docs/cpp/building.md

<!-- Part of the Crubit project, under the Apache License v2.0 with LLVM -->
<!-- Exceptions. See /LICENSE for license information. -->
<!-- SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception -->

# Building the tool

> [!WARNING] Building `rs_bindings_from_cc` from source is experimental. Further
> support is coming soon.

## Cargo

### Prerequisites

*   Requires LLVM and Clang libraries to be built and installed.
    *   They must be built with support for compression (zlib), which is the
        default build config.
*   Requires Abseil libraries to be built and installed.
*   Requires Protobuf libraries and protoc compiler (v27.1+) to be installed.
*   Requires zlib (e.g. libz.so) to be available in the system include and lib
    paths.
*   An up-to-date stable Rust toolchain.

Linux-specific setup:

```sh
# Choice of compiler is optional.
export CC=/path/to/clang
export CXX=/path/to/clang++

# We must use `lld` linker via clang. It must be in the PATH.
export PATH="$PATH:/dir/containing/lld"
export RUSTFLAGS="$RUSTFLAGS -Clinker=/path/to/clang"
export RUSTFLAGS="$RUSTFLAGS -Clink-arg=-fuse-ld=lld"

# If you want to use a sysroot.
# SYSROOT_FLAG=--sysroot=$SYSROOT
# export CXXFLAGS="$CXXFLAGS $SYSROOT_FLAG"
# export RUSTFLAGS="$RUSTFLAGS -Clink-arg=$SYSROOT_FLAG"
```

MacOS-specific setup:

```sh
export CC=clang
export CXX=clang++
export RUSTFLAGS="$RUSTFLAGS -Clinker=clang"
export RUSTFLAGS="$RUSTFLAGS -Clink-arg=-fuse-ld=lld"

# Point to the Xcode sysroot.
export CXXFLAGS="$CXXFLAGS -isysroot $(xcrun --show-sdk-path)"
export RUSTFLAGS="$RUSTFLAGS -Clink-arg=-isysroot -Clink-arg=$(xcrun --show-sdk-path)"
```

Windows-specific setup:

*   **Windows is currently unsupported**, and the APIs generated by Crubit may
    not compile and will change over time.
*   All commands must be run from a development shell, where MSVC environment
    variables are set up.

```sh
# We use clang compiler (clang-cl); MSVC may work too but is unsupported.
export CC=clang-cl
export CXX=clang-cl
# We must use lld to link, which is spelt lld-link. So user-specified linker
# flags must be in MSVC format.
export RUSTFLAGS="$RUSTFLAGS -Clinker=/path/to/lld-link"

# LLVM was built with Zlib support. Point Crubit to the same library.
export CXXFLAGS="$CXXFLAGS /I/path/to/zlib"
export RUSTFLAGS="$RUSTFLAGS -Clink-arg=/LIBPATH:/path/to/zlib"

# Avoid deprecation warnings.
export CXXFLAGS="$CXXFLAGS /D_CRT_SECURE_NO_DEPRECATE"

# If LLVM (-DCMAKE_MSVC_RUNTIME_LIBRARY) and Abseil (-DABSL_MSVC_STATIC_RUNTIME)
# are built against static CRT, then Rust needs to match, or vice-versa.
# export RUSTFLAGS="$RUSTFLAGS -Ctarget-feature=+crt-static"
```

Run the build step via cargo:

```sh
# Paths for Crubit's cargo to use.
## This path contains clang/ and llvm/ dirs with their respective headers.
export CLANG_INCLUDE_PATH=/path/to/llvm/and/clang/headers
## This path contains libLLVM*.a and libclang*.a.
export CLANG_LIB_STATIC_PATH=/path/to/llvm/and/clang/libs
## This path contains absl/ dir with all the includes.
export ABSL_INCLUDE_PATH=/path/to/absl/include/dir
## This path contains libabsl_*
export ABSL_LIB_STATIC_PATH=/path/to/absl/libs
## This path contains google/protobuf/ dir with all the includes.
export PROTOBUF_INCLUDE_PATH=/path/to/protobuf/include/dir
## This path contains libprotobuf.a
export PROTOBUF_LIB_STATIC_PATH=/path/to/protobuf/libs

cargo build --bin rs_bindings_from_cc
```

## Bazel

```sh
apt install clang lld bazel
git clone git@github.com:google/crubit.git
cd crubit
bazel build --linkopt=-fuse-ld=/usr/bin/ld.lld //rs_bindings_from_cc:rs_bindings_from_cc_impl
```

### Using a prebuilt LLVM tree

```sh
git clone https://github.com/llvm/llvm-project
cd llvm-project
CC=clang CXX=clang++ cmake -S llvm -B build -DLLVM_ENABLE_PROJECTS='clang' -DCMAKE_BUILD_TYPE=Release -DCMAKE_INSTALL_PREFIX=install
cmake --build build -j
# wait...
cmake --install build
cd ../crubit
LLVM_INSTALL_PATH=../llvm-project/install bazel build //rs_bindings_from_cc:rs_bindings_from_cc_impl
```