flesh out documentation

guide/src/migration.md

@@ -206,16 +206,72 @@ PyO3 0.23 introduces preliminary support for the new free-threaded build of
 CPython 3.13. PyO3 features that implicitly assumed the existence of the GIL
 are not exposed in the free-threaded build, since they are no longer safe.
 
-If you make use of these features then you will need to use conditional
-compilation to replace them for extensions built for free-threaded
-Python. Extensions built for the free-threaded build will have the
-`Py_GIL_DISABLED` attribute defined.
+If you make use of these features then you will need to account for the
+unavailability of this API in the free-threaded build. One way to handle it is
+via conditional compilation -- extensions built for the free-threaded build will
+have the `Py_GIL_DISABLED` attribute defined.
 
 ### `GILProtected`
 
 `GILProtected` allows mutable access to static data by leveraging the GIL to
 lock concurrent access from other threads. In free-threaded python there is no
-GIL, so you will need to replace this type with some other form of locking.
+GIL, so you will need to replace this type with some other form of locking. In
+many cases, `std::sync::Atomic` or `std::sync::Mutex` will be sufficient. If the
+locks do not guard the execution of arbitrary Python code or use of the CPython
+C API then conditional compilation is likely unnecessary since `GILProtected`
+was not needed in the first place.
+
+Before:
+
+```rust,ignore
+# use pyo3::prelude::*;
+use pyo3::sync::GILProtected;
+use pyo3::types::{PyDict, PyNone};
+use std::cell::RefCell;
+
+static OBJECTS: GILProtected<RefCell<Vec<Py<PyDict>>>> =
+    GILProtected::new(RefCell::new(Vec::new()));
+
+fn main() {
+    Python::with_gil(|py: Python| {
+        let d = PyDict::new(py);
+	    // stand-in for something that executes arbitrary python code
+	    d.set_item(PyNone::get(py), PyNone::get(py)).unwrap();
+	    OBJECTS.get(py).borrow_mut().push(d.unbind());
+    });
+}
+```
+
+After:
+
+```rust
+use pyo3::prelude::*;
+#[cfg(not(Py_GIL_DISABLED))]
+use pyo3::sync::GILProtected;
+use pyo3::types::{PyDict, PyNone};
+#[cfg(not(Py_GIL_DISABLED))]
+use std::cell::RefCell;
+#[cfg(Py_GIL_DISABLED)]
+use std::sync::Mutex;
+
+#[cfg(not(Py_GIL_DISABLED))]
+static OBJECTS: GILProtected<RefCell<Vec<Py<PyDict>>>> =
+    GILProtected::new(RefCell::new(Vec::new()));
+#[cfg(Py_GIL_DISABLED)]
+static OBJECTS: Mutex<Vec<Py<PyDict>>> = Mutex::new(Vec::new());
+
+fn main() {
+    Python::with_gil(|py| {
+	    let d = PyDict::new(py);
+	    // stand-in for something that executes arbitrary python code
+	    d.set_item(PyNone::get(py), PyNone::get(py)).unwrap();
+	    #[cfg(not(Py_GIL_DISABLED))]
+        OBJECTS.get(py).borrow_mut().push(d.unbind());
+	    #[cfg(Py_GIL_DISABLED)]
+	    OBJECTS.lock().unwrap().push(d.unbind());
+    });
+}
+```
 
 </details>
 

src/sync.rs

@@ -19,6 +19,8 @@ use crate::PyVisit;
 /// It ensures that only one thread at a time can access the inner value via shared references.
 /// It can be combined with interior mutability to obtain mutable references.
 ///
+/// This type is not defined for extensions built against the free-threaded CPython ABI.
+///
 /// # Example
 ///
 /// Combining `GILProtected` with `RefCell` enables mutable access to static data: