JS: optimize destructuring declaration when possible

See KT-8285

js/js.tests/test/org/jetbrains/kotlin/js/test/semantics/BoxJsTestGenerated.java

@@ -2332,6 +2332,12 @@ public class BoxJsTestGenerated extends AbstractBoxJsTest {
                 doTest(fileName);
             }
 
+            @TestMetadata("singleComponentDestructuring.kt")
+            public void testSingleComponentDestructuring() throws Exception {
+                String fileName = KotlinTestUtils.navigationMetadata("js/js.translator/testData/box/expression/evaluationOrder/singleComponentDestructuring.kt");
+                doTest(fileName);
+            }
+
             @TestMetadata("whenAsMinusArgument.kt")
             public void testWhenAsMinusArgument() throws Exception {
                 String fileName = KotlinTestUtils.navigationMetadata("js/js.translator/testData/box/expression/evaluationOrder/whenAsMinusArgument.kt");

js/js.translator/src/org/jetbrains/kotlin/js/translate/expression/ExpressionVisitor.java

@@ -122,7 +122,9 @@ public final class ExpressionVisitor extends TranslatorVisitor<JsNode> {
         assert jetInitializer != null : "Initializer for multi declaration must be not null";
         JsExpression initializer = Translation.translateAsExpression(jetInitializer, context);
         JsName parameterName = JsScope.declareTemporary();
-        context.addStatementToCurrentBlock(JsAstUtils.newVar(parameterName, initializer));
+        JsVars tempVarDeclaration = JsAstUtils.newVar(parameterName, initializer);
+        MetadataProperties.setSynthetic(tempVarDeclaration, true);
+        context.addStatementToCurrentBlock(tempVarDeclaration);
         return DestructuringDeclarationTranslator.translate(multiDeclaration, JsAstUtils.pureFqn(parameterName, null), context);
     }
 

js/js.translator/testData/box/expression/evaluationOrder/singleComponentDestructuring.kt

@@ -0,0 +1,20 @@
+// EXPECTED_REACHABLE_NODES: 1029
+// CHECK_VARS_COUNT: function=test count=2
+var log = ""
+
+fun test() {
+    val (a, _) = idAndLog(Pair(idAndLog(1), idAndLog(2)))
+    val (_, b) = idAndLog(Pair(idAndLog(3), idAndLog(4)))
+    log += "result:$a,$b;"
+}
+
+fun <T> idAndLog(x: T): T {
+    log += "$x;"
+    return x
+}
+
+fun box(): String {
+    test()
+    if (log != "1;2;(1, 2);3;4;(3, 4);result:1,4;") return "fail: $log"
+    return "OK"
+}