stdenv: make inputDerivation never require system features

This change removes a limitation with inputDerivations. See
stdenv.tests.inputDerivationRequiredSystemFeatures.meta.longDescription
for details.

I had originally ran into this limitation when trying to write a test
for pull request #319420. That pull request adds a setup hook that can
only be used with derivations that have "uid-range" in their
requiredSystemFeatures. I want to be able to build the inputDerivation
for part of the test on systems that don’t have that system feature.

pkgs/stdenv/generic/default.nix

@@ -214,6 +214,9 @@ let
       shellDryRun = "${stdenv.shell} -n -O extglob";
 
       tests = {
+        inputDerivationRequiredSystemFeatures = import ../tests/inputDerivationRequiredSystemFeatures.nix {
+          inherit lib stdenv;
+        };
         succeedOnFailure = import ../tests/succeedOnFailure.nix { inherit stdenv; };
       };
       passthru.tests = lib.warn "Use `stdenv.tests` instead. `passthru` is a `mkDerivation` detail." stdenv.tests;

pkgs/stdenv/generic/make-derivation.nix

@@ -882,6 +882,9 @@ let
             name = derivationArg.name or "inputDerivation";
             # This always only has one output
             outputs = [ "out" ];
+            # This doesn’t require any system features even if the original
+            # derivation did.
+            requiredSystemFeatures = [ ];
 
             # Propagate the original builder and arguments, since we override
             # them and they might contain references to build inputs

pkgs/stdenv/tests/inputDerivationRequiredSystemFeatures.nix

@@ -0,0 +1,40 @@
+{ lib, stdenv }:
+let
+  impossibleToBuildPackage = stdenv.mkDerivation {
+    name = "impossible-to-build-package";
+    # According to the Nix manual, the "apple-virt" system feature is
+    # Darwin-only and the "kvm" system feature is Linux-only [1].
+    #
+    # [1]: <https://nix.dev/manual/nix/2.32/command-ref/conf-file.html#conf-system-features>
+    requiredSystemFeatures = if stdenv.buildPlatform.isLinux then "apple-virt" else "kvm";
+
+    buildCommand = ''
+      echo ERROR: It’s supposed to be impossible to start building this package. >&2
+      exit 1
+    '';
+  };
+in
+stdenv.mkDerivation {
+  name = "stdenv-test-inputDerivationRequiredSystemFeatures";
+
+  impossibleToBuildPackageInputDerivation = impossibleToBuildPackage.inputDerivation;
+  buildCommand = ''
+    ln --symbolic "$impossibleToBuildPackageInputDerivation" "$out"
+  '';
+
+  meta = {
+    longDescription = ''
+      In previous versions of Nixpkgs,
+      `<originalDerivation>.inputDerivation.requiredSystemFeatures` would
+      always be the same as `<originalDerivation>.requiredSystemFeatures`. This
+      meant that if a builder wasn’t able to build `<originalDerivation>` due
+      to a lack of system features, then that builder would also not be able to
+      build `<originalDerivation>.inputDerivation`.
+
+      That was an aribtrary (and probably accidental) limitation. Building
+      input derivations never requires any system features. This test checks to
+      make sure that that limitation no longer exists.
+    '';
+    maintainers = [ lib.maintainers.jayman2000 ];
+  };
+}