Add a note for ? on future in sync function

compiler/rustc_trait_selection/src/error_reporting/traits/suggestions.rs

@@ -3594,9 +3594,6 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
         trait_pred: ty::PolyTraitPredicate<'tcx>,
         span: Span,
     ) {
-        if let Some(hir::CoroutineKind::Desugared(hir::CoroutineDesugaring::Async, _)) =
-            self.tcx.coroutine_kind(obligation.cause.body_id)
-        {
         let future_trait = self.tcx.require_lang_item(LangItem::Future, None);
 
         let self_ty = self.resolve_vars_if_possible(trait_pred.self_ty());
@@ -3634,6 +3631,8 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
             && let Ok(snippet) = self.tcx.sess.source_map().span_to_snippet(span)
             && snippet.ends_with('?')
         {
+            match self.tcx.coroutine_kind(obligation.cause.body_id) {
+                Some(hir::CoroutineKind::Desugared(hir::CoroutineDesugaring::Async, _)) => {
                     err.span_suggestion_verbose(
                         span.with_hi(span.hi() - BytePos(1)).shrink_to_hi(),
                         "consider `await`ing on the `Future`",
@@ -3641,6 +3640,19 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
                         Applicability::MaybeIncorrect,
                     );
                 }
+                _ => {
+                    let mut span: MultiSpan = span.with_lo(span.hi() - BytePos(1)).into();
+                    span.push_span_label(
+                        self.tcx.def_span(obligation.cause.body_id),
+                        "this is not `async`",
+                    );
+                    err.span_note(
+                        span,
+                        "this implements `Future` and its output type supports \
+                        `?`, but the future cannot be awaited in a synchronous function",
+                    );
+                }
+            }
         }
     }
 

tests/ui/async-await/try-in-sync.rs

@@ -0,0 +1,9 @@
+//@ edition: 2021
+
+async fn foo() -> Result<(), ()> { todo!() }
+
+fn main() -> Result<(), ()> {
+    foo()?;
+    //~^ ERROR the `?` operator can only be applied to values that implement `Try`
+    Ok(())
+}

tests/ui/async-await/try-in-sync.stderr

@@ -0,0 +1,18 @@
+error[E0277]: the `?` operator can only be applied to values that implement `Try`
+  --> $DIR/try-in-sync.rs:6:5
+   |
+LL |     foo()?;
+   |     ^^^^^^ the `?` operator cannot be applied to type `impl Future<Output = Result<(), ()>>`
+   |
+   = help: the trait `Try` is not implemented for `impl Future<Output = Result<(), ()>>`
+note: this implements `Future` and its output type supports `?`, but the future cannot be awaited in a synchronous function
+  --> $DIR/try-in-sync.rs:6:10
+   |
+LL | fn main() -> Result<(), ()> {
+   | --------------------------- this is not `async`
+LL |     foo()?;
+   |          ^
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0277`.