Scope Hoisting: Don't insert unused requires that aren't registered anywhere

[gorakong]

↪️ Pull Request

In getSymbolResolution, unused modules can end up in an asset's hoisted requires. A parcelRequire is then inserted for the unused module, and results in a Cannot find module runtime error (see example below).

Proposed fix is to check that an asset's exportSymbols are actually used before inserting a parcelRequire for it.

Tested on a large app.

💻 Examples

$parcel$export(module.exports, "foo", () => (parcelRequire("jDXqh")).foo);

var $jDXqh = parcelRequire("jDXqh");
var $5oNdt = parcelRequire("5oNdt");                      // 5oNdt isn't registered anywhere
eval('');

});
parcelRequire.register("jDXqh", function(module, exports) {

$parcel$export(module.exports, "foo", () => $e4cfc0f7eb90214f$export$6a5cdcad01c973fa);
const $e4cfc0f7eb90214f$export$6a5cdcad01c973fa = 'foo';

});

✔️ PR Todo

• Added/updated unit tests for this change
• Filled out test instructions (In case there aren't any unit tests)

[height]

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[parcel-benchmark]

Benchmark Results

Kitchen Sink 🚨

Timings

Description	Time	Difference
Cold	FAILED	-0.00ms
Cached	FAILED	-0.00ms

Cold Bundles

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

Cached Bundles

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

React HackerNews ✅

Timings

Description	Time	Difference
Cold	9.32s	+17.00ms
Cached	464.00ms	-17.00ms

Cold Bundles

No bundle changes detected.

Cached Bundles

No bundle changes detected.

AtlasKit Editor 🚨

Timings

Description	Time	Difference
Cold	FAILED	-0.00ms
Cached	FAILED	-0.00ms

Cold Bundles

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

Cached Bundles

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

Three.js ✅

Timings

Description	Time	Difference
Cold	6.58s	-115.00ms
Cached	274.00ms	+12.00ms

Cold Bundles

No bundle changes detected.

Cached Bundles

No bundle changes detected.

Click here to view a detailed benchmark overview.

[wbinnssmith]

The test looks great, but I'll defer to @devongovett or @mischnic to have a look at the scope hoisting logic.

[wbinnssmith]

What's the state of this PR?

[mischnic]

So just checking resolvedAssetUsedSymbols.has(exportSymbol) would also fix the integration test. (And that is also what I would have assumed to work). Can you add tests (or extend the existing one by adding more library folders) so that these conditions are also convered? (Related to #7764 (comment)) Ah, that breaks existing tests. I see...

[mischnic]

This very long condition made me suspicious and indeed the following version passes all test (you probably want to test this in your app though).
The new condition also better corresponds to the cases where it shouldn't insert the statement, which is when no parcelRequire.register was generated (= skipped in this bundle and also not imported from another bundle)

  if (
    isWrapped &&
    dep &&
    !dep?.meta.shouldWrap &&
    symbol !== false &&
    // Only do this if the asset is part of a different bundle (so it was definitely
    // parcelRequire.register'ed there), or if it is indeed registered in this bundle.
    (!this.bundle.hasAsset(resolvedAsset) ||
      !this.shouldSkipAsset(resolvedAsset))
  ) {
    let hoisted = this.hoistedRequires.get(dep.id);
    if (!hoisted) {
      hoisted = new Map();
      this.hoistedRequires.set(dep.id, hoisted);
    }

    hoisted.set(
      resolvedAsset.id,
      `var $${publicId} = parcelRequire(${JSON.stringify(publicId)});`,
    );
  }

[gorakong]

@devongovett updated with @mischnic's suggestion and tested on a large app -- everything looks good 👍