[WIP, not ready for review] PyCell document

Author: kngwyu

guide/src/class.md

@@ -122,10 +122,10 @@ like [std::cell::RefCell](https://doc.rust-lang.org/std/cell/struct.RefCell.html
 
 Users who are familiar with `RefCell` can use `PyCell` just like `RefCell`.
 
-For users who doesn't know `RefCell` well, we repeat the Rust's borrowing rule here:
+For users who doesn't know `RefCell` well, here we repeat the Rust's borrowing rule:
 - At any given time, you can have either (but not both of) one mutable reference or any number of immutable references.
 - References must always be valid.
-`PyCell` ensures these borrowing rules by managing a reference counter.
+`PyCell` ensures these borrowing rules by tracking references at runtime.
 
 TODO: link to the API document
 

src/pycell.rs

@@ -1,4 +1,4 @@
-//! Traits and structs for `#[pyclass]`.
+//! Includes `PyCell` implementation.
 use crate::conversion::{AsPyPointer, FromPyPointer, ToPyObject};
 use crate::pyclass_init::PyClassInitializer;
 use crate::pyclass_slots::{PyClassDict, PyClassWeakRef};
@@ -90,12 +90,19 @@ impl<T: PyClass> PyCellInner<T> {
     }
 }
 
-// TODO(kngwyu): Mutability example
-/// `PyCell` represents the concrete layout of `T: PyClass` when it is converted
-/// to a Python class.
+/// `PyCell` is the container type for [`PyClass`](trait.PyClass.html).
 ///
-/// You can use it to test your `#[pyclass]` correctly works.
+/// From Python side, `PyCell<T>` is the concrete layout of `T: PyClass` in the Python heap,
+/// which means we can convert `*const PyClass<T>` to `*mut ffi::PyObject`.
 ///
+/// From Rust side, `PyCell<T>` is the mutable container of `T`.
+/// Since `PyCell<T: PyClass>` is always on the Python heap, we don't have the ownership of it.
+/// Thus, to mutate the data behind `&PyCell<T>` safely, we employ the
+/// [Interior Mutability Pattern](https://doc.rust-lang.org/book/ch15-05-interior-mutability.html)
+/// like [std::cell::RefCell](https://doc.rust-lang.org/std/cell/struct.RefCell.html).
+///
+/// In most cases, `PyCell` is hidden behind `#[pymethods]`.
+/// However, you can construct `&PyCell` directly to test your pyclass in Rust code.
 /// ```
 /// # use pyo3::prelude::*;
 /// # use pyo3::{py_run, PyCell};
@@ -112,6 +119,7 @@ impl<T: PyClass> PyCellInner<T> {
 ///     author: "Philip Kindred Dick",
 /// };
 /// let book_cell = PyCell::new(py, book).unwrap();
+/// // you can expose PyCell to Python snippets
 /// py_run!(py, book_cell, "assert book_cell.name[-6:] == 'Castle'");
 /// ```
 #[repr(C)]
@@ -254,6 +262,9 @@ impl<T: PyClass + fmt::Debug> fmt::Debug for PyCell<T> {
     }
 }
 
+/// Wraps a borrowed reference to a value in a `PyCell<T>`.
+///
+/// See the [`PyCell`](struct.PyCell.html) documentation for more.
 pub struct PyRef<'p, T: PyClass> {
     inner: &'p PyCellInner<T>,
 }
@@ -319,6 +330,9 @@ impl<T: PyClass + fmt::Debug> fmt::Debug for PyRef<'_, T> {
     }
 }
 
+/// Wraps a mutable borrowed reference to a value in a `PyCell<T>`.
+///
+/// See the [`PyCell`](struct.PyCell.html) documentation for more.
 pub struct PyRefMut<'p, T: PyClass> {
     inner: &'p PyCellInner<T>,
 }