README.md

Compiling the C file:

gcc -o test test.c -I . -I /usr/include -lc

Compiling the Zig file throws an error:

$zig build-exe ztest.zig -I . --library c
Semantic Analysis [690/699] ztest.zig:10:13: error: type '[*c].cimport:4:11.foo_t' does not support field access
    warn(f.?.x.?.y);
            ^
zig-cache/o/KTsLMnJBj_oMpD4ScU0kKunypkMOyrUqGwh01tBqUm6BmJTDGFma2NXIgWNkcfAA/cimport.zig:378:8: error: type '[*c].cimport:4:11.foo_t' does not support field access
    f.?.x = new_bar(y);

test.c

#include <stdio.h>
#include "test.h"

int main(int argc, char* argv[]){
    foo_t *f = new_foo(3);
    printf("%d\n",f->x->y);
}

test.h

#include <stdlib.h>

typedef struct {
    int y;
} bar_t;

typedef struct {
    bar_t * x;
} foo_t;

bar_t *new_bar(int y){
    bar_t *b = (bar_t*)calloc(1,sizeof(bar_t));
    b->y = y;
    return b;
}

foo_t *new_foo(int y){
    foo_t *f = (foo_t*)calloc(1,sizeof(foo_t));
    f->x = new_bar(y);
    return f;
}

ztest.zig

const warn = @import("std").debug.warn;

const c = @cImport({
    @cInclude("test.h");
});

pub fn main() void {
    var f: [*c]c.foo_t = c.new_foo(3);
    warn(f.?.x.?.y);
}