lib/to-lua: fix removing empties nested in lists

- Add options for removing empty/null list entries (default false)
- Fix recursion into attrs that are themselves list entries

Fixes nix-community#1804

lib/to-lua.nix

@@ -84,9 +84,21 @@ rec {
       */
       removeEmptyAttrValues ? true,
 
+      /**
+        If this option is true, lists like [ { } [ ] "" ]
+        will render as { "" }
+      */
+      removeEmptyListEntries ? false,
+
+      /**
+        If this option is true, lists like [ null "" ]
+        will render as { "" }
+      */
+      removeNullListEntries ? false,
+
       /**
         If this option is true, attrsets like { a.__empty = null; }
-        will render as { ["a"] = { } }, ignoring removeEmptyAttrValues and removeNullAttrValues.
+        will render as { ["a"] = { } }, ignoring other filters such as removeEmptyAttrValues.
       */
       allowExplicitEmpty ? true,
 
@@ -111,7 +123,12 @@ rec {
     }:
     let
       # If any of these are options are set, we need to preprocess the value
-      needsPreprocessing = removeNullAttrValues || removeEmptyAttrValues || allowExplicitEmpty;
+      needsPreprocessing =
+        allowExplicitEmpty
+        || removeEmptyAttrValues
+        || removeNullAttrValues
+        || removeEmptyListEntries
+        || removeNullListEntries;
 
       # Slight optimization: only preprocess if we actually need to
       preprocessValue = value: if needsPreprocessing then removeEmptiesRecursive value else value;
@@ -125,23 +142,30 @@ rec {
         else if allowRawValues && value ? __raw then
           value
         else if isList value then
-          map removeEmptiesRecursive value
+          let
+            needsFiltering = removeNullListEntries || removeEmptyListEntries;
+            fn =
+              v: (removeNullListEntries -> (v != null)) && (removeEmptyListEntries -> (v != [ ] && v != { }));
+            v' = map removeEmptiesRecursive value;
+          in
+          if needsFiltering then filter fn v' else v'
         else if isAttrs value then
           concatMapAttrs (
             n: v:
+            let
+              v' = removeEmptiesRecursive v;
+            in
             if removeNullAttrValues && v == null then
               { }
-            else if removeEmptyAttrValues && (v == [ ] || v == { }) then
-              { }
             else if allowExplicitEmpty && v ? __empty then
               { ${n} = { }; }
-            else if isAttrs v then
-              let
-                v' = removeEmptiesRecursive v;
-              in
-              if v' == { } then { } else { ${n} = v'; }
+            # Optimisation: check if v is empty before evaluating v'
+            else if removeEmptyAttrValues && (v == [ ] || v == { }) then
+              { }
+            else if removeEmptyAttrValues && (v' == [ ] || v' == { }) then
+              { }
             else
-              { ${n} = v; }
+              { ${n} = v'; }
           ) value
         else
           value;

tests/lib-tests.nix

@@ -137,6 +137,55 @@ let
       expected = ''{ }'';
     };
 
+    testToLuaObjectAttrListFilters = {
+      expr = helpers.toLuaObject {
+        a = null;
+        b = { };
+        c = [ ];
+        d = {
+          e = null;
+          f = { };
+        };
+        g = [
+          {
+            x = null;
+            y = null;
+            z = null;
+          }
+          {
+            x = { };
+            y = [ ];
+            z = null;
+          }
+        ];
+        __unkeyed-1 = [
+          {
+            x = null;
+            y = null;
+            z = null;
+          }
+          {
+            x = { };
+            y = [ ];
+            z = null;
+          }
+          [
+            {
+              x = null;
+              y = null;
+              z = null;
+            }
+            {
+              x = { };
+              y = [ ];
+              z = null;
+            }
+          ]
+        ];
+      };
+      expected = ''{ { { }, { }, { { }, { } } }, g = { { }, { } } }'';
+    };
+
     testToLuaObjectEmptyTable = {
       expr = helpers.toLuaObject {
         a = null;
@@ -153,6 +202,47 @@ let
       expected = ''{ c = { }, d = { g = { } } }'';
     };
 
+    testToLuaObjectEmptyListEntries = {
+      expr = helpers.lua.toLua' { removeEmptyListEntries = true; } [
+        { }
+        [ ]
+        [ { } ]
+        null
+        1
+        2
+        [
+          null
+          3
+          4
+          { }
+          [ ]
+          [ { } ]
+        ]
+        5
+      ];
+      expected = "{ nil, 1, 2, { nil, 3, 4 }, 5 }";
+    };
+
+    testToLuaObjectNullListEntries = {
+      expr = helpers.lua.toLua' { removeNullListEntries = true; } [
+        null
+        1
+        2
+        [
+          null
+          3
+          4
+          [
+            null
+            5
+            6
+          ]
+        ]
+        7
+      ];
+      expected = "{ 1, 2, { 3, 4, { 5, 6 } }, 7 }";
+    };
+
     testIsLuaKeyword = {
       expr = builtins.mapAttrs (_: builtins.filter helpers.lua.isKeyword) luaNames;
       expected = {