ci: Enable offload tests in CI

src/ci/docker/host-x86_64/optional-x86_64-gnu-offload/Dockerfile

@@ -0,0 +1,38 @@
+FROM ubuntu:24.04
+
+ARG DEBIAN_FRONTEND=noninteractive
+
+RUN apt-get update && apt-get install -y --no-install-recommends \
+    g++ \
+    make \
+    ninja-build \
+    file \
+    curl \
+    ca-certificates \
+    python3 \
+    git \
+    cmake \
+    pkg-config \
+    xz-utils \
+    zlib1g-dev \
+    && rm -rf /var/lib/apt/lists/*
+
+COPY scripts/sccache.sh /scripts/
+RUN sh /scripts/sccache.sh
+
+ENV NO_DOWNLOAD_CI_LLVM 1
+ENV CODEGEN_BACKENDS llvm
+
+ENV RUST_CONFIGURE_ARGS \
+    --build=x86_64-unknown-linux-gnu \
+    --enable-clang \
+    --enable-lld \
+    --enable-llvm-offload \
+    --enable-llvm-link-shared \
+    --enable-ninja \
+    --enable-option-checking \
+    --disable-docs \
+    --set llvm.download-ci-llvm=false
+
+ENV SCRIPT="../x.py test --stage 2 --no-fail-fast \
+    tests/codegen-llvm/gpu_offload"

src/ci/github-actions/jobs.yml

@@ -501,6 +501,11 @@ auto:
     doc_url: https://rustc-dev-guide.rust-lang.org/tests/autodiff-ci-job.html
     <<: *job-linux-4c
 
+  - name: optional-x86_64-gnu-offload
+    continue_on_error: true
+    doc_url: https://rustc-dev-guide.rust-lang.org/tests/offload-ci-job.html
+    <<: *job-linux-4c
+
   ####################
   #  macOS Builders  #
   ####################

src/doc/rustc-dev-guide/src/SUMMARY.md

@@ -36,6 +36,7 @@
         - [GCC codegen backend](./tests/codegen-backend-tests/cg_gcc.md)
     - [Performance testing](./tests/perf.md)
     - [Autodiff CI job](./tests/autodiff-ci-job.md)
+    - [Offload CI job](./tests/offload-ci-job.md)
     - [Pre-stabilization CI job for the next solver and polonius alpha](./tests/x86_64-gnu-next-trait-solver-polonius-ci-job.md)
     - [Standard library semver breakage test](./tests/stdlib-semver-check.md)
     - [Misc info](./tests/misc.md)

src/doc/rustc-dev-guide/src/tests/offload-ci-job.md

@@ -0,0 +1,42 @@
+# Offload CI job
+
+The [`optional-x86_64-gnu-offload`] job provides continuous test coverage for
+the experimental `offload` feature.
+It is an optional [auto job](./ci.md#auto-builds), so a failure does not prevent a
+pull request from being merged.
+
+For more context about the feature, see the [offload tracking issue] and the
+[offload internals](../offload/internals.md) chapter.
+
+## What is tested
+
+The job checks:
+
+- host-side LLVM IR generation;
+- device-side LLVM IR generation for AMDGPU and NVPTX targets;
+- argument lowering in host and device code;
+
+## Running the job
+
+To run the job in a try build, comment on a pull request:
+
+```text
+@bors try jobs=optional-x86_64-gnu-offload
+```
+
+To run the job locally, run this command from a Rust checkout:
+
+```console
+$ cargo run --manifest-path src/ci/citool/Cargo.toml run-local optional-x86_64-gnu-offload
+```
+
+See [Testing with Docker](./docker.md) for more information about running CI jobs locally.
+
+## Point of contact
+
+If you have questions or need help with a failure in this job, open a new topic
+in the [offload Zulip channel].
+
+[offload Zulip channel]: https://rust-lang.zulipchat.com/#narrow/channel/422870-t-compiler.2Fgpgpu-backend
+[offload tracking issue]: https://github.com/rust-lang/rust/issues/131513
+[`optional-x86_64-gnu-offload`]: https://github.com/rust-lang/rust/blob/HEAD/src/ci/github-actions/jobs.yml