No fuckin idea what.

flake.nix

@@ -10,6 +10,7 @@
       let
         pkgs = import nixpkgs { inherit system; };
         naersk-lib = pkgs.callPackage naersk { };
+        PROJECT_ROOT = builtins.toString ./.;
       in
       {
         defaultPackage = naersk-lib.buildPackage ./.;
@@ -42,6 +43,9 @@
                          cargo-tarpaulin
                          deno];
           RUST_SRC_PATH = rustPlatform.rustLibSrc;
+          shellHook = ''
+            export PROJECT_ROOT=${PROJECT_ROOT};
+          '';
         };
       });
 }