x.json2: `@[omitempty]` does not skip empty arrays / nil pointers / empty maps / option structs

[enghitalo]

Describe the bug

@[omitempty] in x.json2's encoder only handles a small set of primitive types (string, int, f32, f64, and the ? versions of those — see check_not_empty[T] in the encoder). It does not handle:

• empty arrays ([]string{} etc.)
• nil pointers (&Number = unsafe { nil })
• empty maps
• option struct (?SomeStruct) when none

The cJSON encoder skips all of those. The result is that ports of code from json produce noisy/incorrect output under x.json2.

V code

import x.json2

struct Number {
	min int
	max int
}

pub struct Resp {
pub:
	options []string @[omitempty]
	number  &Number = unsafe { nil } @[omitempty]
}

fn main() {
	r := Resp{ number: &Number{0, 0} }
	println(json2.encode(r))
	// expected: {"number":{"min":0,"max":0}}
	// actual:   {"options":[],"number":{"min":0,"max":0}}
}

C backend result

// check_not_empty_T_Array_string is never specialised — the comptime
// branches in the encoder only cover string/int/float/?string/?int/?f64/?f32.
// As a result, the encoder writes "options":[] anyway.

Reproduction Steps

import x.json2

struct Number {
	min int
	max int
}

pub struct Resp {
pub:
	options []string @[omitempty]
	number  &Number = unsafe { nil } @[omitempty]
}

fn test_omitempty_skips_empty_array_and_nil_pointer() {
	r1 := Resp{ options: ['first', 'second'] }
	r2 := Resp{ number: &Number{0, 0} }
	assert json2.encode(r1) == '{"options":["first","second"]}'
	assert json2.encode(r2) == '{"number":{"min":0,"max":0}}'
}

Expected Behavior

PASS test_omitempty_skips_empty_array_and_nil_pointer

Current Behavior

> assert json2.encode(r2) == '{"number":{"min":0,"max":0}}'
  Left value (len: 41):  `{"options":[],"number":{"min":0,"max":0}}`
  Right value (len: 28): `{"number":{"min":0,"max":0}}`

Possible Solution

Extend check_not_empty[T] in the encoder with comptime branches for:

• $if T is $array: return val.len != 0
• $if T is $map: return val.len != 0
• $if T.indirections > 0: return !isnil(val)
• $if T is $option: peek inside; for option struct return false when none, otherwise recurse.
• $if T is $struct: arguably never empty — match cJSON's behaviour and treat as non-empty unless explicitly none.

Then in the field encoder loop, ensure the omitempty short-circuit honours the new return value before the field key is even written, including option-of-struct fields that currently bypass the check entirely.

Additional Information/Context

This is also the underlying reason the converted cJSON pointer-encode test fails when targeted at 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

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