--- a/Makefile	Sat Oct 05 10:21:34 2019 -0400
+++ b/Makefile	Mon Oct 14 16:15:19 2019 +0900
@@ -113,7 +113,7 @@
 tests:
         # Run Rust tests if cargo is installed
 	if command -v $(CARGO) >/dev/null 2>&1; then \
-		cd $(HGROOT)/rust/hg-cpython && $(CARGO) test --quiet --all; \
+		$(MAKE) rust-tests; \
 	fi
 	cd tests && $(PYTHON) run-tests.py $(TESTFLAGS)
 
@@ -127,6 +127,13 @@
         $(MAKE) -f $(HGROOT)/contrib/Makefile.python PYTHONVER=$* PREFIX=$(HGPYTHONS)/$* python )
 	cd tests && $(HGPYTHONS)/$*/bin/python run-tests.py $(TESTFLAGS)
 
+rust-tests: py_feature = $(shell $(PYTHON) -c \
+ 'import sys; print(["python27-bin", "python3-bin"][sys.version_info[0] >= 3])')
+rust-tests:
+	cd $(HGROOT)/rust/hg-cpython \
+		&& $(CARGO) test --quiet --all \
+			--no-default-features --features "$(py_feature)"
+
 check-code:
 	hg manifest | xargs python contrib/check-code.py
 
@@ -248,6 +255,7 @@
 
 .PHONY: help all local build doc cleanbutpackages clean install install-bin \
 	install-doc install-home install-home-bin install-home-doc \
-	dist dist-notests check tests check-code format-c update-pot \
+	dist dist-notests check tests rust-tests check-code format-c \
+	update-pot \
 	$(packaging_targets) \
 	osx

--- a/rust/hg-cpython/Cargo.toml	Sat Oct 05 10:21:34 2019 -0400
+++ b/rust/hg-cpython/Cargo.toml	Mon Oct 14 16:15:19 2019 +0900
@@ -11,9 +11,14 @@
 [features]
 default = ["python27"]
 
+# Features to build an extension module:
 python27 = ["cpython/python27-sys", "cpython/extension-module-2-7"]
+python3 = ["cpython/python3-sys", "cpython/extension-module"]
 
-python3 = ["cpython/python3-sys", "cpython/extension-module"]
+# Enable one of these features to build a test executable linked to libpython:
+# e.g. cargo test --no-default-features --features python27-bin
+python27-bin = ["cpython/python27-sys"]
+python3-bin = ["cpython/python3-sys"]
 
 [dependencies]
 hg-core = { path = "../hg-core" }

--- a/rust/hg-cpython/src/lib.rs	Sat Oct 05 10:21:34 2019 -0400
+++ b/rust/hg-cpython/src/lib.rs	Mon Oct 14 16:15:19 2019 +0900
@@ -72,3 +72,10 @@
     )?;
     Ok(())
 });
+
+#[cfg(not(any(feature = "python27-bin", feature = "python3-bin")))]
+#[test]
+#[ignore]
+fn libpython_must_be_linked_to_run_tests() {
+    // stub function to tell that some tests wouldn't run
+}

--- a/rust/hg-cpython/src/ref_sharing.rs	Sat Oct 05 10:21:34 2019 -0400
+++ b/rust/hg-cpython/src/ref_sharing.rs	Mon Oct 14 16:15:19 2019 +0900
@@ -490,3 +490,34 @@
         }
     };
 }
+
+#[cfg(test)]
+#[cfg(any(feature = "python27-bin", feature = "python3-bin"))]
+mod test {
+    use super::*;
+    use cpython::{GILGuard, Python};
+
+    py_class!(class Owner |py| {
+        data string: PySharedRefCell<String>;
+    });
+    py_shared_ref!(Owner, String, string, string_shared);
+
+    fn prepare_env() -> (GILGuard, Owner) {
+        let gil = Python::acquire_gil();
+        let py = gil.python();
+        let owner =
+            Owner::create_instance(py, PySharedRefCell::new("new".to_owned()))
+                .unwrap();
+        (gil, owner)
+    }
+
+    #[test]
+    fn test_borrow_mut_while_leaked() {
+        let (gil, owner) = prepare_env();
+        let py = gil.python();
+        assert!(owner.string_shared(py).borrow_mut().is_ok());
+        let _leaked = owner.string_shared(py).leak_immutable().unwrap();
+        // TODO: will be allowed
+        assert!(owner.string_shared(py).borrow_mut().is_err());
+    }
+}