x.json2: `@[json_null]` field attribute ignored on encode (option fields with `none` always omitted)

[enghitalo]

Describe the bug

The encoder in x.json2 ignores the @[json_null] field attribute. With the legacy json (cJSON) encoder, an Option field annotated with @[json_null] is emitted as "field":null instead of being skipped when the value is none. x.json2 always omits none fields and provides no way to opt-in to writing JSON null.

V code

import x.json2

struct Foo {
	name   ?string @[json_null]
	age    ?int    @[json_null]
	other2 ?string @[json_null]
}

fn main() {
	println(json2.encode(Foo{}))
}

C backend result

// Encoder writes nothing for `none` Option fields — the @[json_null]
// attribute is never read in the encoder.
// Output: {}

Reproduction Steps

import x.json2

struct Bar {
	name ?string @[json_null]
}

struct Foo {
	name   ?string @[json_null]
	age    ?int    @[json_null]
	text   ?string
	other  ?Bar
	other2 ?Bar @[json_null]
}

fn test_json_null_attribute() {
	assert json2.encode(Foo{}) == '{"name":null,"age":null,"other2":null}'
	assert json2.encode(Foo{ name: '' }) == '{"name":"","age":null,"other2":null}'
	assert json2.encode(Foo{ age: 10 }) == '{"name":null,"age":10,"other2":null}'
	assert json2.encode(Foo{
		age:    10
		other2: Bar{
			name: none
		}
	}) == '{"name":null,"age":10,"other2":{"name":null}}'
}

Expected Behavior

PASS test_json_null_attribute

Current Behavior

> assert json2.encode(Foo{}) == '{"name":null,"age":null,"other2":null}'
  Left value (len: 2):  `{}`
  Right value (len: 38): `{"name":null,"age":null,"other2":null}`

Possible Solution

In the struct encoder loop, when iterating fields with $for field in T.fields and detecting field.typ is $option:

1. If the option is none, check whether 'json_null' in field.attrs.
2. If yes, emit "<key>":null, instead of skipping the field.
3. Apply the same rule to embedded option struct values (the other2 ?Bar @[json_null] case in the failing test) and to nested option struct fields (Bar{ name: none } → {"name":null}).

Cache this attribute lookup in cached_field_infos by adding an is_json_null bool field to EncoderFieldInfo so the comptime cost is paid once per type.

Additional Information/Context

This is feature parity with the cJSON json_is_null_attr test, which already passes against the cJSON-based json module. Without this attribute, downstream APIs that distinguish "absent" vs "null" (e.g. JSON Merge Patch, Stripe API style) cannot be expressed in x.json2.

V version

V 0.5.1 1b3385cc34ff783e793d1a26a8ec5be587c80fe0.40b3711

Environment details (OS name and version, etc.)

|V full version      |V 0.5.1 1b3385cc34ff783e793d1a26a8ec5be587c80fe0.40b3711
|:-------------------|:-------------------
|OS                  |linux, Ubuntu 24.04 LTS
|Processor           |16 cpus, 64bit, little endian, AMD Ryzen 7 5800H with Radeon Graphics
|Memory              |8.17GB/30.7GB
|                    |
|V executable        |/home/hitalo/Documents/v/v
|V last modified time|2026-04-18 09:18:00
|                    |
|V home dir          |OK, value: /home/hitalo/Documents/v
|VMODULES            |OK, value: /home/hitalo/.vmodules
|VTMP                |OK, value: /tmp/v_1000
|Current working dir |OK, value: /home/hitalo/Documents/v
|                    |
|Git version         |git version 2.43.0
|V git status        |0.5.1-1006-g40b3711b-dirty
|.git/config present |true
|                    |
|cc version          |cc (GCC) 14.2.0
|gcc version         |gcc (GCC) 14.2.0
|clang version       |Ubuntu clang version 18.1.3 (1)
|tcc version         |tcc version 0.9.28rc 2025-02-13 HEAD@f8bd136d (x86_64 Linux)
|tcc git status      |thirdparty-linux-amd64 696c1d84
|emcc version        |emcc (Emscripten gcc/clang-like replacement + linker emulating GNU ld) 3.1.6 ()
|glibc version       |ldd (Ubuntu GLIBC 2.39-0ubuntu8.3) 2.39

Note

You can use the 👍 reaction to increase the issue's priority for developers.

Please note that only the 👍 reaction to the issue itself counts as a vote.
Other reactions and those to comments will not be taken into account.