When possible point at argument causing item obligation failure

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 +1003,31 @@ 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(", "),
+                            );
+                            // When the obligation error has been ensured to have been caused by
+                            // an argument, the `obligation.cause.span` points at the expression
+                            // of the argument, so we can provide a suggestion. This is signaled
+                            // by `points_at_arg`. Otherwise, we give a more general note.
+                            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 +484,11 @@ EnumTypeFoldableImpl! {
 
 pub struct FulfillmentError<'tcx> {
     pub obligation: PredicateObligation<'tcx>,
-    pub code: FulfillmentErrorCode<'tcx>
+    pub code: FulfillmentErrorCode<'tcx>,
+    /// Diagnostics only: we opportunistically change the `code.span` when we encounter an
+    /// obligation error caused by a call argument. When this is the case, we also signal that in
+    /// this field to ensure accuracy of suggestions.
+    pub points_at_arg_span: bool,
 }
 
 #[derive(Clone)]
@@ -1183,7 +1187,7 @@ 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

@@ -885,12 +885,12 @@ fn typeck_tables_of(tcx: TyCtxt<'_>, def_id: DefId) -> &ty::TypeckTables<'_> {
         };
 
         // All type checking constraints were added, try to fallback unsolved variables.
-        fcx.select_obligations_where_possible(false);
+        fcx.select_obligations_where_possible(false, |_| {});
         let mut fallback_has_occurred = false;
         for ty in &fcx.unsolved_variables() {
             fallback_has_occurred |= fcx.fallback_if_possible(ty);
         }
-        fcx.select_obligations_where_possible(fallback_has_occurred);
+        fcx.select_obligations_where_possible(fallback_has_occurred, |_| {});
 
         // Even though coercion casts provide type hints, we check casts after fallback for
         // backwards compatibility. This makes fallback a stronger type hint than a cast coercion.
@@ -2356,7 +2356,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         // possible. This can help substantially when there are
         // indirect dependencies that don't seem worth tracking
         // precisely.
-        self.select_obligations_where_possible(false);
+        self.select_obligations_where_possible(false, |_| {});
         ty = self.resolve_vars_if_possible(&ty);
 
         debug!("resolve_type_vars_with_obligations: ty={:?}", ty);
@@ -2807,7 +2807,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     fn resolve_generator_interiors(&self, def_id: DefId) {
         let mut generators = self.deferred_generator_interiors.borrow_mut();
         for (body_id, interior, kind) in generators.drain(..) {
-            self.select_obligations_where_possible(false);
+            self.select_obligations_where_possible(false, |_| {});
             generator_interior::resolve_interior(self, def_id, body_id, interior, kind);
         }
     }
@@ -2844,8 +2844,13 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     }
 
     /// Select as many obligations as we can at present.
-    fn select_obligations_where_possible(&self, fallback_has_occurred: bool) {
-        if let Err(errors) = self.fulfillment_cx.borrow_mut().select_where_possible(self) {
+    fn select_obligations_where_possible(
+        &self,
+        fallback_has_occurred: bool,
+        mutate_fullfillment_errors: impl Fn(&mut Vec<traits::FulfillmentError<'tcx>>),
+    ) {
+        if let Err(mut errors) = self.fulfillment_cx.borrow_mut().select_where_possible(self) {
+            mutate_fullfillment_errors(&mut errors);
             self.report_fulfillment_errors(&errors, self.inh.body_id, fallback_has_occurred);
         }
     }
@@ -3253,6 +3258,8 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             formal_tys.clone()
         };
 
+        let mut final_arg_types: Vec<(usize, Ty<'_>)> = vec![];
+
         // Check the arguments.
         // We do this in a pretty awful way: first we type-check any arguments
         // that are not closures, then we type-check the closures. This is so
@@ -3265,7 +3272,14 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             // an "opportunistic" vtable resolution of any trait bounds on
             // the call. This helps coercions.
             if check_closures {
-                self.select_obligations_where_possible(false);
+                self.select_obligations_where_possible(false, |errors| {
+                    self.point_at_arg_instead_of_call_if_possible(
+                        errors,
+                        &final_arg_types[..],
+                        sp,
+                        &args,
+                    );
+                })
             }
 
             // For C-variadic functions, we don't have a declared type for all of
@@ -3311,6 +3325,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 // We're processing function arguments so we definitely want to use
                 // two-phase borrows.
                 self.demand_coerce(&arg, checked_ty, coerce_ty, AllowTwoPhase::Yes);
+                final_arg_types.push((i, coerce_ty));
 
                 // 3. Relate the expected type and the formal one,
                 //    if the expected type was used for the coercion.
@@ -3357,6 +3372,44 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         vec![self.tcx.types.err; len]
     }
 
+    /// Given a vec of evaluated `FullfillmentError`s and an `fn` call argument expressions, we
+    /// walk the resolved types for each argument to see if any of the `FullfillmentError`s
+    /// reference a type argument. If they do, and there's only *one* argument that does, we point
+    /// at the corresponding argument's expression span instead of the `fn` call path span.
+    fn point_at_arg_instead_of_call_if_possible(
+        &self,
+        errors: &mut Vec<traits::FulfillmentError<'_>>,
+        final_arg_types: &[(usize, Ty<'tcx>)],
+        call_sp: Span,
+        args: &'tcx [hir::Expr],
+    ) {
+        if !call_sp.desugaring_kind().is_some() {
+            // We *do not* do this for desugared call spans to keep good diagnostics when involving
+            // the `?` operator.
+            for error in errors {
+                if let ty::Predicate::Trait(predicate) = error.obligation.predicate {
+                    // Collect the argument position for all arguments that could have caused this
+                    // `FullfillmentError`.
+                    let mut referenced_in = final_arg_types.iter()
+                        .flat_map(|(i, ty)| {
+                            let ty = self.resolve_vars_if_possible(ty);
+                            // We walk the argument type because the argument's type could have
+                            // been `Option<T>`, but the `FullfillmentError` references `T`.
+                            ty.walk()
+                                .filter(|&ty| ty == predicate.skip_binder().self_ty())
+                                .map(move |_| *i)
+                        });
+                    if let (Some(ref_in), None) = (referenced_in.next(), referenced_in.next()) {
+                        // We make sure that only *one* argument matches the obligation failure
+                        // and thet the obligation's span to its expression's.
+                        error.obligation.cause.span = args[ref_in].span;
+                        error.points_at_arg_span = true;
+                    }
+                }
+            }
+        }
+    }
+
     // AST fragment checking
     fn check_lit(&self,
                  lit: &hir::Lit,
@@ -3514,8 +3567,11 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 
             // Check bounds on type arguments used in the path.
             let bounds = self.instantiate_bounds(path_span, did, substs);
-            let cause = traits::ObligationCause::new(path_span, self.body_id,
-                                                     traits::ItemObligation(did));
+            let cause = traits::ObligationCause::new(
+                path_span,
+                self.body_id,
+                traits::ItemObligation(did),
+            );
             self.add_obligations_for_parameters(cause, &bounds);
 
             Some((variant, ty))
@@ -4639,7 +4695,8 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         let bounds = self.instantiate_bounds(span, def_id, &substs);
         self.add_obligations_for_parameters(
             traits::ObligationCause::new(span, self.body_id, traits::ItemObligation(def_id)),
-            &bounds);
+            &bounds,
+        );
 
         // Substitute the values for the type parameters into the type of
         // the referenced item.

src/librustc_typeck/check/op.rs

@@ -724,7 +724,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         match method {
             Some(ok) => {
                 let method = self.register_infer_ok_obligations(ok);
-                self.select_obligations_where_possible(false);
+                self.select_obligations_where_possible(false, |_| {});
 
                 Ok(method)
             }

src/librustc_typeck/check/wfcheck.rs

@@ -506,7 +506,7 @@ fn check_where_clauses<'tcx, 'fcx>(
     });
 
     // Now we build the substituted predicates.
-    let default_obligations = predicates.predicates.iter().flat_map(|&(pred, _)| {
+    let default_obligations = predicates.predicates.iter().flat_map(|&(pred, sp)| {
         #[derive(Default)]
         struct CountParams { params: FxHashSet<u32> }
         impl<'tcx> ty::fold::TypeVisitor<'tcx> for CountParams {
@@ -539,9 +539,9 @@ fn check_where_clauses<'tcx, 'fcx>(
             // Avoid duplication of predicates that contain no parameters, for example.
             None
         } else {
-            Some(substituted_pred)
+            Some((substituted_pred, sp))
         }
-    }).map(|pred| {
+    }).map(|(pred, sp)| {
         // Convert each of those into an obligation. So if you have
         // something like `struct Foo<T: Copy = String>`, we would
         // take that predicate `T: Copy`, substitute to `String: Copy`
@@ -551,8 +551,8 @@ fn check_where_clauses<'tcx, 'fcx>(
         // Note the subtle difference from how we handle `predicates`
         // below: there, we are not trying to prove those predicates
         // to be *true* but merely *well-formed*.
-        let pred = fcx.normalize_associated_types_in(span, &pred);
-        let cause = traits::ObligationCause::new(span, fcx.body_id, traits::ItemObligation(def_id));
+        let pred = fcx.normalize_associated_types_in(sp, &pred);
+        let cause = traits::ObligationCause::new(sp, fcx.body_id, traits::ItemObligation(def_id));
         traits::Obligation::new(cause, fcx.param_env, pred)
     });