From 2c2f3ed2c4a99d6d2bc4c1a45890146c7b0c8561 Mon Sep 17 00:00:00 2001 From: =?UTF-8?q?Esteban=20K=C3=BCber?= Date: Thu, 28 Dec 2023 19:09:04 +0000 Subject: [PATCH] Provide more context on recursive `impl` evaluation overflow When an associated type `Self::Assoc` is part of a `where` clause, we end up unable to evaluate the requirement and emit a E0275. We now point at the associated type if specified in the `impl`. If so, we also suggest using that type instead of `Self::Assoc`. Otherwise, we explain that these are not allowed. ``` error[E0275]: overflow evaluating the requirement `<(T,) as Grault>::A == _` --> $DIR/impl-wf-cycle-1.rs:15:1 | LL | / impl Grault for (T,) LL | | LL | | where LL | | Self::A: Baz, LL | | Self::B: Fiz, | |_________________^ LL | { LL | type A = (); | ------ associated type `<(T,) as Grault>::A` is specified here | note: required for `(T,)` to implement `Grault` --> $DIR/impl-wf-cycle-1.rs:15:17 | LL | impl Grault for (T,) | ^^^^^^ ^^^^ ... LL | Self::A: Baz, | --- unsatisfied trait bound introduced here = note: 1 redundant requirement hidden = note: required for `(T,)` to implement `Grault` help: associated type for the current `impl` cannot be restricted in `where` clauses, remove this bound | LL - Self::A: Baz, LL + , | ``` ``` error[E0275]: overflow evaluating the requirement `::Type == ::Type` --> $DIR/impl-wf-cycle-3.rs:7:1 | LL | / impl B for T LL | | where LL | | T: A, | |_____________________^ LL | { LL | type Type = bool; | --------- associated type `::Type` is specified here | note: required for `T` to implement `B` --> $DIR/impl-wf-cycle-3.rs:7:9 | LL | impl B for T | ^ ^ LL | where LL | T: A, | ------------- unsatisfied trait bound introduced here help: replace the associated type with the type specified in this `impl` | LL | T: A, | ~~~~ ``` ``` error[E0275]: overflow evaluating the requirement `::ToMatch == ::ToMatch` --> $DIR/impl-wf-cycle-4.rs:5:1 | LL | / impl Filter for T LL | | where LL | | T: Fn(Self::ToMatch), | |_________________________^ | note: required for `T` to implement `Filter` --> $DIR/impl-wf-cycle-4.rs:5:9 | LL | impl Filter for T | ^^^^^^ ^ LL | where LL | T: Fn(Self::ToMatch), | ----------------- unsatisfied trait bound introduced here note: associated types for the current `impl` cannot be restricted in `where` clauses --> $DIR/impl-wf-cycle-4.rs:7:11 | LL | T: Fn(Self::ToMatch), | ^^^^^^^^^^^^^ ``` Fix #116925 --- .../src/traits/error_reporting/suggestions.rs | 201 ++++++++++++++---- .../associated-types/impl-wf-cycle-1.stderr | 8 + .../associated-types/impl-wf-cycle-2.stderr | 8 + tests/ui/associated-types/impl-wf-cycle-3.rs | 13 ++ .../associated-types/impl-wf-cycle-3.stderr | 27 +++ tests/ui/associated-types/impl-wf-cycle-4.rs | 15 ++ .../associated-types/impl-wf-cycle-4.stderr | 25 +++ 7 files changed, 252 insertions(+), 45 deletions(-) create mode 100644 tests/ui/associated-types/impl-wf-cycle-3.rs create mode 100644 tests/ui/associated-types/impl-wf-cycle-3.stderr create mode 100644 tests/ui/associated-types/impl-wf-cycle-4.rs create mode 100644 tests/ui/associated-types/impl-wf-cycle-4.stderr 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 0e33e9cd790d0..f96392899222c 100644 --- a/compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs +++ b/compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs @@ -7,7 +7,7 @@ use super::{ use crate::errors; use crate::infer::InferCtxt; -use crate::traits::{NormalizeExt, ObligationCtxt}; +use crate::traits::{ImplDerivedObligationCause, NormalizeExt, ObligationCtxt}; use hir::def::CtorOf; use rustc_data_structures::fx::FxHashSet; @@ -2973,7 +2973,7 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> { | ObligationCauseCode::ObjectTypeBound(..) => {} ObligationCauseCode::RustCall => { if let Some(pred) = predicate.to_opt_poly_trait_pred() - && Some(pred.def_id()) == self.tcx.lang_items().sized_trait() + && Some(pred.def_id()) == tcx.lang_items().sized_trait() { err.note("argument required to be sized due to `extern \"rust-call\"` ABI"); } @@ -3022,15 +3022,15 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> { let def_id = trait_pred.def_id(); let visible_item = if let Some(local) = def_id.as_local() { // Check for local traits being reachable. - let vis = &self.tcx.resolutions(()).effective_visibilities; + let vis = &tcx.resolutions(()).effective_visibilities; // Account for non-`pub` traits in the root of the local crate. - let is_locally_reachable = self.tcx.parent(def_id).is_crate_root(); + let is_locally_reachable = tcx.parent(def_id).is_crate_root(); vis.is_reachable(local) || is_locally_reachable } else { // Check for foreign traits being reachable. - self.tcx.visible_parent_map(()).get(&def_id).is_some() + tcx.visible_parent_map(()).get(&def_id).is_some() }; - if Some(def_id) == self.tcx.lang_items().sized_trait() + if Some(def_id) == tcx.lang_items().sized_trait() && let Some(hir::Node::TraitItem(hir::TraitItem { ident, kind: hir::TraitItemKind::Type(bounds, None), @@ -3039,7 +3039,7 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> { // Do not suggest relaxing if there is an explicit `Sized` obligation. && !bounds.iter() .filter_map(|bound| bound.trait_ref()) - .any(|tr| tr.trait_def_id() == self.tcx.lang_items().sized_trait()) + .any(|tr| tr.trait_def_id() == tcx.lang_items().sized_trait()) { let (span, separator) = if let [.., last] = bounds { (last.span().shrink_to_hi(), " +") @@ -3102,10 +3102,8 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> { } ObligationCauseCode::Coercion { source, target } => { let mut file = None; - let source = - self.tcx.short_ty_string(self.resolve_vars_if_possible(source), &mut file); - let target = - self.tcx.short_ty_string(self.resolve_vars_if_possible(target), &mut file); + let source = tcx.short_ty_string(self.resolve_vars_if_possible(source), &mut file); + let target = tcx.short_ty_string(self.resolve_vars_if_possible(target), &mut file); err.note(with_forced_trimmed_paths!(format!( "required for the cast from `{source}` to `{target}`", ))); @@ -3154,7 +3152,7 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> { ); } - if self.tcx.sess.is_nightly_build() + if tcx.sess.is_nightly_build() && matches!(is_constable, IsConstable::Fn | IsConstable::Ctor) { err.help( @@ -3164,8 +3162,8 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> { } } ObligationCauseCode::VariableType(hir_id) => { - let parent_node = self.tcx.hir().parent_id(hir_id); - match self.tcx.opt_hir_node(parent_node) { + let parent_node = tcx.hir().parent_id(hir_id); + match tcx.opt_hir_node(parent_node) { Some(Node::Local(hir::Local { ty: Some(ty), .. })) => { err.span_suggestion_verbose( ty.span.shrink_to_lo(), @@ -3203,7 +3201,7 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> { err.note("all local variables must have a statically known size"); } } - if !self.tcx.features().unsized_locals { + if !tcx.features().unsized_locals { err.help("unsized locals are gated as an unstable feature"); } } @@ -3285,7 +3283,7 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> { err.note("all function arguments must have a statically known size"); } if tcx.sess.opts.unstable_features.is_nightly_build() - && !self.tcx.features().unsized_fn_params + && !tcx.features().unsized_fn_params { err.help("unsized fn params are gated as an unstable feature"); } @@ -3354,7 +3352,7 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> { "all values captured by value by a closure must have a statically known size", ); let hir::ExprKind::Closure(closure) = - self.tcx.hir_node_by_def_id(closure_def_id).expect_expr().kind + tcx.hir_node_by_def_id(closure_def_id).expect_expr().kind else { bug!("expected closure in SizedClosureCapture obligation"); }; @@ -3365,7 +3363,7 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> { } } ObligationCauseCode::SizedCoroutineInterior(coroutine_def_id) => { - let what = match self.tcx.coroutine_kind(coroutine_def_id) { + let what = match tcx.coroutine_kind(coroutine_def_id) { None | Some(hir::CoroutineKind::Coroutine(_)) | Some(hir::CoroutineKind::Desugared(hir::CoroutineDesugaring::Gen, _)) => { @@ -3416,10 +3414,10 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> { 'print: { if !is_upvar_tys_infer_tuple { let mut file = None; - let ty_str = self.tcx.short_ty_string(ty, &mut file); + let ty_str = tcx.short_ty_string(ty, &mut file); let msg = format!("required because it appears within the type `{ty_str}`"); match ty.kind() { - ty::Adt(def, _) => match self.tcx.opt_item_ident(def.did()) { + ty::Adt(def, _) => match tcx.opt_item_ident(def.did()) { Some(ident) => err.span_note(ident.span, msg), None => err.note(msg), }, @@ -3442,7 +3440,7 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> { { break 'print; } - err.span_note(self.tcx.def_span(def_id), msg) + err.span_note(tcx.def_span(def_id), msg) } ty::CoroutineWitness(def_id, args) => { use std::fmt::Write; @@ -3459,7 +3457,7 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> { err.note(msg.trim_end_matches(", ").to_string()) } ty::Coroutine(def_id, _) => { - let sp = self.tcx.def_span(def_id); + let sp = tcx.def_span(def_id); // Special-case this to say "async block" instead of `[static coroutine]`. let kind = tcx.coroutine_kind(def_id).unwrap(); @@ -3471,7 +3469,7 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> { ) } ty::Closure(def_id, _) => err.span_note( - self.tcx.def_span(def_id), + tcx.def_span(def_id), "required because it's used within this closure", ), ty::Str => err.note("`str` is considered to contain a `[u8]` slice for auto trait purposes"), @@ -3515,14 +3513,12 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> { self.resolve_vars_if_possible(data.derived.parent_trait_pred); let parent_def_id = parent_trait_pred.def_id(); let mut file = None; - let self_ty = - self.tcx.short_ty_string(parent_trait_pred.skip_binder().self_ty(), &mut file); - let msg = format!( - "required for `{self_ty}` to implement `{}`", - parent_trait_pred.print_modifiers_and_trait_path() - ); + let self_ty_str = + tcx.short_ty_string(parent_trait_pred.skip_binder().self_ty(), &mut file); + let trait_name = parent_trait_pred.print_modifiers_and_trait_path().to_string(); + let msg = format!("required for `{self_ty_str}` to implement `{trait_name}`"); let mut is_auto_trait = false; - match self.tcx.hir().get_if_local(data.impl_or_alias_def_id) { + match tcx.hir().get_if_local(data.impl_or_alias_def_id) { Some(Node::Item(hir::Item { kind: hir::ItemKind::Trait(is_auto, ..), ident, @@ -3534,7 +3530,7 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> { err.span_note(ident.span, msg); } Some(Node::Item(hir::Item { - kind: hir::ItemKind::Impl(hir::Impl { of_trait, self_ty, .. }), + kind: hir::ItemKind::Impl(hir::Impl { of_trait, self_ty, generics, .. }), .. })) => { let mut spans = Vec::with_capacity(2); @@ -3561,6 +3557,15 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> { ); } err.span_note(spans, msg); + point_at_assoc_type_restriction( + tcx, + err, + &self_ty_str, + &trait_name, + predicate, + &generics, + &data, + ); } _ => { err.note(msg); @@ -3614,9 +3619,8 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> { pluralize!(count) )); let mut file = None; - let self_ty = self - .tcx - .short_ty_string(parent_trait_pred.skip_binder().self_ty(), &mut file); + let self_ty = + tcx.short_ty_string(parent_trait_pred.skip_binder().self_ty(), &mut file); err.note(format!( "required for `{self_ty}` to implement `{}`", parent_trait_pred.print_modifiers_and_trait_path() @@ -3674,10 +3678,7 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> { multispan.push_span_label(span, "required by this bound"); err.span_note( multispan, - format!( - "required by a bound on the type alias `{}`", - self.infcx.tcx.item_name(def_id) - ), + format!("required by a bound on the type alias `{}`", tcx.item_name(def_id)), ); } ObligationCauseCode::FunctionArgumentObligation { @@ -3708,25 +3709,23 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> { }); } ObligationCauseCode::CompareImplItemObligation { trait_item_def_id, kind, .. } => { - let item_name = self.tcx.item_name(trait_item_def_id); + let item_name = tcx.item_name(trait_item_def_id); let msg = format!( "the requirement `{predicate}` appears on the `impl`'s {kind} \ `{item_name}` but not on the corresponding trait's {kind}", ); - let sp = self - .tcx + let sp = tcx .opt_item_ident(trait_item_def_id) .map(|i| i.span) - .unwrap_or_else(|| self.tcx.def_span(trait_item_def_id)); + .unwrap_or_else(|| tcx.def_span(trait_item_def_id)); let mut assoc_span: MultiSpan = sp.into(); assoc_span.push_span_label( sp, format!("this trait's {kind} doesn't have the requirement `{predicate}`"), ); - if let Some(ident) = self - .tcx + if let Some(ident) = tcx .opt_associated_item(trait_item_def_id) - .and_then(|i| self.tcx.opt_item_ident(i.container_id(self.tcx))) + .and_then(|i| tcx.opt_item_ident(i.container_id(tcx))) { assoc_span.push_span_label(ident.span, "in this trait"); } @@ -4816,6 +4815,29 @@ fn hint_missing_borrow<'tcx>( } } +/// Collect all the paths that reference `Self`. +/// Used to suggest replacing associated types with an explicit type in `where` clauses. +#[derive(Debug)] +pub struct SelfVisitor<'v> { + pub paths: Vec<&'v hir::Ty<'v>>, + pub name: Option, +} + +impl<'v> Visitor<'v> for SelfVisitor<'v> { + fn visit_ty(&mut self, ty: &'v hir::Ty<'v>) { + if let hir::TyKind::Path(path) = ty.kind + && let hir::QPath::TypeRelative(inner_ty, segment) = path + && (Some(segment.ident.name) == self.name || self.name.is_none()) + && let hir::TyKind::Path(inner_path) = inner_ty.kind + && let hir::QPath::Resolved(None, inner_path) = inner_path + && let Res::SelfTyAlias { .. } = inner_path.res + { + self.paths.push(ty); + } + hir::intravisit::walk_ty(self, ty); + } +} + /// Collect all the returned expressions within the input expression. /// Used to point at the return spans when we want to suggest some change to them. #[derive(Default)] @@ -5060,6 +5082,95 @@ pub fn suggest_desugaring_async_fn_to_impl_future_in_trait<'tcx>( Some(sugg) } +/// On `impl` evaluation cycles, look for `Self::AssocTy` restrictions in `where` clauses, explain +/// they are not allowed and if possible suggest alternatives. +fn point_at_assoc_type_restriction( + tcx: TyCtxt<'_>, + err: &mut Diagnostic, + self_ty_str: &str, + trait_name: &str, + predicate: ty::Predicate<'_>, + generics: &hir::Generics<'_>, + data: &ImplDerivedObligationCause<'_>, +) { + let ty::PredicateKind::Clause(clause) = predicate.kind().skip_binder() else { + return; + }; + let ty::ClauseKind::Projection(proj) = clause else { + return; + }; + let name = tcx.item_name(proj.projection_ty.def_id); + for pred in generics.predicates { + let hir::WherePredicate::BoundPredicate(pred) = pred else { + continue; + }; + for bound in pred.bounds { + let Some(trait_ref) = bound.trait_ref() else { + continue; + }; + if bound.span() != data.span { + continue; + } + if let hir::TyKind::Path(path) = pred.bounded_ty.kind + && let hir::QPath::TypeRelative(ty, segment) = path + && segment.ident.name == name + && let hir::TyKind::Path(inner_path) = ty.kind + && let hir::QPath::Resolved(None, inner_path) = inner_path + && let Res::SelfTyAlias { .. } = inner_path.res + { + err.span_suggestion_verbose( + pred.span, // FIXME: include the trailing comma. + "associated type for the current `impl` cannot be restricted in `where` \ + clauses, remove this bound", + "", + Applicability::MaybeIncorrect, + ); + } + if let Some(new) = + tcx.associated_items(data.impl_or_alias_def_id).find_by_name_and_kind( + tcx, + Ident::with_dummy_span(name), + ty::AssocKind::Type, + data.impl_or_alias_def_id, + ) + { + // The associated type is specified in the `impl` we're + // looking at. Point at it. + let span = tcx.def_span(new.def_id); + err.span_label( + span, + format!( + "associated type `<{self_ty_str} as {trait_name}>::{name}` is specified \ + here", + ), + ); + // Search for the associated type `Self::{name}`, get + // its type and suggest replacing the bound with it. + let mut visitor = SelfVisitor { paths: vec![], name: Some(name) }; + visitor.visit_trait_ref(trait_ref); + for path in visitor.paths { + err.span_suggestion_verbose( + path.span, + "replace the associated type with the type specified in this `impl`", + format!("{}", tcx.type_of(new.def_id).skip_binder(),), + Applicability::MachineApplicable, + ); + } + } else { + let mut visitor = SelfVisitor { paths: vec![], name: None }; + visitor.visit_trait_ref(trait_ref); + let span: MultiSpan = + visitor.paths.iter().map(|p| p.span).collect::>().into(); + err.span_note( + span, + "associated types for the current `impl` cannot be restricted in `where` \ + clauses", + ); + } + } + } +} + fn get_deref_type_and_refs(mut ty: Ty<'_>) -> (Ty<'_>, Vec) { let mut refs = vec![]; diff --git a/tests/ui/associated-types/impl-wf-cycle-1.stderr b/tests/ui/associated-types/impl-wf-cycle-1.stderr index 5fa36733f6677..b4fc73fc8cfd9 100644 --- a/tests/ui/associated-types/impl-wf-cycle-1.stderr +++ b/tests/ui/associated-types/impl-wf-cycle-1.stderr @@ -7,6 +7,9 @@ LL | | where LL | | Self::A: Baz, LL | | Self::B: Fiz, | |_________________^ +LL | { +LL | type A = (); + | ------ associated type `<(T,) as Grault>::A` is specified here | note: required for `(T,)` to implement `Grault` --> $DIR/impl-wf-cycle-1.rs:15:17 @@ -18,6 +21,11 @@ LL | Self::A: Baz, | --- unsatisfied trait bound introduced here = note: 1 redundant requirement hidden = note: required for `(T,)` to implement `Grault` +help: associated type for the current `impl` cannot be restricted in `where` clauses, remove this bound + | +LL - Self::A: Baz, +LL + , + | error: aborting due to 1 previous error diff --git a/tests/ui/associated-types/impl-wf-cycle-2.stderr b/tests/ui/associated-types/impl-wf-cycle-2.stderr index 17a96bbd93430..b5ece55c1c9ad 100644 --- a/tests/ui/associated-types/impl-wf-cycle-2.stderr +++ b/tests/ui/associated-types/impl-wf-cycle-2.stderr @@ -6,6 +6,9 @@ LL | | LL | | where LL | | Self::A: Copy, | |__________________^ +LL | { +LL | type A = (); + | ------ associated type `<(T,) as Grault>::A` is specified here | note: required for `(T,)` to implement `Grault` --> $DIR/impl-wf-cycle-2.rs:7:17 @@ -15,6 +18,11 @@ LL | impl Grault for (T,) ... LL | Self::A: Copy, | ---- unsatisfied trait bound introduced here +help: associated type for the current `impl` cannot be restricted in `where` clauses, remove this bound + | +LL - Self::A: Copy, +LL + , + | error: aborting due to 1 previous error diff --git a/tests/ui/associated-types/impl-wf-cycle-3.rs b/tests/ui/associated-types/impl-wf-cycle-3.rs new file mode 100644 index 0000000000000..044033c5e8ab1 --- /dev/null +++ b/tests/ui/associated-types/impl-wf-cycle-3.rs @@ -0,0 +1,13 @@ +trait A {} + +trait B { + type Type; +} + +impl B for T //~ ERROR overflow evaluating the requirement +where + T: A, +{ + type Type = bool; +} +fn main() {} diff --git a/tests/ui/associated-types/impl-wf-cycle-3.stderr b/tests/ui/associated-types/impl-wf-cycle-3.stderr new file mode 100644 index 0000000000000..d3ca06f890b34 --- /dev/null +++ b/tests/ui/associated-types/impl-wf-cycle-3.stderr @@ -0,0 +1,27 @@ +error[E0275]: overflow evaluating the requirement `::Type == ::Type` + --> $DIR/impl-wf-cycle-3.rs:7:1 + | +LL | / impl B for T +LL | | where +LL | | T: A, + | |_____________________^ +LL | { +LL | type Type = bool; + | --------- associated type `::Type` is specified here + | +note: required for `T` to implement `B` + --> $DIR/impl-wf-cycle-3.rs:7:9 + | +LL | impl B for T + | ^ ^ +LL | where +LL | T: A, + | ------------- unsatisfied trait bound introduced here +help: replace the associated type with the type specified in this `impl` + | +LL | T: A, + | ~~~~ + +error: aborting due to 1 previous error + +For more information about this error, try `rustc --explain E0275`. diff --git a/tests/ui/associated-types/impl-wf-cycle-4.rs b/tests/ui/associated-types/impl-wf-cycle-4.rs new file mode 100644 index 0000000000000..bfa8adc71a11d --- /dev/null +++ b/tests/ui/associated-types/impl-wf-cycle-4.rs @@ -0,0 +1,15 @@ +trait Filter { + type ToMatch; +} + +impl Filter for T //~ ERROR overflow evaluating the requirement +where + T: Fn(Self::ToMatch), +{ +} + +struct JustFilter { + filter: F, +} + +fn main() {} diff --git a/tests/ui/associated-types/impl-wf-cycle-4.stderr b/tests/ui/associated-types/impl-wf-cycle-4.stderr new file mode 100644 index 0000000000000..cdbac267d34dc --- /dev/null +++ b/tests/ui/associated-types/impl-wf-cycle-4.stderr @@ -0,0 +1,25 @@ +error[E0275]: overflow evaluating the requirement `::ToMatch == ::ToMatch` + --> $DIR/impl-wf-cycle-4.rs:5:1 + | +LL | / impl Filter for T +LL | | where +LL | | T: Fn(Self::ToMatch), + | |_________________________^ + | +note: required for `T` to implement `Filter` + --> $DIR/impl-wf-cycle-4.rs:5:9 + | +LL | impl Filter for T + | ^^^^^^ ^ +LL | where +LL | T: Fn(Self::ToMatch), + | ----------------- unsatisfied trait bound introduced here +note: associated types for the current `impl` cannot be restricted in `where` clauses + --> $DIR/impl-wf-cycle-4.rs:7:11 + | +LL | T: Fn(Self::ToMatch), + | ^^^^^^^^^^^^^ + +error: aborting due to 1 previous error + +For more information about this error, try `rustc --explain E0275`.