diff --git a/fish-rust/src/wutil/fileid.rs b/fish-rust/src/wutil/fileid.rs
new file mode 100644
index 000000000..943a89f38
--- /dev/null
+++ b/fish-rust/src/wutil/fileid.rs
@@ -0,0 +1,85 @@
+use crate::wutil::{wstat, wstr};
+use std::cmp::Ordering;
+use std::fs::{File, Metadata};
+use std::os::fd::RawFd;
+
+use std::os::fd::{FromRawFd, IntoRawFd};
+#[cfg(target_os = "linux")]
+use std::os::linux::fs::MetadataExt;
+#[cfg(target_os = "macos")]
+use std::os::macos::fs::MetadataExt;
+#[cfg(not(any(target_os = "macos", target_os = "linux")))]
+use std::os::unix::fs::MetadataExt;
+
+/// Struct for representing a file's inode. We use this to detect and avoid symlink loops, among
+/// other things. While an inode / dev pair is sufficient to distinguish co-existing files, Linux  
+/// seems to aggressively re-use inodes, so it cannot determine if a file has been deleted (ABA  
+/// problem). Therefore we include richer information.
+#[derive(Debug, Copy, Clone, PartialEq, Eq, PartialOrd, Ord)]
+pub struct FileId {
+    pub device: u64,
+    pub inode: u64,
+    pub size: u64,
+    pub change_seconds: i64,
+    pub change_nanoseconds: i64,
+    pub mod_seconds: i64,
+    pub mod_nanoseconds: i64,
+}
+
+impl FileId {
+    pub fn from_stat(buf: Metadata) -> Self {
+        // These "into()" calls are because the various fields have different types
+        // on different platforms.
+        #[allow(clippy::useless_conversion)]
+        FileId {
+            device: buf.st_dev(),
+            inode: buf.st_ino(),
+            size: buf.st_size(),
+            change_seconds: buf.st_ctime().into(),
+            change_nanoseconds: buf.st_ctime_nsec().into(),
+            mod_seconds: buf.st_mtime().into(),
+            mod_nanoseconds: buf.st_mtime_nsec().into(),
+        }
+    }
+
+    pub fn older_than(&self, rhs: &FileId) -> bool {
+        match (self.change_seconds, self.change_nanoseconds)
+            .cmp(&(rhs.change_seconds, rhs.change_nanoseconds))
+        {
+            Ordering::Less => true,
+            Ordering::Equal | Ordering::Greater => false,
+        }
+    }
+}
+
+pub const INVALID_FILE_ID: FileId = FileId {
+    device: u64::MAX,
+    inode: u64::MAX,
+    size: u64::MAX,
+    change_seconds: i64::MIN,
+    change_nanoseconds: -1,
+    mod_seconds: i64::MIN,
+    mod_nanoseconds: -1,
+};
+
+/// Get a FileID corresponding to a raw file descriptor, or INVALID_FILE_ID if it fails.
+pub fn file_id_for_fd(fd: RawFd) -> FileId {
+    // Safety: we just want fstat(). Rust makes this stupidly hard.
+    // The only way to get fstat from an fd is to use a File as an intermediary,
+    // but File assumes ownership; so we have to use into_raw_fd() to release it.
+    let file = unsafe { File::from_raw_fd(fd) };
+    let res = file
+        .metadata()
+        .map(FileId::from_stat)
+        .unwrap_or(INVALID_FILE_ID);
+    let fd2 = file.into_raw_fd();
+    assert_eq!(fd, fd2);
+    res
+}
+
+/// Get a FileID corresponding to a path, or INVALID_FILE_ID if it fails.
+pub fn file_id_for_path(path: &wstr) -> FileId {
+    wstat(path)
+        .map(FileId::from_stat)
+        .unwrap_or(INVALID_FILE_ID)
+}
diff --git a/fish-rust/src/wutil/mod.rs b/fish-rust/src/wutil/mod.rs
index 63e77b341..c8ac0938c 100644
--- a/fish-rust/src/wutil/mod.rs
+++ b/fish-rust/src/wutil/mod.rs
@@ -1,5 +1,6 @@
 pub mod encoding;
 pub mod errors;
+pub mod fileid;
 pub mod gettext;
 pub mod printf;
 pub mod wcstod;