next-swc shakeExports option does not see vars used as JSX

[overlookmotel]

Verify canary release

• I verified that the issue exists in Next.js canary release

Provide environment information

Operating System:
  Platform: darwin
  Arch: x64
  Version: Darwin Kernel Version 18.7.0: Tue Jun 22 19:37:08 PDT 2021; root:xnu-4903.278.70~1/RELEASE_X86_64
Binaries:
  Node: 16.14.2
  npm: 8.5.5
  Yarn: 1.22.18
  pnpm: N/A
Relevant packages:
  next: 12.1.0
  react: 17.0.2
  react-dom: 17.0.2

PR for failing test based on latest canary: #35735

What browser are you using? (if relevant)

No response

How are you deploying your application? (if relevant)

No response

Describe the Bug

I know that next-swc's shakeExports option is not public, but nonetheless I've found what I think is a bug, so I thought I'd report it.

shakeExports avoids shaking variable declarations in top-level scope where they're used within a function in the ignore list.

However, this breaks if the usage is in a JSX tag.

e.g. input:

function ShouldBeKept() {
  return 'should be kept';
}
let shouldBeKept = 'should be kept';
export default function() {
  return <ShouldBeKept val={shouldBeKept} />;
}

Output with shakeExports: { ignore: ['default'] }:

let shouldBeKept = "should be kept";
export default function() {
    return <ShouldBeKept val={shouldBeKept}/>;
}

ShouldBeKept function has been erroneously shaken out.

My best guess is that the cause is upstream in SWC. Possibly dce expects to be used after JSX->JS transform, so it's not looking for variables used in JSX. That's just a total guess though.

Background on why I'm interested in shakeExports: #35487

Expected Behavior

Please see above.

To Reproduce

PR for failing test: #35735

[balazsorban44]

As explained here #35487 (comment) this API is not public so any usage is not guaranteed to work.

Let's keep the discussion in that thread #35487

[overlookmotel]

Sorry I didn't explain myself well. I didn't open this issue as a demand that it be fixed!

Regardless if it's a public API or not, I'm aware from the discussion that it's in use internally at Vercel, so I thought a bug report and failing test would be helpful.

It turns out that the bug is indeed upstream in SWC's dce transform, so I've raised an issue there and will submit a fix if my meagre Rust skills allow.

[github-actions]

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