styled-jsx transform infinite loop

[mischnic]

Describe the bug

There is an infinite loop when transforming this file with styled-jsx:

Minimal reproduction

Put

export const Probe = (props) => {
  const t = props.theme;
  const Icon = t.icon;
  return (
    <div className="probe">
      <Icon />
      <style jsx>{`
        .probe {
          color: ${t.color};
        }
      `}</style>
    </div>
  );
};

into packages/styled-jsx/transform/tests/fixture/class/input.js

Then you get

> Should transform class correctly
Error: Test timed out in 5000ms.
If this is a long-running test, pass a timeout value as the last argument or configure it globally with "testTimeout".
⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯[1/1]⎯


  Snapshots  1 obsolete
             ↳ __tests__/wasm.test.ts
               · Should transform class correctly 1

 Test Files  1 failed (1)
      Tests  1 failed | 2 passed | 78 skipped (81)
   Start at  21:57:39
   Duration  5.43s (transform 23ms, setup 0ms, collect 44ms, tests 5.23s, environment 0ms, prepare 35ms)

This is on commit a93bcc0

Expected behavior

It should be transformed and not timeout

Additional context
vercel/next.js#97685

[mischnic]

Root cause (confirmed by reproduction + core-dump backtrace)

Reproduced on a93bcc0 (the commit named in the issue) — both the lightningcss and swc fixture
variants hang; RSS stays flat, so it is a genuine spin, not an AST blow-up.

Backtrace of the spinning thread:

styled_jsx::visitor::StyledJSXTransformer::get_jsx_style   visitor.rs:577
 swc_ecma_minifier::eval::Evaluator::eval                 eval.rs:129
     swc_ecma_minifier::eval::Evaluator::eval_tpl           eval.rs:268   -> self.run()
         swc_ecma_minifier::eval::Evaluator::run              eval.rs:89
             swc_ecma_minifier::compress::Compressor::optimize_unit_repeatedly
                 ... Optimizer::merge_sequences_in_exprs          sequences.rs:876  <-- spins

• visitor.rs:458 (fold_module) builds Evaluator::new(module.clone(), …) from the whole,
  still-JSX-containing module.
• get_jsx_style (visitor.rs:576-581) calls evaluator.eval(expr) purely to decide whether a
  dynamic <style jsx> template is is_dynamic (i.e. whether it constant-folds to a literal).
• Evaluator::eval_tpl unconditionally calls run(), which runs the full swc minifier
  compressor over that module copy. swc's minifier expects JSX-free input; on this module its
  merge_sequences_in_exprs loop (sequences.rs:741) keeps reporting changed and never
  converges, so a single compressor pass never returns.

The non-convergence itself is an upstream swc_ecma_minifier bug, but styled-jsx is misusing the
minifier by handing it JSX. The evaluator only ever needs the module to resolve identifiers to
literal values — JSX contributes nothing to that.