Add a "fork" of musl as //fusl.

fusl/src/stdio/vswprintf.c

+#include "stdio_impl.h"
+#include <limits.h>
+#include <string.h>
+#include <errno.h>
+#include <stdint.h>
+#include <wchar.h>
+
+struct cookie {
+        wchar_t *ws;
+        size_t l;
+};
+
+static size_t sw_write(FILE *f, const unsigned char *s, size_t l)
+{
+        size_t l0 = l;
+        int i = 0;
+        struct cookie *c = f->cookie;
+        if (s!=f->wbase && sw_write(f, f->wbase, f->wpos-f->wbase)==-1)
+                return -1;
+        while (c->l && l && (i=mbtowc(c->ws, (void *)s, l))>=0) {
+                s+=i;
+                l-=i;
+                c->l--;
+                c->ws++;
+        }
+        *c->ws = 0;
+        return i<0 ? i : l0;
+}
+
+int vswprintf(wchar_t *restrict s, size_t n, const wchar_t *restrict fmt, va_list ap)
+{
+        int r;
+        FILE f;
+        unsigned char buf[256];
+        struct cookie c = { s, n-1 };
+
+        memset(&f, 0, sizeof(FILE));
+        f.lbf = EOF;
+        f.write = sw_write;
+        f.buf_size = sizeof buf;
+        f.buf = buf;
+        f.lock = -1;
+        f.cookie = &c;
+        if (!n) {
+                return -1;
+        } else if (n > INT_MAX) {
+                errno = EOVERFLOW;
+                return -1;
+        }
+        r = vfwprintf(&f, fmt, ap);
+        sw_write(&f, 0, 0);
+        return r>=n ? -1 : r;
+}