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

fusl/src/stdio/vswscanf.c

+#include "stdio_impl.h"
+#include "libc.h"
+#include <wchar.h>
+
+static size_t wstring_read(FILE *f, unsigned char *buf, size_t len)
+{
+        const wchar_t *src = f->cookie;
+        size_t k;
+
+        if (!src) return 0;
+
+        k = wcsrtombs((void *)f->buf, &src, f->buf_size, 0);
+        if (k==(size_t)-1) {
+                f->rpos = f->rend = 0;
+                return 0;
+        }
+
+        f->rpos = f->buf;
+        f->rend = f->buf + k;
+        f->cookie = (void *)src;
+
+        if (!len || !k) return 0;
+
+        *buf = *f->rpos++;
+        return 1;
+}
+
+int vswscanf(const wchar_t *restrict s, const wchar_t *restrict fmt, va_list ap)
+{
+        unsigned char buf[256];
+        FILE f = {
+                .buf = buf, .buf_size = sizeof buf,
+                .cookie = (void *)s,
+                .read = wstring_read, .lock = -1
+        };
+        return vfwscanf(&f, fmt, ap);
+}
+
+weak_alias(vswscanf,__isoc99_vswscanf);