Add spans to rustpython-stdlib JSON for flamegraph analysis #6722

moreal · 2026-01-13T17:14:57Z

This pull request was extracted from PR #6704. Since it will be squashed and merged, I thought it would be better to submit it separately in case we need to partially revert it later. Of course, there are also parts I'd like to merge first.

Summary by CodeRabbit

New Features
- Added optional performance instrumentation feature that can be enabled to monitor operations.
Chores
- Updated feature flag dependencies and integrated instrumentation support into JSON parsing components for enhanced performance monitoring.

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coderabbitai · 2026-01-13T17:15:26Z

📝 Walkthrough

Walkthrough

The changes introduce flame profiling instrumentation across the codebase by adding a "flame-it" feature flag, creating a conditional flame_guard! macro, and instrumenting the JSON parser with profiling calls to measure execution performance.

Changes

Cohort / File(s)	Summary
Feature and dependency configuration `Cargo.toml`, `crates/stdlib/Cargo.toml`	Added "flame-it" feature flag to workspace manifest and stdlib crate; introduced optional "flame" crate dependency in stdlib; workspace feature now depends on "rustpython-stdlib/flame-it" alongside existing dependencies
Profiling macro infrastructure `crates/stdlib/src/lib.rs`, `crates/stdlib/src/macros.rs`	Declared new `pub(crate) macros` module with `#[macro_use]` attribute; implemented exportable `flame_guard!` macro that conditionally wraps `::flame::start_guard()` when "flame-it" feature is enabled
Profiling instrumentation `crates/stdlib/src/json.rs`	Added `flame_guard!` calls to JsonScanner::parse, JsonScanner::parse_number, encode_string, and scanstring functions for profiling coverage without altering logic or signatures

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

Poem

🔥 A flame so bright, we trace the way,
Through JSON paths, both night and day—
With guards we start, with guards we shine,
Performance secrets, line by line! 🐰✨

🚥 Pre-merge checks | ✅ 2 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 25.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (2 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately summarizes the main change: adding flame profiling spans to the JSON module in rustpython-stdlib for flamegraph analysis, which is reflected throughout all the code changes (macro definition, feature flags, and instrumentation points).

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✨ Finishing touches

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📥 Commits

Reviewing files that changed from the base of the PR and between bb57724 and 9e4ec0d.

⛔ Files ignored due to path filters (1)

Cargo.lock is excluded by !**/*.lock

📒 Files selected for processing (5)

Cargo.toml
crates/stdlib/Cargo.toml
crates/stdlib/src/json.rs
crates/stdlib/src/lib.rs
crates/stdlib/src/macros.rs

🧰 Additional context used

📓 Path-based instructions (1)

**/*.rs

📄 CodeRabbit inference engine (.github/copilot-instructions.md)

**/*.rs: Follow the default rustfmt code style by running cargo fmt to format Rust code
Always run clippy to lint code (cargo clippy) before completing tasks and fix any warnings or lints introduced by changes
Follow Rust best practices for error handling and memory management
Use the macro system (pyclass, pymodule, pyfunction, etc.) when implementing Python functionality in Rust

Files:

crates/stdlib/src/macros.rs
crates/stdlib/src/json.rs
crates/stdlib/src/lib.rs

🧠 Learnings (1)

📚 Learning: 2025-12-27T14:03:49.034Z

Learnt from: CR
Repo: RustPython/RustPython PR: 0
File: .github/copilot-instructions.md:0-0
Timestamp: 2025-12-27T14:03:49.034Z
Learning: Applies to **/*.rs : Use the macro system (`pyclass`, `pymodule`, `pyfunction`, etc.) when implementing Python functionality in Rust

Applied to files:

crates/stdlib/src/lib.rs

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GitHub Check: Run snippets and cpython tests (ubuntu-latest)
GitHub Check: Check Rust code with clippy

🔇 Additional comments (5)

Cargo.toml (1)

18-18: LGTM!

The feature propagation to rustpython-stdlib/flame-it follows the existing pattern used for other features like threading and aligns with the PR's goal of enabling flamegraph analysis for the stdlib.

crates/stdlib/Cargo.toml (1)

25-25: LGTM!

The feature definition and optional dependency follow the established patterns in this manifest. Using workspace-backed versioning ensures consistency across the project.

Also applies to: 37-37

crates/stdlib/src/macros.rs (1)

1-7: LGTM!

The macro correctly uses conditional compilation to emit a flame guard only when the flame-it feature is enabled. The _guard naming convention appropriately suppresses unused variable warnings, and the RAII pattern ensures proper timing capture when the guard goes out of scope.

crates/stdlib/src/lib.rs (1)

11-12: LGTM!

The module declaration with #[macro_use] follows the established pattern in this file and correctly scopes the macro availability to within the crate using pub(crate).

crates/stdlib/src/json.rs (1)

77-77: LGTM!

The profiling guards are strategically placed at key JSON parsing and encoding entry points. The consistent naming convention (Type::method / module::function) will produce clear flamegraph labels, and using static string literals avoids runtime allocation overhead.

Also applies to: 157-157, 218-218, 259-259

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fanninpm · 2026-01-13T17:49:48Z

CI fails for reasons unrelated to this PR.

youknowone

Thank you!

youknowone · 2026-01-14T00:13:58Z

I wished those flaky tests were fixed by yesterday patches 😂

Introduce flame_guard in rustpython-stdlib too

9e4ec0d

youknowone approved these changes Jan 13, 2026

View reviewed changes

youknowone merged commit 49e6931 into RustPython:main Jan 14, 2026
11 of 13 checks passed

terryluan12 pushed a commit to terryluan12/RustPython that referenced this pull request Jan 15, 2026

Introduce flame_guard in rustpython-stdlib too (RustPython#6722)

a46b1b1

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Add spans to rustpython-stdlib JSON for flamegraph analysis #6722

Add spans to rustpython-stdlib JSON for flamegraph analysis #6722

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Add spans to rustpython-stdlib JSON for flamegraph analysis #6722

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