pkgs/overlays/unstable-from-source: allow to provide callPackage

that means that you can build package from unstable,
while using dependencies for a given unstable package from stable

needs more testing with nested packages and package sets,
but I don't see why it wouldn't work

pkgs/overlays/unstable-from-source.nix

@@ -1,10 +1,21 @@
 {
   unstableSource,
   attributeName ? "unstable",
+  # callPackage :: function | boolean
+  callPackage ? false,
 }:
 
 self: super:
 
+assert self.lib.assertMsg (
+  self.lib.isFunction callPackage
+  || self.lib.isBool callPackage
+) ''
+  callPackage argument should be a function or a boolean.
+  If you want to use the callPackage from self, set it to true.
+  If you want to use a custom callPackage, set it to a function (pkgs.callPackage).
+'';
+
 let
   useUnstable = self.config.useUnstable or true;
   sanitizePlatform = platformConfig: self.lib.removeAttrs platformConfig [
@@ -34,7 +45,29 @@ let
     # when crossSystem is passed.
     crossSystem = sanitizePlatform self.stdenv.hostPlatform;
   };
+
+  callPackage' = if builtins.isFunction callPackage then callPackage
+    else if builtins.isBool callPackage && callPackage then self.callPackage
+    else if builtins.isBool callPackage && !callPackage then throw "this should never be evaluated"
+    else throw "callPackage should be a function (callPackage) or a boolean";
+
+  callPackagesUnstablePkgs = self.lib.mapAttrsRecursiveCond (
+    attrset: !(self.lib.hasAttr "override" attrset) && attrset.recurseForDerivations or false
+  ) (
+    _: unstablePackage: callPackage' {
+      # For some reason, override functor of the package has its argument set as required,
+      # which is totally false! the override functor can take all of those arguemnts optionally.
+      __functionArgs = self.lib.mapAttrs (_: _: true) (self.lib.functionArgs unstablePackage.override);
+      __functor =
+        if builtins.isFunction unstablePackage.override then unstablePackage.override
+        else unstablePackage.override.__functor;
+    } {}
+  ) unstablePkgs;
+
 in
 {
-  "${attributeName}" = if useUnstable then unstablePkgs else self;
+  "${attributeName}" = if !useUnstable then self
+    # if callPackage is not false
+    else if builtins.isBool callPackage && !callPackage then callPackagesUnstablePkgs
+    else unstablePkgs;
 }