vimPlugins: generate an overlay

let update.py generates generated.nix as an overlay.
This should help maintaining vim-plugins outside of nixpkgs, as one can
use update.py to generate an out-of-tree vimPlugins overlay.

pkgs/misc/vim-plugins/aliases.nix

@@ -1,8 +1,7 @@
 # Deprecated aliases - for backward compatibility
+lib:
 
-lib: overriden:
-
-with overriden;
+final: prev:
 
 let
   # Removing recurseForDerivation prevents derivations of aliased attribute
@@ -21,12 +20,12 @@ let
 
   # Make sure that we are not shadowing something from
   # all-packages.nix.
-  checkInPkgs = n: alias: if builtins.hasAttr n overriden
+  checkInPkgs = n: alias: if builtins.hasAttr n prev
                           then throw "Alias ${n} is still in vim-plugins"
                           else alias;
 
   mapAliases = aliases:
-     lib.mapAttrs (n: alias: removeDistribute
+    lib.mapAttrs (n: alias: removeDistribute
                              (removeRecurseForDerivations
                               (checkInPkgs n alias)))
                      aliases;
@@ -36,7 +35,7 @@ let
   ) (builtins.fromJSON (builtins.readFile ./deprecated.json));
 
 in
-mapAliases ({
+mapAliases (with prev; {
   airline             = vim-airline;
   alternative         = a-vim; # backwards compat, added 2014-10-21
   bats                = bats-vim;

pkgs/misc/vim-plugins/default.nix

@@ -5,8 +5,12 @@ let
 
   inherit (vimUtils.override {inherit vim;}) buildVimPluginFrom2Nix;
 
+  inherit (lib) extends;
+
+  initialPackages = self: {};
+
   plugins = callPackage ./generated.nix {
-    inherit buildVimPluginFrom2Nix overrides;
+    inherit buildVimPluginFrom2Nix;
   };
 
   # TL;DR
@@ -21,8 +25,13 @@ let
     inherit llvmPackages;
   };
 
-  aliases = lib.optionalAttrs (config.allowAliases or true) (import ./aliases.nix lib plugins);
+  aliases = if (config.allowAliases or true) then final: prev: {} else (import ./aliases.nix lib);
 
+  extensible-self = lib.makeExtensible
+    (extends aliases
+      (extends overrides
+        (extends plugins initialPackages)
+      )
+    );
 in
-
-plugins // aliases
+  extensible-self

pkgs/misc/vim-plugins/update.py

@@ -47,9 +47,9 @@ def generate_nix(plugins: List[Tuple[str, str, pluginupdate.Plugin]], outfile: s
         f.write(HEADER)
         f.write(
             """
-{ lib, buildVimPluginFrom2Nix, fetchFromGitHub, overrides ? (self: super: {}) }:
-let
-  packages = ( self:
+{ lib, buildVimPluginFrom2Nix, fetchFromGitHub }:
+
+final: prev:
 {"""
         )
         for owner, repo, plugin in sorted_plugins:
@@ -75,8 +75,7 @@ let
             )
         f.write(
             """
-});
-in lib.fix' (lib.extends overrides packages)
+}
 """
         )
     print(f"updated {outfile}")