libSrc: introduce a derivation which is the nixpkgs lib source

pkgs/pkgs-lib/default.nix

@@ -1,11 +1,15 @@
 # pkgs-lib is for functions and values that can't be in lib because
 # they depend on some packages. This notably is *not* for supporting package
 # building, instead pkgs/build-support is the place for that.
-{ lib, pkgs }: {
+{ lib, pkgs }:
+
+{
   # setting format types and generators. These do not fit in lib/types.nix,
   # because they depend on pkgs for rendering some formats
   formats = import ./formats.nix {
     inherit lib pkgs;
   };
+
+  libSrc = pkgs.callPackage ./lib-src.nix { };
 }
 

pkgs/pkgs-lib/lib-src.nix

@@ -0,0 +1,22 @@
+{ lib, stdenvNoCC }:
+
+# Produces a derivation which contains **just** the top-level `lib` directory in `nixpkgs`,
+# but maintains the same structure, so it's in the `$out/lib` == `./lib`.
+#
+# This allows Nix-using derivations inside Nixpkgs to be rebuilt when `lib` changes but not
+# for other changes in the tree.
+stdenvNoCC.mkDerivation {
+  name = "nixpkgs-lib";
+
+  src = lib.fileset.toSource {
+    root = ../..;
+    fileset = ../../lib;
+  };
+
+  installPhase = ''
+    runHook preInstall
+    mkdir $out
+    cp -ar $src/* $out
+    runHook postInstall
+  '';
+}

pkgs/top-level/all-packages.nix

@@ -22418,6 +22418,9 @@ with pkgs;
 
   libzdb = callPackage ../development/libraries/libzdb { };
 
+  # The Nixpkgs `lib` as a derivation
+  libSrc = pkgsLib.libSrc;
+
   lightlocker = callPackage ../misc/screensavers/light-locker { };
 
   lightspark = callPackage ../misc/lightspark { };