[ci] Roll pinned nightly toolchain

[google-pr-creation-bot]

Automated changes by create-pull-request GitHub action

[gemini-code-assist]

Summary of Changes

Hello @google-pr-creation-bot, 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 performs an automated update to the Cargo.toml file, specifically rolling forward the pinned nightly Rust toolchain version used for CI builds. This ensures that the project's CI environment remains current with a more recent nightly release, which is a common practice for projects that track the bleeding edge of Rust development.

Highlights

• CI Toolchain Update: The pinned nightly Rust toolchain version used in continuous integration (CI) has been updated from nightly-2025-10-17 to nightly-2025-10-21.

Using Gemini Code Assist

The full guide for Gemini Code Assist can be found on our documentation page, here are some quick tips.

Invoking Gemini

You can request assistance from Gemini at any point by creating a comment using either /gemini <command> or @gemini-code-assist <command>. Below is a summary of the supported commands on the current page.

Feature	Command	Description
Code Review	/gemini review	Performs a code review for the current pull request in its current state.
Pull Request Summary	/gemini summary	Provides a summary of the current pull request in its current state.
Comment	@gemini-code-assist	Responds in comments when explicitly tagged, both in pull request comments and review comments.
Help	/gemini help	Displays a list of available commands.

Customization

To customize Gemini Code Assist for GitHub experience, repository maintainers can create a configuration file and/or provide a custom code review style guide (such as PEP-8 for Python) by creating and adding files to a .gemini/ folder in the base of the repository. Detailed instructions can be found here.

Limitations & Feedback

Gemini Code Assist may make mistakes. Please leave feedback on any instances where its feedback is incorrect or counter productive. You can react with 👍 and 👎 on @gemini-code-assist comments. If you're interested in giving your feedback about your experience with Gemini Code Assist for Github and other Google products, sign up here.

You can also get AI-powered code generation, chat, as well as code reviews directly in the IDE at no cost with the Gemini Code Assist IDE Extension.

Footnotes

1. Review the Privacy Notices, Generative AI Prohibited Use Policy, Terms of Service, and learn how to configure Gemini Code Assist in GitHub here. Gemini can make mistakes, so double check it and use code with caution. ↩

[gemini-code-assist]

Code Review

The pull request updates the pinned nightly toolchain version in the Cargo.toml file. A review comment has been added to highlight the importance of verifying the updated nightly version.

[gemini-code-assist]

Please verify that nightly-2025-10-21 is a valid and existing nightly toolchain. This is critical for ensuring the CI builds are using the correct compiler version.

If the date is incorrect, builds may fail or behave unexpectedly.