pyfunction: better error message with `async fn`

pyo3-macros-backend/src/pyfunction.rs

@@ -8,7 +8,7 @@ use crate::{
     deprecations::Deprecations,
     method::{self, FnArg, FnSpec},
     pymethod::{check_generic, get_arg_names, impl_arg_params},
-    utils,
+    utils::{self, ensure_not_async_fn},
 };
 use proc_macro2::{Span, TokenStream};
 use quote::{format_ident, quote};
@@ -349,6 +349,7 @@ pub fn impl_wrap_pyfunction(
     options: PyFunctionOptions,
 ) -> syn::Result<(Ident, TokenStream)> {
     check_generic(&func.sig)?;
+    ensure_not_async_fn(&func.sig)?;
 
     let python_name = options
         .name

pyo3-macros-backend/src/pymethod.rs

@@ -1,3 +1,4 @@
+use crate::utils::ensure_not_async_fn;
 // Copyright (c) 2017-present PyO3 Project and Contributors
 use crate::{attributes::FromPyWithAttribute, konst::ConstSpec};
 use crate::{deprecations::Deprecations, utils};
@@ -28,6 +29,7 @@ pub fn gen_py_method(
     options: PyFunctionOptions,
 ) -> Result<GeneratedPyMethod> {
     check_generic(sig)?;
+    ensure_not_async_fn(sig)?;
     let spec = FnSpec::parse(sig, &mut *meth_attrs, options)?;
 
     Ok(match &spec.tp {

pyo3-macros-backend/src/utils.rs

@@ -164,3 +164,14 @@ pub fn get_doc(
 
     Ok(syn::LitStr::new(&doc, span))
 }
+
+pub fn ensure_not_async_fn(sig: &syn::Signature) -> syn::Result<()> {
+    if let Some(asyncness) = &sig.asyncness {
+        bail_spanned!(
+            asyncness.span() => "`async fn` is not yet supported for Python functions.\n\n\
+            Additional crates such as `pyo3-asyncio` can be used to integrate async Rust and \
+            Python. For more information, see https://github.com/PyO3/pyo3/issues/1632"
+        );
+    };
+    Ok(())
+}

tests/ui/invalid_pyfunctions.rs

@@ -6,4 +6,7 @@ fn generic_function<T>(value: T) {}
 #[pyfunction]
 fn impl_trait_function(impl_trait: impl AsRef<PyAny>) {}
 
+#[pyfunction]
+async fn async_function() {}
+
 fn main() {}

tests/ui/invalid_pyfunctions.stderr

@@ -9,3 +9,11 @@ error: Python functions cannot have `impl Trait` arguments
   |
 7 | fn impl_trait_function(impl_trait: impl AsRef<PyAny>) {}
   |                                    ^^^^
+
+error: `async fn` is not yet supported for Python functions.
+
+Additional crates such as `pyo3-asyncio` can be used to integrate async Rust and Python. For more information, see https://github.com/PyO3/pyo3/issues/1632
+  --> $DIR/invalid_pyfunctions.rs:10:1
+   |
+10 | async fn async_function() {}
+   | ^^^^^

tests/ui/invalid_pymethods.rs

@@ -103,4 +103,9 @@ impl MyClass {
     fn impl_trait_method_second_arg(&self, impl_trait: impl AsRef<PyAny>) {}
 }
 
+#[pymethods]
+impl MyClass {
+    async fn async_method(&self) {}
+}
+
 fn main() {}

tests/ui/invalid_pymethods.stderr

@@ -87,3 +87,11 @@ error: Python functions cannot have `impl Trait` arguments
     |
 103 |     fn impl_trait_method_second_arg(&self, impl_trait: impl AsRef<PyAny>) {}
     |                                                        ^^^^
+
+error: `async fn` is not yet supported for Python functions.
+
+Additional crates such as `pyo3-asyncio` can be used to integrate async Rust and Python. For more information, see https://github.com/PyO3/pyo3/issues/1632
+   --> $DIR/invalid_pymethods.rs:108:5
+    |
+108 |     async fn async_method(&self) {}
+    |     ^^^^^