fix symlink test on windows

Author: Qelxiros

library/std/src/sys/fs/windows.rs

@@ -1199,12 +1199,16 @@ impl Dir {
         let mut opts = OpenOptions::new();
         opts.write(true);
         let linkfile = File::open(link, &opts)?;
-        let utf16: Vec<u16> = original.iter().chain(original).copied().collect();
         let file_name_len = u16::try_from(original.len()).or(Err(TOO_LONG_ERR))?;
+        let utf16 = b"\\??\\"
+            .iter()
+            .map(|&n| n as u16)
+            .chain(original.iter().copied())
+            .collect::<Vec<u16>>();
         let sym_buffer = c::SYMBOLIC_LINK_REPARSE_BUFFER {
             SubstituteNameOffset: 0,
-            SubstituteNameLength: file_name_len,
-            PrintNameOffset: file_name_len,
+            SubstituteNameLength: file_name_len + 4,
+            PrintNameOffset: 4,
             PrintNameLength: file_name_len,
             Flags: if relative { c::SYMLINK_FLAG_RELATIVE } else { 0 },
             PathBuffer: 0,
@@ -1222,7 +1226,13 @@ impl Dir {
         unsafe {
             buffer.write(c::REPARSE_DATA_BUFFER {
                 ReparseTag: c::IO_REPARSE_TAG_SYMLINK,
-                ReparseDataLength: u16::try_from(size_of_val(&sym_buffer)).or(Err(TOO_LONG_ERR))?,
+                ReparseDataLength: u16::try_from(
+                    size_of::<c::REPARSE_DATA_BUFFER>()
+                        + size_of::<c::SYMBOLIC_LINK_REPARSE_BUFFER>()
+                        + usize::from(file_name_len) * 2
+                        - offset_of!(c::REPARSE_DATA_BUFFER, Reserved),
+                )
+                .or(Err(TOO_LONG_ERR))?,
                 Reserved: 0,
                 rest: (),
             });
@@ -1244,7 +1254,12 @@ impl Dir {
                 linkfile.handle.as_raw_handle(),
                 c::FSCTL_SET_REPARSE_POINT,
                 &raw const buffer as *const c_void,
-                u32::try_from(size_of_val(&buffer)).or(Err(TOO_LONG_ERR))?,
+                u32::try_from(
+                    size_of::<c::REPARSE_DATA_BUFFER>()
+                        + size_of::<c::SYMBOLIC_LINK_REPARSE_BUFFER>()
+                        + usize::from(file_name_len) * 2,
+                )
+                .or(Err(TOO_LONG_ERR))?,
                 ptr::null_mut(),
                 0,
                 ptr::null_mut(),