Rollup merge of #99671 - TaKO8Ki:suggest-dereferencing-index, r=compiler-errors

Suggest dereferencing index when trying to use a reference of usize as index

fixes #96678

2 files changed, 29 insertions, 0 deletions

diff --git a/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs b/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs
index 25ba520ace2..7ee3fe844b5 100644
--- a/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs
+++ b/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs
@@ -531,6 +531,7 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
                         }
 
                         self.suggest_floating_point_literal(&obligation, &mut err, &trait_ref);
+                        self.suggest_dereferencing_index(&obligation, &mut err, trait_predicate);
                         let mut suggested =
                             self.suggest_dereferences(&obligation, &mut err, trait_predicate);
                         suggested |= self.suggest_fn_call(&obligation, &mut err, trait_predicate);
diff --git a/compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs b/compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs
index 6c8faed0df4..1f72c96406d 100644
--- a/compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs
+++ b/compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs
@@ -320,6 +320,13 @@ pub trait InferCtxtExt<'tcx> {
         err: &mut Diagnostic,
         trait_pred: ty::PolyTraitPredicate<'tcx>,
     );
+
+    fn suggest_dereferencing_index(
+        &self,
+        obligation: &PredicateObligation<'tcx>,
+        err: &mut Diagnostic,
+        trait_pred: ty::PolyTraitPredicate<'tcx>,
+    );
 }
 
 fn predicate_constraint(generics: &hir::Generics<'_>, pred: String) -> (Span, String) {
@@ -2895,6 +2902,27 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
             );
         }
     }
+
+    fn suggest_dereferencing_index(
+        &self,
+        obligation: &PredicateObligation<'tcx>,
+        err: &mut Diagnostic,
+        trait_pred: ty::PolyTraitPredicate<'tcx>,
+    ) {
+        if let ObligationCauseCode::ImplDerivedObligation(_) = obligation.cause.code()
+            && self.tcx.is_diagnostic_item(sym::SliceIndex, trait_pred.skip_binder().trait_ref.def_id)
+            && let ty::Slice(_) = trait_pred.skip_binder().trait_ref.substs.type_at(1).kind()
+            && let ty::Ref(_, inner_ty, _) = trait_pred.skip_binder().self_ty().kind()
+            && let ty::Uint(ty::UintTy::Usize) = inner_ty.kind()
+        {
+            err.span_suggestion_verbose(
+                obligation.cause.span.shrink_to_lo(),
+            "dereference this index",
+            '*',
+                Applicability::MachineApplicable,
+            );
+        }
+    }
 }
 
 /// Collect all the returned expressions within the input expression.