Json Stringify option to not write out null optional fields

lib/std/json.zig

@@ -2731,6 +2731,9 @@ pub const StringifyOptions = struct {
 
             /// Should unicode characters be escaped in strings?
             escape_unicode: bool = false,
+
+            /// Should optional fields with null value be written?
+            write_null_optional_fields: bool = true,
         };
     };
 };
@@ -2822,23 +2825,30 @@ pub fn stringify(
                 // don't include void fields
                 if (Field.field_type == void) continue;
 
-                if (!field_output) {
-                    field_output = true;
+                // don't include optional fields that are null when write_null_optional_fields is set to false
+                const write_null_optional_fields = (options.string == .String) and (options.string.String.write_null_optional_fields == false);
+                const is_null = (@typeInfo(Field.field_type) == .Optional) and (@field(value, Field.name) == null);
+                if (write_null_optional_fields and is_null) {
+                    // skip
                 } else {
-                    try out_stream.writeByte(',');
-                }
-                if (child_options.whitespace) |child_whitespace| {
-                    try out_stream.writeByte('\n');
-                    try child_whitespace.outputIndent(out_stream);
-                }
-                try stringify(Field.name, options, out_stream);
-                try out_stream.writeByte(':');
-                if (child_options.whitespace) |child_whitespace| {
-                    if (child_whitespace.separator) {
-                        try out_stream.writeByte(' ');
+                    if (!field_output) {
+                        field_output = true;
+                    } else {
+                        try out_stream.writeByte(',');
                     }
+                    if (child_options.whitespace) |child_whitespace| {
+                        try out_stream.writeByte('\n');
+                        try child_whitespace.outputIndent(out_stream);
+                    }
+                    try stringify(Field.name, options, out_stream);
+                    try out_stream.writeByte(':');
+                    if (child_options.whitespace) |child_whitespace| {
+                        if (child_whitespace.separator) {
+                            try out_stream.writeByte(' ');
+                        }
+                    }
+                    try stringify(@field(value, Field.name), child_options, out_stream);
                 }
-                try stringify(@field(value, Field.name), child_options, out_stream);
             }
             if (field_output) {
                 if (options.whitespace) |whitespace| {
@@ -3127,3 +3137,33 @@ test "stringify struct with custom stringifier" {
 test "stringify vector" {
     try teststringify("[1,1]", @splat(2, @as(u32, 1)), StringifyOptions{});
 }
+
+test "stringify null optional fields" {
+    const MyStruct = struct {
+        optional: ?[]const u8 = null,
+        required: []const u8 = "something",
+        another_optional: ?[]const u8 = null,
+        another_required: []const u8 = "something else",
+    };
+    try teststringify(
+        \\{"optional":null,"required":"something","another_optional":null,"another_required":"something else"}
+    ,
+        MyStruct{},
+        StringifyOptions{},
+    );
+    try teststringify(
+        \\{"required":"something","another_required":"something else"}
+    ,
+        MyStruct{},
+        StringifyOptions{ .string = .{ .String = .{ .write_null_optional_fields = false } } },
+    );
+
+    try std.testing.expect(try parsesTo(
+        MyStruct,
+        MyStruct{},
+        &TokenStream.init(
+            \\{"required":"something","another_required":"something else"}
+        ),
+        .{ .allocator = std.testing.allocator },
+    ));
+}