Optimize rest arguments.

The TypeScript compiler generates the following code for rest arguments:

function foo(x, y, z, ...rest) {
  return;
}

function foo(x, y, z) {
  var rest = [];
  for (var i = 0; i < arguments.length - 3; i++) {
    rest[i] = arguments[i + 3];
  }
  return;
}

Although rest is unused in the body of the function, JS engines can’t easily dead code eliminate the allocation of the rest array and the subsequent for loop due to the possibility of Array.prototype being mutated:

Object.defineProperty(Array.prototype, 42, { set: function() { alert(this[41]); }});

It’s much easier for the TypeScript compiler to avoid emitting this code when it is not necessary.

See more details:

https://bugzilla.mozilla.org/show_bug.cgi?id=1056446

[begincalendar]

As asked by @andy-ms (here), what are the use-cases for having unused rest parameters?

[duncanmak]

Just a reminder, #518 fixes this issue. When will it get merged into master?