Rollup merge of #71948 - csmoe:issue-61076, r=oli-obk

Suggest to await future before ? operator

Closes https://github.com/rust-lang/rust/issues/71811
cc #61076

3 files changed, 143 insertions, 1 deletions

diff --git a/src/librustc_trait_selection/traits/error_reporting/mod.rs b/src/librustc_trait_selection/traits/error_reporting/mod.rs
index 5a63a693652..6bd4455e1e3 100644
--- a/src/librustc_trait_selection/traits/error_reporting/mod.rs
+++ b/src/librustc_trait_selection/traits/error_reporting/mod.rs
@@ -402,6 +402,7 @@ 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;
diff --git a/src/librustc_trait_selection/traits/error_reporting/suggestions.rs b/src/librustc_trait_selection/traits/error_reporting/suggestions.rs
index 5c4ba40bf38..503daf56b49 100644
--- a/src/librustc_trait_selection/traits/error_reporting/suggestions.rs
+++ b/src/librustc_trait_selection/traits/error_reporting/suggestions.rs
@@ -1,14 +1,17 @@
 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;
 use rustc_hir::def::DefKind;
 use rustc_hir::def_id::DefId;
 use rustc_hir::intravisit::Visitor;
+use rustc_hir::lang_items;
 use rustc_hir::{AsyncGeneratorKind, GeneratorKind, Node};
 use rustc_middle::ty::TypeckTables;
 use rustc_middle::ty::{
@@ -150,6 +153,15 @@ 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) {
@@ -1822,6 +1834,95 @@ 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={:?}, trait_ref_self_ty={:?}",
+            obligation,
+            span,
+            trait_ref,
+            trait_ref.self_ty()
+        );
+        let body_hir_id = obligation.cause.body_id;
+        let item_id = self.tcx.hir().get_parent_node(body_hir_id);
+
+        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.require_lang_item(lang_items::FutureTraitLangItem, None);
+
+                let self_ty = self.resolve_vars_if_possible(&trait_ref.self_ty());
+
+                let impls_future = self.tcx.type_implements_trait((
+                    future_trait,
+                    self_ty,
+                    ty::List::empty(),
+                    obligation.param_env,
+                ));
+
+                let item_def_id = self
+                    .tcx
+                    .associated_items(future_trait)
+                    .in_definition_order()
+                    .next()
+                    .unwrap()
+                    .def_id;
+                // `<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) && impls_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.
diff --git a/src/librustc_trait_selection/traits/mod.rs b/src/librustc_trait_selection/traits/mod.rs
index 778430fc2ca..9592f93ce2e 100644
--- a/src/librustc_trait_selection/traits/mod.rs
+++ b/src/librustc_trait_selection/traits/mod.rs
@@ -31,7 +31,9 @@ use rustc_hir::def_id::DefId;
 use rustc_middle::middle::region;
 use rustc_middle::ty::fold::TypeFoldable;
 use rustc_middle::ty::subst::{InternalSubsts, SubstsRef};
-use rustc_middle::ty::{self, GenericParamDefKind, ToPredicate, Ty, TyCtxt, WithConstness};
+use rustc_middle::ty::{
+    self, GenericParamDefKind, ParamEnv, ToPredicate, Ty, TyCtxt, WithConstness,
+};
 use rustc_span::Span;
 
 use std::fmt::Debug;
@@ -523,6 +525,43 @@ fn vtable_methods<'tcx>(
     }))
 }
 
+/// Check whether a `ty` implements given trait(trait_def_id).
+///
+/// NOTE: Always return `false` for a type which needs inference.
+fn type_implements_trait<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    key: (
+        DefId,    // trait_def_id,
+        Ty<'tcx>, // type
+        SubstsRef<'tcx>,
+        ParamEnv<'tcx>,
+    ),
+) -> bool {
+    let (trait_def_id, ty, params, param_env) = key;
+
+    debug!(
+        "type_implements_trait: trait_def_id={:?}, type={:?}, params={:?}, param_env={:?}",
+        trait_def_id, ty, params, param_env
+    );
+
+    // Do not check on infer_types to avoid panic in evaluate_obligation.
+    if ty.has_infer_types() {
+        return false;
+    }
+
+    let ty = tcx.erase_regions(&ty);
+
+    let trait_ref = ty::TraitRef { def_id: trait_def_id, substs: tcx.mk_substs_trait(ty, params) };
+
+    let obligation = Obligation {
+        cause: ObligationCause::dummy(),
+        param_env,
+        recursion_depth: 0,
+        predicate: trait_ref.without_const().to_predicate(),
+    };
+    tcx.infer_ctxt().enter(|infcx| infcx.predicate_must_hold_modulo_regions(&obligation))
+}
+
 pub fn provide(providers: &mut ty::query::Providers<'_>) {
     object_safety::provide(providers);
     *providers = ty::query::Providers {
@@ -531,6 +570,7 @@ pub fn provide(providers: &mut ty::query::Providers<'_>) {
         codegen_fulfill_obligation: codegen::codegen_fulfill_obligation,
         vtable_methods,
         substitute_normalize_and_test_predicates,
+        type_implements_trait,
         ..*providers
     };
 }