nix: Minimize withTools dependencies

default.nix

@@ -47,6 +47,7 @@ let
       allOverlays.postgresql-future
       (allOverlays.haskell-packages { inherit compiler; })
       allOverlays.slocat
+      (allOverlays.minimal { inherit compiler; })
     ];
 
   # Evaluated expression of the Nixpkgs repository.

nix/overlays/default.nix

@@ -3,6 +3,7 @@
   checked-shell-script = import ./checked-shell-script;
   gitignore = import ./gitignore.nix;
   haskell-packages = import ./haskell-packages.nix;
+  minimal = import ./minimal.nix;
   postgresql-libpq = import ./postgresql-libpq.nix;
   postgresql-legacy = import ./postgresql-legacy.nix;
   postgresql-future = import ./postgresql-future.nix;

nix/overlays/minimal.nix

@@ -0,0 +1,61 @@
+# This overlay reduces the closure size of our nix tooling to as much as possible.
+# This makes rebuilding, downloading from cachix and thus CI faster.
+{ compiler }:
+self: super:
+let
+  haskellLib = super.haskell.lib;
+
+  minimizePostgresql = drv: drv.overrideAttrs (finalAttrs: prevAttrs: {
+    buildInputs = [
+      self.readline
+      self.zlib
+      self.openssl
+      self.libxml2
+    ];
+
+    configureFlags = [
+      "--libdir=$(lib)/lib"
+      "--sysconfdir=/etc"
+      "--with-libxml"
+      "--with-openssl"
+      "--with-system-tzdata=${self.tzdata}/share/zoneinfo"
+      "--without-icu"
+    ];
+  });
+in
+{
+  haskell = super.haskell // {
+    packages = super.haskell.packages // {
+      "${compiler}" = super.haskell.packages."${compiler}".override (prev: {
+        ghc = prev.ghc.override {
+          enableDocs = false;
+          enableHaddockProgram = false;
+          enableProfiledLibs = false;
+          enableRelocatedStaticLibs = true;
+          enableShared = false;
+          libffi = null;
+        };
+
+        overrides = self.lib.composeExtensions prev.overrides (self: super: {
+          mkDerivation = args: super.mkDerivation (args // {
+            doCheck = false;
+            doHaddock = false;
+            doHoogle = false;
+            enableExecutableProfiling = false;
+            enableLibraryProfiling = false;
+          });
+
+          # TODO: Remove once https://github.com/NixOS/nixpkgs/pull/287794 is done.
+          aeson = haskellLib.dontCheck super.aeson;
+        });
+      });
+    };
+  };
+
+  postgresql_16 = minimizePostgresql super.postgresql_16;
+  postgresql_15 = minimizePostgresql super.postgresql_15;
+  postgresql_14 = minimizePostgresql super.postgresql_14;
+  postgresql_13 = minimizePostgresql super.postgresql_13;
+  postgresql_12 = minimizePostgresql super.postgresql_12;
+  postgresql_11 = minimizePostgresql super.postgresql_11;
+}