macOS 15 arm64 20250912.392 Image - Rust configuration has changed

[grzegorzkrukowski]

Description

The rust compiler is throwing errors on the latest macOS image on dead code detected.

error: struct `ClipboardWorkspace` is never constructed
Error:   --> data/core/src/clipboard/types.rs:14:8
   |
14 | struct ClipboardWorkspace {
   |        ^^^^^^^^^^^^^^^^^^
   |
   = note: `-D dead-code` implied by `-D warnings`
   = help: to override `-D warnings` add `#[allow(dead_code)]`

On the previous image it was silently ignored as expected.
On the same codebase, the different runners are behaving differently.

Platforms affected

• Azure DevOps
• GitHub Actions - Standard Runners
• GitHub Actions - Larger Runners

Runner images affected

• Ubuntu 22.04
• Ubuntu 24.04
• macOS 13
• macOS 13 Arm64
• macOS 14
• macOS 14 Arm64
• macOS 15
• macOS 15 Arm64
• macOS 26 Arm64
• Windows Server 2019
• Windows Server 2022
• Windows Server 2025

Image version and build link

The working runner:
https://github.com/actions/runner-images/blob/macos-15-arm64/20250911.2324/images/macos/macos-15-arm64-Readme.md

The broken runner:
https://github.com/actions/runner-images/blob/macos-15-arm64/20250916.2338/images/macos/macos-15-arm64-Readme.md

Is it regression?

20250916.2338

Expected behavior

I would expect the same, default behaviour from Rust compiler to not be that strict and not throw an error on dead code, like before.

Actual behavior

Rust compiler is throwing an error causing build failures on the latest image

Repro steps

Compile a Rust code with dead code on the latest runner.

[erik-bershel]

Hey @grzegorzkrukowski!

At first glance, the specified image versions don't appear to contain any changes that could affect Rust. Please provide either the full code of your workflow that broke, or logs for both the last successful and first failed runs. I suspect your workflow contains some step that changes (or reconfigure) the Rust version we install in one way or another.

[erik-bershel]

Yes, I checked. The Rust version in the image is 1.89.0, and yesterday, update 1.90.0 was released (not yet included into our images as not yet Homebrew'ed). You most likely updated to this version on the fly, and it contains the changes mentioned. I recommend checking the version and filing a bug report in the Rust repository if nothing similar found. There are seemingly related issues:

• Function bodies affect compilation of other functions rust-lang/rust#146680
• warning that uninhabited struct is never constructed rust-lang/rust#145936 <--- definitely it

[grzegorzkrukowski]

@erik-bershel thanks for looking into that - of course you are right - we are updating the Rust in our workflow and that probably caused the issue - unlucky timing it was also runner image change, which made me believe this was main reason.

Thanks !

[erik-bershel]

Thank you for highlighting the upcoming regression. 🙇 We'll consider whether it can be prevented.

[programmerjake]

see #13044 (comment) for a response from Rust.