diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index b0a6afd..e7804e5 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -6,17 +6,6 @@ on:
   pull_request:
 
 jobs:
-  pedantry:
-    runs-on: ubuntu-latest
-    steps:
-      - uses: actions/checkout@v2
-      - name: Install Nix
-        uses: cachix/install-nix-action@v12
-        with:
-          nix_path: nixpkgs=channel:nixos-unstable
-      - name: Cargo Pedantry
-        run: nix-shell --run checkPhase -A mozilla-rust-overlay
-  
   checkPhase:
     runs-on: ubuntu-latest
     steps:
@@ -30,8 +19,8 @@ jobs:
           git config --global user.email "ofborg@example.com"
           git config --global user.name "ofborg"
       - name: checkPhase
-        run: nix-shell --run checkPhase
-  
+        run: nix-shell --pure --run checkPhase
+
   nix-build:
     runs-on: ubuntu-latest
     steps:
diff --git a/README.md b/README.md
index 789dd89..fc66195 100644
--- a/README.md
+++ b/README.md
@@ -209,8 +209,7 @@ To test whether or not Continuous Integration will pass with your changes, you
 can run the following commands from the root of your checkout:
 
 ```shell
-$ nix-shell --run checkPhase -A mozilla-rust-overlay # checks rustfmt and clippy
-$ nix-shell --run checkPhase # runs the test suite
+$ nix-shell --pure --run checkPhase # checks rustfmt, clippy & runs the test suite
 $ nix-build -A ofborg.rs # build ofborg
 ```
 
diff --git a/shell.nix b/shell.nix
index ff4ddb5..badfdff 100644
--- a/shell.nix
+++ b/shell.nix
@@ -1,7 +1,6 @@
 { pkgs ? import ./nix {
   overlays = [
     (import ./nix/overlay.nix)
-    (import (builtins.fetchTarball https://github.com/mozilla/nixpkgs-mozilla/archive/master.tar.gz))
   ];
 } }:
 
@@ -24,65 +23,49 @@ let
     # HISTFILE = "${src}/.bash_hist";
   };
 
-  mozilla-rust-overlay = stdenv.mkDerivation {
-    name = "mozilla-rust-overlay";
-    buildInputs = with pkgs; [
-      latest.rustChannels.stable.rust
-      git
+  rustEnv = stdenv.mkDerivation {
+    name = "gh-event-forwarder";
+    nativeBuildInputs = with pkgs; [
+      nix # so in --pure mode we actually find the "correct" nix
+      bash
+      nix-prefetch-git
+      rustPackages.cargo
+      rustPackages.clippy
+      rustPackages.rustfmt
       pkg-config
+      git
+    ];
+    buildInputs = with pkgs; [
       openssl
     ]
       ++ lib.optional stdenv.isDarwin pkgs.darwin.Security;
 
     postHook = ''
       checkPhase() (
-        cd "${builtins.toString ./.}/ofborg"
+          cd "${builtins.toString ./.}/ofborg"
+          set -x
+          cargo fmt
+          git diff --exit-code
+          cargofmtexit=$?
+  
+          cargo clippy
+          cargoclippyexit=$?
+  
 
-        set -x
+          cargo build && cargo test
+          cargotestexit=$?
 
-        cargo fmt
-        git diff --exit-code
-        cargofmtexit=$?
-
-        cargo clippy
-        cargoclippyexit=$?
-
-        sum=$((cargofmtexit + cargoclippyexit))
-        exit $sum
+          sum=$((cargofmtexit + cargoclippyexit + cargotestexit))
+          exit $sum
       )
     '';
 
-    RUSTFLAGS = "-D warnings";
-    RUST_BACKTRACE = "1";
-    NIX_PATH = "nixpkgs=${pkgs.path}";
-  };
-
-  rustEnv = stdenv.mkDerivation {
-    name = "gh-event-forwarder";
-    buildInputs = with pkgs; [
-      bash
-      nix-prefetch-git
-      latest.rustChannels.stable.rust
-      #rustfmt
-      openssl
-      pkg-config
-      git
-    ]
-      ++ lib.optional stdenv.isDarwin pkgs.darwin.Security;
-
-    postHook = ''
-      checkPhase() {
-          ( cd "${builtins.toString ./.}/ofborg" && cargo build && cargo test)
-      }
-    '';
-
     HISTFILE = "${toString ./.}/.bash_hist";
     RUSTFLAGS = "-D warnings";
     RUST_BACKTRACE = "1";
     RUST_LOG = "ofborg=debug";
     NIX_PATH = "nixpkgs=${pkgs.path}";
     passthru.phpEnv = phpEnv;
-    passthru.mozilla-rust-overlay = mozilla-rust-overlay;
   };
 
 in rustEnv