1
+fun baz(x: Int) = "${x.toString()} baz"
+
+// CHECK_STRING_LITERAL_COUNT: function=beer count=2 IGNORED_BACKENDS=JS
+fun beer(x: Int?) = "$x beer"
+
+// CHECK_STRING_LITERAL_COUNT: function=quux count=2 IGNORED_BACKENDS=JS
+fun quux(x: Int?) = "${x?.toString()} quux"
+
+// CHECK_STRING_LITERAL_COUNT: function=test count=2
 fun test(p: String?): String {
     return "${p ?: "Default"} test"
 }
+
 fun box(): String {
     if (test(null) != "Default test") return "fail 1: ${test(null)}"
-    if (test("Good") != "Good test") return "fail 1: ${test("OK")}"
+    if (test("Good") != "Good test") return "fail 2: ${test("Good")}"
+    if (foo(3) != "foo 3") return "fail 3: ${foo(3)}"
+    if (bar(4) != "4 bar") return "fail 4: ${bar(4)}"
+    if (baz(5) != "5 baz") return "fail 5: ${baz(5)}"
+    if (beer(6) != "6 beer") return "fail 6: ${beer(6)}"
+    if (beer(null) != "null beer") return "fail 7: ${beer(null)}"
+    if (quux(8) != "8 quux") return "fail 8: ${quux(8)}"
+    if (quux(null) != "null quux") return "fail 9: ${quux(null)}"
 
     return "OK"
 }

js/js.tests/test/org/jetbrains/kotlin/js/testOld/utils/DirectiveTestUtils.java

@@ -224,6 +224,8 @@ public class DirectiveTestUtils {
 
     private static final DirectiveHandler COUNT_DEBUGGER = new CountNodesDirective<>("CHECK_DEBUGGER_COUNT", JsDebugger.class);
 
+    private static final DirectiveHandler COUNT_STRING_LITERALS = new CountNodesDirective<>("CHECK_STRING_LITERAL_COUNT", JsStringLiteral.class);
+
     private static final DirectiveHandler NOT_REFERENCED = new DirectiveHandler("CHECK_NOT_REFERENCED") {
         @Override
         void processEntry(@NotNull JsNode ast, @NotNull ArgumentsHelper arguments) throws Exception {
@@ -365,6 +367,7 @@ public class DirectiveTestUtils {
             COUNT_CASES,
             COUNT_IF,
             COUNT_DEBUGGER,
+            COUNT_STRING_LITERALS,
             NOT_REFERENCED,
             HAS_INLINE_METADATA,
             HAS_NO_INLINE_METADATA,