Cherry-pick PR #44070 into release-4.3 (#44187)

Component commits:
d1920805b0 Do not incorrectly add line separators for non-synthetic nodes when emitting node list
As of 3c32f6e154, a line separator is
added between nodes if the nodes are not synthetic and on separate
lines. This it push s wrong and previously only happened if the non-synthetic
nodes were on different lines but had the same parent.

Fixes #44068.

Co-authored-by: Paul Gschwendtner <paulgschwendtner@gmail.com>

src/compiler/emitter.ts

@@ -4564,16 +4564,26 @@ namespace ts {
                     // JsxText will be written with its leading whitespace, so don't add more manually.
                     return 0;
                 }
-                else if (preserveSourceNewlines && siblingNodePositionsAreComparable(previousNode, nextNode)) {
-                    return getEffectiveLines(
-                        includeComments => getLinesBetweenRangeEndAndRangeStart(
-                            previousNode,
-                            nextNode,
-                            currentSourceFile!,
-                            includeComments));
-                }
-                else if (!preserveSourceNewlines && !nodeIsSynthesized(previousNode) && !nodeIsSynthesized(nextNode)) {
-                    return rangeEndIsOnSameLineAsRangeStart(previousNode, nextNode, currentSourceFile!) ? 0 : 1;
+                else if (!nodeIsSynthesized(previousNode) && !nodeIsSynthesized(nextNode)) {
+                    if (preserveSourceNewlines && siblingNodePositionsAreComparable(previousNode, nextNode)) {
+                        return getEffectiveLines(
+                            includeComments => getLinesBetweenRangeEndAndRangeStart(
+                                previousNode,
+                                nextNode,
+                                currentSourceFile!,
+                                includeComments));
+                    }
+                    // If `preserveSourceNewlines` is `false` we do not intend to preserve the effective lines between the
+                    // previous and next node. Instead we naively check whether nodes are on separate lines within the
+                    // same node parent. If so, we intend to preserve a single line terminator. This is less precise and
+                    // expensive than checking with `preserveSourceNewlines` as above, but the goal is not to preserve the
+                    // effective source lines between two sibling nodes.
+                    else if (!preserveSourceNewlines && originalNodesHaveSameParent(previousNode, nextNode)) {
+                        return rangeEndIsOnSameLineAsRangeStart(previousNode, nextNode, currentSourceFile!) ? 0 : 1;
+                    }
+                    // If the two nodes are not comparable, add a line terminator based on the format that can indicate
+                    // whether new lines are preferred or not.
+                    return format & ListFormat.PreferNewLine ? 1 : 0;
                 }
                 else if (synthesizedNodeStartsOnNewLine(previousNode, format) || synthesizedNodeStartsOnNewLine(nextNode, format)) {
                     return 1;
@@ -5300,11 +5310,14 @@ namespace ts {
 
         }
 
-        function siblingNodePositionsAreComparable(previousNode: Node, nextNode: Node) {
-            if (nodeIsSynthesized(previousNode) || nodeIsSynthesized(nextNode)) {
-                return false;
-            }
+        function originalNodesHaveSameParent(nodeA: Node, nodeB: Node) {
+            nodeA = getOriginalNode(nodeA);
+            // For performance, do not call `getOriginalNode` for `nodeB` if `nodeA` doesn't even
+            // have a parent node.
+            return nodeA.parent && nodeA.parent === getOriginalNode(nodeB).parent;
+        }
 
+        function siblingNodePositionsAreComparable(previousNode: Node, nextNode: Node) {
             if (nextNode.pos < previousNode.end) {
                 return false;
             }

src/testRunner/unittests/transform.ts

@@ -154,6 +154,26 @@ namespace ts {
             }).outputText;
         });
 
+        testBaseline("issue44068", () => {
+            return transformSourceFile(`
+                const FirstVar = null;
+                const SecondVar = null;
+            `, [
+                context => file => {
+                    const firstVarName = (file.statements[0] as VariableStatement)
+                        .declarationList.declarations[0].name as Identifier;
+                    const secondVarName = (file.statements[0] as VariableStatement)
+                        .declarationList.declarations[0].name as Identifier;
+
+                    return context.factory.updateSourceFile(file, file.statements.concat([
+                        context.factory.createExpressionStatement(
+                            context.factory.createArrayLiteralExpression([firstVarName, secondVarName])
+                        ),
+                    ]));
+                }
+            ]);
+        });
+
         testBaseline("rewrittenNamespace", () => {
             return transpileModule(`namespace Reflect { const x = 1; }`, {
                 transformers: {