guide: note existence of PyFunction::new_closure

Author: davidhewitt

guide/src/function.md

@@ -197,12 +197,6 @@ Docstring: This function adds two unsigned 64-bit integers.
 Type:      builtin_function_or_method
 ```
 
-### Closures
-
-Currently, there are no conversions between `Fn`s in Rust and callables in Python. This would
-definitely be possible and very useful, so contributions are welcome. In the meantime, you can do
-the following:
-
 ### Calling Python functions in Rust
 
 You can pass Python `def`'d functions and built-in functions to Rust functions [`PyFunction`]
@@ -217,13 +211,12 @@ with only positional args.
 
 ### Calling Rust functions in Python
 
-If you have a static function, you can expose it with `#[pyfunction]` and use [`wrap_pyfunction!`]
-to get the corresponding [`PyCFunction`]. For dynamic functions, e.g. lambdas and functions that
-were passed as arguments, you must put them in some kind of owned container, e.g. a `Box`.
-(A long-term solution will be a special container similar to wasm-bindgen's `Closure`). You can
-then use a `#[pyclass]` struct with that container as a field as a way to pass the function over
-the FFI barrier. You can even make that class callable with `__call__` so it looks like a function
-in Python code.
+The ways to convert a Rust function into a Python object vary depending on the function:
+
+- Named functions, e.g. `fn foo()`: add `#[pyfunction]` and then use [`wrap_pyfunction!`] to get the corresponding [`PyCFunction`].
+- Anonymous functions (or closures), e.g. `foo: fn()` either:
+  - use a `#[pyclass]` struct which stores the function as a field and implement `__call__` to call the stored function.
+  - use `PyFunction::new_closure` to create an object directly from the function.
 
 [`PyAny::is_callable`]: {{#PYO3_DOCS_URL}}/pyo3/struct.PyAny.html#tymethod.is_callable
 [`PyAny::call`]: {{#PYO3_DOCS_URL}}/pyo3/struct.PyAny.html#tymethod.call

guide/src/function/error_handling.md

@@ -169,14 +169,14 @@ fn parse_int(s: String) -> PyResult<usize> {
 
 The Rust compiler will not permit implementation of traits for types outside of the crate where the type is defined. (This is known as the "orphan rule".)
 
-Given a type `OtherError` which is defined in thirdparty code, there are two main strategies available to integrate it with PyO3:
+Given a type `OtherError` which is defined in third-party code, there are two main strategies available to integrate it with PyO3:
 
 - Create a newtype wrapper, e.g. `MyOtherError`. Then implement `From<MyOtherError> for PyErr` (or `PyErrArguments`), as well as `From<OtherError>` for `MyOtherError`.
 - Use Rust's Result combinators such as `map_err` to write code freely to convert `OtherError` into whatever is needed. This requires boilerplate at every usage however gives unlimited flexibility.
 
 To detail the newtype strategy a little further, the key trick is to return `Result<T, MyOtherError>` from the `#[pyfunction]`. This means that PyO3 will make use of `From<MyOtherError> for PyErr` to create Python exceptions while the `#[pyfunction]` implementation can use `?` to convert `OtherError` to `MyOtherError` automatically.
 
-The following example demonstrates this for some imaginary thirdparty crate `some_crate` with a function `get_x` returning `Result<i32, OtherError>`:
+The following example demonstrates this for some imaginary third-party crate `some_crate` with a function `get_x` returning `Result<i32, OtherError>`:
 
 ```rust
 # mod some_crate {