fix(runtime): #5586 — RegExp.prototype.exec reads lastIndex once and honors a non-writable lastIndex

[proggeramlug]

Summary

Two RegExp.prototype.exec / RegExpBuiltinExec (ECMA-262 22.2.7.2) spec-compliance fixes toward the test262 built-ins/RegExp tracker (#5586):

1. lastIndex is read exactly once, up front. Step 4 of RegExpBuiltinExec reads lastIndex (Get → ToLength) before the global/sticky branch (step 8). Perry previously skipped the read entirely for a non-global/non-sticky regex, so a coercible lastIndex (re.lastIndex = { valueOf() {…} }) was never observed. The value still only drives the search start for a stateful regex (and is not written back for a non-stateful one).

2. A non-writable lastIndex throws. The lastIndex updates use Set(R, "lastIndex", v, true) (Throw=true), so a lastIndex made non-writable via Object.defineProperty must raise a TypeError on a stateful match instead of silently dropping the write. A new set_last_index_throwing helper replaces the raw header write at every stateful write site.

Tests fixed (test262, pinned 4249661)

• built-ins/RegExp/prototype/exec/success-lastindex-access.js
• built-ins/RegExp/prototype/exec/failure-lastindex-access.js
• built-ins/RegExp/prototype/exec/y-fail-lastindex-no-write.js
• built-ins/RegExp/prototype/test/y-fail-lastindex-no-write.js

Validation

• built-ins/RegExp/prototype subset: 185 pass, 0 compile-fail, 0 diff; the 4 cases above moved from fail → pass, and the remaining 6 failures are pre-existing categorical gaps unrelated to this change (capture-group semantics, lone-surrogate parse, UTF-16-code-unit u-flag lastIndex, and the propertyIsEnumerable-on-RegExp.prototype dispatch bug).
• perry-runtime regex unit tests: 30 pass.
• New node-parity regression test test-files/test_issue_5586_regexp_exec_lastindex.ts (byte-identical under Perry and node --experimental-strip-types); existing regex gap tests still match Node.

Note: version bump + CHANGELOG entry intentionally omitted for the maintainer to fold in at merge time.

🤖 Generated with Claude Code

Summary by CodeRabbit

• Bug Fixes

  • Improved regular expression exec/test behavior to follow standard lastIndex rules more closely.
  • Non-stateful regexes now ignore lastIndex changes, while global and sticky regexes update it correctly after matches and failures.
  • Attempts to update a read-only lastIndex now throw an error instead of failing silently.

• Tests

  • Added coverage for lastIndex reading, updating, and error handling across common regex scenarios.

[coderabbitai]

📝 Walkthrough

Walkthrough

Adds set_last_index_throwing in the regex runtime to throw a TypeError when lastIndex is non-writable. Updates js_regexp_exec to read lastIndex once up-front and force offset to 0 for non-global/non-sticky regexes, replacing all store_last_index_number calls with the new throwing variant. A new test file validates these ECMAScript-conformant semantics.

Changes

RegExp lastIndex ECMAScript conformance

Layer / File(s)	Summary
set_last_index_throwing helper crates/perry-runtime/src/regex.rs	New pub(crate) helper (gated on regex-engine) reads lastIndex writability attribute and either throws a TypeError via js_throw or delegates to store_last_index_number.
js_regexp_exec lastIndex read and throw wiring crates/perry-runtime/src/regex/exec.rs	Reads lastIndex once up-front into last_index_read; computes search start as 0 for non-global/non-sticky. Replaces all store_last_index_number calls (early-out, fancy-regex success/failure, standard-regex success/failure) with set_last_index_throwing.
lastIndex semantics tests test-files/test_issue_5586_regexp_exec_lastindex.ts	Covers: single lastIndex read for non-global/non-sticky (via valueOf counter), global regex advancement and reset, and TypeError on non-writable lastIndex for both exec and test.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~20 minutes

Possibly related issues

• test262 built-ins/RegExp — 94 fails (property-escapes + exec internals) #5586 — The changes directly implement the RegExp.exec/lastIndex spec conformance fixes described in this issue.

Poem

🐰 A bunny checked the index with care,
"Is lastIndex writable? Let's dare!"
If frozen in place, throw a type-fit,
If not global, just start from the pit.
Now exec reads the spec line by line—
The tests all agree, everything's fine! 🎉

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title is specific and matches the main change: RegExp exec lastIndex semantics and non-writable lastIndex handling.
Description check	✅ Passed	The description covers the summary, issue reference, and validation, though it doesn't follow the template's section layout exactly.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch worktree-fix-5586-regexp-test262

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[proggeramlug]

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[coderabbitai]

🧹 Nitpick comments (1)

test-files/test_issue_5586_regexp_exec_lastindex.ts (1)

54-74: 📐 Maintainability & Code Quality | 🔵 Trivial | ⚡ Quick win

Add a success-path non-writable lastIndex regression.

These assertions only cover the reset-on-failure throw path. The PR also changed the successful stateful write sites in crates/perry-runtime/src/regex/exec.rs, so a regression there would still pass this file.

➕ Minimal regression to add

+const g2 = /a/g;
+Object.defineProperty(g2, "lastIndex", { writable: false });
+threw = false;
+try {
+  g2.exec("a");
+} catch (e) {
+  threw = e instanceof TypeError;
+}
+ok("global-nonwritable-exec-throws-on-success", threw);

🤖 Prompt for AI Agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

In `@test-files/test_issue_5586_regexp_exec_lastindex.ts` around lines 54 - 74,
Add a success-path regression for non-writable lastIndex using the existing
RegExp exec/test cases in test_issue_5586_regexp_exec_lastindex.ts, since the
current sticky exec/test assertions only exercise the failure/reset path. Extend
the same style of checks around y/y2 to cover a successful stateful match, so
changes in RegExpBuiltinExec or the lastIndex write logic in
crates/perry-runtime/src/regex/exec.rs are caught even when the match succeeds.

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Nitpick comments:
In `@test-files/test_issue_5586_regexp_exec_lastindex.ts`:
- Around line 54-74: Add a success-path regression for non-writable lastIndex
using the existing RegExp exec/test cases in
test_issue_5586_regexp_exec_lastindex.ts, since the current sticky exec/test
assertions only exercise the failure/reset path. Extend the same style of checks
around y/y2 to cover a successful stateful match, so changes in
RegExpBuiltinExec or the lastIndex write logic in
crates/perry-runtime/src/regex/exec.rs are caught even when the match succeeds.

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

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Run ID: 59706019-a2c5-40ea-a003-645a9f367983

📥 Commits

Reviewing files that changed from the base of the PR and between 1f4a2c5 and cd496a8.

📒 Files selected for processing (3)

• crates/perry-runtime/src/regex.rs
• crates/perry-runtime/src/regex/exec.rs
• test-files/test_issue_5586_regexp_exec_lastindex.ts