--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/rust/hg-cpython/src/pybytes_deref.rs	Mon Sep 06 13:39:54 2021 +0200
@@ -0,0 +1,53 @@
+use cpython::{PyBytes, Python};
+
+/// Safe abstraction over a `PyBytes` together with the `&[u8]` slice
+/// that borrows it. Implements `Deref<Target = [u8]>`.
+///
+/// Calling `PyBytes::data` requires a GIL marker but we want to access the
+/// data in a thread that (ideally) does not need to acquire the GIL.
+/// This type allows separating the call an the use.
+///
+/// It also enables using a (wrapped) `PyBytes` in GIL-unaware generic code.
+pub struct PyBytesDeref {
+    #[allow(unused)]
+    keep_alive: PyBytes,
+
+    /// Borrows the buffer inside `self.keep_alive`,
+    /// but the borrow-checker cannot express self-referential structs.
+    data: *const [u8],
+}
+
+impl PyBytesDeref {
+    pub fn new(py: Python, bytes: PyBytes) -> Self {
+        Self {
+            data: bytes.data(py),
+            keep_alive: bytes,
+        }
+    }
+
+    pub fn unwrap(self) -> PyBytes {
+        self.keep_alive
+    }
+}
+
+impl std::ops::Deref for PyBytesDeref {
+    type Target = [u8];
+
+    fn deref(&self) -> &[u8] {
+        // Safety: the raw pointer is valid as long as the PyBytes is still
+        // alive, and the returned slice borrows `self`.
+        unsafe { &*self.data }
+    }
+}
+
+fn require_send<T: Send>() {}
+
+#[allow(unused)]
+fn static_assert_pybytes_is_send() {
+    require_send::<PyBytes>;
+}
+
+// Safety: PyBytes is Send. Raw pointers are not by default,
+// but here sending one to another thread is fine since we ensure it stays
+// valid.
+unsafe impl Send for PyBytesDeref {}