restore TryFromPyCell for 0.18 branch

src/derive_utils.rs

@@ -4,7 +4,29 @@
 
 //! Functionality for the code generated by the derive backend
 
-use crate::{types::PyModule, Python};
+use crate::{types::PyModule, PyCell, PyClass, PyErr, Python};
+
+/// A trait for types that can be borrowed from a cell.
+///
+/// This serves to unify the use of `PyRef` and `PyRefMut` in automatically
+/// derived code, since both types can be obtained from a `PyCell`.
+#[doc(hidden)]
+pub trait TryFromPyCell<'a, T: PyClass>: Sized {
+    type Error: Into<PyErr>;
+    fn try_from_pycell(cell: &'a crate::PyCell<T>) -> Result<Self, Self::Error>;
+}
+
+impl<'a, T, R> TryFromPyCell<'a, T> for R
+where
+    T: 'a + PyClass,
+    R: std::convert::TryFrom<&'a PyCell<T>>,
+    R::Error: Into<PyErr>,
+{
+    type Error = R::Error;
+    fn try_from_pycell(cell: &'a crate::PyCell<T>) -> Result<Self, Self::Error> {
+        <R as std::convert::TryFrom<&'a PyCell<T>>>::try_from(cell)
+    }
+}
 
 /// Enum to abstract over the arguments of Python function wrappers.
 pub enum PyFunctionArguments<'a> {