7 files changed, 112 insertions, 20 deletions
diff --git a/lib/default.nix b/lib/default.nix
index 2dfe62e82a8..26842253880 100644
--- a/lib/default.nix
+++ b/lib/default.nix
@@ -122,8 +122,9 @@ let
       mkRenamedOptionModule mkMergedOptionModule mkChangedOptionModule
       mkAliasOptionModule mkDerivedConfig doRename;
     inherit (self.options) isOption mkEnableOption mkSinkUndeclaredOptions
-      mergeDefaultOption mergeOneOption mergeEqualOption getValues
-      getFiles optionAttrSetToDocList optionAttrSetToDocList'
+      mergeDefaultOption mergeOneOption mergeEqualOption mergeUniqueOption
+      getValues getFiles
+      optionAttrSetToDocList optionAttrSetToDocList'
       scrubOptionValue literalExpression literalExample literalDocBook
       showOption showFiles unknownModule mkOption;
     inherit (self.types) isType setType defaultTypeMerge defaultFunctor
diff --git a/lib/options.nix b/lib/options.nix
index 794ca5e3394..627aac24d2f 100644
--- a/lib/options.nix
+++ b/lib/options.nix
@@ -172,11 +172,13 @@ rec {
     else if all isInt list && all (x: x == head list) list then head list
     else throw "Cannot merge definitions of `${showOption loc}'. Definition values:${showDefs defs}";
 
-  mergeOneOption = loc: defs:
-    if defs == [] then abort "This case should never happen."
-    else if length defs != 1 then
-      throw "The unique option `${showOption loc}' is defined multiple times. Definition values:${showDefs defs}"
-    else (head defs).value;
+  mergeOneOption = mergeUniqueOption { message = ""; };
+
+  mergeUniqueOption = { message }: loc: defs:
+    if length defs == 1
+    then (head defs).value
+    else assert length defs > 1;
+      throw "The option `${showOption loc}' is defined multiple times.\n${message}\nDefinition values:${showDefs defs}";
 
   /* "Merge" option definitions by checking that they all have the same value. */
   mergeEqualOption = loc: defs:
diff --git a/lib/types.nix b/lib/types.nix
index cc3ac5fdf6f..f2f9b2bca98 100644
--- a/lib/types.nix
+++ b/lib/types.nix
@@ -32,7 +32,6 @@ let
     last
     length
     tail
-    unique
     ;
   inherit (lib.attrsets)
     attrNames
@@ -48,6 +47,7 @@ let
     mergeDefaultOption
     mergeEqualOption
     mergeOneOption
+    mergeUniqueOption
     showFiles
     showOption
     ;
@@ -470,6 +470,18 @@ rec {
       nestedTypes.elemType = elemType;
     };
 
+    unique = { message }: type: mkOptionType rec {
+      name = "unique";
+      inherit (type) description check;
+      merge = mergeUniqueOption { inherit message; };
+      emptyValue = type.emptyValue;
+      getSubOptions = type.getSubOptions;
+      getSubModules = type.getSubModules;
+      substSubModules = m: uniq (type.substSubModules m);
+      functor = (defaultFunctor name) // { wrapped = type; };
+      nestedTypes.elemType = type;
+    };
+
     # Null or value of ...
     nullOr = elemType: mkOptionType rec {
       name = "nullOr";
@@ -599,6 +611,7 @@ rec {
     # A value from a set of allowed ones.
     enum = values:
       let
+        inherit (lib.lists) unique;
         show = v:
                if builtins.isString v then ''"${v}"''
           else if builtins.isInt v then builtins.toString v
diff --git a/nixos/doc/manual/development/option-types.section.md b/nixos/doc/manual/development/option-types.section.md
index ed557206659..56ffa8e9d79 100644
--- a/nixos/doc/manual/development/option-types.section.md
+++ b/nixos/doc/manual/development/option-types.section.md
@@ -250,6 +250,12 @@ Composed types are types that take a type as parameter. `listOf
 :   Ensures that type *`t`* cannot be merged. It is used to ensure option
     definitions are declared only once.
 
+`types.unique` `{ message = m }` *`t`*
+
+:   Ensures that type *`t`* cannot be merged. Prints the message *`m`*, after
+    the line `The option <option path> is defined multiple times.` and before
+    a list of definition locations.
+
 `types.either` *`t1 t2`*
 
 :   Type *`t1`* or type *`t2`*, e.g. `with types; either int str`.
diff --git a/nixos/doc/manual/from_md/development/option-types.section.xml b/nixos/doc/manual/from_md/development/option-types.section.xml
index c83ffa2add5..76ffb6f837c 100644
--- a/nixos/doc/manual/from_md/development/option-types.section.xml
+++ b/nixos/doc/manual/from_md/development/option-types.section.xml
@@ -498,6 +498,22 @@
       </varlistentry>
       <varlistentry>
         <term>
+          <literal>types.unique</literal>
+          <literal>{ message = m }</literal>
+          <emphasis><literal>t</literal></emphasis>
+        </term>
+        <listitem>
+          <para>
+            Ensures that type <emphasis><literal>t</literal></emphasis>
+            cannot be merged. Prints the message
+            <emphasis><literal>m</literal></emphasis>, after the line
+            <literal>The option &lt;option path&gt; is defined multiple times.</literal>
+            and before a list of definition locations.
+          </para>
+        </listitem>
+      </varlistentry>
+      <varlistentry>
+        <term>
           <literal>types.either</literal>
           <emphasis><literal>t1 t2</literal></emphasis>
         </term>
diff --git a/nixos/modules/system/activation/top-level.nix b/nixos/modules/system/activation/top-level.nix
index 8e53ec1ffab..c9fef33c940 100644
--- a/nixos/modules/system/activation/top-level.nix
+++ b/nixos/modules/system/activation/top-level.nix
@@ -109,9 +109,7 @@ let
     utillinux = pkgs.util-linux;
 
     kernelParams = config.boot.kernelParams;
-    installBootLoader =
-      config.system.build.installBootLoader
-      or "echo 'Warning: do not know how to make this configuration bootable; please enable a boot loader.' 1>&2; true";
+    installBootLoader = config.system.build.installBootLoader;
     activationScript = config.system.activationScripts.script;
     dryActivationScript = config.system.dryActivationScript;
     nixosLabel = config.system.nixos.label;
@@ -135,25 +133,27 @@ let
       pkgs.replaceDependency { inherit oldDependency newDependency drv; }
     ) baseSystemAssertWarn config.system.replaceRuntimeDependencies;
 
+  /* Workaround until https://github.com/NixOS/nixpkgs/pull/156533
+     Call can be replaced by argument when that's merged.
+  */
+  tmpFixupSubmoduleBoundary = subopts:
+    lib.mkOption {
+      type = lib.types.submoduleWith {
+        modules = [ { options = subopts; } ];
+      };
+    };
+
 in
 
 {
   imports = [
+    ../build.nix
     (mkRemovedOptionModule [ "nesting" "clone" ] "Use `specialisation.«name» = { inheritParentConfig = true; configuration = { ... }; }` instead.")
     (mkRemovedOptionModule [ "nesting" "children" ] "Use `specialisation.«name».configuration = { ... }` instead.")
   ];
 
   options = {
 
-    system.build = mkOption {
-      internal = true;
-      default = {};
-      type = with types; lazyAttrsOf (uniq unspecified);
-      description = ''
-        Attribute set of derivations used to setup the system.
-      '';
-    };
-
     specialisation = mkOption {
       default = {};
       example = lib.literalExpression "{ fewJobsManyCores.configuration = { nix.buildCores = 0; nix.maxJobs = 1; }; }";
@@ -224,6 +224,39 @@ in
       '';
     };
 
+    system.build = tmpFixupSubmoduleBoundary {
+      installBootLoader = mkOption {
+        internal = true;
+        # "; true" => make the `$out` argument from switch-to-configuration.pl
+        #             go to `true` instead of `echo`, hiding the useless path
+        #             from the log.
+        default = "echo 'Warning: do not know how to make this configuration bootable; please enable a boot loader.' 1>&2; true";
+        description = ''
+          A program that writes a bootloader installation script to the path passed in the first command line argument.
+
+          See <literal>nixos/modules/system/activation/switch-to-configuration.pl</literal>.
+        '';
+        type = types.unique {
+          message = ''
+            Only one bootloader can be enabled at a time. This requirement has not
+            been checked until NixOS 22.05. Earlier versions defaulted to the last
+            definition. Change your configuration to enable only one bootloader.
+          '';
+        } (types.either types.str types.package);
+      };
+
+      toplevel = mkOption {
+        type = types.package;
+        readOnly = true;
+        description = ''
+          This option contains the store path that typically represents a NixOS system.
+
+          You can read this path in a custom deployment tool for example.
+        '';
+      };
+    };
+
+
     system.copySystemConfiguration = mkOption {
       type = types.bool;
       default = false;
diff --git a/nixos/modules/system/build.nix b/nixos/modules/system/build.nix
new file mode 100644
index 00000000000..58dc3f0d411
--- /dev/null
+++ b/nixos/modules/system/build.nix
@@ -0,0 +1,21 @@
+{ lib, ... }:
+let
+  inherit (lib) mkOption types;
+in
+{
+  options = {
+
+    system.build = mkOption {
+      default = {};
+      description = ''
+        Attribute set of derivations used to set up the system.
+      '';
+      type = types.submoduleWith {
+        modules = [{
+          freeformType = with types; lazyAttrsOf (uniq unspecified);
+        }];
+      };
+    };
+
+  };
+}