Documents experimental-declarative-modules feature (#3953)

* Documents experimental-declarative-modules feature

* More details on experimental-declarative-modules progress

guide/src/features.md

@@ -57,6 +57,12 @@ This feature adds support for `async fn` in `#[pyfunction]` and `#[pymethods]`.
 
 The feature has some unfinished refinements and performance improvements. To help finish this off, see [issue #1632](https://github.com/PyO3/pyo3/issues/1632) and its associated draft PRs.
 
+### `experimental-declarative-modules`
+
+This feature allows to declare Python modules using `#[pymodule] mod my_module { ... }` syntax. 
+
+The feature has some unfinished refinements and edge cases. To help finish this off, see [issue #3900](https://github.com/PyO3/pyo3/issues/3900).
+
 ### `experimental-inspect`
 
 This feature adds the `pyo3::inspect` module, as well as `IntoPy::type_output` and `FromPyObject::type_input` APIs to produce Python type "annotations" for Rust types.

guide/src/module.md

@@ -105,3 +105,55 @@ submodules by using `from parent_module import child_module`. For more informati
 [#1517](https://github.com/PyO3/pyo3/issues/1517#issuecomment-808664021).
 
 It is not necessary to add `#[pymodule]` on nested modules, which is only required on the top-level module.
+
+## Declarative modules (experimental)
+
+Another syntax based on Rust inline modules is also available to declare modules.
+The `experimental-declarative-modules` feature must be enabled to use it.
+
+For example:
+```rust
+# #[cfg(feature = "experimental-declarative-modules")]
+# mod declarative_module_test {
+use pyo3::prelude::*;
+
+#[pyfunction]
+fn double(x: usize) -> usize {
+    x * 2
+}
+
+#[pymodule]
+mod my_extension {
+    use super::*;
+
+    #[pymodule_export]
+    use super::double; // Exports the double function as part of the module
+
+    #[pyfunction] // This will be part of the module
+    fn triple(x: usize) -> usize {
+        x * 3
+    }
+
+    #[pyclass] // This will be part of the module
+    struct Unit;
+
+    #[pymodule]
+    mod submodule {
+        // This is a submodule
+    }
+
+    #[pymodule_init]
+    fn init(m: &Bound<'_, PyModule>) -> PyResult<()> {
+        // Arbitrary code to run at the module initialization
+        m.add("double2", m.getattr("double")?)?;
+        Ok(())
+    }
+}
+# }
+```
+
+Some changes are planned to this feature before stabilization, like automatically
+filling submodules into `sys.modules` to allow easier imports (see [issue #759](https://github.com/PyO3/pyo3/issues/759))
+and filling the `module` argument of inlined `#[pyclass]` automatically with the proper module name.
+Macro names might also change.
+See [issue #3900](https://github.com/PyO3/pyo3/issues/3900) to track this feature progress.