Reuse unchanged ambient declarations in incremental parsing (microsoft#32849)

* Incrementally parse unchanged ambient declarations

* Re-parse modifiers and decorators so as not to skip errors

* Revert unnecessary change

* Undo the test change that was changed 2 years ago

Author: andrewbranch

src/compiler/parser.ts

@@ -5492,10 +5492,22 @@ namespace ts {
         }
 
         function parseDeclaration(): Statement {
+            const modifiers = lookAhead(() => (parseDecorators(), parseModifiers()));
+            // `parseListElement` attempted to get the reused node at this position,
+            // but the ambient context flag was not yet set, so the node appeared
+            // not reusable in that context.
+            const isAmbient = some(modifiers, isDeclareModifier);
+            if (isAmbient) {
+                const node = tryReuseAmbientDeclaration();
+                if (node) {
+                    return node;
+                }
+            }
+
             const node = <Statement>createNodeWithJSDoc(SyntaxKind.Unknown);
             node.decorators = parseDecorators();
             node.modifiers = parseModifiers();
-            if (some(node.modifiers, isDeclareModifier)) {
+            if (isAmbient) {
                 for (const m of node.modifiers!) {
                     m.flags |= NodeFlags.Ambient;
                 }
@@ -5506,6 +5518,15 @@ namespace ts {
             }
         }
 
+        function tryReuseAmbientDeclaration(): Statement | undefined {
+            return doInsideOfContext(NodeFlags.Ambient, () => {
+                const node = currentNode(parsingContext);
+                if (node) {
+                    return consumeNode(node) as Statement;
+                }
+            });
+        }
+
         function parseDeclarationWorker(node: Statement): Statement {
             switch (token()) {
                 case SyntaxKind.VarKeyword:

src/testRunner/unittests/incrementalParser.ts

@@ -671,7 +671,7 @@ module m3 { }\
             const oldText = ScriptSnapshot.fromString(source);
             const newTextAndChange = withInsert(oldText, 0, "{");
 
-            compareTrees(oldText, newTextAndChange.text, newTextAndChange.textChangeRange, 4);
+            compareTrees(oldText, newTextAndChange.text, newTextAndChange.textChangeRange, 9);
         });
 
         it("Removing block around function declarations", () => {
@@ -680,7 +680,7 @@ module m3 { }\
             const oldText = ScriptSnapshot.fromString(source);
             const newTextAndChange = withDelete(oldText, 0, "{".length);
 
-            compareTrees(oldText, newTextAndChange.text, newTextAndChange.textChangeRange, 4);
+            compareTrees(oldText, newTextAndChange.text, newTextAndChange.textChangeRange, 9);
         });
 
         it("Moving methods from class to object literal", () => {