THIS IS NOT AN OFFICIALLY-SUPPORTED PLATFORM. Since we don't plan to have it in CI, it'll probably bitrot, but I'll be happy to accept patches to fix.

Since I had a FreeBSD VM set up to test capnproto/capnproto#1555 (which also benefits Mac, but testing on FreeBSD was easier), I thought it'd be fun to try to build the whole of workerd on FreeBSD. I managed to do so! With all tests passing!

However, this PR will NOT work out-of-the-box. You need some extra hacks:

• Add --linkopt='-lm' --linkopt='-lexecinfo' --host_linkopt='-lm' --host_linkopt='-lexecinfo' to your bazel build or bazel test command line.
• Symlink libstdc++.so to libc++.so, i.e. ln -s libc++.so /usr/lib/libstdc++.so, to work around a Bazel bug which will be fixed in 6.0 (but that's not released yet).
• Move /usr/bin/clang to /usr/bin/clang.real and instead replace /usr/bin/clang with this script:

#! /usr/local/bin/bash

set -euo pipefail

FLAGS=("$@")
for I in "${!FLAGS[@]}"; do
  if [ "${FLAGS[$I]}" = '--target=x86_64-unknown-freebsd' ]; then
    unset -v 'FLAGS[$I]'
  fi
done

exec /usr/bin/clang.real "${FLAGS[@]}"

Obviously, the last hack is unreasonable, but without this I had persistent problems building //rust-deps. Specifically, when building the cxx crate, it would try to build the C++ source file scr/cxx.cc, but would not be able to find the libc++ library headers when doing so. It looks like this code is supposed to remove the -target flag, but is not designed to also recognize --target:

https://github.com/bazelbuild/rules_rust/blob/da3d522d59becbec5697dac0b2aa39890656ed4b/cargo/cargo_build_script.bzl#L16-L39

But I have no idea why this is needed or generally what is going on in this code. Moreover, I could not figure out how to tell it to use a different compiler binary (setting CC, CXX, etc. seemed to have no effect; it always used /usr/bin/clang no matter what I did). Otherwise, I could have placed by wrapper script somewhere else and pointed this code at that. Instead, I had to replace clang in its install location, which is awful.

It'd be great if some Bazel and/or FreeBSD expert could figure out what is needed here.