lib/asserts: Factor out NixOS's toplevel assertion / warning logic.

lib/asserts.nix

@@ -1,5 +1,16 @@
 { lib }:
 
+let
+  inherit (lib.strings)
+    concatStringsSep
+    ;
+  inherit (lib.lists)
+    filter
+    ;
+  inherit (lib.trivial)
+    showWarnings
+    ;
+in
 rec {
 
   /**
@@ -131,4 +142,61 @@ rec {
       "each element in ${name} must be one of ${lib.generators.toPretty { } xs}, but is: ${
         lib.generators.toPretty { } vals
       }";
+
+  /**
+    Wrap a value with logic that throws an error when assertions
+    fail and emits any warnings.
+
+    # Inputs
+
+    `assertions`
+
+    : A list of assertions. If any of their `assertion` attrs is `false`, their `message` attrs will be emitted in a `throw`.
+
+    `warnings`
+
+    : A list of strings to emit as warnings. This function does no filtering on this list.
+
+    `val`
+
+    : A value to return, wrapped in `warn`, if a `throw` is not necessary.
+
+    # Type
+
+    ```
+    checkAssertWarn :: [ { assertion :: Bool; message :: String } ] -> [ String ] -> Any -> Any
+    ```
+
+    # Examples
+    :::{.example}
+    ## `lib.asserts.checkAssertWarn` usage example
+    ```nix
+    checkAssertWarn
+      [ { assertion = false; message = "Will fail"; } ]
+      [ ]
+      null
+    stderr>        error:
+    stderr>        Failed assertions:
+    stderr>        - Will fail
+
+    checkAssertWarn
+      [ { assertion = true; message = "Will not fail"; } ]
+      [ "Will warn" ]
+      null
+    stderr> evaluation warning: Will warn
+    null
+    ```
+
+    :::
+  */
+  checkAssertWarn =
+    assertions: warnings: val:
+    let
+      failedAssertions = map (x: x.message) (filter (x: !x.assertion) assertions);
+    in
+    if failedAssertions != [ ] then
+      throw "\nFailed assertions:\n${concatStringsSep "\n" (map (x: "- ${x}") failedAssertions)}"
+    else
+      showWarnings warnings val;
+
 }

nixos/modules/system/activation/top-level.nix

@@ -77,14 +77,7 @@ let
   );
 
   # Handle assertions and warnings
-
+  baseSystemAssertWarn = lib.asserts.checkAssertWarn config.assertions config.warnings baseSystem;
-  failedAssertions = map (x: x.message) (filter (x: !x.assertion) config.assertions);
-
-  baseSystemAssertWarn =
-    if failedAssertions != [ ] then
-      throw "\nFailed assertions:\n${concatStringsSep "\n" (map (x: "- ${x}") failedAssertions)}"
-    else
-      showWarnings config.warnings baseSystem;
 
   # Replace runtime dependencies
   system =