lib/trivial: add a few examples of usage of assertMsg/assertOneOf

lib/strings.nix

@@ -410,7 +410,7 @@ rec {
       components = splitString "/" url;
       filename = lib.last components;
       name = builtins.head (splitString sep filename);
-    in assert name !=  filename; name;
+    in assert name != filename; name;
 
   /* Create an --{enable,disable}-<feat> string that can be passed to
      standard GNU Autoconf scripts.
@@ -468,7 +468,10 @@ rec {
       strw = lib.stringLength str;
       reqWidth = width - (lib.stringLength filler);
     in
-      assert strw <= width;
+      assert lib.assertMsg (strw <= width)
+        "fixedWidthString: requested string length (${
+          toString width}) must not be shorter than actual length (${
+            toString strw})";
       if strw == width then str else filler + fixedWidthString reqWidth filler str;
 
   /* Format a number adding leading zeroes up to fixed width.