rustbuild ignores the llvm-config setting

[nagisa]

I got in my src/bootstrap/config.toml:

[target.x86_64-unknown-linux-gnu]
…
llvm-config = "/home/nagisa/local/bin/llvm-config"

but rustc_llvm/build.rs still seems to attempt using /usr/bin/llvm-config

$ cat build/x86_64-unknown-linux-gnu/stage0-rustc/x86_64-unknown-linux-gnu/release/build/rustc_llvm-9ee423514f5f4196/output 
cargo:rustc-cfg=cargobuild
cargo:rerun-if-changed=/usr/bin/llvm-config

cc @alexcrichton

[alexcrichton]

I think that the actual behavior (from IRC) that happened here was that llvm-config configuration change in cargo.toml, but the build script wasn't re-run.

The build script should probably print out rerun-if-changed pointing at config.toml if it exists, passed through via rustbuild.

[Mark-Simulacrum]

@alexcrichton I think the build script doesn't point to config.toml with rerun-if-changed; is it possible we could make this into an E-easy issue? I get the feeling that this would also make a decent intro into rustbuild.

[alexcrichton]

Certainly!

I think we'll need to pass the path of config.toml via an env var, but that shouldn't be too onerous to do in rustbuild either. If anyone's got any questions please feel free to ask!

[venkatagiri]

I would like to work on this. Can I use the env var CFG_TOML to pass the path of config.toml from bootstrap::compile::rustc? Or LLVM_CFG_TOML or CONFIG_TOML.

[Mark-Simulacrum]

I think an env var would probably be the best approach. CFG_LLVM_TOML seems good, but that can be bikeshedded in the PR. Let us know if we can do anything to help!