hir_typeck: Propagate async closure output into its body

compiler/rustc_hir_typeck/src/closure.rs

@@ -62,6 +62,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         let (expected_sig, expected_kind) = match expected.to_option(self) {
             Some(ty) => self.deduce_closure_signature(
                 self.deeply_resolve_ignoring_regions_with_obligations(ty),
+                closure.def_id,
                 closure.kind,
             ),
             None => (None, None),
@@ -291,9 +292,28 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     fn deduce_closure_signature(
         &self,
         expected_ty: Ty<'tcx>,
+        closure_def_id: LocalDefId,
         closure_kind: hir::ClosureKind,
     ) -> (Option<ExpectedSig<'tcx>>, Option<ty::ClosureKind>) {
         match *expected_ty.kind() {
+            ty::Coroutine(def_id, args)
+                if def_id == closure_def_id.to_def_id()
+                    && matches!(
+                        closure_kind,
+                        hir::ClosureKind::Coroutine(hir::CoroutineKind::Desugared(
+                            hir::CoroutineDesugaring::Async,
+                            hir::CoroutineSource::Closure,
+                        ))
+                    ) =>
+            {
+                // An async closure's body is itself a coroutine. Propagate the signature
+                // inferred for that coroutine into its body.
+                let args = args.as_coroutine();
+                let sig = ty::Binder::dummy(
+                    self.tcx.mk_fn_sig_safe_rust_abi([args.resume_ty()], args.return_ty()),
+                );
+                (Some(ExpectedSig { cause_span: None, sig }), None)
+            }
             ty::Alias(_, ty::AliasTy { kind: ty::Opaque { def_id }, args, .. }) => self
                 .deduce_closure_signature_from_predicates(
                     expected_ty,

tests/ui/async-await/async-closures/async-closure-output-mismatch-issue-161619.current.stderr

@@ -0,0 +1,21 @@
+error[E0308]: mismatched types
+  --> $DIR/async-closure-output-mismatch-issue-161619.rs:16:27
+   |
+LL |     for_each(async |_| Ok(String::new()));
+   |                        -- ^^^^^^^^^^^^^ expected `()`, found `String`
+   |                        |
+   |                        arguments to this enum variant are incorrect
+   |
+help: the type constructed contains `String` due to the type of the argument passed
+  --> $DIR/async-closure-output-mismatch-issue-161619.rs:16:24
+   |
+LL |     for_each(async |_| Ok(String::new()));
+   |                        ^^^-------------^
+   |                           |
+   |                           this argument influences the type of `Ok`
+note: tuple variant defined here
+  --> $SRC_DIR/core/src/result.rs:LL:COL
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0308`.

tests/ui/async-await/async-closures/async-closure-output-mismatch-issue-161619.next.stderr

@@ -0,0 +1,21 @@
+error[E0308]: mismatched types
+  --> $DIR/async-closure-output-mismatch-issue-161619.rs:16:27
+   |
+LL |     for_each(async |_| Ok(String::new()));
+   |                        -- ^^^^^^^^^^^^^ expected `()`, found `String`
+   |                        |
+   |                        arguments to this enum variant are incorrect
+   |
+help: the type constructed contains `String` due to the type of the argument passed
+  --> $DIR/async-closure-output-mismatch-issue-161619.rs:16:24
+   |
+LL |     for_each(async |_| Ok(String::new()));
+   |                        ^^^-------------^
+   |                           |
+   |                           this argument influences the type of `Ok`
+note: tuple variant defined here
+  --> $SRC_DIR/core/src/result.rs:LL:COL
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0308`.

tests/ui/async-await/async-closures/async-closure-output-mismatch-issue-161619.rs

@@ -0,0 +1,18 @@
+//@ edition: 2024
+//@ revisions: current next
+//@ ignore-compare-mode-next-solver (explicit revisions)
+//@[next] compile-flags: -Znext-solver
+
+use std::future::Future;
+
+fn for_each<F, Fut>(_: F)
+where
+    F: FnMut(()) -> Fut,
+    Fut: Future<Output = Result<(), ()>>,
+{
+}
+
+fn main() {
+    for_each(async |_| Ok(String::new()));
+    //~^ ERROR mismatched types
+}

tests/ui/suggestions/suggest-create-closure-issue-150701.fixed

@@ -9,7 +9,6 @@ fn f(_c: impl Future<Output = ()>) {}
 
 fn main() {
     f((async || {})()); //~ ERROR: expected function, found `()`
-    //~^ ERROR: is not a future
     f(async {});
     //~^ ERROR: is not a future
 }

tests/ui/suggestions/suggest-create-closure-issue-150701.rs

@@ -9,7 +9,6 @@ fn f(_c: impl Future<Output = ()>) {}
 
 fn main() {
     f(async || {}()); //~ ERROR: expected function, found `()`
-    //~^ ERROR: is not a future
     f(async || {});
     //~^ ERROR: is not a future
 }

tests/ui/suggestions/suggest-create-closure-issue-150701.stderr

@@ -11,30 +11,15 @@ help: if you meant to create this closure and immediately call it, surround the
 LL |     f((async || {})());
    |       +           +
 
-error[E0277]: `{async closure@$DIR/suggest-create-closure-issue-150701.rs:11:7: 11:15}` is not a future
-  --> $DIR/suggest-create-closure-issue-150701.rs:11:7
-   |
-LL |     f(async || {}());
-   |     - ^^^^^^^^^^^^^ `{async closure@$DIR/suggest-create-closure-issue-150701.rs:11:7: 11:15}` is not a future
-   |     |
-   |     required by a bound introduced by this call
-   |
-   = help: the trait `Future` is not implemented for `{async closure@$DIR/suggest-create-closure-issue-150701.rs:11:7: 11:15}`
-note: required by a bound in `f`
-  --> $DIR/suggest-create-closure-issue-150701.rs:8:15
-   |
-LL | fn f(_c: impl Future<Output = ()>) {}
-   |               ^^^^^^^^^^^^^^^^^^^ required by this bound in `f`
-
-error[E0277]: `{async closure@$DIR/suggest-create-closure-issue-150701.rs:13:7: 13:15}` is not a future
-  --> $DIR/suggest-create-closure-issue-150701.rs:13:7
+error[E0277]: `{async closure@$DIR/suggest-create-closure-issue-150701.rs:12:7: 12:15}` is not a future
+  --> $DIR/suggest-create-closure-issue-150701.rs:12:7
    |
 LL |     f(async || {});
-   |     - ^^^^^^^^^^^ `{async closure@$DIR/suggest-create-closure-issue-150701.rs:13:7: 13:15}` is not a future
+   |     - ^^^^^^^^^^^ `{async closure@$DIR/suggest-create-closure-issue-150701.rs:12:7: 12:15}` is not a future
    |     |
    |     required by a bound introduced by this call
    |
-   = help: the trait `Future` is not implemented for `{async closure@$DIR/suggest-create-closure-issue-150701.rs:13:7: 13:15}`
+   = help: the trait `Future` is not implemented for `{async closure@$DIR/suggest-create-closure-issue-150701.rs:12:7: 12:15}`
 note: required by a bound in `f`
   --> $DIR/suggest-create-closure-issue-150701.rs:8:15
    |
@@ -46,7 +31,7 @@ LL -     f(async || {});
 LL +     f(async {});
    |
 
-error: aborting due to 3 previous errors
+error: aborting due to 2 previous errors
 
 Some errors have detailed explanations: E0277, E0618.
 For more information about an error, try `rustc --explain E0277`.