feat(godbolt): add Compiler Explorer integration for rust-cuda kernels

contrib/godbolt/README.md

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+# Compiler Explorer (Godbolt) Integration for rust-cuda
+
+This directory contains everything needed to add rust-cuda as a compiler on
+[Compiler Explorer](https://compiler-explorer.com/) so that users can type
+Rust GPU kernel code and see the resulting PTX assembly.
+
+## How it works
+
+Compiler Explorer expects a single "compiler" binary that reads source on
+stdin or from a file and writes assembly to stdout.  Since rust-cuda has no
+standalone compiler (the pipeline is `rustc` with a custom codegen backend
+plus `cargo` for dependency resolution), the integration uses a wrapper
+script that:
+
+1. Accepts a `.rs` file containing `#[kernel]` functions.
+2. Creates a temporary Cargo project that depends on `cuda_std`.
+3. Sets `CARGO_ENCODED_RUSTFLAGS` with the same flags `cuda_builder` uses
+   (codegen backend path, `no_std` injection, `nvptx64-nvidia-cuda` target,
+   `-Zbuild-std=core,alloc`, etc.).
+4. Runs `cargo build` and parses the JSON output to locate the `.ptx` artifact.
+5. Prints the PTX to stdout (or LLVM IR if `--emit=llvm-ir` is passed).
+6. Forwards compiler diagnostics to stderr so CE displays them.
+
+## Files
+
+| File | Purpose |
+|------|---------|
+| `rust-cuda-wrapper.sh` | The wrapper script CE invokes as the "compiler" |
+| `rust-cuda.defaults.properties` | CE configuration (compiler type, flags, defaults) |
+| `rust-cuda.amazon.properties` | CE instance-specific overrides for the AWS fleet |
+| `install.sh` | Installs the pinned nightly, builds the codegen backend, and lays out the prefix |
+| `test-kernel.rs` | Sample kernel with shared memory and thread indexing |
+
+## Supported flags
+
+| Flag | Description |
+|------|-------------|
+| `--emit=ptx` | Output PTX assembly (default) |
+| `--emit=llvm-ir` | Output LLVM IR before libnvvm conversion |
+| `--opt-level=0` | Disable optimisations |
+| `--opt-level=3` | Enable optimisations (default) |
+| `--gpu-arch=sm_XX` | Target GPU compute capability (default `sm_75` / Turing) |
+| `--version` | Print version info |
+
+## Testing locally
+
+### Prerequisites
+
+- CUDA toolkit installed (need `libnvvm` in `$CUDA_PATH/nvvm/lib64/`)
+- The Rust nightly pinned in `rust-toolchain.toml` (`nightly-2026-04-02`)
+- A built `librustc_codegen_nvvm.so`
+
+### Quick test
+
+```bash
+# From the rust-cuda repo root, after building the codegen backend:
+export RUST_CUDA_ROOT=/opt/compiler-explorer/rust-cuda   # or your install prefix
+export CUDA_PATH=/usr/local/cuda
+
+# Run install.sh first (or manually arrange the prefix):
+./contrib/godbolt/install.sh
+
+# Then test:
+./contrib/godbolt/rust-cuda-wrapper.sh contrib/godbolt/test-kernel.rs
+```
+
+You should see PTX assembly printed to stdout.
+
+### Without install.sh
+
+If you already have the codegen backend built in the workspace, you can
+point the wrapper at the repo tree directly:
+
+```bash
+export RUST_CUDA_ROOT=/path/to/rust-cuda
+# Ensure lib/librustc_codegen_nvvm.so exists at that path, or adjust
+# CODEGEN_SO in the script.
+```
+
+## Submitting to Compiler Explorer
+
+1. Open an issue on [compiler-explorer/compiler-explorer](https://github.com/compiler-explorer/compiler-explorer)
+   proposing the new compiler, linking to this directory.
+2. Open a PR on [compiler-explorer/infra](https://github.com/compiler-explorer/infra)
+   that adds `install.sh` to the builder configuration.
+3. Copy `rust-cuda.defaults.properties` into
+   `etc/config/` in the compiler-explorer repo.
+4. Copy `rust-cuda.amazon.properties` into the appropriate instance
+   config directory.
+
+Key things CE maintainers will want to verify:
+
+- The wrapper is sandboxed (it only writes to `$TMPDIR` and cleans up).
+- Build times are acceptable (first build is slow due to `-Zbuild-std`; subsequent
+  builds reuse the sysroot cache).
+- The CUDA toolkit / `libnvvm` licence permits redistribution on CE's
+  infrastructure (NVIDIA's EULA generally allows this for development tools).
+
+## Compilation pipeline
+
+For reference, the full pipeline that the wrapper reproduces:
+
+```
+  User's .rs file
+       |
+       v
+  [cargo build]
+       |  --target=nvptx64-nvidia-cuda
+       |  -Zbuild-std=core,alloc
+       |  CARGO_ENCODED_RUSTFLAGS with -Zcodegen-backend=...
+       v
+  [rustc + rustc_codegen_nvvm]
+       |  Compiles Rust -> NVVM IR (LLVM 7 bitcode dialect)
+       v
+  [libnvvm]  (from CUDA toolkit)
+       |  Optimises NVVM IR -> PTX
+       v
+  .ptx file  (stdout)
+```

contrib/godbolt/install.sh

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+#!/usr/bin/env bash
+#
+# install.sh - Install the rust-cuda toolchain for Compiler Explorer.
+#
+# This script is meant to be run on a CE builder node (or locally for
+# testing).  It performs the following:
+#
+#   1. Installs the pinned Rust nightly with required components.
+#   2. Clones the rust-cuda repository (or uses a local checkout).
+#   3. Builds the rustc_codegen_nvvm codegen backend against libnvvm.
+#   4. Copies the backend, cuda_std sources, and the wrapper script into
+#      a self-contained prefix under /opt/compiler-explorer/rust-cuda/.
+#
+# Prerequisites:
+#   - CUDA toolkit installed (CUDA_PATH or /usr/local/cuda)
+#   - cmake, ninja-build, clang, pkg-config, libssl-dev, zlib1g-dev
+#   - For LLVM 7 path: the prebuilt LLVM archive is downloaded automatically
+#     by the codegen's build.rs, or you can pre-install LLVM 7 and export
+#     LLVM_CONFIG=/path/to/llvm-config-7.
+#
+# Environment variables:
+#   INSTALL_PREFIX   - Where to install (default: /opt/compiler-explorer/rust-cuda)
+#   CUDA_PATH        - CUDA toolkit root  (default: /usr/local/cuda)
+#   RUST_CUDA_REPO   - Path to an existing rust-cuda checkout (skips git clone)
+#   RUST_CUDA_REF    - Git ref to check out (default: main)
+
+set -euo pipefail
+
+INSTALL_PREFIX="${INSTALL_PREFIX:-/opt/compiler-explorer/rust-cuda}"
+CUDA_PATH="${CUDA_PATH:-/usr/local/cuda}"
+RUST_CUDA_REF="${RUST_CUDA_REF:-main}"
+
+NIGHTLY="nightly-2026-04-02"
+COMPONENTS="rust-src,rustc-dev,llvm-tools-preview"
+
+echo "==> rust-cuda Compiler Explorer installer"
+echo "    prefix:  ${INSTALL_PREFIX}"
+echo "    CUDA:    ${CUDA_PATH}"
+echo "    nightly: ${NIGHTLY}"
+
+# ---------------------------------------------------------------------------
+# 1. Install the pinned Rust nightly
+# ---------------------------------------------------------------------------
+echo "==> Installing Rust ${NIGHTLY} ..."
+if ! command -v rustup &>/dev/null; then
+    curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh -s -- -y --profile minimal --default-toolchain none
+    export PATH="${HOME}/.cargo/bin:${PATH}"
+fi
+
+rustup toolchain install "${NIGHTLY}" --component "${COMPONENTS}"
+rustup default "${NIGHTLY}"
+
+echo "    rustc: $(rustc --version)"
+
+# ---------------------------------------------------------------------------
+# 2. Get the rust-cuda source
+# ---------------------------------------------------------------------------
+if [[ -n "${RUST_CUDA_REPO:-}" ]]; then
+    REPO_DIR="${RUST_CUDA_REPO}"
+    echo "==> Using existing checkout at ${REPO_DIR}"
+else
+    REPO_DIR="$(mktemp -d -t rust-cuda-src.XXXXXXXXXX)"
+    echo "==> Cloning rust-cuda (ref: ${RUST_CUDA_REF}) into ${REPO_DIR} ..."
+    git clone --depth 1 --branch "${RUST_CUDA_REF}" \
+        https://github.com/Rust-GPU/rust-cuda.git "${REPO_DIR}"
+fi
+
+# ---------------------------------------------------------------------------
+# 3. Build the codegen backend
+# ---------------------------------------------------------------------------
+echo "==> Building rustc_codegen_nvvm ..."
+export LD_LIBRARY_PATH="${CUDA_PATH}/nvvm/lib64:${CUDA_PATH}/lib64:${LD_LIBRARY_PATH:-}"
+
+cd "${REPO_DIR}"
+cargo build -p rustc_codegen_nvvm --release
+
+# Find the built .so
+CODEGEN_SO="$(find target/release -maxdepth 2 -name 'librustc_codegen_nvvm.so' -print -quit 2>/dev/null || true)"
+if [[ -z "${CODEGEN_SO}" ]]; then
+    # Try the deps directory with hash suffix.
+    CODEGEN_SO="$(find target/release/deps -maxdepth 1 -name 'librustc_codegen_nvvm-*.so' -print -quit 2>/dev/null || true)"
+fi
+if [[ -z "${CODEGEN_SO}" ]]; then
+    echo "error: could not find librustc_codegen_nvvm.so after build" >&2
+    exit 1
+fi
+echo "    codegen backend: ${CODEGEN_SO}"
+
+# ---------------------------------------------------------------------------
+# 4. Install into the prefix
+# ---------------------------------------------------------------------------
+echo "==> Installing to ${INSTALL_PREFIX} ..."
+mkdir -p "${INSTALL_PREFIX}"/{bin,lib,crates}
+
+# Backend shared library.
+cp "${CODEGEN_SO}" "${INSTALL_PREFIX}/lib/librustc_codegen_nvvm.so"
+
+# Copy the crates that kernel code depends on at build time.
+for crate in cuda_std cuda_std_macros; do
+    cp -a "${REPO_DIR}/crates/${crate}" "${INSTALL_PREFIX}/crates/${crate}"
+done
+
+# Copy workspace-level files needed by cargo (Cargo.lock is especially
+# important so dependency resolution is reproducible).
+cp "${REPO_DIR}/Cargo.lock" "${INSTALL_PREFIX}/" 2>/dev/null || true
+
+# The wrapper script.
+cp "${REPO_DIR}/contrib/godbolt/rust-cuda-wrapper.sh" "${INSTALL_PREFIX}/bin/"
+chmod +x "${INSTALL_PREFIX}/bin/rust-cuda-wrapper.sh"
+
+# Version marker.
+echo "${NIGHTLY}" > "${INSTALL_PREFIX}/rust-toolchain-version"
+
+# Also copy any native libs the codegen may need at link time (from the
+# same build directory).
+for lib in "${REPO_DIR}"/target/release/deps/lib*.so; do
+    [[ -f "${lib}" ]] && cp "${lib}" "${INSTALL_PREFIX}/lib/" 2>/dev/null || true
+done
+
+echo "==> Installation complete."
+echo ""
+echo "Test with:"
+echo "  RUST_CUDA_ROOT=${INSTALL_PREFIX} CUDA_PATH=${CUDA_PATH} \\"
+echo "    ${INSTALL_PREFIX}/bin/rust-cuda-wrapper.sh contrib/godbolt/test-kernel.rs"