pkgs.format.toml: introduce test for checking semantics

pkgs/pkgs-lib/tests/formats.nix

@@ -219,6 +219,35 @@ in runBuildTests {
     '';
   };
 
+  # see https://github.com/NixOS/nixpkgs/issues/237521 for this particular test
+  testTomlSemantics = {
+    drv = evalFormat formats.toml {} {
+      processors = {
+        override = [
+          { tags = { cluster = "staging"; }; }
+        ];
+        rename = [
+          {
+            replace = [
+              { dest = "ceph_telegraf_check"; measurement = "exec"; }
+            ];
+          }
+        ];
+      };
+    };
+    expected = ''
+      [processors]
+      [[processors.override]]
+      [processors.override.tags]
+      cluster = "staging"
+
+      [[processors.rename]]
+      [[processors.rename.replace]]
+      dest = "ceph_telegraf_check"
+      measurement = "exec"
+    '';
+  };
+
   # This test is responsible for
   #   1. testing type coercions
   #   2. providing a more readable example test