Let terser eliminate transpiled classes

[mischnic]

↪️ Pull Request

A statement like this doesn't get tree shaken because it does look pure (without looking at the comment).

var $fa7e8a5a8249ace53223ae89975fffbd$export$default =
/*#__PURE__*/
function () {
  function XYZ() {
    $fa7e8a5a8249ace53223ae89975fffbd$var$_classCallCheck(this, XYZ);
  }

  $fa7e8a5a8249ace53223ae89975fffbd$var$_createClass(XYZ, [{
    key: "method",
    value: function method() {
      return "abc";
    }
  }]);
  return XYZ;
}();

By outputting comments when generating the code after scopehoisting (and before terser), we can leverage terser to remove everything.

This also makes it possible to allow certain comments to be preserved by terser (e.g. license headers): fixes #3322

---

We could a handle these comments in treeShake, but I'm not sure if there is an advantage to doing that?
+ Smaller cache size, less data to serialize (both to cache and AST->code)
- larger error surface, would need more tests

diff --git a/packages/shared/scope-hoisting/src/shake.js b/packages/shared/scope-hoisting/src/shake.js
index c3b3d09a..7a87f25d 100644
--- a/packages/shared/scope-hoisting/src/shake.js
+++ b/packages/shared/scope-hoisting/src/shake.js
@@ -67,7 +67,9 @@ function isPure(binding) {
       init.isPure() ||
       init.isIdentifier() ||
       init.isThisExpression() ||
-      binding.path.node.id.name === '$parcel$global'
+      binding.path.node.id.name === '$parcel$global' ||
+      (init.isCallExpression() &&
+        init.node.leadingComments.filter(v => v.value === '#__PURE__'))
     );
   }

(it might be more complicated than that: rollup/rollup#2429)

[parcel-benchmark]

Benchmark Results

packages/benchmarks/kitchen-sink

Timings

Description	Time	Difference
Cold	10.28s	-427.00ms
Cached	8.20s	+283.00ms

Cold Bundles

Bundle	Size	Difference	Time	Difference
dist/legacy/parcel.ff215454.webp	102.94kb	+0.00b	172.00ms	-39.00ms 🚀
dist/modern/parcel.76ee4591.webp	102.94kb	+0.00b	171.00ms	-39.00ms 🚀
dist/modern/index.js	794.00b	+0.00b	462.00ms	-24.00ms 🚀
dist/legacy/index.js	252.00b	+0.00b	457.00ms	-32.00ms 🚀

Cached Bundles

Bundle	Size	Difference	Time	Difference
dist/legacy/parcel.ff215454.webp	102.94kb	+0.00b	12.00ms	-1.00ms 🚀
dist/modern/parcel.76ee4591.webp	102.94kb	+0.00b	10.00ms	-2.00ms 🚀
dist/legacy/index.js	252.00b	+0.00b	23.00ms	-3.00ms 🚀
dist/legacy/index.css	29.00b	+0.00b	10.00ms	-2.00ms 🚀
dist/modern/index.css	29.00b	+0.00b	10.00ms	-1.00ms 🚀

packages/benchmarks/react-hn

Timings

Description	Time	Difference
Cold	14.71s	-90.00ms
Cached	FAILED	-0.00ms

Cold Bundles

No bundle changes detected.

Cached Bundles

No bundles found, this is probably a failed build...

Click here to view a detailed benchmark overview.

[wbinnssmith]

@mischnic what does this tell babel to do? Should it also just be left to terser?

[mischnic]

AFAICT, it only compresses whitespaces:

var generate = require("@babel/generator").default
var parse = require("@babel/parser").parse

const AST = parse(`const ABCDEF = "hel" + "lo";`);
console.log(generate(AST, {minified: true}).code);
// const ABCDEF="hel"+"lo";