nixos/lib: Add featureFlags, use it for minimal modules

nixos/lib/default.nix

@@ -1,5 +1,25 @@
-{ lib ? import ../../lib, ... }:
 let
+  # The warning is in a top-level let binding so it is only printed once.
+  minimalModulesWarning = warn "lib.nixos.evalModules is experimental and subject to change. See nixos/lib/default.nix" null;
+  inherit (nonExtendedLib) warn;
+  nonExtendedLib = import ../../lib;
+in
+{ # Optional. Allows an extended `lib` to be used instead of the regular Nixpkgs lib.
+  lib ? nonExtendedLib,
+
+  # Feature flags allow you to opt in to unfinished code. These may change some
+  # behavior or disable warnings.
+  featureFlags ? {},
+
+  # This file itself is rather new, so we accept unknown parameters to be forward
+  # compatible. This is generally not recommended, because typos go undetected.
+  ...
+}:
+let
+  seqIf = cond: if cond then builtins.seq else a: b: b;
+  # If cond, force `a` before returning any attr
+  seqAttrsIf = cond: a: lib.mapAttrs (_: v: seqIf cond a v);
+
   eval-config-minimal = import ./eval-config-minimal.nix { inherit lib; };
 in
 /*
@@ -7,7 +27,7 @@ in
   using a binding like `nixosLib = import (nixpkgs + "/nixos/lib") { }`.
 */
 {
-  inherit (eval-config-minimal)
+  inherit (seqAttrsIf (!featureFlags?minimalModules) minimalModulesWarning eval-config-minimal)
     evalModules
     ;
 }

nixos/lib/eval-config-minimal.nix

@@ -1,10 +1,7 @@
-let
-  # The warning is in a top-level let binding so it is only printed once.
-  experimentalWarning = warn "lib.nixos.evalModules is experimental and subject to change. See nixos/lib/default.nix" null;
-  inherit (nonExtendedLib) warn;
-  nonExtendedLib = import ../../lib;
-in
-{ lib ? nonExtendedLib, bypassEvalModulesWarning ? false, ... }:
+
+# DO NOT IMPORT. Use nixpkgsFlake.lib.nixos, or import (nixpkgs + "/nixos/lib")
+{ lib }: # read -^
+
 let
 
   /*
@@ -43,5 +40,5 @@ let
 
 in
 {
-  evalModules = builtins.seq (if bypassEvalModulesWarning then null else experimentalWarning) evalModules;
+  inherit evalModules;
 }

nixos/tests/all-tests.nix

@@ -20,7 +20,11 @@ let
     if elem system systems then handleTest path args
     else {};
 
-  nixosLib = import ../lib {};
+  nixosLib = import ../lib {
+    # Experimental features need testing too, but there's no point in warning
+    # about it, so we enable the feature flag.
+    featureFlags.minimalModules = {};
+  };
   evalMinimalConfig = module: nixosLib.evalModules { modules = [ module ]; };
 in
 {