fix(stim-parser): address proptest findings by Roger-luo · Pull Request #74 · QuEraComputing/ppvm

Roger-luo · 2026-05-17T16:46:51Z

Summary

PR #73 added a proptest suite that surfaced two issues, both of which were worked around in the tests rather than fixed in the code. This PR removes the workarounds by fixing the underlying parser/AST.

Finding 1 — stack overflow on deeply nested REPEAT

The chumsky grammar uses recursive(...) for REPEAT bodies, and the interpret pass recurses through Repeat bodies as well. Both pieces descend recursively at parse time, and the default thread stack (~2–8 MiB) overflowed somewhere past 24 nesting levels in debug builds.

Fix: run `parse` and `parse_extended` on a dedicated 16 MiB parser thread via `std::thread::scope`. Covers both the chumsky combinator stack and the interpret pass without touching the grammar itself.

`proptest_parse.rs`'s `nested_repeats_never_panic` cap is raised from 16 to 128.

Finding 2 — `Raw(MPad)` never round-trips through `parse_extended`

`parse_extended` always lowers `MPAD …` into the typed `ExtendedInstruction::MPad`, so `Raw(RawInstruction::MPad{..})` and `Raw(RawInstruction::Repeat{..})` were states the parser never produced but the type system still allowed — causing runtime `unreachable!()` paths in `prepare` / `executor` and a workaround in the proptest generator.

Fix: introduce `RawPassthrough` (sub-enum of `RawInstruction` covering only `Gate` / `Noise` / `Measure` / `Annotation`) and change `ExtendedInstruction::Raw(RawInstruction)` to `Raw(RawPassthrough)`. The bad states are now compile-time unrepresentable.

Downstream impact:

`ExecError::Malformed` dropped — it can no longer be reached.
Two `unreachable!()` arms in the executor removed.
`proptest_ast.rs` drops `ext_raw_passthrough` and generates `Raw(RawPassthrough::*)` directly across all four passthrough variants.
The stale regression seed file is removed (the case it captured no longer compiles).
`tests/extended.rs` pattern matches updated to the new variant.

Test plan

`cargo test --workspace --exclude ppvm-python-native` — all green
`pytest ppvm-python/test/generalized_tableau/test_stim.py` — 31 pass
proptest_parse, proptest_roundtrip, proptest_ast all pass with the workarounds removed
`nested_repeats_never_panic` now exercises depths up to 128

🤖 Generated with Claude Code

Resolves the two issues surfaced by the fuzz suite in #73 by fixing the underlying parser/AST, not the tests. The workarounds that were present in the proptest files are removed. **Finding 1: stack overflow on deeply nested REPEAT.** The chumsky grammar uses `recursive(...)` for REPEAT bodies and the interpret pass recurses through `Repeat` bodies as well. Both pieces descend recursively at parse time, and the default thread stack (≈2–8 MiB) overflowed somewhere past 24 nesting levels in debug builds. Fixed by running `parse` and `parse_extended` on a dedicated 16 MiB parser thread via `std::thread::scope`. This covers both the chumsky combinator stack and the interpret pass without touching the grammar itself. The proptest cap on `nested_repeats_never_panic` is raised from 16 to 128. **Finding 2: `Raw(MPad)` never round-trips through `parse_extended`.** `parse_extended` always lowers `MPAD …` into the typed `ExtendedInstruction::MPad`, so `ExtendedInstruction::Raw(RawInstruction::MPad{..})` and `Raw(RawInstruction::Repeat{..})` were states that the parser never produced but the type system still allowed — causing runtime `unreachable!()` paths in `prepare` / `executor` and a workaround in the proptest generator. Fixed by introducing `RawPassthrough` (a sub-enum of `RawInstruction` covering only `Gate`/`Noise`/`Measure`/`Annotation`) and changing `ExtendedInstruction::Raw(RawInstruction)` to `Raw(RawPassthrough)`. The bad states are now compile-time unrepresentable: - `prepare` drops the `ExecError::Malformed` variant — it can no longer be reached. - `executor` drops the two `unreachable!()` arms. - `proptest_ast.rs` drops the `ext_raw_passthrough` workaround and generates `Raw(RawPassthrough::*)` directly across all four passthrough variants. The stale regression seed is removed (the case it captured no longer compiles). - `tests/extended.rs` pattern-matches are updated to the new variant. Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

github-actions · 2026-05-17T16:49:37Z

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2026-05-17 17:31 UTC

Copilot

Pull request overview

This PR fixes STIM parser fuzz findings by avoiding parser stack overflows on deeply nested REPEAT blocks and making invalid extended raw states unrepresentable.

Changes:

Runs parse / parse_extended on a dedicated 16 MiB parser stack.
Introduces RawPassthrough for extended raw instructions and updates parser, display, executor, preparation, and tests.
Updates/removes proptest workarounds and regression seed tied to the old invalid AST state.

Reviewed changes

Copilot reviewed 13 out of 13 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
`crates/stim-parser/src/parser.rs`	Adds dedicated parser-stack execution and splits `parse_impl`.
`crates/stim-parser/src/extended/parser.rs`	Runs parse + interpret on the parser stack.
`crates/stim-parser/src/extended/ast.rs`	Adds `RawPassthrough` and updates extended measurement counting.
`crates/stim-parser/src/extended/interpret.rs`	Converts raw AST variants into `RawPassthrough`.
`crates/stim-parser/src/extended/mod.rs`	Re-exports `RawPassthrough`.
`crates/stim-parser/src/lib.rs`	Adds `RawPassthrough` to the prelude.
`crates/stim-parser/src/display.rs`	Prints `RawPassthrough` through raw formatting.
`crates/ppvm-stim/src/prepare.rs`	Updates validation for `RawPassthrough` and removes malformed raw states.
`crates/ppvm-stim/src/executor.rs`	Updates execution matches for `RawPassthrough`.
`crates/stim-parser/tests/extended.rs`	Updates extended parser assertions to match `RawPassthrough`.
`crates/stim-parser/tests/proptest_ast.rs`	Updates extended AST generators and line normalization for `RawPassthrough`.
`crates/stim-parser/tests/proptest_parse.rs`	Raises nested repeat fuzz depth.
`crates/stim-parser/tests/proptest_ast.proptest-regressions`	Removes stale regression seed for no-longer-representable AST.

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

    match i {
-        ExtendedInstruction::Raw(r) => return fmt_raw(r, f, depth),
+        ExtendedInstruction::Raw(r) => {
+            let owned: RawInstruction = r.clone().into_raw();


+    /// program.
    #[test]
-    fn nested_repeats_never_panic(depth in 0usize..16) {
+    fn nested_repeats_never_panic(depth in 0usize..128) {


+            args: vec![p],
+            targets,
+            line: 0,
+        }),


Roger-luo · 2026-05-17T17:00:48Z

@copilot apply changes based on the comments in this thread

Agent-Logs-Url: https://github.com/QuEraComputing/ppvm/sessions/3ac90f00-66d8-4d7f-9f7d-f2227bde1f48 Co-authored-by: Roger-luo <8445510+Roger-luo@users.noreply.github.com>

Copilot · 2026-05-17T17:06:13Z

@copilot apply changes based on the comments in this thread

Implemented the requested thread follow-ups in commit a2e2985:

removed clone-based formatting for RawPassthrough in Display
made nested repeat proptest depth inclusive up to 128
added Y_ERROR and Z_ERROR to extended passthrough noise proptest generation

No UI changes were made, so there is no UI screenshot to share.

Take main's versions of stim-parser, ppvm-stim, and lockfiles (superseded by #73, #74, #75 landing on main). Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Copilot AI review requested due to automatic review settings May 17, 2026 16:46

Copilot started reviewing on behalf of Roger-luo May 17, 2026 16:47 View session

Copilot AI reviewed May 17, 2026

View reviewed changes

Copilot started work on behalf of Roger-luo May 17, 2026 17:00 View session

fix(stim-parser): address PR review follow-ups

a2e2985

Agent-Logs-Url: https://github.com/QuEraComputing/ppvm/sessions/3ac90f00-66d8-4d7f-9f7d-f2227bde1f48 Co-authored-by: Roger-luo <8445510+Roger-luo@users.noreply.github.com>

Copilot finished work on behalf of Roger-luo May 17, 2026 17:06

Roger-luo merged commit 88a16a6 into main May 17, 2026
7 checks passed

Roger-luo deleted the roger/stim-parser-fix-fuzz-findings branch May 17, 2026 17:30

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fix(stim-parser): address proptest findings - #74

fix(stim-parser): address proptest findings#74
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Roger-luo commented May 17, 2026

Summary

Finding 1 — stack overflow on deeply nested REPEAT

Finding 2 — `Raw(MPad)` never round-trips through `parse_extended`

Test plan

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github-actions Bot commented May 17, 2026 •

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