perf(core): split monolithic worker bundle into lazy-loaded chunks#1085
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perf(core): split monolithic worker bundle into lazy-loaded chunks#1085
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Convert snapshot and fakeTimers from static imports to dynamic imports, and extract assertion vendor deps into a stable named chunk. This reduces Worker startup cost by deferring code that is only needed on first use (snapshot matching, fake timers) and improves V8 compile cache hit rate via stable chunk filenames.
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9aoy
approved these changes
Mar 24, 2026
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Benchmark
tested in Rstest self codebase.
unit test
sliced E2E test (exclude noisy cases like browser mode)
Summary
Background
The Worker process loaded a single monolithic 742KB chunk (
1949.js) at startup, containing all runtime dependencies including snapshot utilities and fake timers — even when they weren't needed for every test run. This added unnecessary parse/compile overhead to Worker initialization.Implementation
SnapshotPluginfrom a static import to a dynamicimport()increateRstestRuntime(), parallelized withcreateRunner()viaPromise.allFakeTimersfrom a static import to a dynamicimport()increateRstestUtilities()snapshotPluginan optional parameter oncreateExpect()(passed fromcreateRstestRuntimeafter dynamic load)vendor-assertionsplitChunks cache group to extract chai/@vitest/expect/spy/utils into a stable-named chunk for better V8 compile cache reuseUser Impact
Reduced Worker startup overhead by deferring ~165KB of code (snapshot + fake-timers) to first use. The stable
vendor-assertion.jschunk name improves V8 compile cache hit rate across rebuilds.Checklist