Do not suggest borrowing enclosing calls for nested where-clause obligations

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

@@ -349,6 +349,7 @@ pub(super) fn maybe_report_ambiguity(
                     if impl_candidates.len() < 40 {
                         self.report_similar_impl_candidates(
                             impl_candidates.as_slice(),
+                            obligation,
                             trait_pred,
                             obligation.cause.body_id,
                             &mut err,

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

@@ -2008,6 +2008,7 @@ pub(super) fn find_similar_impl_candidates(
     pub(super) fn report_similar_impl_candidates(
         &self,
         impl_candidates: &[ImplCandidate<'tcx>],
+        obligation: &PredicateObligation<'tcx>,
         trait_pred: ty::PolyTraitPredicate<'tcx>,
         body_def_id: LocalDefId,
         err: &mut Diag<'_>,
@@ -2072,6 +2073,20 @@ pub(super) fn report_similar_impl_candidates(
         };
 
         if let [single] = &impl_candidates {
+            let self_ty = trait_pred.skip_binder().self_ty();
+            if !self_ty.has_escaping_bound_vars() {
+                let self_ty = self.tcx.instantiate_bound_regions_with_erased(trait_pred.self_ty());
+                if let ty::Ref(_, inner_ty, _) = self_ty.kind()
+                    && self.can_eq(param_env, single.trait_ref.self_ty(), *inner_ty)
+                    && !self.where_clause_expr_matches_failed_self_ty(obligation, self_ty)
+                {
+                    // Avoid pointing at a nearby impl like `String: Borrow<str>` when the
+                    // failing obligation comes from something nested inside an enclosing call
+                    // expression such as `foo(&[String::from("a")])`.
+                    return true;
+                }
+            }
+
             // If we have a single implementation, try to unify it with the trait ref
             // that failed. This should uncover a better hint for what *is* implemented.
             if self.probe(|_| {
@@ -2491,6 +2506,7 @@ fn report_similar_impl_candidates_for_root_obligation(
             let impl_candidates = self.find_similar_impl_candidates(trait_pred);
             self.report_similar_impl_candidates(
                 &impl_candidates,
+                obligation,
                 trait_pred,
                 body_def_id,
                 err,
@@ -3165,6 +3181,7 @@ fn try_to_add_help_message(
             let impl_candidates = self.find_similar_impl_candidates(trait_predicate);
             if !self.report_similar_impl_candidates(
                 &impl_candidates,
+                obligation,
                 trait_predicate,
                 body_def_id,
                 err,

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

@@ -1467,6 +1467,39 @@ pub fn extract_callable_info(
         if output.is_ty_var() { None } else { Some((def_id_or_name, output, inputs)) }
     }
 
+    pub(super) fn where_clause_expr_matches_failed_self_ty(
+        &self,
+        obligation: &PredicateObligation<'tcx>,
+        old_self_ty: Ty<'tcx>,
+    ) -> bool {
+        let ObligationCauseCode::WhereClauseInExpr(..) = obligation.cause.code() else {
+            return true;
+        };
+        let (Some(typeck_results), Some(body)) = (
+            self.typeck_results.as_ref(),
+            self.tcx.hir_maybe_body_owned_by(obligation.cause.body_id),
+        ) else {
+            return true;
+        };
+
+        let mut expr_finder = FindExprBySpan::new(obligation.cause.span, self.tcx);
+        expr_finder.visit_expr(body.value);
+        let Some(expr) = expr_finder.result else {
+            return true;
+        };
+
+        let inner_old_self_ty = match old_self_ty.kind() {
+            ty::Ref(_, inner_ty, _) => Some(*inner_ty),
+            _ => None,
+        };
+
+        [typeck_results.expr_ty_adjusted_opt(expr)].into_iter().flatten().any(|expr_ty| {
+            self.can_eq(obligation.param_env, expr_ty, old_self_ty)
+                || inner_old_self_ty
+                    .is_some_and(|inner_ty| self.can_eq(obligation.param_env, expr_ty, inner_ty))
+        })
+    }
+
     pub(super) fn suggest_add_reference_to_arg(
         &self,
         obligation: &PredicateObligation<'tcx>,
@@ -1740,6 +1773,15 @@ pub(super) fn suggest_add_reference_to_arg(
             if let hir::ExprKind::AddrOf(_, _, _) = expr.kind {
                 return false;
             }
+            let old_self_ty = old_pred.skip_binder().self_ty();
+            if !old_self_ty.has_escaping_bound_vars()
+                && !self.where_clause_expr_matches_failed_self_ty(
+                    obligation,
+                    self.tcx.instantiate_bound_regions_with_erased(old_pred.self_ty()),
+                )
+            {
+                return false;
+            }
             let needs_parens_post = expr_needs_parens(expr);
             let needs_parens_pre = match self.tcx.parent_hir_node(expr.hir_id) {
                 Node::Expr(e)

tests/ui/suggestions/dont-suggest-borrow-whole-call-issue-155088.rs

@@ -0,0 +1,8 @@
+use std::borrow::Borrow;
+
+fn foo(_v: impl IntoIterator<Item = impl Borrow<str>>) {}
+
+fn main() {
+    foo(&[String::from("a")]);
+    //~^ ERROR the trait bound `&String: Borrow<str>` is not satisfied
+}

tests/ui/suggestions/dont-suggest-borrow-whole-call-issue-155088.stderr

@@ -0,0 +1,15 @@
+error[E0277]: the trait bound `&String: Borrow<str>` is not satisfied
+  --> $DIR/dont-suggest-borrow-whole-call-issue-155088.rs:6:5
+   |
+LL |     foo(&[String::from("a")]);
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Borrow<str>` is not implemented for `&String`
+   |
+note: required by a bound in `foo`
+  --> $DIR/dont-suggest-borrow-whole-call-issue-155088.rs:3:42
+   |
+LL | fn foo(_v: impl IntoIterator<Item = impl Borrow<str>>) {}
+   |                                          ^^^^^^^^^^^ required by this bound in `foo`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0277`.