Infer async block return type from future expectation

compiler/rustc_hir_typeck/src/closure.rs

@@ -293,7 +293,10 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         let gen_trait = tcx.lang_items().gen_trait();
         let is_gen = gen_trait == Some(trait_def_id);
 
-        if !is_fn && !is_gen {
+        let future_trait = tcx.lang_items().future_trait();
+        let is_future = future_trait == Some(trait_def_id);
+
+        if !(is_fn || is_gen || is_future) {
             debug!("not fn or generator");
             return None;
         }
@@ -305,6 +308,11 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             return None;
         }
 
+        // Since this is a return parameter type it is safe to unwrap.
+        let ret_param_ty = projection.skip_binder().term.ty().unwrap();
+        let ret_param_ty = self.resolve_vars_if_possible(ret_param_ty);
+        debug!(?ret_param_ty);
+
         let input_tys = if is_fn {
             let arg_param_ty = projection.skip_binder().projection_ty.substs.type_at(1);
             let arg_param_ty = self.resolve_vars_if_possible(arg_param_ty);
@@ -314,17 +322,27 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 &ty::Tuple(tys) => tys,
                 _ => return None,
             }
+        } else if is_future {
+            // HACK: Skip infer vars to `ui/generic-associated-types/issue-89008.rs` pass.
+            // Otherwise, we end up with inferring the closure signature to be
+            // `fn() -> Empty<Repr>` instead of `fn() -> Self::LineStream<'a, Repr>` and
+            // opaque type inference gets bungled. Similarly, skip opaques, because we don't
+            // replace them with infer vars, and opaque type inference gets bungled in
+            // `async fn ..() -> impl Trait {}` cases.
+            if ret_param_ty.is_ty_var() || ret_param_ty.has_opaque_types() {
+                return None;
+            }
+
+            let resume_ty_def_id = self.tcx.require_lang_item(hir::LangItem::ResumeTy, cause_span);
+            self.tcx.mk_type_list(&[self
+                .tcx
+                .mk_adt(self.tcx.adt_def(resume_ty_def_id), ty::List::empty())])
         } else {
             // Generators with a `()` resume type may be defined with 0 or 1 explicit arguments,
             // else they must have exactly 1 argument. For now though, just give up in this case.
             return None;
         };
 
-        // Since this is a return parameter type it is safe to unwrap.
-        let ret_param_ty = projection.skip_binder().term.ty().unwrap();
-        let ret_param_ty = self.resolve_vars_if_possible(ret_param_ty);
-        debug!(?ret_param_ty);
-
         let sig = projection.rebind(self.tcx.mk_fn_sig(
             input_tys,
             ret_param_ty,

tests/ui/async-await/expectation.rs

@@ -0,0 +1,11 @@
+// check-pass
+// edition: 2021
+
+use std::fmt::Debug;
+use std::future::Future;
+
+fn needs_future(_: impl Future<Output = Box<dyn Debug>>) {}
+
+fn main() {
+    needs_future(async { Box::new(()) })
+}

tests/ui/impl-trait/issues/issue-78722.rs

@@ -7,8 +7,8 @@ type F = impl core::future::Future<Output = u8>;
 struct Bug {
     V1: [(); {
         fn concrete_use() -> F {
-            //~^ ERROR to be a future that resolves to `u8`, but it resolves to `()`
             async {}
+            //~^ ERROR mismatched types
         }
         let f: F = async { 1 };
         //~^ ERROR `async` blocks are not allowed in constants

tests/ui/impl-trait/issues/issue-78722.stderr

@@ -7,13 +7,13 @@ LL |         let f: F = async { 1 };
    = note: see issue #85368 <https://github.com/rust-lang/rust/issues/85368> for more information
    = help: add `#![feature(const_async_blocks)]` to the crate attributes to enable
 
-error[E0271]: expected `[async block@$DIR/issue-78722.rs:11:13: 11:21]` to be a future that resolves to `u8`, but it resolves to `()`
-  --> $DIR/issue-78722.rs:9:30
+error[E0308]: mismatched types
+  --> $DIR/issue-78722.rs:10:19
    |
-LL |         fn concrete_use() -> F {
-   |                              ^ expected `()`, found `u8`
+LL |             async {}
+   |                   ^^ expected `u8`, found `()`
 
 error: aborting due to 2 previous errors
 
-Some errors have detailed explanations: E0271, E0658.
-For more information about an error, try `rustc --explain E0271`.
+Some errors have detailed explanations: E0308, E0658.
+For more information about an error, try `rustc --explain E0308`.

tests/ui/traits/new-solver/async.fail.stderr

@@ -1,17 +1,9 @@
-error[E0271]: expected `[async block@$DIR/async.rs:12:17: 12:25]` to be a future that resolves to `i32`, but it resolves to `()`
-  --> $DIR/async.rs:12:17
+error[E0308]: mismatched types
+  --> $DIR/async.rs:12:23
    |
 LL |     needs_async(async {});
-   |     ----------- ^^^^^^^^ expected `i32`, found `()`
-   |     |
-   |     required by a bound introduced by this call
-   |
-note: required by a bound in `needs_async`
-  --> $DIR/async.rs:8:31
-   |
-LL | fn needs_async(_: impl Future<Output = i32>) {}
-   |                               ^^^^^^^^^^^^ required by this bound in `needs_async`
+   |                       ^^ expected `i32`, found `()`
 
 error: aborting due to previous error
 
-For more information about this error, try `rustc --explain E0271`.
+For more information about this error, try `rustc --explain E0308`.

tests/ui/traits/new-solver/async.rs

@@ -10,7 +10,7 @@ fn needs_async(_: impl Future<Output = i32>) {}
 #[cfg(fail)]
 fn main() {
     needs_async(async {});
-    //[fail]~^ ERROR to be a future that resolves to `i32`, but it resolves to `()`
+    //[fail]~^ mismatched types
 }
 
 #[cfg(pass)]