Preserve backtrace through JsError → JsValue → JsError round-trip

[AlbertMarashi]

Summary

Closes #4608
Supersedes #4607

• Errors caught by internal exception handlers (e.g. async module evaluation) now preserve their backtrace through the JsError → JsValue → JsError round-trip via promise rejection
• Stores the backtrace on the Error object in JsError::into_opaque (both Native and Opaque variants) and recovers it in JsError::from_opaque
• Moves backtrace capture in handle_error before the exception handler check, so catchable errors also get a backtrace before being intercepted by internal handlers
• Adds regression tests

Details

Previously, the backtrace was only captured for uncatchable errors (inside if !err.is_catchable()) and for catchable errors that reached handle_throw(). Errors intercepted by internal handlers (e.g. the module async wrapper) were stored as pending_exception before either path could capture a backtrace. When these errors were later converted to JsValue for promise rejection, the backtrace was lost entirely.

This PR:

1. Captures the backtrace unconditionally in handle_error, before the exception handler check
2. Stores the backtrace on the Error object during into_opaque (for both Repr::Native and Repr::Opaque errors) so it survives conversion to JsValue
3. Recovers the backtrace in from_opaque by reading it back from the Error object

This approach was suggested by @jedel1043 in #4607 — extending the Error type with a backtrace field rather than injecting positions, which would be incorrect with nested try blocks.

Test plan

• test_call_error_preserves_backtrace — calls undefined() at module top level, verifies backtrace with exact string match
• test_nested_call_error_preserves_backtrace — nested foo → baz → import.meta.non_existent(), verifies full multi-frame backtrace
• test_explicit_throw_preserves_backtrace — explicit throw new Error("test") inside a function, verifies backtrace frames are present

Made with Cursor

[AlbertMarashi]

@jedel1043 requesting review

[github-actions]

Test262 conformance changes

Test result	main count	PR count	difference
Total	52,862	52,862	0
Passed	49,497	49,497	0
Ignored	2,261	2,261	0
Failed	1,104	1,104	0
Panics	0	0	0
Conformance	93.63%	93.63%	0.00%

[randomboi404]

I recommend you to fix the issues in your code and make it pass the CI otherwise it can't be merged.

[AlbertMarashi]

@randomboi404 @jedel1043 done

[AlbertMarashi]

interesting. it appears that backtraces (maybe a feature) seems to differ on the CI...

to avoid flakiness with source code changes in the (native) part of the backtraces, I might strip them out or use another approach - (so ideally when the source code is changed it doesn't painstakingly cause this test to fail also)

---

Reproduced it. So:

• cargo test -p boa_engine — only boa_engine's own features, native-backtrace is off → tests pass
• cargo test --workspace (what CI does) — Cargo unifies features across the workspace, the CLI crate enables native-backtrace on boa_engine → (native at file:line:col) appears instead of (native) → exact string match fails

[codecov]

Codecov Report

❌ Patch coverage is 92.45283% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 56.91%. Comparing base (6ddc2b4) to head (18373eb).
⚠️ Report is 672 commits behind head on main.

Files with missing lines	Patch %	Lines
core/engine/src/error/tests.rs	89.28%	3 Missing ⚠️
core/engine/src/builtins/error/mod.rs	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4609      +/-   ##
==========================================
+ Coverage   47.24%   56.91%   +9.66%     
==========================================
  Files         476      549      +73     
  Lines       46892    60129   +13237     
==========================================
+ Hits        22154    34222   +12068     
- Misses      24738    25907    +1169     

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[AlbertMarashi]

@jedel1043 should be ready to merge now.

[jedel1043]

Very nice, thank you for the fix!