Make it easy to use clangd (i.e. an IDE) with the engine (especially post-RBE)

[matanlurey]

It was and still is possible to use clangd (i.e. with the VSCode clangd plugin) while working in the engine.

For example, here is an excerpt of my workspace settings (JSON):

"clangd.path": "${workspaceFolder}/buildtools/mac-arm64/clang/bin/clangd",
"clangd.arguments": [
  "--compile-commands-dir=${workspaceFolder}/../out/host_debug_unopt_arm64"
],
"clang-format.executable": "${workspaceRoot}/buildtools/mac-arm64/clang/bin/clang-format",

Even before RBE, this required manual configuration, and requires re-configuration as you change platforms (for example, editing C++ code that uses Android-only dependencies requires the --compile-commands-dir to point to a ../out/android_* target).

Post RBE, the default --compile-commands-dir for Google employees uses flags (wrappers) unrecognized by clangd:

One workaround is to intentionally create and use a target output that opts out of RBE:

./tools/gn --unopt --mac-cpu=arm64 --target-dir=host_debug_unopt_arm64_ide --no-rbe

Problems to address (outside of having to do all this manually):

1. By default, this produces many broken links as a result of (mostly Impeller?) generators that need to run. One such example:

   # There are about ~3 sets of targets that need to be built so that clangd finds all generated files.
   ninja -C ../out/host_debug_unopt_arm64_ide shader_fixtures

2. Because this target is not actually used, it will frequently get out of date. That is, you will need to need to gclient sync -D
   and reset GN arguments for both the target you're actually working on (i.e. host_debug_unopt_arm64, using RBE), and this stub target (host_debug_unopt_arm64_ide).

I'm opening this issue to solicit ideas for addressing this. Incremental is good, automated is good, tested is better.

[bc-lee]

In my opinion, consuming raw compile_commands.json generated by gn isn't feasible for tools like clangd, since gn dumps raw commands with all the flags used to build the target. I don't work at Google and therefore don't use Goma or RBE, but I encountered the same issue a long time ago and developed a solution to parse compile_commands.json and filter out flags unnecessary for consuming tools.

You can find my script here: https://gist.github.com/bc-lee/8707d1e62d14f8bb6bd30ef6e67f1625

The script modifies the compile_commands.json file by removing flags that clangd doesn't need. It's not perfect, but it works for me. At least it's much easier than maintaining a separate output folder to avoid using unnecessary flags with clangd.

Example usage:
$ gn gen --export-compile-commands out/out_dir && parse-compile-commands out/out_dir

[matanlurey]

Thanks @bc-lee, that certainly is an option, maybe even the best one.

I'll let the rest of the engine team weigh in, but I appreciate the contribution!