feat(3p/nix): Add a derivation to launch clangd r/816

This wrapper derivation (which assumes that the depot is available at
~/depot) can be used to actually get clangd working with
//third_party/nix.

In my setup I can launch this with M-x eglot, followed by

  env
    CLANGD_FLAGS='--compile-commands-dir=/home/tazjin/projects/nix-build'
    nix-shell -A third_party.nix --run 'nix-clangd' /home/tazjin/depot

1 files changed, 30 insertions, 0 deletions

diff --git a/third_party/nix/clangd.nix b/third_party/nix/clangd.nix
new file mode 100644
index 000000000000..9494e756b16f
--- /dev/null
+++ b/third_party/nix/clangd.nix
@@ -0,0 +1,30 @@
+# Create a clangd wrapper script that can be used with this project.
+# The default Nix wrapper only works with C projects, not C++
+# projects.
+#
+# The CPATH construction logic is lifted from the original wrapper
+# script.
+
+pkgs:
+
+pkgs.writeShellScriptBin "nix-clangd" ''
+  buildcpath() {
+    local path
+    while (( $# )); do
+      case $1 in
+          -isystem)
+              shift
+              path=$path''${path:+':'}$1
+      esac
+      shift
+    done
+    echo $path
+  }
+
+  export CPATH=''${CPATH}''${CPATH:+':'}:$(buildcpath ''${NIX_CFLAGS_COMPILE})
+  export CPATH=${pkgs.glibc.dev}/include''${CPATH:+':'}''${CPATH}
+  export CPLUS_INCLUDE_PATH=${pkgs.llvmPackages.libcxx}/include/c++/v1:''${CPATH}
+
+  # TODO(tazjin): Configurable commands directory?
+  exec -a clangd ${pkgs.llvmPackages.clang-unwrapped}/bin/clangd $@
+''