added RUST_SRC_PATH in shell.nix for better support for IDEs

shell.nix

@@ -6,7 +6,7 @@ let
   nixpkgs = versions.nixpkgs;
   env = versions.stdenv;
   project = import ./default.nix ( buildOptions // { inherit versions; } );
-  rust = nixpkgs.rust.override {
+  rust = versions.rustToolchain.rust.override {
     extensions = [ "rust-src" ];
   };
   nativeBuildInputs = [rust nixpkgs.cacert nixpkgs.openssl] ++ project.nativeBuildInputs;
@@ -15,4 +15,5 @@ nixpkgs.mkShell.override { stdenv = env; }
   {
     inherit nativeBuildInputs;
     inherit (project) buildInputs shellHook;
+    RUST_SRC_PATH = "${versions.rustToolchain.rust-src}/lib/rustlib/src/rust/library/";
   }