Fix windows verbatim paths (#1295)

crates/memofs/Cargo.toml

@@ -16,6 +16,7 @@ homepage = "https://github.com/rojo-rbx/rojo/tree/master/memofs"
 
 [dependencies]
 crossbeam-channel = "0.5.12"
+dunce = "1.0.5"
 fs-err = "2.11.0"
 notify = "4.0.17"
 serde = { version = "1.0.197", features = ["derive"] }

crates/memofs/src/lib.rs

@@ -644,13 +644,49 @@ mod test {
 
         let vfs = Vfs::new(StdBackend::new().unwrap());
         let canonicalized = vfs.canonicalize(&file_path).unwrap();
-        assert_eq!(canonicalized, file_path.canonicalize().unwrap());
+        assert_eq!(canonicalized, dunce::canonicalize(&file_path).unwrap());
         assert_eq!(
             vfs.read_to_string(&canonicalized).unwrap().to_string(),
             contents.to_string()
         );
     }
 
+    #[test]
+    #[cfg(windows)]
+    fn canonicalize_std_backend_not_verbatim() {
+        use std::path::{Component, Prefix};
+
+        let dir = tempfile::tempdir().unwrap();
+        let file_path = dir.path().join("file.txt");
+        fs_err::write(&file_path, "hello").unwrap();
+
+        let vfs = Vfs::new(StdBackend::new().unwrap());
+        let canonicalized = vfs.canonicalize(&file_path).unwrap();
+
+        let is_verbatim = matches!(
+            canonicalized.components().next(),
+            Some(Component::Prefix(prefix)) if matches!(
+                prefix.kind(),
+                Prefix::Verbatim(_) | Prefix::VerbatimDisk(_) | Prefix::VerbatimUNC(_, _)
+            )
+        );
+        assert!(
+            !is_verbatim,
+            "expected a non-verbatim path, got {:?}",
+            canonicalized
+        );
+
+        // Joining a relative parent path must preserve the `..` segment. On a
+        // verbatim path Rust would drop it lexically, which is the root cause
+        // of the bug.
+        let joined = canonicalized.join("..").join("sibling");
+        assert!(
+            joined.components().any(|c| c == Component::ParentDir),
+            "`..` should be preserved when joining onto {:?}",
+            canonicalized
+        );
+    }
+
     #[test]
     fn canonicalize_std_backend_missing_errors() {
         let dir = tempfile::tempdir().unwrap();

crates/memofs/src/std_backend.rs

@@ -107,7 +107,7 @@ impl VfsBackend for StdBackend {
     }
 
     fn canonicalize(&mut self, path: &Path) -> io::Result<PathBuf> {
-        fs_err::canonicalize(path)
+        dunce::canonicalize(path)
     }
 
     fn event_receiver(&self) -> crossbeam_channel::Receiver<VfsEvent> {