suggest adding a reference to a trait assoc item

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

@@ -882,6 +882,8 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
             obligation.cause.code()
         {
             &parent_code
+        } else if let ObligationCauseCode::ItemObligation(_) = obligation.cause.code() {
+            obligation.cause.code()
         } else if let ExpnKind::Desugaring(DesugaringKind::ForLoop) =
             span.ctxt().outer_expn_data().kind
         {
@@ -906,102 +908,116 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
         let param_env = obligation.param_env;
 
         // Try to apply the original trait binding obligation by borrowing.
-        let mut try_borrowing =
+        let mut try_borrowing = |old_pred: ty::PolyTraitPredicate<'tcx>,
-            |old_pred: ty::PolyTraitPredicate<'tcx>, blacklist: &[DefId]| -> bool {
+                                 blacklist: &[DefId]|
-                if blacklist.contains(&old_pred.def_id()) {
+         -> bool {
-                    return false;
+            if blacklist.contains(&old_pred.def_id()) {
-                }
-                // We map bounds to `&T` and `&mut T`
-                let trait_pred_and_imm_ref = old_pred.map_bound(|trait_pred| {
-                    (
-                        trait_pred,
-                        self.tcx.mk_imm_ref(self.tcx.lifetimes.re_static, trait_pred.self_ty()),
-                    )
-                });
-                let trait_pred_and_mut_ref = old_pred.map_bound(|trait_pred| {
-                    (
-                        trait_pred,
-                        self.tcx.mk_mut_ref(self.tcx.lifetimes.re_static, trait_pred.self_ty()),
-                    )
-                });
-
-                let mk_result = |trait_pred_and_new_ty| {
-                    let obligation =
-                        self.mk_trait_obligation_with_new_self_ty(param_env, trait_pred_and_new_ty);
-                    self.predicate_must_hold_modulo_regions(&obligation)
-                };
-                let imm_result = mk_result(trait_pred_and_imm_ref);
-                let mut_result = mk_result(trait_pred_and_mut_ref);
-
-                if imm_result || mut_result {
-                    if let Ok(snippet) = self.tcx.sess.source_map().span_to_snippet(span) {
-                        // We have a very specific type of error, where just borrowing this argument
-                        // might solve the problem. In cases like this, the important part is the
-                        // original type obligation, not the last one that failed, which is arbitrary.
-                        // Because of this, we modify the error to refer to the original obligation and
-                        // return early in the caller.
-
-                        let msg = format!("the trait bound `{}` is not satisfied", old_pred);
-                        if has_custom_message {
-                            err.note(&msg);
-                        } else {
-                            err.message =
-                                vec![(rustc_errors::DiagnosticMessage::Str(msg), Style::NoStyle)];
-                        }
-                        if snippet.starts_with('&') {
-                            // This is already a literal borrow and the obligation is failing
-                            // somewhere else in the obligation chain. Do not suggest non-sense.
-                            return false;
-                        }
-                        err.span_label(
-                            span,
-                            &format!(
-                                "expected an implementor of trait `{}`",
-                                old_pred.print_modifiers_and_trait_path(),
-                            ),
-                        );
-
-                        // This if is to prevent a special edge-case
-                        if matches!(
-                            span.ctxt().outer_expn_data().kind,
-                            ExpnKind::Root | ExpnKind::Desugaring(DesugaringKind::ForLoop)
-                        ) {
-                            // We don't want a borrowing suggestion on the fields in structs,
-                            // ```
-                            // struct Foo {
-                            //  the_foos: Vec<Foo>
-                            // }
-                            // ```
-
-                            if imm_result && mut_result {
-                                err.span_suggestions(
-                                    span.shrink_to_lo(),
-                                    "consider borrowing here",
-                                    ["&".to_string(), "&mut ".to_string()].into_iter(),
-                                    Applicability::MaybeIncorrect,
-                                );
-                            } else {
-                                err.span_suggestion_verbose(
-                                    span.shrink_to_lo(),
-                                    &format!(
-                                        "consider{} borrowing here",
-                                        if mut_result { " mutably" } else { "" }
-                                    ),
-                                    format!("&{}", if mut_result { "mut " } else { "" }),
-                                    Applicability::MaybeIncorrect,
-                                );
-                            }
-                        }
-                        return true;
-                    }
-                }
                 return false;
+            }
+            // We map bounds to `&T` and `&mut T`
+            let trait_pred_and_imm_ref = old_pred.map_bound(|trait_pred| {
+                (
+                    trait_pred,
+                    self.tcx.mk_imm_ref(self.tcx.lifetimes.re_static, trait_pred.self_ty()),
+                )
+            });
+            let trait_pred_and_mut_ref = old_pred.map_bound(|trait_pred| {
+                (
+                    trait_pred,
+                    self.tcx.mk_mut_ref(self.tcx.lifetimes.re_static, trait_pred.self_ty()),
+                )
+            });
+
+            let mk_result = |trait_pred_and_new_ty| {
+                let obligation =
+                    self.mk_trait_obligation_with_new_self_ty(param_env, trait_pred_and_new_ty);
+                self.predicate_must_hold_modulo_regions(&obligation)
             };
+            let imm_result = mk_result(trait_pred_and_imm_ref);
+            let mut_result = mk_result(trait_pred_and_mut_ref);
+
+            let ref_inner_ty_result =
+                if let ObligationCauseCode::ItemObligation(_) = obligation.cause.code()
+                    && let ty::Ref(_, ty, mutability) = old_pred.self_ty().skip_binder().kind()
+                {
+                    Some((mk_result(old_pred.map_bound(|trait_pred| (trait_pred, *ty))), mutability))
+                } else {
+                    None
+                };
+
+            if imm_result || mut_result || ref_inner_ty_result.map_or(false, |(result, _)| result) {
+                if let Ok(snippet) = self.tcx.sess.source_map().span_to_snippet(span) {
+                    // We have a very specific type of error, where just borrowing this argument
+                    // might solve the problem. In cases like this, the important part is the
+                    // original type obligation, not the last one that failed, which is arbitrary.
+                    // Because of this, we modify the error to refer to the original obligation and
+                    // return early in the caller.
+
+                    let msg = format!("the trait bound `{}` is not satisfied", old_pred);
+                    if has_custom_message {
+                        err.note(&msg);
+                    } else {
+                        err.message =
+                            vec![(rustc_errors::DiagnosticMessage::Str(msg), Style::NoStyle)];
+                    }
+                    if snippet.starts_with('&') {
+                        // This is already a literal borrow and the obligation is failing
+                        // somewhere else in the obligation chain. Do not suggest non-sense.
+                        return false;
+                    }
+                    err.span_label(
+                        span,
+                        &format!(
+                            "expected an implementor of trait `{}`",
+                            old_pred.print_modifiers_and_trait_path(),
+                        ),
+                    );
+
+                    // This if is to prevent a special edge-case
+                    if matches!(
+                        span.ctxt().outer_expn_data().kind,
+                        ExpnKind::Root | ExpnKind::Desugaring(DesugaringKind::ForLoop)
+                    ) {
+                        // We don't want a borrowing suggestion on the fields in structs,
+                        // ```
+                        // struct Foo {
+                        //  the_foos: Vec<Foo>
+                        // }
+                        // ```
+
+                        if imm_result && mut_result {
+                            err.span_suggestions(
+                                span.shrink_to_lo(),
+                                "consider borrowing here",
+                                ["&".to_string(), "&mut ".to_string()].into_iter(),
+                                Applicability::MaybeIncorrect,
+                            );
+                        } else {
+                            let is_mut = mut_result
+                                || ref_inner_ty_result.map_or(false, |(_, mutabl)| {
+                                    matches!(mutabl, hir::Mutability::Mut)
+                                });
+                            err.span_suggestion_verbose(
+                                span.shrink_to_lo(),
+                                &format!(
+                                    "consider{} borrowing here",
+                                    if is_mut { " mutably" } else { "" }
+                                ),
+                                format!("&{}", if is_mut { "mut " } else { "" }),
+                                Applicability::MaybeIncorrect,
+                            );
+                        }
+                    }
+                    return true;
+                }
+            }
+            return false;
+        };
 
         if let ObligationCauseCode::ImplDerivedObligation(cause) = &*code {
             try_borrowing(cause.derived.parent_trait_pred, &[])
         } else if let ObligationCauseCode::BindingObligation(_, _)
-        | ObligationCauseCode::ItemObligation(_) = code
+        | ObligationCauseCode::ItemObligation(..) = code
         {
             try_borrowing(poly_trait_pred, &never_suggest_borrow)
         } else {

src/test/ui/suggestions/suggest-adding-reference-to-trait-assoc-item.fixed

@@ -0,0 +1,15 @@
+// run-rustfix
+#![allow(unused_variables)]
+
+fn foo(foo: &mut usize) {
+    todo!()
+}
+
+fn bar(bar: &usize) {
+    todo!()
+}
+
+fn main() {
+    foo(&mut Default::default()); //~ the trait bound `&mut usize: Default` is not satisfied
+    bar(&Default::default()); //~ the trait bound `&usize: Default` is not satisfied
+}

src/test/ui/suggestions/suggest-adding-reference-to-trait-assoc-item.rs

@@ -0,0 +1,15 @@
+// run-rustfix
+#![allow(unused_variables)]
+
+fn foo(foo: &mut usize) {
+    todo!()
+}
+
+fn bar(bar: &usize) {
+    todo!()
+}
+
+fn main() {
+    foo(Default::default()); //~ the trait bound `&mut usize: Default` is not satisfied
+    bar(Default::default()); //~ the trait bound `&usize: Default` is not satisfied
+}

src/test/ui/suggestions/suggest-adding-reference-to-trait-assoc-item.stderr

@@ -0,0 +1,25 @@
+error[E0277]: the trait bound `&mut usize: Default` is not satisfied
+  --> $DIR/suggest-adding-reference-to-trait-assoc-item.rs:13:9
+   |
+LL |     foo(Default::default());
+   |         ^^^^^^^^^^^^^^^^ expected an implementor of trait `Default`
+   |
+help: consider mutably borrowing here
+   |
+LL |     foo(&mut Default::default());
+   |         ++++
+
+error[E0277]: the trait bound `&usize: Default` is not satisfied
+  --> $DIR/suggest-adding-reference-to-trait-assoc-item.rs:14:9
+   |
+LL |     bar(Default::default());
+   |         ^^^^^^^^^^^^^^^^ expected an implementor of trait `Default`
+   |
+help: consider borrowing here
+   |
+LL |     bar(&Default::default());
+   |         +
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0277`.