add async: true case to Rust codegen_tests

This ensures that all the codegen test WIT files produce compile-able bindings
with `async: true` (i.e. all imports lowered and all exports lifted using the
async ABI).  That revealed some issues involving resource methods and
constructors, as well as missing stub support, which I've resolved.

Signed-off-by: Joel Dice <joel.dice@fermyon.com>

crates/core/src/abi.rs

@@ -1030,7 +1030,7 @@ impl<'a, B: Bindgen> Generator<'a, B> {
                         // `self.return_pointer`) so we use that to read
                         // the result of the function from memory.
                         AbiVariant::GuestImport => {
-                            assert!(sig.results.is_empty());
+                            assert!(sig.results.is_empty() || self.async_);
                             self.return_pointer.take().unwrap()
                         }
 

crates/guest-rust/rt/src/async_support.rs

@@ -1,4 +1,5 @@
 #![deny(missing_docs)]
+#![allow(static_mut_refs)]
 
 use {
     futures::{
@@ -21,6 +22,8 @@ use {
     },
 };
 
+pub use futures;
+
 type BoxFuture = Pin<Box<dyn Future<Output = ()> + 'static>>;
 
 /// Represents a task created by either a call to an async-lifted export or a

crates/rust/Cargo.toml

@@ -28,6 +28,7 @@ prettyplease = { workspace = true }
 
 [dev-dependencies]
 wit-bindgen = { path = '../guest-rust' }
+wit-bindgen-rt = { path = '../guest-rust/rt' }
 test-helpers = { path = '../test-helpers' }
 # For use with the custom attributes test
 serde = { version = "1.0", features = ["derive"] }

crates/rust/src/bindgen.rs

@@ -859,24 +859,31 @@ impl Bindgen for FunctionBindgen<'_, '_> {
                 } else {
                     self.let_results(func.results.len(), results);
                 };
-                match &func.kind {
+                let constructor_type = match &func.kind {
                     FunctionKind::Freestanding => {
                         self.push_str(&format!("T::{}", to_rust_ident(&func.name)));
+                        None
                     }
                     FunctionKind::Method(_) | FunctionKind::Static(_) => {
                         self.push_str(&format!("T::{}", to_rust_ident(func.item_name())));
+                        None
                     }
                     FunctionKind::Constructor(ty) => {
-                        self.push_str(&format!(
-                            "{}::new(T::new",
-                            resolve.types[*ty]
-                                .name
-                                .as_deref()
-                                .unwrap()
-                                .to_upper_camel_case()
-                        ));
+                        let ty = resolve.types[*ty]
+                            .name
+                            .as_deref()
+                            .unwrap()
+                            .to_upper_camel_case();
+                        let call = if self.async_ {
+                            let async_support = self.gen.path_to_async_support();
+                            format!("{async_support}::futures::FutureExt::map(T::new")
+                        } else {
+                            format!("{ty}::new(T::new",)
+                        };
+                        self.push_str(&call);
+                        Some(ty)
                     }
-                }
+                };
                 self.push_str("(");
                 for (i, operand) in operands.iter().enumerate() {
                     if i > 0 {
@@ -893,8 +900,12 @@ impl Bindgen for FunctionBindgen<'_, '_> {
                     }
                 }
                 self.push_str(")");
-                if let FunctionKind::Constructor(_) = &func.kind {
-                    self.push_str(")");
+                if let Some(ty) = constructor_type {
+                    self.push_str(&if self.async_ {
+                        format!(", {ty}::new)")
+                    } else {
+                        ")".into()
+                    });
                 }
                 self.push_str(";\n");
             }

crates/rust/src/interface.rs

@@ -1169,7 +1169,7 @@ impl {stream_and_future_support}::StreamPayload for {name} {{
 
                     let resource_methods = funcs.remove(&Some(*id)).unwrap_or(Vec::new());
                     let trait_name = format!("{path}::Guest{camel}");
-                    self.generate_stub_impl(&trait_name, "", &resource_methods);
+                    self.generate_stub_impl(&trait_name, "", &resource_methods, interface);
                 }
                 format!("{path}::Guest")
             }
@@ -1180,7 +1180,7 @@ impl {stream_and_future_support}::StreamPayload for {name} {{
         };
 
         if !root_methods.is_empty() || !extra_trait_items.is_empty() {
-            self.generate_stub_impl(&guest_trait, &extra_trait_items, &root_methods);
+            self.generate_stub_impl(&guest_trait, &extra_trait_items, &root_methods, interface);
         }
     }
 
@@ -1189,6 +1189,7 @@ impl {stream_and_future_support}::StreamPayload for {name} {{
         trait_name: &str,
         extra_trait_items: &str,
         funcs: &[&Function],
+        interface: Option<(InterfaceId, &WorldKey)>,
     ) {
         uwriteln!(self.src, "impl {trait_name} for Stub {{");
         self.src.push_str(extra_trait_items);
@@ -1197,7 +1198,19 @@ impl {stream_and_future_support}::StreamPayload for {name} {{
             if self.gen.skip.contains(&func.name) {
                 continue;
             }
+            let async_ = match &self.gen.opts.async_ {
+                AsyncConfig::None => false,
+                AsyncConfig::All => true,
+                AsyncConfig::Some { exports, .. } => {
+                    exports.contains(&if let Some((_, key)) = interface {
+                        format!("{}#{}", self.resolve.name_world_key(key), func.name)
+                    } else {
+                        func.name.clone()
+                    })
+                }
+            };
             let mut sig = FnSig {
+                async_,
                 use_item_name: true,
                 private: true,
                 ..Default::default()
@@ -1206,8 +1219,14 @@ impl {stream_and_future_support}::StreamPayload for {name} {{
                 sig.self_arg = Some("&self".into());
                 sig.self_is_first_param = true;
             }
-            self.print_signature(func, true, &sig, true);
-            self.src.push_str("{ unreachable!() }\n");
+            self.print_signature(func, true, &sig, false);
+            let call = if async_ {
+                let async_support = self.path_to_async_support();
+                format!("{{ #[allow(unreachable_code)]{async_support}::futures::future::ready(unreachable!()) }}\n")
+            } else {
+                "{ unreachable!() }\n".into()
+            };
+            self.src.push_str(&call);
         }
 
         self.src.push_str("}\n");
@@ -1273,7 +1292,11 @@ impl {stream_and_future_support}::StreamPayload for {name} {{
     ) -> Vec<String> {
         let params = self.print_docs_and_params(func, params_owned, sig, use_async_sugar);
         if let FunctionKind::Constructor(_) = &func.kind {
-            self.push_str(" -> Self")
+            self.push_str(if sig.async_ && !use_async_sugar {
+                " -> impl ::core::future::Future<Output = Self>"
+            } else {
+                " -> Self"
+            })
         } else {
             self.print_results(&func.results, sig.async_ && !use_async_sugar);
         }

crates/rust/tests/codegen.rs

@@ -51,6 +51,20 @@ mod codegen_tests {
                     #[test]
                     fn works() {}
                 }
+
+                #[cfg(feature = "async")]
+                mod async_ {
+                    wit_bindgen::generate!({
+                        path: $test,
+                        stubs,
+                        export_prefix: "[async]",
+                        generate_all,
+                        async: true
+                    });
+
+                    #[test]
+                    fn works() {}
+                }
             }
 
         };