[Bug]: `jsc.experimental.cacheRoot` destroys SWC performance

[walkerburgin]

What happened?

Transpilation using SWC is incredibly slow if swc_plugin() is used.

Something about setting jsc.experimental.cacheRoot inside of the Bazel sandbox when invoking SWC is absolutely destroying performance.

I put together a standalone repro here: walkerburgin/rules_swc-plugin-cache-repro using the @swc/plugin-noop.

➜  hyperfine --runs 5 'bazel clean && bazel build //...'
Benchmark 1: bazel clean && bazel build //...
  Time (mean ± σ):     26.342 s ±  0.855 s    [User: 0.182 s, System: 0.076 s]
  Range (min … max):   25.613 s … 27.800 s    5 runs

If I patch aspect_rules_swc to stop specifying jsc.experimental.cacheRoot:

Benchmark 1: bazel clean && bazel build //...
  Time (mean ± σ):      8.647 s ±  0.229 s    [User: 0.177 s, System: 0.070 s]
  Range (min … max):    8.346 s …  8.939 s    5 runs

The effect in our codebase using an actual plugin is much, much more pronounced. It slows typescript transpilation down by something like +500x and slows my machine to a crawl.

It feels like there's an SWC bug related to symlinks at the heart of this…

Version

Development (host) and target OS/architectures: darwin-arm64

Output of bazel --version:

➜  rules_swc-plugin-cache-repro git:(master) bazel --version
aspect 2025.05.46+36b9b384e

Version of the Aspect rules, or other relevant rules from your WORKSPACE or MODULE.bazel file:

bazel_dep(name = "aspect_rules_js", version = "2.1.1")
bazel_dep(name = "aspect_rules_ts", version = "3.3.1")
bazel_dep(name = "aspect_rules_swc", version = "2.3.0")

Language(s) and/or frameworks involved:

• SWC
• TypeScript

How to reproduce

See https://github.com/walkerburgin/rules_swc-plugin-cache-repro

Any other information?

No response