hir_ty: use async ret type for inference inside async bodies

crates/hir_ty/src/infer.rs

@@ -558,7 +558,13 @@ impl<'a> InferenceContext<'a> {
 
             self.infer_pat(*pat, &ty, BindingMode::default());
         }
-        let return_ty = self.make_ty_with_mode(&data.ret_type, ImplTraitLoweringMode::Disallowed); // FIXME implement RPIT
+        let error_ty = &TypeRef::Error;
+        let return_ty = if data.is_async() {
+            data.async_ret_type.as_deref().unwrap_or(error_ty)
+        } else {
+            &*data.ret_type
+        };
+        let return_ty = self.make_ty_with_mode(return_ty, ImplTraitLoweringMode::Disallowed); // FIXME implement RPIT
         self.return_ty = return_ty;
     }
 

crates/hir_ty/src/tests/traits.rs

@@ -3660,3 +3660,52 @@ impl foo::Foo for u32 {
         "#]],
     );
 }
+
+#[test]
+fn infer_async_ret_type() {
+    check_types(
+        r#"
+//- /main.rs crate:main deps:core
+
+enum Result<T, E> {
+    Ok(T),
+    Err(E),
+}
+
+use Result::*;
+
+
+struct Fooey;
+
+impl Fooey {
+    fn collect<B: Convert>(self) -> B {
+        B::new()
+    }
+}
+
+trait Convert {
+    fn new() -> Self;
+}
+impl Convert for u32 {
+    fn new() -> Self {
+        0
+    }
+}
+
+async fn get_accounts() -> Result<u32, ()> {
+    let ret = Fooey.collect();
+    //                      ^ u32
+    Ok(ret)
+}
+
+//- /core.rs crate:core
+#[prelude_import] use future::*;
+mod future {
+    #[lang = "future_trait"]
+    trait Future {
+        type Output;
+    }
+}
+"#,
+    );
+}