Exploring Ruby with clangd

July 21, 2020

I’ve managed to get LSP-based IDE features powered by clangd working for the Ruby VM’s source code (in my case, in Vim). Here’s how I did it!

I’ve been making a point to learn more about things I depend on recently. Today, that means learning about Ruby. And what better way to learn than to check out the source code, and jump around?

clangd is an editor-agnostic language server that uses the Language Server Protocol to power IDE-like features in your preferred text editor. All it needs is a compile_commands.json, which is basically a mapping of filename to options to pass to clang so that it knows things like which warnings to enable and where to search for header files.

clangd works best for projects built using cmake, but the Ruby VM doesn’t use cmake. Regardless, we can make a compile_commands.json file by using Bear to trace the execution of a Ruby build, and use the trace information to write out a compile_commands.json file.


I could only get these steps to work for Linux, as the Bear README mentions that on macOS you have to disable System Integrity Protection to get it to work.

1. Install Bear

I describe how I built Bear from source in the Appendix.

2. Clone the Ruby source code.

git clone https://github.com/ruby/ruby
cd ruby

3. Configure the Ruby build.

We have to tell the configure script to use Clang to compile (or if you’re confident that your system compiler toolchain is Clang, you can just run ./configure).

# Create the ./configure file
# This only works when using clang to build Ruby
./configure CC=clang

4. Use bear to invoke make

Bear will use a dynamically preloaded library to trace system calls that exec clang processes, looking at things like the command line arguments given to Clang.

bear make

5. That’s it!

The output is ./compile_commands.json, which should be non-empty. If it’s empty or just has [], it didn’t work. There’s some troubleshooting in the Bear README.

The compile_commands.json file will be consumed by clangd in your editor. Check https://langserver.org to find an LSP client for your preferred editor, and follow its setup instructions.

Once you’ve built the compile_commands.json file and configured your editor to use LSP with clangd, you should be able to do things like Jump to Definition and Hover on the Ruby source code!

Appendix: Building Bear from source

This is probably common knowledge for people who use cmake regularly, but this is how I built Bear from source, because I built it on a machine where I didn’t have root so I couldn’t write to /usr/local.

git clone https://github.com/rizsotto/Bear
cd Bear
mkdir build
cd build

# Install to $HOME/.local/bin instead of /usr/local/bin
cmake .. "-DCMAKE_INSTALL_PREFIX=$HOME/.local"
make -j$(nproc)
make install

# → $HOME/.local/bin/bear exists now

Appendix: LSP in Neovim with LanguageClient-neovim

I use Neovim. My preferred LSP client is LanguageClient-neovim. Here’s the parts of my Neovim config files that setup clangd:

vim/plug-settings.vim in jez/dotfiles

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