Link to the code that reproduces this issue

https://github.com/katanabe/nextjs-mdx-multibyte-bug

To Reproduce

1. Clone the reproduction repo
2. Run npm install
3. Run npm run build

The MDX file (app/test/page.mdx) contains Japanese text with full-width parentheses:

export const metadata = {
  title: "Test",
};

# Test

これはテスト（サンプル）です。

The issue is triggered by multibyte characters in the MDX body (e.g., （）), not by export const metadata content.

Current vs. Expected behavior

Current behavior:
Build fails with SWC panic:

thread '<unnamed>' panicked at swc_common-18.0.0/src/source_map.rs:657:62:
byte index 88 is not a char boundary; it is inside '（' (bytes 87..90)

Expected behavior:
Build should succeed, as it does in Next.js 16.0.10.

Provide environment information

Operating System:
  Platform: darwin
  Arch: arm64
  Version: Darwin 25.1.0
  Available memory (MB): 65536
  Available CPU cores: 12
Binaries:
  Node: 24.11.1
  npm: 11.3.0
  pnpm: 9.15.9
Relevant Packages:
  next: 16.1.1
  react: 19.0.0
  react-dom: 19.0.0

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

SWC (minification/transpilation), Turbopack

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

next build (default compiler)

Additional context

Version comparison:

Likely cause:
This regression was likely introduced by the SWC version bump in 16.1.0:

• Bump swc to 48 #86240 (Bump to SWC 48)
• Bump to swc 49 #86689 (Bump to SWC 49)

The issue occurs when Rust string slicing attempts to access a byte index that falls in the middle of a multibyte UTF-8 character (Japanese full-width parenthesis （ is 3 bytes).