Preserve lone surrogates across WASM conversions by HyoJongPark · Pull Request #8508 · RustPython/RustPython

HyoJongPark · 2026-08-13T05:18:26Z

Closes Define surrogate-safe Python↔JavaScript string conversion at the WASM boundary #8410
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Summary

RustPython str and JavaScript strings can both represent lone surrogates, but parts of the WASM conversion layer passed strings and property names through Rust String or serde's UTF-8 str representation. This replaced lone surrogates with U+FFFD or could panic when the generic serializer encountered a surrogate-containing Python string, including in exception arguments.

This PR converts directly between JavaScript UTF-16 code units and RustPython's WTF-8 representation, preserving lone surrogates in both directions.

Changes

Convert directly between JavaScript UTF-16 and RustPython WTF-8.
Apply the conversion to strings, nested containers, object keys, kwargs, and module injection.
Return a serialization error instead of panicking if a surrogate reaches the generic serde path.
Preserve the previous all-or-nothing behavior for unsupported nested values: the entire container still converts to undefined.

Testing

Local Chrome/Selenium regression checks cover both conversion directions:

JavaScript String.fromCharCode(0xd800) → Python ord(...) == 0xd800
Python chr(0xd800) → JavaScript charCodeAt(0) == 0xd800

Before this change, both directions above produced U+FFFD (0xfffd). The checks also cover lone lead and trail surrogates, nested containers, object and kwargs keys, module injection, exception handling, and the previous undefined behavior for unsupported nested values.

Result: 12 passed

Summary by CodeRabbit

Bug Fixes
- Improved handling of JavaScript and Python strings containing unusual Unicode characters.
- Preserved string contents when converting JavaScript object keys, keyword arguments, and Python values.
- Added clearer serialization errors when unsupported surrogate characters are encountered.
- Improved conversion of Python dictionaries to JavaScript maps while maintaining existing byte and container behavior.
- Improved handling of JavaScript maps when converting them back to Python dictionaries.
- Preserved expected behavior for byte arrays and unsupported values during conversion.

coderabbitai · 2026-08-13T05:18:57Z

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📥 Commits

Reviewing files that changed from the base of the PR and between cfa3eaf and 253d7b3.

📒 Files selected for processing (1)

crates/wasm/src/convert.rs

🚧 Files skipped from review as they are similar to previous changes (1)

crates/wasm/src/convert.rs

📝 Walkthrough

Walkthrough

The WASM bridge now preserves WTF-8 strings across JavaScript and Python conversions. Python string serialization rejects surrogate-containing values with a custom error. JavaScript module property names and keyword keys avoid lossy Rust String conversion.

Changes

WTF-8 conversion

Layer / File(s)	Summary
String conversion and serialization `crates/wasm/src/convert.rs`, `crates/vm/src/py_serde.rs`	JavaScript strings use WTF-8-aware conversion. `PyStr` serialization reports `"str contains surrogates"` when conversion fails.
WASM boundary conversion paths `crates/wasm/src/convert.rs`	Keyword names, object keys, Python strings, lists, tuples, and dictionaries use explicit conversion paths. Dictionaries become JavaScript `Map` objects, and byte values remain copied `Uint8Array` data.
JavaScript module property injection `crates/wasm/src/vm_class.rs`	Imported JavaScript property names are converted to Python attributes without passing through Rust `String`.

Estimated code review effort: 3 (Moderate) | ~20 minutes

Mergeability Score: ⚪ Minimal · up to 253d7

The PR preserves lone surrogates across WASM string conversions and reports serialization errors instead of panicking; no actionable merge-blocking risk remains after normal checks and review.

Sequence Diagram(s)

sequenceDiagram
  participant JavaScript
  participant WASMConvert
  participant PythonVM
  JavaScript->>WASMConvert: pass JsString, keys, and values
  WASMConvert->>PythonVM: create WTF-8 Python strings and keys
  PythonVM->>WASMConvert: return PyStr and containers
  WASMConvert-->>JavaScript: return JsString, Map, or Uint8Array

Possibly related PRs

RustPython/RustPython#7673: Updates related WTF-8 handling in serialization paths.
RustPython/RustPython#7675: Addresses related surrogate-preservation paths.
RustPython/RustPython#8409: Covers related surrogate-safe WASM and JavaScript conversion paths.

Suggested labels: z-ca-2026

Suggested reviewers: youknowone

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly summarizes the primary change: preserving lone surrogates across WASM conversions.
Linked Issues check	✅ Passed	The changes address issue `#8410` by preserving surrogates across string, key, kwargs, container, module, and serialization paths.
Out of Scope Changes check	✅ Passed	The changes remain within issue `#8410` and support consistent surrogate-safe WASM conversion behavior.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

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Actionable comments posted: 1

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Inline comments:
In `@crates/wasm/src/convert.rs`:
- Around line 204-209: Update js_to_py to handle JavaScript Map values before
the generic object conversion, iterating Map entries and recursively converting
each key and value back to Python; preserve the existing generic object path for
non-Map values.

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ℹ️ Review info

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Configuration used: Path: .coderabbit.yml

Review profile: CHILL

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Run ID: a367ff28-0546-4e28-adab-2613e05f2be2

📥 Commits

Reviewing files that changed from the base of the PR and between 525ba8c and cfa3eaf.

📒 Files selected for processing (3)

crates/vm/src/py_serde.rs
crates/wasm/src/convert.rs
crates/wasm/src/vm_class.rs

Assisted-by: Codex:GPT-5

youknowone

looks good, thanks!

coderabbitai Bot reviewed Aug 13, 2026

View reviewed changes

Comment thread crates/wasm/src/convert.rs

Preserve lone surrogates across WASM conversions

253d7b3

Assisted-by: Codex:GPT-5

HyoJongPark force-pushed the fix/wasm-surrogate-conversion branch from cfa3eaf to 253d7b3 Compare August 13, 2026 05:41

youknowone added the z-ca-2026 Tag to track Contribution Academy 2026 label Aug 13, 2026

youknowone approved these changes Aug 14, 2026

View reviewed changes

youknowone merged commit d0baa1c into RustPython:main Aug 14, 2026
27 checks passed

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Preserve lone surrogates across WASM conversions - #8508

Preserve lone surrogates across WASM conversions#8508
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HyoJongPark:fix/wasm-surrogate-conversion

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