Add regression test for recursive BackwardDifferentiable hang (#10066)

[rkoivunen-sw]

Adds regression test for #10066

The test uses //TEST_IGNORE_FILE: because the unfixed compiler hangs. It will be ignored by CI until the bug is fixed. To verify manually:

timeout 5 slangc tests/bugs/recursive-backward-diff-hang.slang -target glsl -entry computeMain -stage compute -o /dev/null
# Exit 124 = timeout (confirming hang)

[CLAassistant]

All committers have signed the CLA.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

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📝 Walkthrough

Walkthrough

Adds a new ignored regression test that reproduces a hang in the backward-differentiation path by invoking the backward-diff extension on a recursive BackwardDifferentiable function.

Changes

Cohort / File(s)	Summary
Regression Test tests/bugs/recursive-backward-diff-hang.slang	Added a new TEST_IGNORE_FILE regression test that defines BackwardDifferentiable recursiveSum(float x, int n) and invokes __bwd_diff on a DifferentialPair<float> to reproduce a hang; includes manual timeout-based reproduction steps.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~2 minutes

Poem

🐰 I hopped through calls both near and far,
A halting trace beneath the star,
I planted a test to stop and stare,
A timeout carrot hidden there. 🥕

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly identifies the main change: adding a regression test for a specific bug (recursive BackwardDifferentiable hang #10066).
Description check	✅ Passed	The description is directly related to the changeset, explaining the purpose of the regression test, why it uses TEST_IGNORE_FILE, and how to manually verify the bug.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment

---

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[coderabbitai]

Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

Verify each finding against the current code and only fix it if needed.

Inline comments:
In `@tests/bugs/recursive-backward-diff-hang.slang`:
- Around line 7-8: Update the manual reproduction comment to use the correct
filename: replace the incorrect `bug-report-03-recursive-hang.slang` string with
`recursive-backward-diff-hang.slang` in the reproduction command comment so the
timeout example matches the actual file name; look for the reproduced command
line containing `timeout 10 ... -entry computeMain -stage compute` and edit that
filename.

[coderabbitai]

Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

Verify each finding against the current code and only fix it if needed.

Inline comments:
In `@tests/bugs/recursive-backward-diff-hang.slang`:
- Around line 1-8: The two manual repro command lines in the comment block are
inconsistent; update them so both use the same timeout and target to avoid
confusion (e.g., change "timeout 5 slangc recursive-backward-diff-hang.slang
-target glsl -entry computeMain -stage compute" to match "timeout 10 slangc
recursive-backward-diff-hang.slang -target spirv -entry computeMain -stage
compute" or vice versa). Locate the two command strings ("timeout 5 slangc ..."
and "timeout 10 slangc ...") in the file and make them identical (same timeout
value and same -target) so the documentation is unambiguous.

[coderabbitai]

⚠️ Potential issue | 🟡 Minor

Align the two manual repro commands to avoid confusion.

You currently document two different timeouts/targets. Consider consolidating or making them identical so the instructions are unambiguous.

💡 Suggested alignment

-// Run manually: timeout 5 slangc recursive-backward-diff-hang.slang -target glsl -entry computeMain -stage compute
+// Run manually: timeout 5 slangc recursive-backward-diff-hang.slang -target glsl -entry computeMain -stage compute
 ...
-// To reproduce the hang manually (with timeout):
-//   timeout 10 slangc recursive-backward-diff-hang.slang -target spirv -entry computeMain -stage compute
+// To reproduce the hang manually (with timeout):
+//   timeout 5 slangc recursive-backward-diff-hang.slang -target glsl -entry computeMain -stage compute

🤖 Prompt for AI Agents

Verify each finding against the current code and only fix it if needed.

In `@tests/bugs/recursive-backward-diff-hang.slang` around lines 1 - 8, The two
manual repro command lines in the comment block are inconsistent; update them so
both use the same timeout and target to avoid confusion (e.g., change "timeout 5
slangc recursive-backward-diff-hang.slang -target glsl -entry computeMain -stage
compute" to match "timeout 10 slangc recursive-backward-diff-hang.slang -target
spirv -entry computeMain -stage compute" or vice versa). Locate the two command
strings ("timeout 5 slangc ..." and "timeout 10 slangc ...") in the file and
make them identical (same timeout value and same -target) so the documentation
is unambiguous.