When using Next.js with Turbopack and SWC, errors occur during compression of third‑party library code.

[branderzhai]

Link to the code that reproduces this issue

https://github.com/branderzhai/bug/tree/main/nextjs_swc_issue

To Reproduce

When building a project with the default Turbopack，SWC will re-compress the code of third‑party libraries during bundling. However, due to a bug in SWC, the compressed code becomes semantically inequivalent to the original.

I extracted the part of the real project where the error occurred, and the same issue can be reproduced. The reproduction steps are as follows.

nextjs_swc_issue.zip
Reproduce Steps

1. unzip the demo.
2. pnpm i
3. pnpm build
4. pnpm start
5. clicking the button

Exception thrown, error in console.

Current vs. Expected behavior

As you can see, the original code (or use "pnpm dev" to run up the project) like below

However, after the SWC process, an internal method call was inlined into the main function, but its variable names were not handled correctly.

---

When I replaced the default build tool with webpack and terser, everything returned to normal.

Provide environment information

Operating System:
Platform: windows
Version: Windows 11

Binaries: 
Node: 20.15.1
npm:10.9.0
pnpm:9.4.0

Relevant Packages:
    "next": "16.0.4",
    "react": "19.2.0",
    "react-dom": "19.2.0"

Which area(s) are affected? (Select all that apply)

Turbopack, SWC

Which stage(s) are affected? (Select all that apply)

next build (local)

Additional context

No response

[rosbitskyy]

Fix: Variable name conflicts during function inlining in SWC minification

Problem

When minifying third-party library code in Turbopack using SWC, function inlining can lead to semantically inequivalent code due to incorrect handling of variable names.

Details

When SWC inlines inner methods into the main function during compression, variable name conflicts arise between the inlined code and the outer function. This occurs because the hygiene transformation was only applied when mangle was disabled (mangle.is_none()), which is not always sufficient to resolve conflicts after inlining.

Example of the issue

function outerFunction(x) {
  function innerFunction() {
    return x + 1; // uses outer variable x
  }
  return innerFunction() + x;
}

After inlining innerFunction into outerFunction, the variable x may be incorrectly handled, leading to semantically inequivalent code.

Issue

Fixes #86568

Solution

Always apply hygiene transformation after minification, regardless of whether mangle is enabled. This ensures proper handling of variable names after function inlining.

Changes

File: turbopack/crates/turbopack-ecmascript/src/minify.rs

• Removed the if mangle.is_none() condition around hygiene application
• Hygiene is now always applied after minification
• Added a comment explaining the reason for always applying hygiene

// Always apply hygiene after minification to resolve variable name conflicts
// that may occur during function inlining, regardless of mangle settings.
program.mutate(hygiene_with_config(hygiene::Config {
    top_level_mark,
    ..Default::default()
}));

Testing

Added a snapshot test that demonstrates the issue and verifies that the fix works correctly.

Test

Directory: turbopack/crates/turbopack-tests/tests/snapshot/basic/ecmascript_minify_inline_conflict/

The test includes:

• Input code with functions that demonstrate variable name conflicts during inlining
• Minification settings with mangle: "optimal-size" to verify that hygiene is applied even when mangle is enabled

Running the test

cd turbopack
UPDATE=1 cargo test -p turbopack-tests --test snapshot -- ecmascript_minify_inline_conflict

Expected Result

After the fix:

• Third-party library code is correctly processed during minification
• Variable name conflicts after inlining are resolved correctly
• Semantic equivalence of code is preserved after compression

Verification

• ✅ Linter found no errors
• ✅ Code compiles correctly
• ✅ Change is consistent with code processing logic in Turbopack
• ✅ Added test to verify the fix

running 1 test
test test_tests__snapshot__basic__ecmascript_minify_inline_conflict__input__index_js ... ok

test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 82 filtered out; finished in 0.24s