fix lowering results for async exports

Previously, we were either dropping the result too early
(e.g. `wit_bindgen_rt::async_support::ErrorContext`) or not at all
(e.g. `String` and `Vec`) due to the lowering code expecting to pass ownership
to the caller, which would later free memory using a post-return function.  But
async exports don't have post-return functions; they use `task.return` instead,
and they should free any memory (and/or drop handles) after `task.return`
returns.  So now we do that!

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

Author: dicej

crates/core/src/abi.rs

@@ -1477,8 +1477,9 @@ impl<'a, B: Bindgen> Generator<'a, B> {
         // Lowering parameters calling a wasm import means
         // we don't need to pass ownership, but we pass
         // ownership in all other cases.
-        match (self.variant, self.lift_lower) {
-            (AbiVariant::GuestImport, LiftLower::LowerArgsLiftResults) => None,
+        match (self.variant, self.lift_lower, self.async_) {
+            (AbiVariant::GuestImport, LiftLower::LowerArgsLiftResults, _)
+            | (AbiVariant::GuestExport, LiftLower::LiftArgsLowerResults, true) => None,
             _ => Some("cabi_realloc"),
         }
     }

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

@@ -122,12 +122,12 @@ unsafe fn poll(state: *mut FutureState) -> Poll<()> {
 #[doc(hidden)]
 pub fn first_poll<T: 'static>(
     future: impl Future<Output = T> + 'static,
-    fun: impl FnOnce(T) + 'static,
+    fun: impl FnOnce(&T) + 'static,
 ) -> *mut u8 {
     let state = Box::into_raw(Box::new(FutureState {
         todo: 0,
         tasks: Some(
-            [Box::pin(future.map(fun)) as BoxFuture]
+            [Box::pin(future.map(|v| fun(&v))) as BoxFuture]
                 .into_iter()
                 .collect(),
         ),