fix(swc-plugin): count destructuring-default references in DCE usage analysis#2398
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…analysis
The DCE usage collector skipped the entire variable name pattern when
visiting a `VarDeclarator` (to avoid marking the binding name as "used").
But default-value initializers inside destructuring patterns live in that
pattern — e.g. the `TTL` in `const { ttl = TTL } = options;` — so those
references were invisible to the collector. A module-scope `const`
referenced only through such a default was treated as unused and stripped,
while the surviving code kept reading it, producing a runtime
`ReferenceError` when the default fired.
Traverse the default-value initializer expressions (and computed keys)
within destructuring patterns while still not marking the binding names
themselves, so the referenced declaration is preserved. Function-parameter
defaults were already covered (params are visited in full).
Fixes #2396.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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Pull request overview
Fixes the SWC plugin’s DCE usage analysis so module-scope declarations referenced only via destructuring default initializers (e.g. const { ttl = TTL } = options) are correctly counted as “used”, preventing runtime ReferenceErrors when defaults fire.
Changes:
- Extend
ComprehensiveUsageCollectorto traverse destructuring patterns and visit only default-value initializers (and computed keys), without marking binding identifiers as used. - Add a regression fixture covering module-scope const preservation in both step and workflow modes (and confirming truly-unused consts are still removed).
- Update the SWC plugin spec and add a changeset for the patch release.
Reviewed changes
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Show a summary per file
| File | Description |
|---|---|
packages/swc-plugin-workflow/transform/src/lib.rs |
Adds visit_pat_default_initializers and wires it into visit_mut_var_declarator so destructuring-default references are counted for DCE. |
packages/swc-plugin-workflow/transform/tests/fixture/destructuring-default-references-module-const/input.js |
New fixture input reproducing the destructuring-default-only reference pattern and an unused const. |
packages/swc-plugin-workflow/transform/tests/fixture/destructuring-default-references-module-const/output-step.js |
Step-mode snapshot asserting referenced consts survive and unused const is removed. |
packages/swc-plugin-workflow/transform/tests/fixture/destructuring-default-references-module-const/output-workflow.js |
Workflow-mode snapshot asserting referenced consts survive and unused const is removed. |
packages/swc-plugin-workflow/spec.md |
Documents that destructuring-default initializers count as references for DCE in both modes. |
.changeset/swc-destructuring-default-dce.md |
Patch changeset describing the DCE fix and the prevented runtime error. |
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* origin/main: docs: document run idempotency (#2011) Render attr_set events and run attributes in observability UI (#2393) [ci] Fix backport job model slug (#2403) [ci] Comment on PR when backport fails, revert to use opus 4.8 (#2400) Update queue client to 0.3.1 (#2399) fix(deps): upgrade esbuild to 0.28.1 (GHSA-gv7w-rqvm-qjhr) (#2395)
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Fixes #2396.
Problem
The SWC plugin's dead-code-elimination (DCE) usage analysis did not count identifier references that appear as default values inside a destructuring pattern (e.g.
const { ttl = TTL } = options;). When a module-scopeconstwas referenced only through such a destructuring default, the DCE pass treated it as unused and stripped it from the emitted bundle — while keeping the code that referenced it. The result was a runtimeReferenceError: <const> is not definedwhen the default fired.This is mode-independent (reproduces in both step and workflow mode) and class-independent (a plain top-level function exhibits it too). It shipped as a worked-around bug in the kill-switch pattern (#1858) and remains latent in
workbench/vitest/workflows/cookbook/distributed-abort-controller.ts.Root cause
In
ComprehensiveUsageCollector::visit_mut_var_declarator, the collector visited only the initializer and deliberately skipped the entirevar_decl.namepattern (to avoid marking the binding name as "used"). For destructuring patterns, the default-value initializer expressions live insidevar_decl.name— so references like theTTLin{ ttl = TTL }were never added toused_identifiers, andremove_dead_codethen pruned the still-referenced declaration.Fix
Added
visit_pat_default_initializers, which walks a binding pattern and visits only the default-value initializer expressions (and computed keys) —ObjectPatProp::KeyValuevalues,ObjectPatProp::Assign.value,AssignPat.right, array element defaults, rest args — while still not marking the binding names themselves as used.visit_mut_var_declaratornow calls it onvar_decl.name.Function-parameter destructuring defaults were already covered:
visit_mut_fn_declvisits params in full (confirmed by the existingdefault-parameter-usagefixture), so no change was needed there.Tests
destructuring-default-references-module-const/asserts that two module-scope consts referenced only via destructuring defaults — one inside a class static method, one inside a plain exported function — survive in both step and workflow mode. A third, genuinely-unused const is still stripped, confirming DCE is not over-broadened.main(consts stripped) and passes with the fix.cargo test -p swc_workflowsuite green (128 fixture tests + error tests).Spec
Updated
packages/swc-plugin-workflow/spec.md(both the step-mode and workflow-mode DCE sections) to note that a reference counts even when it appears only inside a destructuring-default initializer.Relation to prior fixes
Same symptom class as #1944 (DCE removing a declaration that surviving code references) but a distinct root cause: #1944 was about DCE ordering relative to step hoisting; this is about the usage collector never traversing destructuring-default initializers at all, and reproduces with no nested/hoisted steps involved.
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