CMake correctness; streamline nix-shell environment

[bb010g]

These allow nix-shell --pure --show-trace --keep-going --run 'cargo build --release' from a clean checkout to succeed.

Don't assume LLVM & Clang share the same lib dir

Also, don't assume llvm-config --libdir shares the same lib/ root as llvm-config --cmakedir.

streamline nix-shell environment

Rustup is no longer required when using nix-shell. Instead, the required Rust nightly is already available.

[flosse]

awesome! I did a nix-shell + cargo install --path c2rust --locked and it worked out of the box 🎉

[fw-immunant]

This looks good overall and improves error reporting, which is always a plus. I'm not a nix expert but the nix changes look plausible.

[fw-immunant]

As far as I can tell, with these changes this string becomes dead. But I'm not opposed to keeping it around as a reminder to add better error handling for this situation.

[kkysen]

@bb010g @fw-immunant, can this be merged now? @fw-immunant approved it already and I just fixed the merge conflicts.

@thedataking, the c2rust-testsuite CI is still failing, though, even though c2rust-testsuite is now public. I think that's because the .github/workflows/internal-testsuite.yml still uses a secret token, which it seems is what the CI is failing on. Then there's also the slack fail in the same CI workflow.

[kkysen]

CI is now failing after I re-triggered it with the merge:

CI failure on Ubuntu 18

          Couldn't find Clang cmake dir. Try setting the `CLANG_CMAKE_DIR` environment
          variable or make sure `llvm-config` is on $PATH then re-build. For example:

            $ export CLANG_CMAKE_DIR=/usr/local/opt/llvm/lib/cmake/clang
          : Os { code: 2, kind: NotFound, message: "No such file or directory" }', c2rust-ast-exporter/build.rs:343:22

This might be flaky CI again since it only happened on Ubuntu 18.

[kkysen]

Ubuntu 18 CI failed again, so it's probably not flaky. I'm not sure why it's only happening in Ubuntu 18, and not even in Ubuntu 20. The build scripts for the docker containers for those should be exactly the same.

[kkysen]

c54f7ca, removing the token field, did fix the immunant/c2rust-testsuite checkout issue in GitHub Actions.

[kkysen]

It's failing in Ubuntu 18 because Ubuntu 18 uses llvm 6, which doesn't have a $(llvm-config --cmake-dir)/../clang directory, and in fact, llvm doesn't have this directory until llvm 9. We'll have to find a fix for that. @bb010g, do llvm and clang need separate directories before llvm 9? The changes were in 289d44a. Should we have a fallback to the old behavior?

[bb010g]

@kkysen A fallback sounds reasonable. I don't think I tested on older LLVMs.