Setting cxx in cargo.toml doesn't pass through to build.rs scripts such as in rustc_llvm

[mstewartgallus]

rustc_llvm has a custom build.rs script that calls the gcc crate and compiles bindings to LLVM. However, this doesn't call the C++ compiler passed in the cxx option in the cargo.toml configuration. Instead it just calls the default of c++. This ends up breaking builds for me because I want to use an up to date clang to compile rust instead of my distribution provided gcc which is too old. My current hacky workaround is to run env CXX=clang++-3.8 CC=clang-3.8 ./x.py build.

[Mark-Simulacrum]

This should be fixed if/when #42206 is merged. Also, cc #42255.

[jyn514]

This seems to have been fixed:

rust/src/bootstrap/native.rs

Line 645 in 9ef8407

None => (builder.cc(target), builder.cxx(target).unwrap()),