Turbopack panics in ModuleGraphImportTracer: 'entered unreachable code: there must be a path to a root' (module_graph/mod.rs:750)

[agentHits]

Link to the code that reproduces this issue

https://github.com/agentHits/next-prerender-workstore-reproduction

To Reproduce

1. Clone reproduction repository:

git clone https://github.com/agentHits/next-prerender-workstore-reproduction.git
cd next-prerender-workstore-reproduction
bun install # or npm install

2. Run a cold static export build without experimental.inlineCss: true:

rm -rf .next
bun run build # or next build

3. On projects with standalone output and complex CSS/SCSS modules, Turbopack panics in the Rust runtime:

thread 'tokio-rt-worker' panicked at turbopack/crates/turbopack-core/src/module_graph/mod.rs:750:25:
internal error: entered unreachable code: there must be a path to a root

Debug info:
- Execution of emit_all_output_assets_once_with_issues_operation failed
- Execution of PlainIssue::from_issue failed
- Execution of <ModuleGraphImportTracer as ImportTracer>::get_traces failed
- internal error: entered unreachable code: there must be a path to a root

Current vs. Expected behavior

Current Behavior:
When formatting an issue/warning trace during CSS chunk emission (emit_all_output_assets_once_with_issues), ModuleGraphImportTracer::get_traces executes astar searching for a node with no incoming edges in the reversed graph (|n| reversed_graph.neighbors(n).next().is_none()).
If the module subgraph contains cyclic references or disconnected nodes, astar returns None, triggering the panic unreachable!("there must be a path to a root") at line 750, terminating the entire build process.

To avoid this crash, developers are forced to enable experimental.inlineCss: true, which bypasses external CSS chunking by serializing all CSS text chunks directly into the RSC Flight stream (self.__next_f), inflating document HTML size from ~150 KB to >700 KB and causing significant Core Web Vitals regressions (LCP: 8.9s vs 2.1s).

Expected Behavior:
Turbopack should handle cyclic or rootless subgraphs gracefully by falling back to returning the current module index instead of panicking.

Provide environment information

Operating System:
  Platform: darwin
  Arch: arm64
  Version: Darwin Kernel Version 25.6.0
Binaries:
  Node: 24.20.0
  npm: 11.19.0
  Yarn: N/A
  pnpm: 11.19.0
Relevant Packages:
  next: 16.3.4 (and canary 16.4.0-canary.15)
  react: 19.2.8
  react-dom: 19.2.8
  typescript: 6.0.3
Next.js Config:
  output: N/A

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

Turbopack, CSS, Performance

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

next build (local)

Additional context

A complete Rust pull request with the fix has been submitted:
👉 PR: #98204

Fix in turbopack/crates/turbopack-core/src/module_graph/mod.rs:

let path = match petgraph::algo::astar(
    &reversed_graph,
    module_idx,
    |n| reversed_graph.neighbors(n).next().is_none(),
    |e| match e.weight().chunking_type {
        ChunkingType::Parallel { .. } => 0,
        _ => 1,
    },
    |_| 0,
) {
    Some((_, path)) => path,
    None => vec![module_idx],
};