using this in godbolt to check stuff for metamod:source

packed_size_explorer.cpp

#include <stdio.h>

#ifdef _MSC_VER
#define PACKED
#pragma pack(push, 1)
#else
#define PACKED __attribute__((packed))
#endif
typedef struct PACKED thing {
    thing() : a(10), b(15) { printf("hello %d %d\n", a, b); }
    thing& operator=(thing& other) { this->a = other.a; this->b = other.b; }
    ~thing() { puts("bye"); }
    int a;
    short b;
} thing;
#ifdef _MSC_VER
#pragma pack(pop)
#endif

thing getthing(int a, int b) {
    //static_assert(sizeof(thing) == 5, "fuck!");
    thing t;
    t.a = a;
    t.b = b;
    //t.c = 5;
    //t.d = 7;
    //for (int i = 0; i < sizeof(t.b); i++) t.b[i] = b;
    return t;
}

void waaa() {
    auto t = getthing(1,2);
    printf("%d %d", t.a, t.b);
}

packed_size_explorer2.cpp

#include <stdio.h>

class Test {
public:
    Test() : Z(100) {}
    float Add(
        int a, int b,
        float c, float d, float e, float f, 
        int g, int p, int q,
        float h, float i, float j, float k,
        int l, int m, float r, int n,
        float o, float s
    ) {
        return Z + a + b + c + d + e + f + g + h + i + j + k + l + m + n + o + p + q + r + s;
    }
    int Z;
};

void waaa() {
    Test t;
    float res = t.Add(1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19);
    printf("%f\n", res);
}

packed_size_explorer3.rs

#[repr(C)]
pub struct Vectorf3 {
    pub a: f32,
    pub b: f32,
    pub c: f32,
}

#[no_mangle]
pub extern "C" fn get_vectorf3(a: i32, b: i32) -> Vectorf3 {
    Vectorf3 { a: a as f32, b: b as f32, c: 3.0 }
}

#[no_mangle]
pub extern "C" fn use_vectorf3(v: Vectorf3) -> f32 {
    v.a + v.b + v.c
}

#[no_mangle]
pub extern "C" fn thing() {
    let v = get_vectorf3(1,2);
    let sum = v.a + v.b + v.c;
    //println!("{}", sum);
}