[red-knot]: Add a VendoredFileSystem implementation (#11863)

Co-authored-by: Micha Reiser <micha@reiser.io>

crates/ruff_db/src/file_revision.rs

@@ -0,0 +1,69 @@
+/// A number representing the revision of a file.
+///
+/// Two revisions that don't compare equal signify that the file has been modified.
+/// Revisions aren't guaranteed to be monotonically increasing or in any specific order.
+///
+/// Possible revisions are:
+/// * The last modification time of the file.
+/// * The hash of the file's content.
+/// * The revision as it comes from an external system, for example the LSP.
+#[derive(Copy, Clone, Debug, Eq, PartialEq)]
+pub struct FileRevision(u128);
+
+impl FileRevision {
+    pub fn new(value: u128) -> Self {
+        Self(value)
+    }
+
+    pub const fn zero() -> Self {
+        Self(0)
+    }
+
+    #[must_use]
+    pub fn as_u128(self) -> u128 {
+        self.0
+    }
+}
+
+impl From<u128> for FileRevision {
+    fn from(value: u128) -> Self {
+        FileRevision(value)
+    }
+}
+
+impl From<u64> for FileRevision {
+    fn from(value: u64) -> Self {
+        FileRevision(u128::from(value))
+    }
+}
+
+impl From<filetime::FileTime> for FileRevision {
+    fn from(value: filetime::FileTime) -> Self {
+        let seconds = value.seconds() as u128;
+        let seconds = seconds << 64;
+        let nanos = u128::from(value.nanoseconds());
+
+        FileRevision(seconds | nanos)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use filetime::FileTime;
+
+    use super::*;
+
+    #[test]
+    fn revision_from_file_time() {
+        let file_time = FileTime::now();
+        let revision = FileRevision::from(file_time);
+
+        let revision = revision.as_u128();
+
+        let nano = revision & 0xFFFF_FFFF_FFFF_FFFF;
+        let seconds = revision >> 64;
+
+        assert_eq!(file_time.nanoseconds(), nano as u32);
+        assert_eq!(file_time.seconds(), seconds as i64);
+    }
+}