meta.tests: rename into passthru.tests

Nix currently rejects derivations in `meta` values. This works around
that limitation by using `passthru` instead.

Closes https://github.com/NixOS/nixpkgs/issues/50230

doc/meta.xml

@@ -255,6 +255,17 @@ meta.platforms = stdenv.lib.platforms.linux;
      <varname>tests</varname>
     </term>
     <listitem>
+     <warning>
+      <para>
+       This attribute is special in that it is not actually under the
+       <literal>meta</literal> attribute set but rather under the
+       <literal>passthru</literal> attribute set. This is due to a current
+       limitation of Nix, and will change as soon as Nixpkgs will be able to
+       depend on a new enough version of Nix. See
+       <link xlink:href="https://github.com/NixOS/nix/issues/2532">the relevant
+       issue</link> for more details.
+      </para>
+     </warning>
      <para>
       An attribute set with as values tests. A test is a derivation, which
       builds successfully when the test passes, and fails to build otherwise. A
@@ -269,7 +280,7 @@ meta.platforms = stdenv.lib.platforms.linux;
 { /* ... */, nixosTests }:
 {
   # ...
-  meta.tests = {
+  passthru.tests = {
     basic-functionality-and-dovecot-integration = nixosTests.opensmtpd;
   };
 }

nixos/tests/all-tests.nix

@@ -2,7 +2,7 @@
 # The return value of this function will be an attrset with arbitrary depth and
 # the `anything` returned by callTest at its test leafs.
 # The tests not supported by `system` will be replaced with `{}`, so that
-# `meta.tests` can contain links to those without breaking on architectures
+# `passthru.tests` can contain links to those without breaking on architectures
 # where said tests are unsupported.
 # Example callTest that just extracts the derivation from the test:
 #   callTest = t: t.test;

pkgs/servers/mail/dovecot/default.nix

@@ -68,11 +68,8 @@ stdenv.mkDerivation rec {
     description = "Open source IMAP and POP3 email server written with security primarily in mind";
     maintainers = with stdenv.lib.maintainers; [ peti rickynils fpletz ];
     platforms = stdenv.lib.platforms.unix;
-    # https://github.com/NixOS/nixpkgs/issues/50230 must be resolved before
-    # this can be enabled.
-    #
-    # tests = {
-    #   opensmtpd-interaction = nixosTests.opensmtpd;
-    # };
+  };
+  passthru.tests = {
+    opensmtpd-interaction = nixosTests.opensmtpd;
   };
 }

pkgs/servers/mail/opensmtpd/default.nix

@@ -61,11 +61,8 @@ stdenv.mkDerivation rec {
     license = licenses.isc;
     platforms = platforms.linux;
     maintainers = with maintainers; [ rickynils obadz ekleog ];
-    # https://github.com/NixOS/nixpkgs/issues/50230 must be resolved before
-    # this can be enabled.
-    #
-    # tests = {
-    #   basic-functionality-and-dovecot-interaction = nixosTests.opensmtpd;
-    # };
+  };
+  passthru.tests = {
+    basic-functionality-and-dovecot-interaction = nixosTests.opensmtpd;
   };
 }

pkgs/stdenv/generic/check-meta.nix

@@ -166,6 +166,7 @@ let
     hydraPlatforms = listOf str;
     broken = bool;
     # TODO: refactor once something like Profpatsch's types-simple will land
+    # This is currently dead code due to https://github.com/NixOS/nix/issues/2532
     tests = attrsOf (mkOptionType {
       name = "test";
       check = x: x == {} || ( # Accept {} for tests that are unsupported