bazel_rules_qt VS toolchains_llvm

I migrated a mid-size polyglot project from Makefiles to Bazel and C++ was a large component of the project.

Some obstacles:

1. Building with QT5 MOC & UI files. There is a great library[0] for it but it has hardcoded paths to the QT binaries and header files assuming a system-wide installation. I had to patch the rule to point to our QT location. Then it worked fine.

2. There is no rule to build a fully static library[1]. Since we were shipping a static library in our Makefile system, that was somewhat annoying.

3. We were using system links like `$PROJECT_ROOT/links/GCC/vX.Y.Z/ -> /opt/gcc/...` to point to all the build tools, but these didn't work in Bazel I think because it required absolute paths for any binaries it calls. We ended up putting them in a .bazelrc but we would need a different one for Windows and Linux.

4. Not good integration with IDEs

Ultimately we did not keep using Bazel because we were building Python binaries and py_binary was too slow on Windows. And we didn't have enough time to write a PyInstaller rule.

[0]: https://github.com/justbuchanan/bazel_rules_qt

[1]: https://github.com/bazelbuild/bazel/issues/1920

load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive") BAZEL_TOOLCHAIN_TAG = "0.7.2" BAZEL_TOOLCHAIN_SHA = "f7aa8e59c9d3cafde6edb372d9bd25fb4ee7293ab20b916d867cd0baaa642529" http_archive( name = "com_grail_bazel_toolchain", sha256 = BAZEL_TOOLCHAIN_SHA, strip_prefix = "bazel-toolchain-{tag}".format(tag = BAZEL_TOOLCHAIN_TAG), canonical_id = BAZEL_TOOLCHAIN_TAG, url = "https://github.com/grailbio/bazel-toolchain/archive/{tag}.tar.gz".format(tag = BAZEL_TOOLCHAIN_TAG), ) load("@com_grail_bazel_toolchain//toolchain:deps.bzl", "bazel_toolchain_dependencies") bazel_toolchain_dependencies() load("@com_grail_bazel_toolchain//toolchain:rules.bzl", "llvm_toolchain") llvm_toolchain( name = "llvm_toolchain", llvm_version = "15.0.5", ) load("@llvm_toolchain//:toolchains.bzl", "llvm_register_toolchains") llvm_register_toolchains() http_archive( name = "com_google_googletest", urls = ["https://github.com/google/googletest/archive/609281088cfefc76f9d0ce82e1ff6c30cc3591e5.zip"], strip_prefix = "googletest-609281088cfefc76f9d0ce82e1ff6c30cc3591e5", )

Yeah the cross-compilation thing is definitely a rough spot. I have one project that's able to work around it via extensive hacks with macros, but at some point I'll need to do it "the right way."

Honestly if the docs had a canonical example of e.g. using unix_cc_toolchain_config (example: [0]) + Bootlin to compile for aarch64, it'd probably go a long way to making things understandable. Because say what you will about the old CROSSTOOL approach, at least there was a nice tutorial for it.

[0] https://github.com/grailbio/bazel-toolchain/blob/f14a8a5de8f...

I'm really not familiar with this issue or Go nor C++ overall, but if all you need is to set up a C++ toolchain, this should be quite simple and solve your issue.

The trick is that to provide Bazel with a custom toolchain involves way more than just setting an environment variable, because Bazel wants to control installing and making available the compiler reliably (e.g., what if `emcc` is not present on the system where Bazel was invoked? Bazel solves that problem by fetching it and building it for that system)

There are projects that provide drop-in support for custom toolchains (e.g., we use this project[0] in Sorbet to fetch and build a custom LLVM/Clang toolchain for every host we build on (rather than relying on the system toolchain). But I'm not aware of a project that has done that for Emscripten. Maybe it would be as easy as plucking out what we've done in our project into a project that others could depend on, but to quote a colleague:

> Setting up a cc toolchain in Bazel is a unique sort of pain.

[0] https://github.com/grailbio/bazel-toolchain/