When possible, suggest fn call

src/librustc/traits/chalk_fulfill.rs

@@ -81,6 +81,7 @@ impl TraitEngine<'tcx> for FulfillmentContext<'tcx> {
                 .map(|obligation| FulfillmentError {
                     obligation: obligation.goal.clone(),
                     code: FulfillmentErrorCode::CodeAmbiguity,
+                    points_at_arg_span: false,
                 })
                 .collect();
             Err(errors)
@@ -129,6 +130,7 @@ impl TraitEngine<'tcx> for FulfillmentContext<'tcx> {
                                     code: FulfillmentErrorCode::CodeSelectionError(
                                         SelectionError::Unimplemented
                                     ),
+                                    points_at_arg_span: false,
                                 }),
                             }
                         } else {
@@ -142,6 +144,7 @@ impl TraitEngine<'tcx> for FulfillmentContext<'tcx> {
                         code: FulfillmentErrorCode::CodeSelectionError(
                             SelectionError::Unimplemented
                         ),
+                        points_at_arg_span: false,
                     })
                 }
             }

src/librustc/traits/error_reporting.rs

@@ -119,11 +119,11 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
 
     // returns if `cond` not occurring implies that `error` does not occur - i.e., that
     // `error` occurring implies that `cond` occurs.
-    fn error_implies(&self,
-                     cond: &ty::Predicate<'tcx>,
-                     error: &ty::Predicate<'tcx>)
-                     -> bool
-    {
+    fn error_implies(
+        &self,
+        cond: &ty::Predicate<'tcx>,
+        error: &ty::Predicate<'tcx>,
+    ) -> bool {
         if cond == error {
             return true
         }
@@ -155,13 +155,21 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
         false
     }
 
-    fn report_fulfillment_error(&self, error: &FulfillmentError<'tcx>,
-                                body_id: Option<hir::BodyId>,
-                                fallback_has_occurred: bool) {
+    fn report_fulfillment_error(
+        &self,
+        error: &FulfillmentError<'tcx>,
+        body_id: Option<hir::BodyId>,
+        fallback_has_occurred: bool,
+    ) {
         debug!("report_fulfillment_errors({:?})", error);
         match error.code {
-            FulfillmentErrorCode::CodeSelectionError(ref e) => {
-                self.report_selection_error(&error.obligation, e, fallback_has_occurred);
+            FulfillmentErrorCode::CodeSelectionError(ref selection_error) => {
+                self.report_selection_error(
+                    &error.obligation,
+                    selection_error,
+                    fallback_has_occurred,
+                    error.points_at_arg_span,
+                );
             }
             FulfillmentErrorCode::CodeProjectionError(ref e) => {
                 self.report_projection_error(&error.obligation, e);
@@ -170,19 +178,21 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                 self.maybe_report_ambiguity(&error.obligation, body_id);
             }
             FulfillmentErrorCode::CodeSubtypeError(ref expected_found, ref err) => {
-                self.report_mismatched_types(&error.obligation.cause,
-                                             expected_found.expected,
-                                             expected_found.found,
-                                             err.clone())
-                    .emit();
+                self.report_mismatched_types(
+                    &error.obligation.cause,
+                    expected_found.expected,
+                    expected_found.found,
+                    err.clone(),
+                ).emit();
             }
         }
     }
 
-    fn report_projection_error(&self,
-                               obligation: &PredicateObligation<'tcx>,
-                               error: &MismatchedProjectionTypes<'tcx>)
-    {
+    fn report_projection_error(
+        &self,
+        obligation: &PredicateObligation<'tcx>,
+        error: &MismatchedProjectionTypes<'tcx>,
+    ) {
         let predicate =
             self.resolve_vars_if_possible(&obligation.predicate);
 
@@ -603,6 +613,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
         obligation: &PredicateObligation<'tcx>,
         error: &SelectionError<'tcx>,
         fallback_has_occurred: bool,
+        points_at_arg: bool,
     ) {
         let span = obligation.cause.span;
 
@@ -690,7 +701,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                         }
 
                         self.suggest_borrow_on_unsized_slice(&obligation.cause.code, &mut err);
-                        self.suggest_fn_call(&obligation, &mut err, &trait_ref);
+                        self.suggest_fn_call(&obligation, &mut err, &trait_ref, points_at_arg);
                         self.suggest_remove_reference(&obligation, &mut err, &trait_ref);
                         self.suggest_semicolon_removal(&obligation, &mut err, span, &trait_ref);
 
@@ -963,6 +974,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
         obligation: &PredicateObligation<'tcx>,
         err: &mut DiagnosticBuilder<'tcx>,
         trait_ref: &ty::Binder<ty::TraitRef<'tcx>>,
+        points_at_arg: bool,
     ) {
         let self_ty = trait_ref.self_ty();
         match self_ty.sty {
@@ -991,15 +1003,27 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                             ..
                         })) = self.tcx.hir().get_if_local(def_id) {
                             let body = self.tcx.hir().body(*body_id);
-                            err.help(&format!(
-                                "use parentheses to call the function: `{}({})`",
+                            let msg = "use parentheses to call the function";
+                            let snippet = format!(
+                                "{}({})",
                                 ident,
                                 body.params.iter()
                                     .map(|arg| match &arg.pat.node {
                                         hir::PatKind::Binding(_, _, ident, None)
                                         if ident.name != kw::SelfLower => ident.to_string(),
                                         _ => "_".to_string(),
-                                    }).collect::<Vec<_>>().join(", ")));
+                                    }).collect::<Vec<_>>().join(", "),
+                            );
+                            if points_at_arg {
+                                err.span_suggestion(
+                                    obligation.cause.span,
+                                    msg,
+                                    snippet,
+                                    Applicability::HasPlaceholders,
+                                );
+                            } else {
+                                err.help(&format!("{}: `{}`", msg, snippet));
+                            }
                         }
                     }
                     _ => {}

src/librustc/traits/mod.rs

@@ -484,7 +484,8 @@ EnumTypeFoldableImpl! {
 
 pub struct FulfillmentError<'tcx> {
     pub obligation: PredicateObligation<'tcx>,
-    pub code: FulfillmentErrorCode<'tcx>
+    pub code: FulfillmentErrorCode<'tcx>,
+    pub points_at_arg_span: bool,
 }
 
 #[derive(Clone)]
@@ -1183,7 +1184,7 @@ impl<'tcx> FulfillmentError<'tcx> {
            code: FulfillmentErrorCode<'tcx>)
            -> FulfillmentError<'tcx>
     {
-        FulfillmentError { obligation: obligation, code: code }
+        FulfillmentError { obligation: obligation, code: code, points_at_arg_span: false }
     }
 }
 

src/librustc_mir/borrow_check/nll/type_check/mod.rs

@@ -1999,6 +1999,7 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
                             ),
                             &traits::SelectionError::Unimplemented,
                             false,
+                            false,
                         );
                     }
                 }

src/librustc_typeck/check/coercion.rs

@@ -617,7 +617,7 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
 
                 // Object safety violations or miscellaneous.
                 Err(err) => {
-                    self.report_selection_error(&obligation, &err, false);
+                    self.report_selection_error(&obligation, &err, false, false);
                     // Treat this like an obligation and follow through
                     // with the unsizing - the lack of a coercion should
                     // be silent, as it causes a type mismatch later.

src/librustc_typeck/check/mod.rs

@@ -3396,6 +3396,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                     }
                     if referenced_in.len() == 1 {
                         error.obligation.cause.span = args[referenced_in[0]].span;
+                        error.points_at_arg_span = true;
                     }
                 }
             }

src/test/ui/suggestions/async-fn-ctor-passed-as-arg-where-it-should-have-been-called.stderr

@@ -5,9 +5,10 @@ LL | fn bar(f: impl Future<Output=()>) {}
    | --------------------------------- required by `bar`
 ...
 LL |     bar(foo);
-   |         ^^^ the trait `std::future::Future` is not implemented for `fn() -> impl std::future::Future {foo}`
-   |
-   = help: use parentheses to call the function: `foo()`
+   |         ^^^
+   |         |
+   |         the trait `std::future::Future` is not implemented for `fn() -> impl std::future::Future {foo}`
+   |         help: use parentheses to call the function: `foo()`
 
 error: aborting due to previous error
 

src/test/ui/suggestions/fn-ctor-passed-as-arg-where-it-should-have-been-called.stderr

@@ -5,9 +5,10 @@ LL | fn bar(f: impl T<O=()>) {}
    | ----------------------- required by `bar`
 ...
 LL |     bar(foo);
-   |         ^^^ the trait `T` is not implemented for `fn() -> impl T {foo}`
-   |
-   = help: use parentheses to call the function: `foo()`
+   |         ^^^
+   |         |
+   |         the trait `T` is not implemented for `fn() -> impl T {foo}`
+   |         help: use parentheses to call the function: `foo()`
 
 error: aborting due to previous error