Suggest adding a return type for async functions

compiler/rustc_hir_typeck/src/fn_ctxt/_impl.rs

@@ -921,6 +921,22 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 kind: hir::ImplItemKind::Fn(ref sig, ..),
                 ..
             }) => Some((&sig.decl, ident, false)),
+            Node::Expr(&hir::Expr {
+                hir_id,
+                kind: hir::ExprKind::Closure(..),
+                ..
+            }) if let Some(Node::Expr(&hir::Expr {
+                hir_id,
+                kind: hir::ExprKind::Call(..),
+                ..
+            })) = self.tcx.hir().find_parent(hir_id) &&
+            let Some(Node::Item(&hir::Item {
+                ident,
+                kind: hir::ItemKind::Fn(ref sig, ..),
+                ..
+            })) = self.tcx.hir().find_parent(hir_id) => {
+                Some((&sig.decl, ident, ident.name != sym::main))
+            },
             _ => None,
         }
     }

compiler/rustc_hir_typeck/src/fn_ctxt/suggestions.rs

@@ -704,10 +704,38 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 }
             }
             hir::FnRetTy::Return(ty) => {
+                let span = ty.span;
+
+                if let hir::TyKind::OpaqueDef(item_id, ..) = ty.kind
+                && let hir::Node::Item(hir::Item {
+                    kind: hir::ItemKind::OpaqueTy(op_ty),
+                    ..
+                }) = self.tcx.hir().get(item_id.hir_id())
+                && let hir::OpaqueTy {
+                    bounds: [bound], ..
+                } = op_ty
+                && let hir::GenericBound::LangItemTrait(
+                    hir::LangItem::Future, _, _, generic_args) = bound
+                && let hir::GenericArgs { bindings: [ty_binding], .. } = generic_args
+                && let hir::TypeBinding { kind, .. } = ty_binding
+                && let hir::TypeBindingKind::Equality { term } = kind
+                && let hir::Term::Ty(term_ty) = term {
+                    // Check if async function's return type was omitted.
+                    // Don't emit suggestions if the found type is `impl Future<...>`.
+                    debug!("suggest_missing_return_type: found = {:?}", found);
+                    if found.is_suggestable(self.tcx, false) {
+                        if term_ty.span.is_empty() {
+                            err.subdiagnostic(AddReturnTypeSuggestion::Add { span, found: found.to_string() });
+                            return true;
+                        } else {
+                            err.subdiagnostic(ExpectedReturnTypeLabel::Other { span, expected });
+                        }
+                    }
+                }
+
                 // Only point to return type if the expected type is the return type, as if they
                 // are not, the expectation must have been caused by something else.
                 debug!("suggest_missing_return_type: return type {:?} node {:?}", ty, ty.kind);
-                let span = ty.span;
                 let ty = self.astconv().ast_ty_to_ty(ty);
                 debug!("suggest_missing_return_type: return type {:?}", ty);
                 debug!("suggest_missing_return_type: expected type {:?}", ty);

tests/ui/typeck/issue-90027-async-fn-return-suggestion.rs

@@ -0,0 +1,21 @@
+// edition:2018
+
+async fn hello() { //~ HELP try adding a return type
+    0
+    //~^ ERROR [E0308]
+}
+
+async fn world() -> () {
+    0
+    //~^ ERROR [E0308]
+}
+
+async fn suggest_await_in_async_fn_return() {
+    hello()
+    //~^ ERROR mismatched types [E0308]
+    //~| HELP consider `await`ing on the `Future`
+    //~| HELP consider using a semicolon here
+    //~| SUGGESTION .await
+}
+
+fn main() {}

tests/ui/typeck/issue-90027-async-fn-return-suggestion.stderr

@@ -0,0 +1,36 @@
+error[E0308]: mismatched types
+  --> $DIR/issue-90027-async-fn-return-suggestion.rs:4:5
+   |
+LL | async fn hello() {
+   |                  - help: try adding a return type: `-> i32`
+LL |     0
+   |     ^ expected `()`, found integer
+
+error[E0308]: mismatched types
+  --> $DIR/issue-90027-async-fn-return-suggestion.rs:9:5
+   |
+LL | async fn world() -> () {
+   |                     -- expected `()` because of return type
+LL |     0
+   |     ^ expected `()`, found integer
+
+error[E0308]: mismatched types
+  --> $DIR/issue-90027-async-fn-return-suggestion.rs:14:5
+   |
+LL |     hello()
+   |     ^^^^^^^ expected `()`, found opaque type
+   |
+   = note: expected unit type `()`
+            found opaque type `impl Future<Output = ()>`
+help: consider `await`ing on the `Future`
+   |
+LL |     hello().await
+   |            ++++++
+help: consider using a semicolon here
+   |
+LL |     hello();
+   |            +
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0308`.