refactor: fix warnings

src/nvim/buffer.c

@@ -4086,10 +4086,8 @@ void fname_expand(buf_T *buf, char_u **ffname, char_u **sfname)
 
 #ifdef WIN32
   if (!buf->b_p_bin) {
-    char_u  *rfname;
-
     // If the file name is a shortcut file, use the file it links to.
-    rfname = os_resolve_shortcut(*ffname);
+    char_u *rfname = (char_u *)os_resolve_shortcut(*ffname);
     if (rfname != NULL) {
       xfree(*ffname);
       *ffname = rfname;

src/nvim/eval.c

@@ -14137,11 +14137,9 @@ static void f_resolve(typval_T *argvars, typval_T *rettv, FunPtr fptr)
   p = get_tv_string(&argvars[0]);
 #ifdef WIN32
   {
-    char_u  *v = NULL;
-
-    v = os_resolve_shortcut(p);
+    char *v = os_resolve_shortcut(p);
     if (v != NULL) {
-      rettv->vval.v_string = v;
+      rettv->vval.v_string = (char_u *)v;
     } else {
       rettv->vval.v_string = vim_strsave(p);
     }

src/nvim/mbyte.c

@@ -1517,7 +1517,7 @@ int utf16_to_utf8(const WCHAR *strw, char **str)
                               0,
                               strw,
                               -1,
-                              (LPSTR *)pos,
+                              pos,
                               utf8_len,
                               NULL,
                               NULL);

src/nvim/os/fs.c

@@ -424,11 +424,11 @@ ptrdiff_t os_read(const int fd, bool *ret_eof, char *const ret_buf,
   size_t read_bytes = 0;
   bool did_try_to_free = false;
   while (read_bytes != size) {
+    assert(size >= read_bytes);
     const ptrdiff_t cur_read_bytes = read(fd, ret_buf + read_bytes,
                                           size - read_bytes);
     if (cur_read_bytes > 0) {
       read_bytes += (size_t)cur_read_bytes;
-      assert(read_bytes <= size);
     }
     if (cur_read_bytes < 0) {
       const int error = os_translate_sys_error(errno);
@@ -527,6 +527,7 @@ ptrdiff_t os_write(const int fd, const char *const buf, const size_t size)
   }
   size_t written_bytes = 0;
   while (written_bytes != size) {
+    assert(size >= written_bytes);
     const ptrdiff_t cur_written_bytes = write(fd, buf + written_bytes,
                                               size - written_bytes);
     if (cur_written_bytes > 0) {
@@ -949,12 +950,12 @@ bool os_fileid_equal_fileinfo(const FileID *file_id,
 /// When "fname" is the name of a shortcut (*.lnk) resolve the file it points
 /// to and return that name in allocated memory.
 /// Otherwise NULL is returned.
-char_u * os_resolve_shortcut(char_u *fname)
+char *os_resolve_shortcut(char_u *fname)
 {
   HRESULT hr;
   IPersistFile *ppf = NULL;
   OLECHAR wsz[MAX_PATH];
-  char_u *rfname = NULL;
+  char *rfname = NULL;
   int len;
   IShellLinkW *pslw = NULL;
   WIN32_FIND_DATAW ffdw;