Proof-of-concept Rust wrapper

This contains Rust wrappers around enough of the Python API to implement Rust versions of the `battery_client` and `run_scanner` examples. The goal is to gather feedback on the approach, and of course to show that it is possible.

The module structure mirrors that of the Python. The Rust API is not optimally Rust-y, but given the constraints of everything having to delegate to Python, it's at least usable.

Notably, this does not yet solve the packaging problem: users must have an appropriate virtualenv, libpython, etc. [PyOxidizer](https://github.com/indygreg/PyOxidizer) may be a viable path there.

rust/src/wrapper/transport.rs

@@ -0,0 +1,72 @@
+// Copyright 2023 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+//! HCI packet transport
+
+use pyo3::{intern, types::PyModule, PyObject, PyResult, Python};
+
+/// A source/sink pair for HCI packet I/O.
+///
+/// See <https://google.github.io/bumble/transports/index.html>.
+pub struct Transport(PyObject);
+
+impl Transport {
+    /// Open a new Transport for the provided spec, e.g. `"usb:0"` or `"android-netsim"`.
+    pub async fn open(transport_spec: impl Into<String>) -> PyResult<Self> {
+        Python::with_gil(|py| {
+            PyModule::import(py, intern!(py, "bumble.transport"))?
+                .call_method1(intern!(py, "open_transport"), (transport_spec.into(),))
+                .and_then(pyo3_asyncio::tokio::into_future)
+        })?
+        .await
+        .map(Self)
+    }
+
+    /// Close the transport.
+    pub async fn close(&mut self) -> PyResult<()> {
+        Python::with_gil(|py| {
+            self.0
+                .call_method0(py, intern!(py, "close"))
+                .and_then(|coroutine| pyo3_asyncio::tokio::into_future(coroutine.as_ref(py)))
+        })?
+        .await
+        .map(|_| ())
+    }
+
+    /// Returns the source half of the transport.
+    pub fn source(&self) -> PyResult<Source> {
+        Python::with_gil(|py| self.0.getattr(py, intern!(py, "source"))).map(Source)
+    }
+
+    /// Returns the sink half of the transport.
+    pub fn sink(&self) -> PyResult<Sink> {
+        Python::with_gil(|py| self.0.getattr(py, intern!(py, "sink"))).map(Sink)
+    }
+}
+
+impl Drop for Transport {
+    fn drop(&mut self) {
+        // can't await in a Drop impl, but we can at least spawn a task to do it
+        let obj = self.0.clone();
+        tokio::spawn(async move { Self(obj).close().await });
+    }
+}
+
+/// The source side of a [Transport].
+#[derive(Clone)]
+pub struct Source(pub(crate) PyObject);
+
+/// The sink side of a [Transport].
+#[derive(Clone)]
+pub struct Sink(pub(crate) PyObject);