Improve documented workaround for Tokio integration (#5736)

Document the two constraints of using Tokio futures in Slint, and how to work around them.

Fixes #5733

Co-authored-by: Olivier Goffart <olivier.goffart@slint.dev>

api/rs/slint/Cargo.toml

@@ -200,6 +200,8 @@ slint-build = { path = "../build" }
 i-slint-backend-testing = {  path = "../../../internal/backends/testing", features = ["internal"] }
 serde_json = { workspace = true }
 serde = { workspace = true }
+tokio = { version = "1", features = ["rt-multi-thread", "macros", "sync", "net", "io-util"]}
+async-compat = { version = "0.2.4" }
 
 [target.'cfg(target_os = "linux")'.dependencies]
 # this line is there to add the "enable" feature by default, but only on linux

api/rs/slint/tests/tokio.rs

@@ -0,0 +1,34 @@
+// Copyright © SixtyFPS GmbH <info@slint.dev>
+// SPDX-License-Identifier: GPL-3.0-only OR LicenseRef-Slint-Royalty-free-2.0 OR LicenseRef-Slint-Software-3.0
+
+#[test]
+fn tokio_poll_with_compat() {
+    i_slint_backend_testing::init_integration_test_with_mock_time();
+    use std::io::Write;
+
+    let listener = std::net::TcpListener::bind("127.0.0.1:0").unwrap();
+    let local_addr = listener.local_addr().unwrap();
+    let server = std::thread::spawn(move || {
+        let mut stream = listener.incoming().next().unwrap().unwrap();
+        stream.write("Hello World".as_bytes()).unwrap();
+    });
+
+    let slint_future = async move {
+        for _ in 0..1000 {
+            tokio::task::consume_budget().await;
+        }
+
+        use tokio::io::AsyncReadExt;
+        let mut stream = tokio::net::TcpStream::connect(local_addr).await.unwrap();
+        let mut data = Vec::new();
+        stream.read_to_end(&mut data).await.unwrap();
+        assert_eq!(data, "Hello World".as_bytes());
+        slint::quit_event_loop().unwrap();
+    };
+
+    slint::spawn_local(async_compat::Compat::new(slint_future)).unwrap();
+
+    slint::run_event_loop_until_quit().unwrap();
+
+    server.join().unwrap();
+}

api/rs/slint/tests/tokio_block_in_place.rs

@@ -0,0 +1,34 @@
+// Copyright © SixtyFPS GmbH <info@slint.dev>
+// SPDX-License-Identifier: GPL-3.0-only OR LicenseRef-Slint-Royalty-free-2.0 OR LicenseRef-Slint-Software-3.0
+
+#[tokio::test(flavor = "multi_thread", worker_threads = 1)]
+async fn tokio_poll_with_block_in_place() {
+    i_slint_backend_testing::init_integration_test_with_mock_time();
+    use std::io::Write;
+
+    let listener = std::net::TcpListener::bind("127.0.0.1:0").unwrap();
+    let local_addr = listener.local_addr().unwrap();
+    let server = std::thread::spawn(move || {
+        let mut stream = listener.incoming().next().unwrap().unwrap();
+        stream.write("Hello World".as_bytes()).unwrap();
+    });
+
+    let slint_future = async move {
+        for _ in 0..1000 {
+            tokio::task::consume_budget().await;
+        }
+
+        use tokio::io::AsyncReadExt;
+        let mut stream = tokio::net::TcpStream::connect(local_addr).await.unwrap();
+        let mut data = Vec::new();
+        stream.read_to_end(&mut data).await.unwrap();
+        assert_eq!(data, "Hello World".as_bytes());
+        slint::quit_event_loop().unwrap();
+    };
+
+    slint::spawn_local(slint_future).unwrap();
+
+    tokio::task::block_in_place(slint::run_event_loop_until_quit).unwrap();
+
+    server.join().unwrap();
+}