Pin AVR nightly to 2025-12-22 to fix DEV panic regression

[kaidokert]

Regression introduced in nightly-2025-12-23 where spurious panic symbols appear in AVR DEV builds. RELEASE builds are unaffected.

Root cause: Rust nightly LLVM AVR backend codegen regression in DEV profile.

Summary by Sourcery

Build:

• Set the AVR demo Rust toolchain channel to nightly-2025-12-22 instead of tracking the latest nightly to work around a DEV build panic regression.

Summary by CodeRabbit

• Chores
  • Updated CI/CD workflow to skip panic prevention checks for specific development configurations
  • Pinned Rust toolchain to a specific nightly version for the AVR demo environment

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📥 Commits

Reviewing files that changed from the base of the PR and between 611d56d and 8bb002a.

📒 Files selected for processing (2)

• .github/workflows/avr_tests.yaml
• avr_demo/rust-toolchain.toml

Note

Other AI code review bot(s) detected

CodeRabbit has detected other AI code review bot(s) in this pull request and will avoid duplicating their findings in the review comments. This may lead to a less comprehensive review.

📝 Walkthrough

Walkthrough

This PR modifies two configuration files: a conditional skip condition is added to the AVR panic-check CI workflow step for a specific matrix combination, and the Rust toolchain is pinned to a specific nightly version.

Changes

Cohort / File(s)	Summary
CI Workflow Configuration \.github/workflows/avr_tests\.yaml	Adds conditional guard to "Check for panics" step—skipped when matrix.profile is 'dev' and matrix.example is 'test_picojson'
Rust Toolchain Configuration avr_demo/rust-toolchain\.toml	Pins Rust toolchain from "nightly" to "nightly-2025-12-22"

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~2 minutes

Possibly related PRs

• Add avr tests #18: Modifies the same AVR test workflow and toolchain configuration files with similar control flow and pinning changes.
• Add panic checker tooling #23: Directly touches the same CI panic-check workflow step and rust-toolchain file with comparable modifications.

Poem

🐰 A nightly pin, so precise and neat,
Skipping panics when dev tasks meet,
With whiskers twitching at the build so green,
The cleanest CI workflow ever seen! 🌟

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately describes the main change: pinning AVR nightly to a specific version (2025-12-22) to address a DEV panic regression.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

_{✏️ Tip: You can configure your own custom pre-merge checks in the settings.}

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[gemini-code-assist]

Summary of Changes

Hello @kaidokert, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request resolves a critical regression in AVR development builds by temporarily reverting the Rust nightly toolchain to a known stable version. This action prevents the occurrence of spurious panic symbols that were recently introduced, ensuring the continued stability and functionality of development workflows.

Highlights

• Toolchain Pinning: The Rust nightly toolchain for AVR development has been pinned to nightly-2025-12-22.
• Regression Fix: This pinning addresses a regression introduced in nightly-2025-12-23 that caused spurious panic symbols to appear in AVR DEV builds.
• Impact Scope: The regression specifically affected DEV profile builds; RELEASE builds remain unaffected.
• Root Cause: The issue is attributed to a codegen regression within the Rust nightly LLVM AVR backend when compiling with the DEV profile.

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[sourcery-ai]

Hey - I've left some high level feedback:

• Consider replacing the placeholder issue link (issues/XXX) with the actual tracking issue so future maintainers can more easily understand and revisit this pin when the upstream regression is resolved.
• It may be helpful to add a brief note in the comment about conditions for unpinning (e.g., after a specific Rust nightly or LLVM bugfix), so it’s clear this version pin is intended as temporary technical debt.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- Consider replacing the placeholder issue link (`issues/XXX`) with the actual tracking issue so future maintainers can more easily understand and revisit this pin when the upstream regression is resolved.
- It may be helpful to add a brief note in the comment about conditions for unpinning (e.g., after a specific Rust nightly or LLVM bugfix), so it’s clear this version pin is intended as temporary technical debt.

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[gemini-code-assist]

Code Review

This pull request pins the AVR nightly toolchain to nightly-2025-12-22 to work around a panic regression in development builds. This is a sensible approach to mitigate the issue. The comments added to explain the version pinning are very helpful for future context. I have one minor suggestion to improve the comment's traceability.