diff --git a/mercurial/cext/revlog.c b/mercurial/cext/revlog.c
--- a/mercurial/cext/revlog.c
+++ b/mercurial/cext/revlog.c
@@ -2878,6 +2878,12 @@
 	if (nullentry)
 		PyObject_GC_UnTrack(nullentry);
 
+	void *caps = PyCapsule_New(
+	    HgRevlogIndex_GetParents,
+	    "mercurial.cext.parsers.index_get_parents_CAPI", NULL);
+	if (caps != NULL)
+		PyModule_AddObject(mod, "index_get_parents_CAPI", caps);
+
 #ifdef WITH_RUST
 	rustlazyancestorsType.tp_new = PyType_GenericNew;
 	if (PyType_Ready(&rustlazyancestorsType) < 0)
diff --git a/rust/hg-cpython/src/cindex.rs b/rust/hg-cpython/src/cindex.rs
new file mode 100644
--- /dev/null
+++ b/rust/hg-cpython/src/cindex.rs
@@ -0,0 +1,95 @@
+// cindex.rs
+//
+// Copyright 2018 Georges Racinet <gracinet@anybox.fr>
+//
+// This software may be used and distributed according to the terms of the
+// GNU General Public License version 2 or any later version.
+
+//! Bindings to use the Index defined by the parsers C extension
+//!
+//! Ideally, we should use an Index entirely implemented in Rust,
+//! but this will take some time to get there.
+#[cfg(feature = "python27")]
+extern crate python27_sys as python_sys;
+#[cfg(feature = "python3")]
+extern crate python3_sys as python_sys;
+
+use self::python_sys::PyCapsule_Import;
+use cpython::{PyErr, PyObject, PyResult, Python};
+use hg::{Graph, GraphError, Revision};
+use libc::{c_int, ssize_t};
+use std::ffi::CStr;
+use std::mem::transmute;
+
+type IndexParentsFn = unsafe extern "C" fn(
+    index: *mut python_sys::PyObject,
+    rev: ssize_t,
+    ps: *mut [c_int; 2],
+    max_rev: c_int,
+) -> c_int;
+
+/// A Graph backed up by objects and functions from revlog.c
+///
+/// This implementation of the Graph trait, relies on (pointers to)
+/// - the C index object (`index` member)
+/// - the `index_get_parents()` function (`parents` member)
+pub struct Index {
+    index: PyObject,
+    parents: IndexParentsFn,
+}
+
+impl Index {
+    pub fn new(py: Python, index: PyObject) -> PyResult<Self> {
+        Ok(Index {
+            index: index,
+            parents: decapsule_parents_fn(py)?,
+        })
+    }
+}
+
+impl Graph for Index {
+    /// wrap a call to the C extern parents function
+    fn parents(&self, rev: Revision) -> Result<[Revision; 2], GraphError> {
+        let mut res: [c_int; 2] = [0; 2];
+        let code = unsafe {
+            (self.parents)(
+                self.index.as_ptr(),
+                rev as ssize_t,
+                &mut res as *mut [c_int; 2],
+                rev,
+            )
+        };
+        match code {
+            0 => Ok(res),
+            _ => Err(GraphError::ParentOutOfRange(rev)),
+        }
+    }
+}
+
+/// Return the `index_get_parents` function of the parsers C Extension module.
+///
+/// A pointer to the function is stored in the `parsers` module as a
+/// standard [Python capsule](https://docs.python.org/2/c-api/capsule.html).
+///
+/// This function retrieves the capsule and casts the function pointer
+///
+/// Casting function pointers is one of the rare cases of
+/// legitimate use cases of `mem::transmute()` (see
+/// https://doc.rust-lang.org/std/mem/fn.transmute.html of
+/// `mem::transmute()`.
+/// It is inappropriate for architectures where
+/// function and data pointer sizes differ (so-called "Harvard
+/// architectures"), but these are nowadays mostly DSPs
+/// and microcontrollers, hence out of our scope.
+fn decapsule_parents_fn(py: Python) -> PyResult<IndexParentsFn> {
+    unsafe {
+        let caps_name = CStr::from_bytes_with_nul_unchecked(
+            b"mercurial.cext.parsers.index_get_parents_CAPI\0",
+        );
+        let from_caps = PyCapsule_Import(caps_name.as_ptr(), 0);
+        if from_caps.is_null() {
+            return Err(PyErr::fetch(py));
+        }
+        Ok(transmute(from_caps))
+    }
+}
diff --git a/rust/hg-cpython/src/lib.rs b/rust/hg-cpython/src/lib.rs
--- a/rust/hg-cpython/src/lib.rs
+++ b/rust/hg-cpython/src/lib.rs
@@ -21,6 +21,7 @@
 #[macro_use]
 extern crate cpython;
 extern crate hg;
+extern crate libc;
 
 mod ancestors;
 mod exceptions;