man-db: add assertion for man.man-db.extraConfig when man.generateCaches is false

modules/programs/man/man-db.nix

@@ -33,6 +33,10 @@ in
   };
 
   config = mkIf (cfg.enable && cfgManDb.enable) {
+    warnings = lib.optional (
+      cfgManDb.extraConfig != "" && !cfg.generateCaches
+    ) "programs.man.man-db.extraConfig has no effect when programs.man.generateCaches is false";
+
     # This is mostly copy/pasted/adapted from NixOS' documentation.nix.
     home.file = mkIf (cfg.generateCaches && cfg.package != null) {
       ".manpath".text =

tests/modules/programs/man/default.nix

@@ -6,6 +6,7 @@
   man-disabled-man-db = ./disabled-man-db.nix;
   man-disabled-mandoc = ./disabled-mandoc.nix;
   man-extra-config = ./extra-config.nix;
+  man-extra-config-and-no-cache = ./extra-config-and-no-cache.nix;
   man-mandoc = ./mandoc.nix;
   man-no-manpath = ./no-manpath.nix;
   man-no-caches-without-package = ./no-caches-without-package.nix;

tests/modules/programs/man/extra-config-and-no-cache.nix

@@ -0,0 +1,16 @@
+{
+  config = {
+    test.asserts.warnings.expected = [
+      "programs.man.man-db.extraConfig has no effect when programs.man.generateCaches is false"
+    ];
+
+    programs.man = {
+      enable = true;
+      generateCaches = false;
+      man-db.extraConfig = ''
+        MANDATORY_MANPATH /usr/man
+        SECTION 1 n l 8
+      '';
+    };
+  };
+}