Auto merge of #47956 - retep998:is-nibbles, r=BurntSushi

This is the ideal FileType on Windows. You may not like it, but this is what peak performance looks like.

Theoretically this would fix https://github.com/rust-lang/rust/issues/46484

The current iteration of this PR should not cause existing code to break, but instead merely improves handling around reparse points. Specifically...

* Reparse points are considered to be symbolic links if they have the name surrogate bit set. Name surrogates are reparse points that effectively act like symbolic links, redirecting you to a different directory/file. By checking for this bit instead of specific tags, we become much more general in our handling of reparse points, including those added by third parties.
* If something is a reparse point but does not have the name surrogate bit set, then we ignore the fact that it is a reparse point because it is actually a file or directory directly there, despite having additional handling by drivers due to the reparse point.
* For everything which is not a symbolic link (including non-surrogate reparse points) we report whether it is a directory or a file based on the presence of the directory attribute bit.
* Notably this still preserves invariant that when `is_symlink` returns `true`, both `is_dir` and `is_file` will return `false`. The potential for breakage was far too high.
* Adds an unstable `FileTypeExt` to allow users to determine whether a symbolic link is a directory or a file, since `FileType` by design is incapable of reporting this information.

src/libstd/sys/windows/ext/fs.rs

@@ -445,6 +445,24 @@ impl MetadataExt for Metadata {
     fn file_size(&self) -> u64 { self.as_inner().size() }
 }
 
+/// Add support for the Windows specific fact that a symbolic link knows whether it is a file
+/// or directory.
+#[unstable(feature = "windows_file_type_ext", issue = "0")]
+pub trait FileTypeExt {
+    /// Returns whether this file type is a symbolic link that is also a directory.
+    #[unstable(feature = "windows_file_type_ext", issue = "0")]
+    fn is_symlink_dir(&self) -> bool;
+    /// Returns whether this file type is a symbolic link that is also a file.
+    #[unstable(feature = "windows_file_type_ext", issue = "0")]
+    fn is_symlink_file(&self) -> bool;
+}
+
+#[unstable(feature = "windows_file_type_ext", issue = "0")]
+impl FileTypeExt for fs::FileType {
+    fn is_symlink_dir(&self) -> bool { self.as_inner().is_symlink_dir() }
+    fn is_symlink_file(&self) -> bool { self.as_inner().is_symlink_file() }
+}
+
 /// Creates a new file symbolic link on the filesystem.
 ///
 /// The `dst` path will be a file symbolic link pointing to the `src`

src/libstd/sys/windows/fs.rs

@@ -38,8 +38,9 @@ pub struct FileAttr {
 }
 
 #[derive(Copy, Clone, PartialEq, Eq, Hash, Debug)]
-pub enum FileType {
-    Dir, File, SymlinkFile, SymlinkDir, ReparsePoint, MountPoint,
+pub struct FileType {
+    attributes: c::DWORD,
+    reparse_tag: c::DWORD,
 }
 
 pub struct ReadDir {
@@ -516,30 +517,34 @@ impl FilePermissions {
 
 impl FileType {
     fn new(attrs: c::DWORD, reparse_tag: c::DWORD) -> FileType {
-        match (attrs & c::FILE_ATTRIBUTE_DIRECTORY != 0,
-               attrs & c::FILE_ATTRIBUTE_REPARSE_POINT != 0,
-               reparse_tag) {
-            (false, false, _) => FileType::File,
-            (true, false, _) => FileType::Dir,
-            (false, true, c::IO_REPARSE_TAG_SYMLINK) => FileType::SymlinkFile,
-            (true, true, c::IO_REPARSE_TAG_SYMLINK) => FileType::SymlinkDir,
-            (true, true, c::IO_REPARSE_TAG_MOUNT_POINT) => FileType::MountPoint,
-            (_, true, _) => FileType::ReparsePoint,
-            // Note: if a _file_ has a reparse tag of the type IO_REPARSE_TAG_MOUNT_POINT it is
-            // invalid, as junctions always have to be dirs. We set the filetype to ReparsePoint
-            // to indicate it is something symlink-like, but not something you can follow.
+        FileType {
+            attributes: attrs,
+            reparse_tag: reparse_tag,
         }
     }
-
-    pub fn is_dir(&self) -> bool { *self == FileType::Dir }
-    pub fn is_file(&self) -> bool { *self == FileType::File }
+    pub fn is_dir(&self) -> bool {
+        !self.is_symlink() && self.is_directory()
+    }
+    pub fn is_file(&self) -> bool {
+        !self.is_symlink() && !self.is_directory()
+    }
     pub fn is_symlink(&self) -> bool {
-        *self == FileType::SymlinkFile ||
-        *self == FileType::SymlinkDir ||
-        *self == FileType::MountPoint
+        self.is_reparse_point() && self.is_reparse_tag_name_surrogate()
     }
     pub fn is_symlink_dir(&self) -> bool {
-        *self == FileType::SymlinkDir || *self == FileType::MountPoint
+        self.is_symlink() && self.is_directory()
+    }
+    pub fn is_symlink_file(&self) -> bool {
+        self.is_symlink() && !self.is_directory()
+    }
+    fn is_directory(&self) -> bool {
+        self.attributes & c::FILE_ATTRIBUTE_DIRECTORY != 0
+    }
+    fn is_reparse_point(&self) -> bool {
+        self.attributes & c::FILE_ATTRIBUTE_REPARSE_POINT != 0
+    }
+    fn is_reparse_tag_name_surrogate(&self) -> bool {
+        self.reparse_tag & 0x20000000 != 0
     }
 }