Building `mos-rust` from source

mos-rust build instructions

Note: `mos-rust` is available via https://hub.docker.com.  But if you're not on `amd64` and don't mind potentially stale builds
(the docker image is 8 months old and 19,980 commits behind `llvm-mos` as of the time of this writing).

# Installation
## 1. Build & install `llvm-mos` to `/opt/llvm-mos`
### macOS
```fish
sudo mkdir /opt/llvm-mos
sudo chwown "$(whoami)" /opt/llvm-mos
git clone https://github.com/llvm-mos/llvm-mos
cd llvm-mos
cmake -C clang/cmake/caches/MOS.cmake -G "Ninja" -S llvm -B build -DCMAKE_INSTALL_PREFIX=/opt/llvm-mos -DLLVM_INSTALL_TOOLCHAIN_ONLY=OFF -DLLVM_BUILD_LLVM_DYLIB=ON -DLLVM_LINK_LLVM_DYLIB=ON -DLLVM_INSTALL_UTILS=ON -DLLVM_BUILD_UTILS=ON -DLLVM_TOOLCHAIN_UTILITIES=FileCheck -DLLVM_TOOLCHAIN_TOOLS="llvm-addr2line;llvm-ar;llvm-cxxfilt;llvm-dwarfdump;llvm-mc;llvm-nm;llvm-objcopy;llvm-objdump;llvm-ranlib;llvm-readelf;llvm-readobj;llvm-size;llvm-strings;llvm-strip;llvm-symbolizer;llvm-config;llc" -DLIBXML2_LIBRARY=/usr/local/opt/libxml2/lib/libxml2.dylib -DLLVM_TARGETS_TO_BUILD="MOS;AArch64" -DCMAKE_BUILD_TYPE=Release
cmake --build build -t install
# Wait about 8 minutes; there will be a few warnings
fish_add_path -a "/opt/llvm-mos/bin" "/opt/llvm-mos"
```

### Linux (incomplete)
```bash
sudo mkdir /opt/llvm-mos
sudo chwown "$(whoami)" /opt/llvm-mos
git clone https://github.com/llvm-mos/llvm-mos
cd llvm-mos
cmake -C clang/cmake/caches/MOS.cmake -G "Ninja" -S llvm -B build \
   -DCMAKE_INSTALL_PREFIX=/opt/llvm-mos \
   -DLLVM_INSTALL_TOOLCHAIN_ONLY=OFF \
   -DLLVM_BUILD_LLVM_DYLIB=ON -DLLVM_LINK_LLVM_DYLIB=ON \
   -DLLVM_INSTALL_UTILS=ON -DLLVM_BUILD_UTILS=ON -DLLVM_TOOLCHAIN_UTILITIES=FileCheck \
   -DLLVM_TOOLCHAIN_TOOLS="llvm-addr2line;llvm-ar;llvm-cxxfilt;llvm-dwarfdump;llvm-mc;llvm-nm;llvm-objcopy;llvm-objdump;llvm-ranlib;llvm-readelf;llvm-readobj;llvm-size;llvm-strings;llvm-strip;llvm-symbolizer;llvm-config;llc" \
   -DLIBXML2_LIBRARY=/usr/lib/x86_64-linux-gnu/libxml2.so \
   -DLLVM_TARGETS_TO_BUILD="MOS;X86" \
   -DLLVM_ENABLE_PROJECTS="clang;lld;lldb" # remove this line if you see strange errors
cmake --build build -t install
fish_add_path -a "/opt/llvm-mos/bin" "/opt/llvm-mos"
```

## 2. Build & install `llvm-mos-sdk` to `llvm-mos`
### All Unix platforms
```fish
git clone https://github.com/llvm-mos/llvm-mos-sdk
cd llvm-mos-sdk
cmake -S . -B build -G "Ninja" -DCMAKE_INSTALL_PREFIX=/opt/llvm-mos
cmake --build build -t install
```

## 3. Build & install `rust-mos` Rust compiler
export RUST_TARGET_PATH=/opt/rust-mos-targets/
cp config.toml.example config.toml # even though there's a (smaller) `config.toml` there already
# in config.toml adjust path to llvm-config
llvm-config = "/opt/llvm-mos/bin/llvm-config"
# add 'download-ci-llvm = false' commenting out existing `download-ci-live` line (currently l.53
# comment out '# llvm-has-rust-patches = if llvm-config { false } else { true }' line (currently l.777)
./x.py build -i --stage 0 src/tools/cargo # seems like `rust-mos` builds its own llvm here; is `llvm-mos` even needed?
./x.py build -i
ln -s ../../stage0-tools-bin/cargo build/aarch64-apple-darwin/stage1/bin/cargo
rustup toolchain link mos build/aarch64-apple-darwin/stage1
rustup default mos
mkdir -p $RUST_TARGET_PATH # may require `sudo` if `$RUST_TARGET_PATH` is in a protected folder
sudo chown u007d $RUST_TARGET_PATH # if `mkdir -p $RUST_TARGET_PATH` required `sudo`
python3 create_mos_targets.py $RUST_TARGET_PATH # <-- failing here