Improve swap copy testing and add new failing copy test

inputs/failing/copyCounting/arraySwap.ts

@@ -1,20 +0,0 @@
-//! test_output(2)
-// Should be: 0
-
-/// <reference path="../../../concept-code/vs.d.ts" />
-
-export default function main() {
-  return measure(true) - measure(false);
-}
-
-function measure(swap: boolean) {
-  const x = Debug.makeCopyCounter("x");
-
-  let arr = [x, "y", "z"];
-
-  if (swap) {
-    [arr[1], arr[2]] = [arr[2], arr[1]];
-  }
-
-  return x.count;
-}

inputs/failing/copyCounting/externalMethod.ts

@@ -0,0 +1,29 @@
+//! test_output(1)
+// Should be: 0
+
+/// <reference path="../../../concept-code/vs.d.ts" />
+
+export default function main() {
+  return measure(true) - measure(false);
+}
+
+function measure(doPush: boolean) {
+  const x = Debug.makeCopyCounter("x");
+
+  let arr: unknown[] = echo([x]);
+
+  if (doPush) {
+    arr = push(arr, "y");
+  }
+
+  return x.count;
+}
+
+function push<T>(x: T[], value: T) {
+  x.push(value);
+  return x;
+}
+
+function echo<T>(x: T) {
+  return x;
+}

inputs/passing/copyCounting/arraySwap.ts

@@ -0,0 +1,28 @@
+//! test_output(0)
+
+/// <reference path="../../../concept-code/vs.d.ts" />
+
+export default function main() {
+  return measure(true) - measure(false);
+}
+
+function measure(doSwap: boolean) {
+  const x = Debug.makeCopyCounter("x");
+
+  let arr: unknown[] = [x, "y", "z"];
+  arr = swapFn(arr, 1, 2, doSwap);
+
+  return len(arr) + x.count;
+}
+
+function swapFn(arr: unknown[], i: number, j: number, doSwap: boolean) {
+  if (doSwap) {
+    [arr[i], arr[j]] = [arr[j], arr[i]];
+  }
+
+  return arr;
+}
+
+function len(arr: unknown[]) {
+  return arr.length;
+}