This simple gem generates a clangd-compatible compile_commands.json file from the extconf.rb file of a native gem. It's purpose is to enable clangd/LSP to work better when developing native Ruby extension gems.

Just add this to your extconf.rb file after your create_makefile invocation:

require 'extconf_compile_commands_json'
ExtconfCompileCommandsJson.generate!

That will write out a compile_commands.json file to the same directory that mkmf generated your Makefile in.

If you're using rake-compiler to compile your project, that will be in tmp/${arch}/${gem_name}/${ruby_version}/compile_commands.json; you will probably want to symlink that file into the root of your project so that clangd can find it. This can be done by calling symlink!; so:

require 'extconf_compile_commands_json'
ExtconfCompileCommandsJson.generate!
ExtconfCompileCommandsJson.symlink!

Modifying extconf.rb is fine if you own all the build config for your project, but if, for example, you're working on somebody else's project and don't want to modify their extconf.rb, you can still use extconf_compile_commands_json! Something like the following should work, if using rake-compiler:

RUBYOPT="$RUBYOPT -r$(gem which extconf_compile_commands_json/autoload)" bundle exec rake compile

That will load up extconf_compile_commands_json as a patch on mkmf and inject it into any Ruby process that your Rake invocation spawns.