Turbopack never passes comments to SWC wasm plugins unless a file has both leading AND trailing comments

Link to the code that reproduces this issue

https://github.com/instagibb/turbopack-comments-repro

To Reproduce

With Turbopack (NextJS default)

1. Run npm run dev
2. Browse to http://localhost:3000/
3. The page reads istanbul's __coverage__ global store and renders the result (see DevTools console if preferred). The istanbul ignore hint comment in leading-only.ts has not been processed and the function was instrumented while with-trailing.ts which is identical except for one trailing comment has the hint honoured and there is no instrumenting

For comparison with Webpack

1. Run npm run dev:webpack
2. Browse to http://localhost:3000/
3. The output shows that the istanbul ignore hint comment has been honoured in both instances

Or for a fully automatic test

1. Run npm run check to build with both Turbopack and Webpack and print the comparison

Current vs. Expected behavior

Current behaviour: Turbopack only passes the comments to SWC plugins when a file contains both leading AND trailing comments. Otherwise the plugin receives an empty comment store, so any comment-driven behaviour silently does nothing (no error)

See: swc_ecma_transform_plugins.rs lines 136–141 on canary

let should_enable_comments_proxy =
    !ctx.comments.leading.is_empty() && !ctx.comments.trailing.is_empty();

//[TODO]: as same as swc/core does, we should set should_enable_comments_proxy
// depends on the src's comments availability. For now, check naively if leading
// / trailing comments are empty.

Expected behaviour: Turbopack should pass comments to SWC plugins when a file contains any comments (leading OR trailing)

let should_enable_comments_proxy =
    !ctx.comments.leading.is_empty() || !ctx.comments.trailing.is_empty();

Provide environment information

Operating System:
  Platform: darwin
  Arch: arm64
  Version: Darwin Kernel Version 25.5.0: Tue Jun  9 22:28:34 PDT 2026; root:xnu-12377.121.10~1/RELEASE_ARM64_T6041
  Available memory (MB): 49152
  Available CPU cores: 12
Binaries:
  Node: 24.14.1
  npm: 11.11.0
  Yarn: N/A
  pnpm: N/A
Relevant Packages:
  next: 16.4.0-canary.6 // Latest available version is detected (16.4.0-canary.6).
  eslint-config-next: N/A
  react: 19.2.8
  react-dom: 19.2.8
  typescript: 5.9.3
Next.js Config:
  output: N/A

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

Turbopack, SWC

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

next dev (local), next build (local)

Additional context

No response

Start in turbopack/crates/turbopack-ecmascript-plugins/src/transform/swc_ecma_transform_plugins.rs around lines 136–141 and inspect how comment availability enables the SWC plugin proxy. Run the linked reproduction with npm run check, then verify that files with only leading or only trailing comments receive comments and match the Webpack comparison.