Rollup merge of rust-lang#101260 - ChrisDenton:attribute-tag, r=thomcc

Use `FILE_ATTRIBUTE_TAG_INFO` to get reparse tag

I've been looking at this code recently and it just occurred to me we don't actually use the full reparse data at this point, only the tag. [`GetFileInformationByHandleEx`](https://docs.microsoft.com/en-us/windows/win32/api/winbase/nf-winbase-getfileinformationbyhandleex) can do exactly that by filling a [`FILE_ATTRIBUTE_TAG_INFO`](https://docs.microsoft.com/en-us/windows/win32/api/winbase/ns-winbase-file_attribute_tag_info) struct.

r? `````@thomcc````` since you've made changes here recently (which is why I have this code on my mind atm)

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

@@ -454,6 +454,12 @@ pub enum FILE_INFO_BY_HANDLE_CLASS {
     MaximumFileInfoByHandlesClass,
 }
 
+#[repr(C)]
+pub struct FILE_ATTRIBUTE_TAG_INFO {
+    pub FileAttributes: DWORD,
+    pub ReparseTag: DWORD,
+}
+
 #[repr(C)]
 pub struct FILE_DISPOSITION_INFO {
     pub DeleteFile: BOOLEAN,

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

@@ -326,10 +326,15 @@ impl File {
             cvt(c::GetFileInformationByHandle(self.handle.as_raw_handle(), &mut info))?;
             let mut reparse_tag = 0;
             if info.dwFileAttributes & c::FILE_ATTRIBUTE_REPARSE_POINT != 0 {
-                let mut b =
-                    Align8([MaybeUninit::<u8>::uninit(); c::MAXIMUM_REPARSE_DATA_BUFFER_SIZE]);
-                if let Ok((_, buf)) = self.reparse_point(&mut b) {
-                    reparse_tag = (*buf).ReparseTag;
+                let mut attr_tag: c::FILE_ATTRIBUTE_TAG_INFO = mem::zeroed();
+                cvt(c::GetFileInformationByHandleEx(
+                    self.handle.as_raw_handle(),
+                    c::FileAttributeTagInfo,
+                    ptr::addr_of_mut!(attr_tag).cast(),
+                    mem::size_of::<c::FILE_ATTRIBUTE_TAG_INFO>().try_into().unwrap(),
+                ))?;
+                if attr_tag.FileAttributes & c::FILE_ATTRIBUTE_REPARSE_POINT != 0 {
+                    reparse_tag = attr_tag.ReparseTag;
                 }
             }
             Ok(FileAttr {
@@ -390,10 +395,15 @@ impl File {
             attr.file_size = info.AllocationSize as u64;
             attr.number_of_links = Some(info.NumberOfLinks);
             if attr.file_type().is_reparse_point() {
-                let mut b =
-                    Align8([MaybeUninit::<u8>::uninit(); c::MAXIMUM_REPARSE_DATA_BUFFER_SIZE]);
-                if let Ok((_, buf)) = self.reparse_point(&mut b) {
-                    attr.reparse_tag = (*buf).ReparseTag;
+                let mut attr_tag: c::FILE_ATTRIBUTE_TAG_INFO = mem::zeroed();
+                cvt(c::GetFileInformationByHandleEx(
+                    self.handle.as_raw_handle(),
+                    c::FileAttributeTagInfo,
+                    ptr::addr_of_mut!(attr_tag).cast(),
+                    mem::size_of::<c::FILE_ATTRIBUTE_TAG_INFO>().try_into().unwrap(),
+                ))?;
+                if attr_tag.FileAttributes & c::FILE_ATTRIBUTE_REPARSE_POINT != 0 {
+                    reparse_tag = attr_tag.ReparseTag;
                 }
             }
             Ok(attr)