https://twitter.com/hikari_no_yume/status/1373048051212894215

who-needs-glsl-anyway.c

#include "who-needs-glsl-anyway.h"
#include <stdio.h>
int main(void)
{
    vec4 v = { {1.5, 2.5, 3.5, 4.5} };
    printf("%f, %f, %f, %f\n", v.rg.x, v.xy.t, v.stp.z, v.w);
    for (int i = 3; i >= 0; i--) {
        printf("%f\n", v._[i]);
    }
}

who-needs-glsl-anyway.h

#define DEF_VEC2(_0, _1) \
    struct { \
        float _0; \
        float _1; \
    };

typedef union {
    float _[2];
    union {
        DEF_VEC2(r, g)
        DEF_VEC2(x, y)
        DEF_VEC2(s, t)
    };
} vec2;

#define DEF_VEC3(_0, _1, _2) \
    struct { \
        union { \
            DEF_VEC2(_0, _1) \
            vec2 _0##_1; \
        }; \
        float _2; \
    };

typedef union {
    float _[3];
    union {
        DEF_VEC3(r, g, b)
        DEF_VEC3(x, y, z)
        DEF_VEC3(s, t, p)
    };
} vec3;

#define DEF_VEC4(_0, _1, _2, _3) \
    struct { \
        union { \
            DEF_VEC3(_0, _1, _2) \
            vec3 _0##_1##_2; \
        }; \
        float _3; \
    };

typedef union {
    float _[4];
    union {
        DEF_VEC4(r, g, b, a)
        DEF_VEC4(x, y, z, w)
        DEF_VEC4(s, t, p, q)
    };
} vec4;