Remove redundant notes in E0275

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

@@ -200,6 +200,7 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
             &obligation.predicate,
             &obligation.cause.code,
             &mut vec![],
+            &mut Default::default(),
         );
 
         err.emit();
@@ -1700,6 +1701,7 @@ impl<'a, 'tcx> InferCtxtPrivExt<'tcx> for InferCtxt<'a, 'tcx> {
                 &obligation.predicate,
                 &obligation.cause.code,
                 &mut vec![],
+                &mut Default::default(),
             );
             self.suggest_unsized_bound_if_applicable(err, obligation);
         }

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

@@ -7,6 +7,7 @@ use crate::autoderef::Autoderef;
 use crate::infer::InferCtxt;
 use crate::traits::normalize_projection_type;
 
+use rustc_data_structures::fx::FxHashSet;
 use rustc_data_structures::stack::ensure_sufficient_stack;
 use rustc_errors::{error_code, struct_span_err, Applicability, DiagnosticBuilder, Style};
 use rustc_hir as hir;
@@ -158,6 +159,7 @@ pub trait InferCtxtExt<'tcx> {
         predicate: &T,
         cause_code: &ObligationCauseCode<'tcx>,
         obligated_types: &mut Vec<&ty::TyS<'tcx>>,
+        seen_requirements: &mut FxHashSet<DefId>,
     ) where
         T: fmt::Display;
 
@@ -1787,6 +1789,7 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
             &obligation.predicate,
             next_code.unwrap(),
             &mut Vec::new(),
+            &mut Default::default(),
         );
     }
 
@@ -1796,6 +1799,7 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
         predicate: &T,
         cause_code: &ObligationCauseCode<'tcx>,
         obligated_types: &mut Vec<&ty::TyS<'tcx>>,
+        seen_requirements: &mut FxHashSet<DefId>,
     ) where
         T: fmt::Display,
     {
@@ -2050,25 +2054,52 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
                             &parent_predicate,
                             &data.parent_code,
                             obligated_types,
+                            seen_requirements,
                         )
                     });
                 }
             }
             ObligationCauseCode::ImplDerivedObligation(ref data) => {
-                let parent_trait_ref = self.resolve_vars_if_possible(data.parent_trait_ref);
+                let mut parent_trait_ref = self.resolve_vars_if_possible(data.parent_trait_ref);
+                let parent_def_id = parent_trait_ref.def_id();
                 err.note(&format!(
                     "required because of the requirements on the impl of `{}` for `{}`",
                     parent_trait_ref.print_only_trait_path(),
                     parent_trait_ref.skip_binder().self_ty()
                 ));
-                let parent_predicate = parent_trait_ref.without_const().to_predicate(tcx);
+
+                let mut parent_predicate = parent_trait_ref.without_const().to_predicate(tcx);
+                let mut data = data;
+                let mut count = 0;
+                seen_requirements.insert(parent_def_id);
+                while let ObligationCauseCode::ImplDerivedObligation(child) = &*data.parent_code {
+                    // Skip redundant recursive obligation notes. See `ui/issue-20413.rs`.
+                    let child_trait_ref = self.resolve_vars_if_possible(child.parent_trait_ref);
+                    let child_def_id = child_trait_ref.def_id();
+                    if seen_requirements.insert(child_def_id) {
+                        break;
+                    }
+                    count += 1;
+                    data = child;
+                    parent_predicate = child_trait_ref.without_const().to_predicate(tcx);
+                    parent_trait_ref = child_trait_ref;
+                }
+                if count > 0 {
+                    err.note(&format!("{} redundant requirements hidden", count));
+                    err.note(&format!(
+                        "required because of the requirements on the impl of `{}` for `{}`",
+                        parent_trait_ref.print_only_trait_path(),
+                        parent_trait_ref.skip_binder().self_ty()
+                    ));
+                }
                 // #74711: avoid a stack overflow
                 ensure_sufficient_stack(|| {
                     self.note_obligation_cause_code(
                         err,
                         &parent_predicate,
                         &data.parent_code,
                         obligated_types,
+                        seen_requirements,
                     )
                 });
             }
@@ -2082,20 +2113,21 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
                         &parent_predicate,
                         &data.parent_code,
                         obligated_types,
+                        seen_requirements,
                     )
                 });
             }
             ObligationCauseCode::CompareImplMethodObligation { .. } => {
                 err.note(&format!(
-                    "the requirement `{}` appears on the impl method \
-                     but not on the corresponding trait method",
+                    "the requirement `{}` appears on the impl method but not on the corresponding \
+                     trait method",
                     predicate
                 ));
             }
             ObligationCauseCode::CompareImplTypeObligation { .. } => {
                 err.note(&format!(
-                    "the requirement `{}` appears on the associated impl type \
-                     but not on the corresponding associated trait type",
+                    "the requirement `{}` appears on the associated impl type but not on the \
+                     corresponding associated trait type",
                     predicate
                 ));
             }