picom: remove experimentalBackends, add extraArgs (#3423)

`--experimental-backends` flag was removed in the recent released picom
v10. Using it now will result in the program exiting.

v10 also introduces its counter-part, `--legacy-backends`. However this
will be removed soon. Instead of adding this as an separate option, add
`extraArgs` option so for those that they want they can pass it manuall.
It is also more future proof.

modules/services/picom.nix

@@ -54,6 +54,8 @@ in {
   imports = [
     (mkRemovedOptionModule [ "services" "picom" "refreshRate" ]
       "The option `refresh-rate` has been deprecated by upstream.")
+    (mkRemovedOptionModule [ "services" "picom" "experimentalBackends" ]
+      "The option `--experimental-backends` has been removed by upstream.")
     (mkRemovedOptionModule [ "services" "picom" "extraOptions" ]
       "This option has been replaced by `services.picom.settings`.")
     (mkRenamedOptionModule [ "services" "picom" "opacityRule" ] [
@@ -66,8 +68,6 @@ in {
   options.services.picom = {
     enable = mkEnableOption "Picom X11 compositor";
 
-    experimentalBackends = mkEnableOption "the new experimental backends";
-
     fade = mkOption {
       type = types.bool;
       default = false;
@@ -213,6 +213,15 @@ in {
       '';
     };
 
+    extraArgs = mkOption {
+      type = with types; listOf str;
+      default = [ ];
+      example = literalExpression ''[ "--legacy-backends" ]'';
+      description = ''
+        Extra arguments to be passed to the picom executable.
+      '';
+    };
+
     package = mkOption {
       type = types.package;
       default = pkgs.picom;
@@ -306,7 +315,7 @@ in {
         ExecStart = concatStringsSep " " ([
           "${cfg.package}/bin/picom"
           "--config ${config.xdg.configFile."picom/picom.conf".source}"
-        ] ++ optional cfg.experimentalBackends "--experimental-backends");
+        ] ++ cfg.extraArgs);
         Restart = "always";
         RestartSec = 3;
       };

tests/modules/services/picom/picom-basic-configuration-expected.service

@@ -2,7 +2,7 @@
 WantedBy=graphical-session.target
 
 [Service]
-ExecStart=@picom@/bin/picom --config /nix/store/00000000000000000000000000000000-hm_picompicom.conf --experimental-backends
+ExecStart=@picom@/bin/picom --config /nix/store/00000000000000000000000000000000-hm_picompicom.conf --legacy-backends
 Restart=always
 RestartSec=3
 

tests/modules/services/picom/picom-basic-configuration.nix

@@ -19,7 +19,7 @@
       "unredir-if-possible" = true;
       "dbe" = true;
     };
-    experimentalBackends = true;
+    extraArgs = [ "--legacy-backends" ];
   };
 
   test.stubs.picom = { };