BridgeJS: Remove JSException storage boxing once swiftlang/swift#89715 lands
@krodak is already working on this.
Since Jun 11, 2026.
Assessment
This issue has not been assessed yet.
Description
Tracks removal of the JSException storage boxing introduced in #766.
Context
swiftlang/swift#89320 miscompiles Wasm calls to captureless async throws(JSException) closure values when the typed error exceeds the direct error convention: the thrown error is corrupted across the async unwind, so the rejected Promise receives garbage. JSException (~36 bytes) always took the affected indirect-error path.
#766 works around this at the library level by boxing JSException's stored properties (thrownValue, description, stack) into a private final class, shrinking the struct to a single stored reference so it travels in the direct error convention and the broken path is never taken. The public API is unchanged; the cost is one heap allocation per thrown exception. The original exploration is in https://github.com/PassiveLogic/JavaScriptKit/pull/13.
The codegen-level counterpart for zero-parameter async throwing exports is tracked separately in #761.
What to do once the compiler fix ships
The proper fix is swiftlang/swift#89715 (IRGen: fix async typed throws miscompiles on Wasm). Once it is available in the oldest Swift toolchain JavaScriptKit supports:
- Move
thrownValue,description, andstackback to stored properties onJSExceptionand delete theStorageclass. This is a library-internal layout change only, not a breaking API change. - Keep the async closure reject end-to-end tests as the regression guard that the reject path still works without the boxing.
- Dominant language
- Swift
- Stars
- 986
- Forks
- 76
- Avg merge
- 21h 11m
- Merged PRs (30d)
- 4
Contributor guide
First steps
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- Open a pull request that references the issue number.
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