Pass -Werror when building the LLVM wrapper

[rperier]

cc #109712

[rustbot]

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[cuviper]

I disagree with applying -Werror wholesale, because that can break anyone who happens to use a different C++ compiler than what we already tested with, especially for new warnings that may not even be real problems, or may be caused by external headers out of our control.

Maybe we could inject -Werror only for Rust CI, where we control the entire build environment.

[rperier]

Even if warnings are not enabled ? Well, even if you can still get warnings that are not parts of -Wall and -Wextra...
I understand now what @jyn514 meant by "in CI". I gonna to investigate to find how to inject this only for Rust CI.

[Noratrieb]

note that this here isn't actually building llvm, it's just building rustc's wrapper. the actual LLVM build happens in src/bootstrap

[cuviper]

I gonna to investigate to find how to inject this only for Rust CI.

I don't think it needs to be sophisticated -- maybe just check for the CI environment variable, which has precedent:

rust/compiler/rustc_codegen_llvm/src/llvm_util.rs

Line 55 in 855e0fe

if std::env::var_os("CI").is_some() {

[Noratrieb]

Bootstrap has existing methods for checking whether it's in rust-lang/rust CI, maybe you could reuse those by piping through environment variables or something like that

[rperier]

Boostrap has existing types and methods for checking whether it's in rust-lang/rust CI, that's the CiEnv enum. However, it seems that .github/workflows/ci.yml calls src/ci/docker/run.sh at some point. The docker script wrapper set the environment variable CI before running the container. That's the variable that is used later by compiler/rustc_codegen_llvm/src/llvm_util.rs. Checking for the presence of the CI environment variable looks simpler, as we already have everything we need, in my humble opinion.