nixos/dovecot2: refactor mailboxes option

Specifying mailboxes as a list isn't a good approach since this makes it
impossible to override values. For backwards-compatibility, it's still
possible to declare a list of mailboxes, but a deprecation warning will
be shown.

nixos/doc/manual/release-notes/rl-2009.xml

@@ -614,6 +614,29 @@ systemd.services.nginx.serviceConfig.ReadWritePaths = [ "/var/www" ];
      queued on the kernel side of the netlink socket.
     </para>
    </listitem>
+   <listitem>
+    <para>
+     Specifying <link linkend="opt-services.dovecot2.mailboxes">mailboxes</link> in the <package>dovecot2</package> module
+     as a list is deprecated and will break eval in 21.03. Instead, an attribute-set should be specified where the <literal>name</literal>
+     should be the key of the attribute.
+    </para>
+    <para>
+     This means that a configuration like this
+<programlisting>{
+  <link linkend="opt-services.dovecot2.mailboxes">services.dovecot2.mailboxes</link> = [
+    { name = "Junk";
+      auto = "create";
+    }
+  ];
+}</programlisting>
+    should now look like this:
+<programlisting>{
+  <link linkend="opt-services.dovecot2.mailboxes">services.dovecot2.mailboxes</link> = {
+    Junk.auto = "create";
+  };
+}</programlisting>
+    </para>
+   </listitem>
   </itemizedlist>
  </section>
 </section>

nixos/modules/services/mail/dovecot.nix

@@ -134,8 +134,9 @@ let
   mailboxes = { ... }: {
     options = {
       name = mkOption {
-        type = types.strMatching ''[^"]+'';
+        type = types.nullOr (types.strMatching ''[^"]+'');
         example = "Spam";
+        default = null;
         description = "The name of the mailbox.";
       };
       auto = mkOption {
@@ -334,9 +335,24 @@ in
     };
 
     mailboxes = mkOption {
-      type = types.listOf (types.submodule mailboxes);
+      type = with types; let m = submodule mailboxes; in either (listOf m) (attrsOf m);
-      default = [];
+      default = {};
-      example = [ { name = "Spam"; specialUse = "Junk"; auto = "create"; } ];
+      apply = x:
+        if isList x then warn "Declaring `services.dovecot2.mailboxes' as a list is deprecated and will break eval in 21.03!" x
+        else mapAttrsToList (name: value:
+          if value.name != null
+            then throw ''
+              When specifying dovecot2 mailboxes as attributes, declaring
+              a `name'-attribute is prohibited! The name ${value.name} should
+              be the attribute key!
+            ''
+          else value // { inherit name; }
+        ) x;
+      example = literalExample ''
+        {
+          Spam = { specialUse = "Junk"; auto = "create"; };
+        }
+      '';
       description = "Configure mailboxes and auto create or subscribe them.";
     };