Suggest to await future before ? operator

src/librustc_middle/hir/map/mod.rs

@@ -390,11 +390,7 @@ impl<'hir> Map<'hir> {
     /// Given a `HirId`, returns the `BodyId` associated with it,
     /// if the node is a body owner, otherwise returns `None`.
     pub fn maybe_body_owned_by(&self, hir_id: HirId) -> Option<BodyId> {
-        if let Some(node) = self.find(hir_id) {
-            associated_body(node)
-        } else {
-            bug!("no entry for id `{}`", hir_id)
-        }
+        if let Some(node) = self.find(hir_id) { associated_body(node) } else { None }
     }
 
     /// Given a body owner's id, returns the `BodyId` associated with it.

src/librustc_trait_selection/traits/error_reporting/mod.rs

@@ -400,6 +400,7 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
                         self.suggest_remove_reference(&obligation, &mut err, &trait_ref);
                         self.suggest_semicolon_removal(&obligation, &mut err, span, &trait_ref);
                         self.note_version_mismatch(&mut err, &trait_ref);
+                        self.suggest_await_before_try(&mut err, &obligation, &trait_ref, span);
                         if self.suggest_impl_trait(&mut err, span, &obligation, &trait_ref) {
                             err.emit();
                             return;

src/librustc_trait_selection/traits/error_reporting/suggestions.rs

@@ -1,8 +1,10 @@
 use super::{
     EvaluationResult, Obligation, ObligationCause, ObligationCauseCode, PredicateObligation,
+    SelectionContext,
 };
 
 use crate::infer::InferCtxt;
+use crate::traits::normalize_projection_type;
 
 use rustc_errors::{error_code, struct_span_err, Applicability, DiagnosticBuilder, Style};
 use rustc_hir as hir;
@@ -150,6 +152,15 @@ pub trait InferCtxtExt<'tcx> {
         T: fmt::Display;
 
     fn suggest_new_overflow_limit(&self, err: &mut DiagnosticBuilder<'_>);
+
+    /// Suggest to await before try: future? => future.await?
+    fn suggest_await_before_try(
+        &self,
+        err: &mut DiagnosticBuilder<'_>,
+        obligation: &PredicateObligation<'tcx>,
+        trait_ref: &ty::Binder<ty::TraitRef<'tcx>>,
+        span: Span,
+    );
 }
 
 fn predicate_constraint(generics: &hir::Generics<'_>, pred: String) -> (Span, String) {
@@ -1765,6 +1776,95 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
             suggested_limit, self.tcx.crate_name,
         ));
     }
+
+    fn suggest_await_before_try(
+        &self,
+        err: &mut DiagnosticBuilder<'_>,
+        obligation: &PredicateObligation<'tcx>,
+        trait_ref: &ty::Binder<ty::TraitRef<'tcx>>,
+        span: Span,
+    ) {
+        debug!(
+            "suggest_await_befor_try: obligation={:?}, span={:?}, trait_ref={:?}",
+            obligation, span, trait_ref
+        );
+        let body_hir_id = obligation.cause.body_id;
+        let item_id = self.tcx.hir().get_parent_node(body_hir_id);
+
+        let mut is_future = false;
+        if let ty::Opaque(def_id, substs) = trait_ref.self_ty().kind {
+            let preds = self.tcx.predicates_of(def_id).instantiate(self.tcx, substs);
+            for p in preds.predicates {
+                if let Some(trait_ref) = p.to_opt_poly_trait_ref() {
+                    if Some(trait_ref.def_id()) == self.tcx.lang_items().future_trait() {
+                        is_future = true;
+                        break;
+                    }
+                }
+            }
+        }
+
+        if let Some(body_id) = self.tcx.hir().maybe_body_owned_by(item_id) {
+            let body = self.tcx.hir().body(body_id);
+            if let Some(hir::GeneratorKind::Async(_)) = body.generator_kind {
+                let future_trait = self.tcx.lang_items().future_trait().unwrap();
+                let item_def_id = self
+                    .tcx
+                    .associated_items(future_trait)
+                    .in_definition_order()
+                    .next()
+                    .unwrap()
+                    .def_id;
+                debug!("trait_ref_self_ty: {:?}", trait_ref.self_ty());
+                // `<T as Future>::Output`
+                let projection_ty = ty::ProjectionTy {
+                    // `T`
+                    substs: self.tcx.mk_substs_trait(
+                        trait_ref.self_ty(),
+                        self.fresh_substs_for_item(span, item_def_id),
+                    ),
+                    // `Future::Output`
+                    item_def_id,
+                };
+
+                let mut selcx = SelectionContext::new(self);
+
+                let mut obligations = vec![];
+                let normalized_ty = normalize_projection_type(
+                    &mut selcx,
+                    obligation.param_env,
+                    projection_ty,
+                    obligation.cause.clone(),
+                    0,
+                    &mut obligations,
+                );
+
+                debug!(
+                    "suggest_await_befor_try: normalized_projection_type {:?}",
+                    self.resolve_vars_if_possible(&normalized_ty)
+                );
+                let try_obligation = self.mk_obligation_for_def_id(
+                    trait_ref.def_id(),
+                    normalized_ty,
+                    obligation.cause.clone(),
+                    obligation.param_env,
+                );
+                debug!("suggest_await_befor_try: try_trait_obligation {:?}", try_obligation);
+                if self.predicate_may_hold(&try_obligation) && is_future {
+                    if let Ok(snippet) = self.tcx.sess.source_map().span_to_snippet(span) {
+                        if snippet.ends_with('?') {
+                            err.span_suggestion(
+                                span,
+                                "consider using `.await` here",
+                                format!("{}.await?", snippet.trim_end_matches('?')),
+                                Applicability::MaybeIncorrect,
+                            );
+                        }
+                    }
+                }
+            }
+        }
+    }
 }
 
 /// Collect all the returned expressions within the input expression.

src/librustc_typeck/check/mod.rs

@@ -5289,6 +5289,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         expected: Ty<'tcx>,
         found: Ty<'tcx>,
     ) {
+        debug!("suggest_missing_await: expr={:?} expected={:?}, found={:?}", expr, expected, found);
         // `.await` is not permitted outside of `async` bodies, so don't bother to suggest if the
         // body isn't `async`.
         let item_id = self.tcx().hir().get_parent_node(self.body_id);
@@ -5306,22 +5307,25 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                     .next()
                     .unwrap()
                     .def_id;
+                // `<T as Future>::Output`
+                let projection_ty = ty::ProjectionTy {
+                    // `T`
+                    substs: self
+                        .tcx
+                        .mk_substs_trait(found, self.fresh_substs_for_item(sp, item_def_id)),
+                    // `Future::Output`
+                    item_def_id,
+                };
+
                 let predicate =
                     ty::Predicate::Projection(ty::Binder::bind(ty::ProjectionPredicate {
-                        // `<T as Future>::Output`
-                        projection_ty: ty::ProjectionTy {
-                            // `T`
-                            substs: self.tcx.mk_substs_trait(
-                                found,
-                                self.fresh_substs_for_item(sp, item_def_id),
-                            ),
-                            // `Future::Output`
-                            item_def_id,
-                        },
+                        projection_ty,
                         ty: expected,
                     }));
                 let obligation = traits::Obligation::new(self.misc(sp), self.param_env, predicate);
+
                 debug!("suggest_missing_await: trying obligation {:?}", obligation);
+
                 if self.infcx.predicate_may_hold(&obligation) {
                     debug!("suggest_missing_await: obligation held: {:?}", obligation);
                     if let Ok(code) = self.sess().source_map().span_to_snippet(sp) {

src/test/ui/async-await/await-keyword/incorrect-syntax-suggestions.stderr

@@ -237,7 +237,10 @@ error[E0277]: the `?` operator can only be applied to values that implement `std
   --> $DIR/incorrect-syntax-suggestions.rs:16:19
    |
 LL |     let _ = await bar()?;
-   |                   ^^^^^^ the `?` operator cannot be applied to type `impl std::future::Future`
+   |                   ^^^^^^
+   |                   |
+   |                   the `?` operator cannot be applied to type `impl std::future::Future`
+   |                   help: consider using `.await` here: `bar().await?`
    |
    = help: the trait `std::ops::Try` is not implemented for `impl std::future::Future`
    = note: required by `std::ops::Try::into_result`

src/test/ui/async-await/issue-61076.rs

@@ -0,0 +1,12 @@
+// edition:2018
+
+async fn foo() -> Result<(), ()> {
+    Ok(())
+}
+
+async fn bar() -> Result<(), ()> {
+    foo()?; //~ ERROR the `?` operator can only be applied to values that implement `std::ops::Try`
+    Ok(())
+}
+
+fn main() {}

src/test/ui/async-await/issue-61076.stderr

@@ -0,0 +1,15 @@
+error[E0277]: the `?` operator can only be applied to values that implement `std::ops::Try`
+  --> $DIR/issue-61076.rs:8:5
+   |
+LL |     foo()?;
+   |     ^^^^^^
+   |     |
+   |     the `?` operator cannot be applied to type `impl std::future::Future`
+   |     help: consider using `.await` here: `foo().await?`
+   |
+   = help: the trait `std::ops::Try` is not implemented for `impl std::future::Future`
+   = note: required by `std::ops::Try::into_result`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0277`.