lib/minfeatures: init from minver.nix (#433057)

default.nix

@@ -1,13 +1,15 @@
 let
-  requiredVersion = import ./lib/minver.nix;
+  missingFeatures = map ({ description, ... }: description) (import ./lib/minfeatures.nix).missing;
 in
 
-if !builtins ? nixVersion || builtins.compareVersions requiredVersion builtins.nixVersion == 1 then
+if missingFeatures != [ ] then
 
   abort ''
 
-    This version of Nixpkgs requires Nix >= ${requiredVersion} but it is being
-    evaluated with Nix ${builtins.nixVersion or "(too old to know)"}, please upgrade:
+    This version of Nixpkgs requires an implementation of Nix with the following features:
+    - ${builtins.concatStringsSep "\n- " missingFeatures}
+
+    Your are evaluating with Nix ${builtins.nixVersion or "(too old to know)"}, please upgrade:
 
     - If you are running NixOS, `nixos-rebuild' can be used to upgrade your system.
 

lib/README.md

@@ -19,7 +19,7 @@ This file evaluates to an attribute set containing two separate kinds of attribu
   Example: `lib.take` is an alias for `lib.lists.take`.
 
 Most files in this directory are definitions of sub-libraries, but there are a few others:
-- [`minver.nix`](minver.nix): A string of the minimum version of Nix that is required to evaluate Nixpkgs.
+- [`minfeatures.nix`](minfeatures.nix): A list of conditions for the used Nix version to match that are required to evaluate Nixpkgs.
 - [`tests`](tests): Tests, see [Running tests](#running-tests)
   - [`release.nix`](tests/release.nix): A derivation aggregating all tests
   - [`misc.nix`](tests/misc.nix): Evaluation unit tests for most sub-libraries

lib/minfeatures.nix

@@ -0,0 +1,19 @@
+let
+  features = [
+    {
+      description = "the `nixVersion` builtin";
+      condition = builtins ? nixVersion;
+    }
+    {
+      description = "`builtins.nixVersion` reports at least 2.18";
+      condition = builtins ? nixVersion && builtins.compareVersions "2.18" builtins.nixVersion != 1;
+    }
+  ];
+
+  evaluated = builtins.partition ({ condition, ... }: condition) features;
+in
+{
+  all = features;
+  supported = evaluated.right;
+  missing = evaluated.wrong;
+}

lib/minver.nix

@@ -1,2 +0,0 @@
-# Expose the minimum required version for evaluating Nixpkgs
-"2.18"