Fix IIFE optimization removing const/let declarations causing undefined variable errors

[Copilot]

The IIFE (Immediately Invoked Function Expression) optimization was incorrectly handling const and let declarations, causing undefined variable errors in the minified output.

Issue

When minifying code with IIFE containing const or let declarations, the optimizer would:

1. Extract initializers from variable declarations and convert them to assignments
2. Add the now-empty declarations to prepend_stmts
3. This created invalid declarations like const obj; (not allowed in JavaScript)

Example input:

(function () {
    const obj = { value: 42 };
    console.log(obj === null || obj === void 0 ? void 0 : obj.value)
})()

Before fix (invalid output):

console.log(null==obj?void 0:42);
// ReferenceError: obj is not defined

Fix

Modified the inline_fn_like function in crates/swc_ecma_minifier/src/compress/optimize/iife.rs to only add var declarations to prepend_stmts, since:

• var declarations can exist without initializers: var obj; ✓
• const and let declarations cannot: const obj; ✗

After fix (valid output):

!function(){const obj={value:42};console.log(null==obj?void 0:obj.value)}();
// Works correctly, outputs: 42

Testing

Added comprehensive test case in tests/fixture/iife_const_bug/ that verifies:

• The minified output doesn't create undefined variable references
• The generated code executes without errors
• The optimization still provides benefits while maintaining correctness

Fixes #10849.

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Latest commit: 2bd1ceb

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[codspeed-hq]

CodSpeed Performance Report

Merging #10855 will not alter performance

_{Comparing copilot/fix-10849 (2bd1ceb) with main (93b318f)}

Summary

✅ 140 untouched benchmarks

[kdy1]

@copilot Fix CI

[kdy1]

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