Fix lifetime safety bug of AsPyRef::as_ref(). (#876)

* Fix lifetime safety bug of AsPyRef::as_ref().

Fixes #875.

* Add test for AsPyRef's lifetimes.

src/instance.rs

@@ -167,7 +167,7 @@ impl<T> Py<T> {
 pub trait AsPyRef: Sized {
     type Target;
     /// Return reference to object.
-    fn as_ref(&self, py: Python<'_>) -> &Self::Target;
+    fn as_ref<'p>(&'p self, py: Python<'p>) -> &'p Self::Target;
 }
 
 impl<T> AsPyRef for Py<T>
@@ -175,7 +175,7 @@ where
     T: PyTypeInfo,
 {
     type Target = T::AsRefTarget;
-    fn as_ref(&self, _py: Python) -> &Self::Target {
+    fn as_ref<'p>(&'p self, _py: Python<'p>) -> &'p Self::Target {
         let any = self as *const Py<T> as *const PyAny;
         unsafe { PyDowncastImpl::unchecked_downcast(&*any) }
     }

src/object.rs

@@ -267,7 +267,7 @@ impl PyObject {
 
 impl AsPyRef for PyObject {
     type Target = PyAny;
-    fn as_ref(&self, _py: Python) -> &PyAny {
+    fn as_ref<'p>(&'p self, _py: Python<'p>) -> &'p PyAny {
         unsafe { &*(self as *const _ as *const PyAny) }
     }
 }

tests/test_compile_error.rs

@@ -7,6 +7,7 @@ fn test_compile_errors() {
     t.compile_fail("tests/ui/invalid_pymethod_names.rs");
     t.compile_fail("tests/ui/missing_clone.rs");
     t.compile_fail("tests/ui/reject_generics.rs");
+    t.compile_fail("tests/ui/wrong_aspyref_lifetimes.rs");
     // Since the current minimum nightly(2020-01-20) has a different error message,
     // we skip this test.
     // TODO(kngwyu): Remove this `if` when we update minimum nightly.

tests/ui/wrong_aspyref_lifetimes.rs

@@ -0,0 +1,10 @@
+use pyo3::{types::PyDict, AsPyRef, Py, PyNativeType, Python};
+
+fn main() {
+    let gil = Python::acquire_gil();
+    let dict: Py<PyDict> = PyDict::new(gil.python()).into();
+    let dict: &PyDict = dict.as_ref(gil.python());
+    drop(gil);
+
+    let _py: Python = dict.py(); // Obtain a Python<'p> without GIL.
+}

tests/ui/wrong_aspyref_lifetimes.stderr

@@ -0,0 +1,10 @@
+error[E0505]: cannot move out of `gil` because it is borrowed
+ --> $DIR/wrong_aspyref_lifetimes.rs:7:10
+  |
+6 |     let dict: &PyDict = dict.as_ref(gil.python());
+  |                                     --- borrow of `gil` occurs here
+7 |     drop(gil);
+  |          ^^^ move out of `gil` occurs here
+8 | 
+9 |     let _py: Python = dict.py(); // Obtain a Python<'p> without GIL.
+  |                       ---- borrow later used here