Make generated rust async futures Send (#1405)

The futures for generated rust async bindings were not always Send,
because the LoweredParams held a non-Send `*mut u8`. To fix this,
explicitly implement Send for LoweredParams. This makes the futures
compatible with more of the Rust ecosystem, eg. an axum webserver where
all handlers must be Send. This is safe, at least for now, because
the generated code is single threaded.

To make LoweredParams Send, change the type from a tuple to a struct
which we can explicitly mark as Send. This also requires moving it up a
little bit in the generated code outside of the trait impl.

Add a test that asserts the generated binding is Send.

Author: jellevandenhooff

crates/rust/src/interface.rs

@@ -812,6 +812,32 @@ pub mod vtable{ordinal} {{
         let sig = self
             .resolve
             .wasm_signature(AbiVariant::GuestImportAsync, func);
+
+        // Generate `type ParamsLower`
+        //
+        uwrite!(
+            self.src,
+            "
+#[derive(Copy, Clone)]
+struct ParamsLower(
+            "
+        );
+        let mut params_lower = sig.params.as_slice();
+        if sig.retptr {
+            params_lower = &params_lower[..params_lower.len() - 1];
+        }
+        for ty in params_lower {
+            self.src.push_str(wasm_type(*ty));
+            self.src.push_str(", ");
+        }
+        uwriteln!(
+            self.src,
+            "
+);
+unsafe impl Send for ParamsLower {{}}
+            "
+        );
+
         uwriteln!(
             self.src,
             "
@@ -844,16 +870,7 @@ unsafe impl<'a> _Subtask for _MySubtask<'a> {{
         }
 
         // Generate `type ParamsLower`
-        uwrite!(self.src, "type ParamsLower = (");
-        let mut params_lower = sig.params.as_slice();
-        if sig.retptr {
-            params_lower = &params_lower[..params_lower.len() - 1];
-        }
-        for ty in params_lower {
-            self.src.push_str(wasm_type(*ty));
-            self.src.push_str(", ");
-        }
-        uwriteln!(self.src, ");");
+        uwrite!(self.src, "type ParamsLower = ParamsLower;");
 
         // Generate `const ABI_LAYOUT`
         let mut heap_types = Vec::new();
@@ -962,7 +979,7 @@ unsafe fn call_import(&self, _params: Self::ParamsLower, _results: *mut u8) -> u
                 lowers.push(start);
                 param_lowers.push(name);
             }
-            lowers.push("(_ptr,)".to_string());
+            lowers.push("ParamsLower(_ptr,)".to_string());
         } else {
             let mut f = FunctionBindgen::new(self, Vec::new(), module, true);
             let mut results = Vec::new();
@@ -974,7 +991,7 @@ unsafe fn call_import(&self, _params: Self::ParamsLower, _results: *mut u8) -> u
             for result in results.iter_mut() {
                 result.push_str(",");
             }
-            let result = format!("({})", results.join(" "));
+            let result = format!("ParamsLower({})", results.join(" "));
             lowers.push(format!("unsafe {{ {} {result} }}", String::from(f.src)));
         }
 

tests/runtime-async/async/rust-lowered-send/runner.rs

@@ -0,0 +1,21 @@
+include!(env!("BINDINGS"));
+
+use std::future::Future;
+
+use crate::a::b::i::*;
+
+// Explicitly require Send.
+#[allow(dead_code)]
+fn require_send<T: Send>(_t: &T) {}
+
+// This is the type of block_on with a Send requirement added.
+pub fn block_on_require_send<T: 'static>(future: impl Future<Output = T> + Send + 'static) -> T {
+    require_send(&future);
+    wit_bindgen::block_on(future)
+}
+
+fn main() {
+    block_on_require_send(async {
+        one_argument("hello".into()).await;
+    });
+}

tests/runtime-async/async/rust-lowered-send/test.rs

@@ -0,0 +1,11 @@
+include!(env!("BINDINGS"));
+
+struct Component;
+
+export!(Component);
+
+impl crate::exports::a::b::i::Guest for Component {
+    async fn one_argument(x: String) {
+        assert_eq!(&x, "hello");
+    }
+}

tests/runtime-async/async/rust-lowered-send/test.wit

@@ -0,0 +1,13 @@
+package a:b;
+
+interface i {
+  one-argument: async func(x: string);
+}
+
+world test {
+  export i;
+}
+
+world runner {
+  import i;
+}