Rust 1.89.0 is Demoting x86_64-apple-darwin to Tier 2 with host tools.

GitHub will soon discontinue providing free macOS x86_64 runners for public repositories. Apple has also announced their plans for discontinuing support for the x86_64 architecture.

This means the x86_64-apple-darwin toolchain will be guaranteed to build, but no longer guaranteed to work.

The direct impact will be GitHub runners. Currently we do minimal tests with the following configurations:

  minimal-tests-core:
    needs: setup-test-matrix
    strategy:
      fail-fast: false
      matrix:
        target:
          - { os: ubuntu-22.04, triple: x86_64-unknown-linux-gnu }
          - { os: ubuntu-22.04, triple: i686-unknown-linux-gnu }
          - { os: macos-15, triple: x86_64-apple-darwin }
        rust: ${{ fromJson(needs.setup-test-matrix.outputs.rust )}}

    name: minimal-tests-core/${{ matrix.target.triple }}/${{ matrix.rust }}
    runs-on: ${{ matrix.target.os }}

According to the list of available runners, the identifier macos-15 is already MacOS 15 ARM64. Our recent CI test logs clearly indicates that it is running on arm64. But even though we set the triplet to x86_64-apple-darwin, it somehow managed to run successfully. It installed the aarch64-apple-darwin host toolchain, added the x86_64 target, compiled mmtk-core for the x86_64 target and ran. I guess some compatibility layers on MacOS allowed it to run x86_64 binaries on ARM64 machines.

But anyway we should migrate to aarch64-apple-darwin because Apple is also dropping support for x86_64.

If needed, we may test on aarch64-unknown-linux-gnu, too, to catch aarch64-specific bugs.