cgen: error for unused functions returning fixed arrays of custom structs

[demizer]

Describe the bug

When skip_unused is enabled (default), functions that return fixed-size arrays of custom structs (e.g., [4]MyStruct) but are never called cause C compilation errors. The V compiler generates a wrapper struct for the return type that references the custom struct, but the custom struct itself is not emitted because it's considered "unused".

This only occurs in larger codebases where skip_unused optimization has an effect.

Reproduction Steps

struct QuadVertex {
    pos [2]f32
    uv  [2]f32
}

// This function is never called
pub fn make_quad_vertices(x f32, y f32, w f32, h f32) [4]QuadVertex {
    return [
        QuadVertex{pos: [x, y]!, uv: [f32(0), 0]!},
        QuadVertex{pos: [x + w, y]!, uv: [f32(1), 0]!},
        QuadVertex{pos: [x + w, y + h]!, uv: [f32(1), 1]!},
        QuadVertex{pos: [x, y + h]!, uv: [f32(0), 1]!},
    ]!
}

fn main() {
    // make_quad_vertices is intentionally not called
    println('hello')
}

Note: This may not reproduce in a small isolated file. The bug manifests in larger codebases (e.g., 57k+ lines) where skip_unused actively removes unused code.

Expected Behavior

The code should compile successfully. If a function is unused, its return type wrapper struct should either:

1. Not be generated, OR
2. Have its element type also generated

Current Behavior

C compilation fails with:

error: ';' expected (got "main__QuadVertex")

The generated C code contains:

// In BEGIN_array_fixed_return_structs section:
struct _v_Array_fixed_main__QuadVertex_4 {
    main__QuadVertex ret_arr[4];  // ERROR: main__QuadVertex not defined!
};

But main__QuadVertex is never defined because skip_unused determined it was unused.

Possible Solution

In write_array_fixed_return_types() in vlib/v/gen/c/cgen.v, skip generating wrapper structs when the element type is a non-builtin struct that isn't marked as used:

elem_sym := g.table.sym(info.elem_type)
if g.pref.skip_unused && elem_sym.kind == .struct && !elem_sym.is_builtin()
    && elem_sym.idx !in g.table.used_features.used_syms {
    continue
}

Additional Information/Context

• The bug only affects functions that RETURN fixed arrays of custom structs
• Functions that take fixed arrays as PARAMETERS work fine
• Using -no-skip-unused flag works around the issue
• Builtin element types (e.g., [4]int, [4]f32) are not affected

V version

V 0.5.0 c3b924c

Environment details (OS name and version, etc.)

V full version	V 0.5.0 c3b924c.c2aa96a
OS	linux, "Arch Linux" (VM)
Processor	12 cpus, 64bit, little endian, AMD Ryzen Threadripper 7960X 24-Cores
Memory	1.95GB/47GB
V executable	/home/jesusa/code/v/v/v
V last modified time	2026-01-25 17:52:50
V home dir	OK, value: /home/jesusa/code/v/v
VMODULES	OK, value: /home/jesusa/.vmodules
VTMP	OK, value: /tmp/v_1000
Current working dir	OK, value: /home/jesusa/code/v/vrasta
Git version	git version 2.52.0
V git status	weekly.2026.04-20-gc2aa96a2-dirty
.git/config present	true
cc version	cc (GCC) 15.2.1 20260103
gcc version	gcc (GCC) 15.2.1 20260103
clang version	clang version 21.1.6
tcc version	tcc version 0.9.28rc 2025-02-13 HEAD@f8bd136d (x86_64 Linux)
tcc git status	thirdparty-linux-amd64 696c1d84
emcc version	N/A
glibc version	ldd (GNU libc) 2.42

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