Nix / NixOS

Because C++ doesn't have a single well-defined build system, clangd doesn't know how exactly to compile the files you are asking it to - in this case, it doesn't know what headers to include. On some other distros it might "just work" because all headers are lumped together somewhere in /usr/include or something, and something somewhere tells clangd to just look in there. This isn't the case on NixOS (in fact the headers are not installed at all with your system), and so you have to tell clangd where to look for them.

Typically, your build system will have that information somehow. The process can vary depending on the build system you are using - some (like CMake or meson) can do it natively, for others you have to resort to a very useful hack called bear. Judging by your shell.nix, I would guess it's make, which doesn't have native compile_commands.json support, so what you have to do is:

1. Add bear to your nativeBuildInputs
2. Enter the shell again so that you have it in $PATH
3. Run make clean (or otherwise ensure there are no build artifacts already present - maybe git clean -fx but be careful with that)
4. Run bear -- make from the project root.

make should then build your entire project, compiling every file in the process. bear will inspect the system calls that make is executing and determine which commands (and hence which command arguments) it used to compile each file, and create a file called compile_commands.json recording this information. clangd should then automatically find that file and use it to figure out which headers (and other arguments) it needs to compile each file.

This is the setup I personally use to hack on Nix itself BTW, and it works great. Although I'm using helix and not neovim, but that shouldn't matter.

i have a .clangd file with

CompileFlags:
    Add: [
    CompilationDatabase: /src/
-Wall,
-I/nix/store/vm10zh43xgfxvgrqs8brz6v6xyrq9qin-glibc-2.40-66-dev/include,
-I/nix/store/64819v2lcdbdqqb52gyqpic3khn4hvyf-libcxx-19.1.7-dev/include,
-I/nix/store/f4x7z33ymmqn0m0pvi5ydxifsfmic1jw-raylib-5.5/include,
-I/nix/store/wakcjb523m43qbm4xblyqm7rgg9l9s32-glu-9.0.3-dev/include,
-I/nix/store/8xbbdx4ckcdj76ldb0cbym965whipq72-libglvnd-1.7.0-dev/include,
-I/nix/store/b5i8r0l5bnaj8khjz4lsmsdph8dkha3s-libX11-1.8.12-dev/include,
-I/nix/store/l1qkka1svxk8s0myynmb6kgl0ni19mjk-xorgproto-2024.1/include,
-I/nix/store/y91x77rjip3i5zdza2ikf2lj80qc0286-libxcb-1.17.0-dev/include,
-I/nix/store/95hqc5scvz3vxhs0p1crx9rr5r1sfs1i-compiler-rt-libc-19.1.7-dev/include,
-I/nix/store/64819v2lcdbdqqb52gyqpic3khn4hvyf-libcxx-19.1.7-dev/include,
-I/nix/store/f4x7z33ymmqn0m0pvi5ydxifsfmic1jw-raylib-5.5/include,
-I/nix/store/wakcjb523m43qbm4xblyqm7rgg9l9s32-glu-9.0.3-dev/include,
-I/nix/store/8xbbdx4ckcdj76ldb0cbym965whipq72-libglvnd-1.7.0-dev/include,
-I/nix/store/b5i8r0l5bnaj8khjz4lsmsdph8dkha3s-libX11-1.8.12-dev/include,
]

and a create-clangd.py

   3 includes = []
   4 with open("./src/compile_commands.json", 'r') as f:
   5     lines = f.readlines()
   6     for line in lines:
   7         if "include" in line:
   8             includes += [line[1:-2].strip()]
   9
  10 s = """
  11 CompileFlags:
  12     Add: [
  13     CompilationDatabase: /src/
  14 -Wall,
  15 """
  16
  17 for include in includes:
  18     s += "-I" + include[1:-1] + ",\n"
  19
  20 s += "]"
  21 print(s)
  22
  23 with open(".clangd", 'w') as file:
  24     file.write(s)

I forgot how I made the compile-commands.json file. I think I used a program called bear.

I also don't code cpp normally, I just did this for one small project, so it might be a bad solution

Try launching nvim from the same shell where you register your build tools

Not really confident, but is the LSP running from a separate child process? Is that child process inheriting all of the anticipated environment variables from the shell that launched Neovim?