Rspack support

[ngranander]

Adds Rspack as an alternative bundler for webpack_bundle and webpack_devserver, addressing #159.

A new bundler parameter ("webpack" or "rspack", default "webpack") controls which bundler is used. When set to "rspack", the rules use @rspack/core and @rspack/cli instead of webpack and webpack-cli, via a new rspack_binary macro that mirrors the existing webpack_binary.

Config merging:
Rspack CLI doesn't support --merge, so when a custom webpack_config is provided it replaces the generated base config entirely. Without a custom config the generated base config is used as-is. This is documented in the bundler param docstrings and the README.

Performance:
Rspack's Rust-based architecture provides significant speedups over webpack. In published benchmarks rspack is 4-6x faster for production builds and up to 15x+ faster for dev startup on larger projects. In a real-world production web application (~10k modules with SWC loaders, CSS extraction, and code splitting), we measured approximately 2.3x (dev) and 3.2x (prod) end-to-end Bazel build speedup.

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Changes are visible to end-users: yes

• Searched for relevant documentation and updated as needed: yes
• Breaking change (forces users to change their own code or config): no
• Suggested release notes appear below: yes

New: webpack_bundle and webpack_devserver now support Rspack as a drop-in alternative bundler. Set bundler = "rspack" and ensure @rspack/core and @rspack/cli are linked in your node_modules. See the README for details.

Test plan

• Covered by existing test cases
• New test cases added
  • webpack/tests/simple: rspack with custom config and with generated config
  • webpack/tests/devtool-override: rspack with devtool override
  • webpack/tests/worker: rspack with workers enabled
  • webpack/tests/devserver: rspack devserver target
  • e2e/smoke: rspack end-to-end smoke test with production-mode output comparison

[CLAassistant]

All committers have signed the CLA.

[aspect-workflows]

Bazel 7 (Test)

19 test targets passed

Targets

//.github/workflows:bazel7.update_test [k8-fastbuild]                          92ms
//docs:update_test [k8-fastbuild]                                              21ms
//tools:preset.update_test [k8-fastbuild]                                      140ms
//webpack/tests/create_configs:tests_0_test [k8-fastbuild]                     96ms
//webpack/tests/create_configs:tests_1_test [k8-fastbuild]                     164ms
//webpack/tests/create_configs:tests_2_test [k8-fastbuild]                     95ms
//webpack/tests/create_configs:tests_3_test [k8-fastbuild]                     60ms
//webpack/tests/create_configs:tests_4_test [k8-fastbuild]                     60ms
//webpack/tests/devtool-override:rspack_bundle_matches_expected [k8-fastbuild] 86ms
//webpack/tests/devtool-override:write_bundle_test [k8-fastbuild]              85ms
//webpack/tests/devtool-override:write_rspack_bundle_test [k8-fastbuild]       74ms
//webpack/tests/simple:bundle_no_execroot_entry_point_matches [k8-fastbuild]   111ms
//webpack/tests/simple:entries_match [k8-fastbuild]                            133ms
//webpack/tests/simple:rspack_bundle_matches_expected [k8-fastbuild]           71ms
//webpack/tests/simple:write_bundle_test [k8-fastbuild]                        96ms
//webpack/tests/simple:write_rspack_bundle_test [k8-fastbuild]                 72ms
//webpack/tests/worker:rspack_bundle_matches_expected [k8-fastbuild]           59ms
//webpack/tests/worker:write_bundle_test [k8-fastbuild]                        100ms
//webpack/tests/worker:write_rspack_bundle_test [k8-fastbuild]                 91ms

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Bazel 8 (Test)

All tests were cache hits

16 tests (100.0%) were fully cached saving 2s.

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Bazel 9 (Test)

All tests were cache hits

16 tests (100.0%) were fully cached saving 2s.

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Bazel 7 (Test)

e2e/loaders

1 test target passed

Targets

//:test [k8-fastbuild]                                                         19ms

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Bazel 8 (Test)

e2e/loaders

All tests were cache hits

1 test (100.0%) was fully cached saving 36ms.

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Bazel 9 (Test)

e2e/loaders

All tests were cache hits

1 test (100.0%) was fully cached saving 31ms.

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Bazel 7 (Test)

e2e/loaders_jslib

1 test target passed

Targets

//:test [k8-fastbuild]                                                         19ms

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Bazel 8 (Test)

e2e/loaders_jslib

All tests were cache hits

1 test (100.0%) was fully cached saving 62ms.

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Bazel 9 (Test)

e2e/loaders_jslib

All tests were cache hits

1 test (100.0%) was fully cached saving 32ms.

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Bazel 7 (Test)

e2e/smoke

3 test targets passed

Targets

//src:bundles_0_test [k8-fastbuild]                                            20ms
//src:bundles_1_test [k8-fastbuild]                                            19ms
//src:bundles_2_test [k8-fastbuild]                                            46ms

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Bazel 8 (Test)

e2e/smoke

All tests were cache hits

3 tests (100.0%) were fully cached saving 98ms.

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Bazel 9 (Test)

e2e/smoke

All tests were cache hits

3 tests (100.0%) were fully cached saving 150ms.

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Bazel 7 (Test)

e2e/worker

2 test targets passed

Targets

//:bundles_match [k8-fastbuild]                                                23ms
//:write_bundle_test [k8-fastbuild]                                            27ms

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Bazel 8 (Test)

e2e/worker

All tests were cache hits

2 tests (100.0%) were fully cached saving 64ms.

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Bazel 9 (Test)

e2e/worker

All tests were cache hits

2 tests (100.0%) were fully cached saving 67ms.

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Buildifier

[ngranander]

@alexeagle We're looking into Rspack as an alternative to Webpack under Bazel. What do you think of something like this?