Split item bounds and item super predicates

compiler/rustc_borrowck/src/diagnostics/conflict_errors.rs

@@ -716,7 +716,7 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
                 .copied()
                 .find_map(find_fn_kind_from_did),
             ty::Alias(ty::Opaque, ty::AliasTy { def_id, args, .. }) => tcx
-                .explicit_item_bounds(def_id)
+                .explicit_item_super_predicates(def_id)
                 .iter_instantiated_copied(tcx, args)
                 .find_map(|(clause, span)| find_fn_kind_from_did((clause, span))),
             ty::Closure(_, args) => match args.as_closure().kind() {

compiler/rustc_hir_analysis/src/check/mod.rs

@@ -431,7 +431,7 @@ fn fn_sig_suggestion<'tcx>(
 
     let asyncness = if tcx.asyncness(assoc.def_id).is_async() {
         output = if let ty::Alias(_, alias_ty) = *output.kind() {
-            tcx.explicit_item_bounds(alias_ty.def_id)
+            tcx.explicit_item_super_predicates(alias_ty.def_id)
                 .iter_instantiated_copied(tcx, alias_ty.args)
                 .find_map(|(bound, _)| bound.as_projection_clause()?.no_bound_vars()?.term.ty())
                 .unwrap_or_else(|| {

compiler/rustc_hir_analysis/src/collect.rs

@@ -61,6 +61,9 @@ pub fn provide(providers: &mut Providers) {
         type_alias_is_lazy: type_of::type_alias_is_lazy,
         item_bounds: item_bounds::item_bounds,
         explicit_item_bounds: item_bounds::explicit_item_bounds,
+        item_super_predicates: item_bounds::item_super_predicates,
+        explicit_item_super_predicates: item_bounds::explicit_item_super_predicates,
+        item_non_self_assumptions: item_bounds::item_non_self_assumptions,
         generics_of: generics_of::generics_of,
         predicates_of: predicates_of::predicates_of,
         predicates_defined_on,
@@ -633,7 +636,9 @@ fn convert_item(tcx: TyCtxt<'_>, item_id: hir::ItemId) {
             tcx.ensure().generics_of(def_id);
             tcx.ensure().predicates_of(def_id);
             tcx.ensure().explicit_item_bounds(def_id);
+            tcx.ensure().explicit_item_super_predicates(def_id);
             tcx.ensure().item_bounds(def_id);
+            tcx.ensure().item_super_predicates(def_id);
         }
 
         hir::ItemKind::TyAlias(..) => {
@@ -689,6 +694,7 @@ fn convert_trait_item(tcx: TyCtxt<'_>, trait_item_id: hir::TraitItemId) {
 
         hir::TraitItemKind::Type(_, Some(_)) => {
             tcx.ensure().item_bounds(def_id);
+            tcx.ensure().item_super_predicates(def_id);
             tcx.ensure().type_of(def_id);
             // Account for `type T = _;`.
             let mut visitor = HirPlaceholderCollector::default();
@@ -698,6 +704,7 @@ fn convert_trait_item(tcx: TyCtxt<'_>, trait_item_id: hir::TraitItemId) {
 
         hir::TraitItemKind::Type(_, None) => {
             tcx.ensure().item_bounds(def_id);
+            tcx.ensure().item_super_predicates(def_id);
             // #74612: Visit and try to find bad placeholders
             // even if there is no concrete type.
             let mut visitor = HirPlaceholderCollector::default();

compiler/rustc_hir_analysis/src/collect/item_bounds.rs

@@ -1,5 +1,6 @@
 use super::ItemCtxt;
 use crate::astconv::{AstConv, PredicateFilter};
+use rustc_data_structures::fx::FxIndexSet;
 use rustc_hir as hir;
 use rustc_infer::traits::util;
 use rustc_middle::ty::GenericArgs;
@@ -19,6 +20,7 @@ fn associated_type_bounds<'tcx>(
     assoc_item_def_id: LocalDefId,
     ast_bounds: &'tcx [hir::GenericBound<'tcx>],
     span: Span,
+    filter: PredicateFilter,
 ) -> &'tcx [(ty::Clause<'tcx>, Span)] {
     let item_ty = Ty::new_projection(
         tcx,
@@ -27,7 +29,7 @@ fn associated_type_bounds<'tcx>(
     );
 
     let icx = ItemCtxt::new(tcx, assoc_item_def_id);
-    let mut bounds = icx.astconv().compute_bounds(item_ty, ast_bounds, PredicateFilter::All);
+    let mut bounds = icx.astconv().compute_bounds(item_ty, ast_bounds, filter);
     // Associated types are implicitly sized unless a `?Sized` bound is found
     icx.astconv().add_implicitly_sized(&mut bounds, item_ty, ast_bounds, None, span);
 
@@ -63,10 +65,11 @@ fn opaque_type_bounds<'tcx>(
     ast_bounds: &'tcx [hir::GenericBound<'tcx>],
     item_ty: Ty<'tcx>,
     span: Span,
+    filter: PredicateFilter,
 ) -> &'tcx [(ty::Clause<'tcx>, Span)] {
     ty::print::with_reduced_queries!({
         let icx = ItemCtxt::new(tcx, opaque_def_id);
-        let mut bounds = icx.astconv().compute_bounds(item_ty, ast_bounds, PredicateFilter::All);
+        let mut bounds = icx.astconv().compute_bounds(item_ty, ast_bounds, filter);
         // Opaque types are implicitly sized unless a `?Sized` bound is found
         icx.astconv().add_implicitly_sized(&mut bounds, item_ty, ast_bounds, None, span);
         debug!(?bounds);
@@ -78,6 +81,21 @@ fn opaque_type_bounds<'tcx>(
 pub(super) fn explicit_item_bounds(
     tcx: TyCtxt<'_>,
     def_id: LocalDefId,
+) -> ty::EarlyBinder<&'_ [(ty::Clause<'_>, Span)]> {
+    explicit_item_bounds_with_filter(tcx, def_id, PredicateFilter::All)
+}
+
+pub(super) fn explicit_item_super_predicates(
+    tcx: TyCtxt<'_>,
+    def_id: LocalDefId,
+) -> ty::EarlyBinder<&'_ [(ty::Clause<'_>, Span)]> {
+    explicit_item_bounds_with_filter(tcx, def_id, PredicateFilter::SelfOnly)
+}
+
+pub(super) fn explicit_item_bounds_with_filter(
+    tcx: TyCtxt<'_>,
+    def_id: LocalDefId,
+    filter: PredicateFilter,
 ) -> ty::EarlyBinder<&'_ [(ty::Clause<'_>, Span)]> {
     match tcx.opt_rpitit_info(def_id.to_def_id()) {
         // RPITIT's bounds are the same as opaque type bounds, but with
@@ -95,6 +113,7 @@ pub(super) fn explicit_item_bounds(
                     ty::GenericArgs::identity_for_item(tcx, def_id),
                 ),
                 item.span,
+                filter,
             ));
         }
         Some(ty::ImplTraitInTraitData::Impl { .. }) => span_bug!(
@@ -109,7 +128,7 @@ pub(super) fn explicit_item_bounds(
             kind: hir::TraitItemKind::Type(bounds, _),
             span,
             ..
-        }) => associated_type_bounds(tcx, def_id, bounds, *span),
+        }) => associated_type_bounds(tcx, def_id, bounds, *span, filter),
         hir::Node::Item(hir::Item {
             kind: hir::ItemKind::OpaqueTy(hir::OpaqueTy { bounds, in_trait: false, .. }),
             span,
@@ -117,7 +136,7 @@ pub(super) fn explicit_item_bounds(
         }) => {
             let args = GenericArgs::identity_for_item(tcx, def_id);
             let item_ty = Ty::new_opaque(tcx, def_id.to_def_id(), args);
-            opaque_type_bounds(tcx, def_id, bounds, item_ty, *span)
+            opaque_type_bounds(tcx, def_id, bounds, item_ty, *span, filter)
         }
         // Since RPITITs are astconv'd as projections in `ast_ty_to_ty`, when we're asking
         // for the item bounds of the *opaques* in a trait's default method signature, we
@@ -135,7 +154,7 @@ pub(super) fn explicit_item_bounds(
             let args = GenericArgs::identity_for_item(tcx, def_id);
             let item_ty = Ty::new_opaque(tcx, def_id.to_def_id(), args);
             tcx.arena.alloc_slice(
-                &opaque_type_bounds(tcx, def_id, bounds, item_ty, *span)
+                &opaque_type_bounds(tcx, def_id, bounds, item_ty, *span, filter)
                     .to_vec()
                     .fold_with(&mut AssocTyToOpaque { tcx, fn_def_id: fn_def_id.to_def_id() }),
             )
@@ -155,6 +174,31 @@ pub(super) fn item_bounds(
     })
 }
 
+pub(super) fn item_super_predicates(
+    tcx: TyCtxt<'_>,
+    def_id: DefId,
+) -> ty::EarlyBinder<&'_ ty::List<ty::Clause<'_>>> {
+    tcx.explicit_item_super_predicates(def_id).map_bound(|bounds| {
+        tcx.mk_clauses_from_iter(
+            util::elaborate(tcx, bounds.iter().map(|&(bound, _span)| bound)).filter_only_self(),
+        )
+    })
+}
+
+pub(super) fn item_non_self_assumptions(
+    tcx: TyCtxt<'_>,
+    def_id: DefId,
+) -> ty::EarlyBinder<&'_ ty::List<ty::Clause<'_>>> {
+    let all_bounds: FxIndexSet<_> = tcx.item_bounds(def_id).skip_binder().iter().collect();
+    let own_bounds: FxIndexSet<_> =
+        tcx.item_super_predicates(def_id).skip_binder().iter().collect();
+    if all_bounds.len() == own_bounds.len() {
+        ty::EarlyBinder::bind(ty::List::empty())
+    } else {
+        ty::EarlyBinder::bind(tcx.mk_clauses_from_iter(all_bounds.difference(&own_bounds).copied()))
+    }
+}
+
 struct AssocTyToOpaque<'tcx> {
     tcx: TyCtxt<'tcx>,
     fn_def_id: DefId,

compiler/rustc_hir_typeck/src/_match.rs

@@ -645,7 +645,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 for ty in [first_ty, second_ty] {
                     for (clause, _) in self
                         .tcx
-                        .explicit_item_bounds(rpit_def_id)
+                        .explicit_item_super_predicates(rpit_def_id)
                         .iter_instantiated_copied(self.tcx, args)
                     {
                         let pred = clause.kind().rebind(match clause.kind().skip_binder() {

compiler/rustc_hir_typeck/src/closure.rs

@@ -329,7 +329,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                     expected_ty,
                     closure_kind,
                     self.tcx
-                        .explicit_item_bounds(def_id)
+                        .explicit_item_super_predicates(def_id)
                         .iter_instantiated_copied(self.tcx, args)
                         .map(|(c, s)| (c.as_predicate(), s)),
                 ),
@@ -906,7 +906,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             }
             ty::Alias(ty::Opaque, ty::AliasTy { def_id, args, .. }) => self
                 .tcx
-                .explicit_item_bounds(def_id)
+                .explicit_item_super_predicates(def_id)
                 .iter_instantiated_copied(self.tcx, args)
                 .find_map(|(p, s)| get_future_output(p.as_predicate(), s))?,
             ty::Error(_) => return Some(ret_ty),

compiler/rustc_infer/src/infer/error_reporting/mod.rs

@@ -403,8 +403,10 @@ impl<'tcx> InferCtxt<'tcx> {
         let future_trait = self.tcx.require_lang_item(LangItem::Future, None);
         let item_def_id = self.tcx.associated_item_def_ids(future_trait)[0];
 
-        self.tcx.explicit_item_bounds(def_id).iter_instantiated_copied(self.tcx, args).find_map(
+        self.tcx
-            |(predicate, _)| {
+            .explicit_item_super_predicates(def_id)
+            .iter_instantiated_copied(self.tcx, args)
+            .find_map(|(predicate, _)| {
                 predicate
                     .kind()
                     .map_bound(|kind| match kind {
@@ -417,8 +419,7 @@ impl<'tcx> InferCtxt<'tcx> {
                     })
                     .no_bound_vars()
                     .flatten()
-            },
+            })
-        )
     }
 }
 

compiler/rustc_infer/src/infer/error_reporting/note_and_explain.rs

@@ -299,8 +299,11 @@ impl<T> Trait<T> for X {
                     }
                     (ty::Dynamic(t, _, ty::DynKind::Dyn), ty::Alias(ty::Opaque, alias))
                         if let Some(def_id) = t.principal_def_id()
-                            && tcx.explicit_item_bounds(alias.def_id).skip_binder().iter().any(
+                            && tcx
-                                |(pred, _span)| match pred.kind().skip_binder() {
+                                .explicit_item_super_predicates(alias.def_id)
+                                .skip_binder()
+                                .iter()
+                                .any(|(pred, _span)| match pred.kind().skip_binder() {
                                     ty::ClauseKind::Trait(trait_predicate)
                                         if trait_predicate.polarity
                                             == ty::ImplPolarity::Positive =>
@@ -308,8 +311,7 @@ impl<T> Trait<T> for X {
                                         trait_predicate.def_id() == def_id
                                     }
                                     _ => false,
-                                },
+                                }) =>
-                            ) =>
                     {
                         diag.help(format!(
                             "you can box the `{}` to coerce it to `Box<{}>`, but you'll have to \
@@ -412,7 +414,7 @@ impl<T> Trait<T> for X {
                             ty::Alias(..) => values.expected,
                             _ => values.found,
                         };
-                        let preds = tcx.explicit_item_bounds(opaque_ty.def_id);
+                        let preds = tcx.explicit_item_super_predicates(opaque_ty.def_id);
                         for (pred, _span) in preds.skip_binder() {
                             let ty::ClauseKind::Trait(trait_predicate) = pred.kind().skip_binder()
                             else {

compiler/rustc_infer/src/infer/outlives/verify.rs

@@ -300,7 +300,7 @@ impl<'cx, 'tcx> VerifyBoundCx<'cx, 'tcx> {
         alias_ty: ty::AliasTy<'tcx>,
     ) -> impl Iterator<Item = ty::Region<'tcx>> {
         let tcx = self.tcx;
-        let bounds = tcx.item_bounds(alias_ty.def_id);
+        let bounds = tcx.item_super_predicates(alias_ty.def_id);
         trace!("{:#?}", bounds.skip_binder());
         bounds
             .iter_instantiated(tcx, alias_ty.args)

compiler/rustc_lint/src/unused.rs

@@ -295,7 +295,9 @@ impl<'tcx> LateLintPass<'tcx> for UnusedResults {
                 ty::Alias(ty::Opaque | ty::Projection, ty::AliasTy { def_id: def, .. }) => {
                     elaborate(
                         cx.tcx,
-                        cx.tcx.explicit_item_bounds(def).instantiate_identity_iter_copied(),
+                        cx.tcx
+                            .explicit_item_super_predicates(def)
+                            .instantiate_identity_iter_copied(),
                     )
                     // We only care about self bounds for the impl-trait
                     .filter_only_self()

compiler/rustc_metadata/src/rmeta/decoder.rs

@@ -1063,6 +1063,20 @@ impl<'a, 'tcx> CrateMetadataRef<'a> {
         ty::EarlyBinder::bind(&*output)
     }
 
+    fn get_explicit_item_super_predicates(
+        self,
+        index: DefIndex,
+        tcx: TyCtxt<'tcx>,
+    ) -> ty::EarlyBinder<&'tcx [(ty::Clause<'tcx>, Span)]> {
+        let lazy = self.root.tables.explicit_item_super_predicates.get(self, index);
+        let output = if lazy.is_default() {
+            &mut []
+        } else {
+            tcx.arena.alloc_from_iter(lazy.decode((self, tcx)))
+        };
+        ty::EarlyBinder::bind(&*output)
+    }
+
     fn get_variant(
         self,
         kind: DefKind,

compiler/rustc_metadata/src/rmeta/decoder/cstore_impl.rs

@@ -206,6 +206,7 @@ impl IntoArgs for (CrateNum, SimplifiedType) {
 
 provide! { tcx, def_id, other, cdata,
     explicit_item_bounds => { cdata.get_explicit_item_bounds(def_id.index, tcx) }
+    explicit_item_super_predicates => { cdata.get_explicit_item_super_predicates(def_id.index, tcx) }
     explicit_predicates_of => { table }
     generics_of => { table }
     inferred_outlives_of => { table_defaulted_array }

compiler/rustc_metadata/src/rmeta/encoder.rs

@@ -1491,6 +1491,7 @@ impl<'a, 'tcx> EncodeContext<'a, 'tcx> {
             }
             if let DefKind::OpaqueTy = def_kind {
                 self.encode_explicit_item_bounds(def_id);
+                self.encode_explicit_item_super_predicates(def_id);
                 self.tables
                     .is_type_alias_impl_trait
                     .set(def_id.index, self.tcx.is_type_alias_impl_trait(def_id));
@@ -1599,6 +1600,12 @@ impl<'a, 'tcx> EncodeContext<'a, 'tcx> {
         record_defaulted_array!(self.tables.explicit_item_bounds[def_id] <- bounds);
     }
 
+    fn encode_explicit_item_super_predicates(&mut self, def_id: DefId) {
+        debug!("EncodeContext::encode_explicit_item_super_predicates({:?})", def_id);
+        let bounds = self.tcx.explicit_item_super_predicates(def_id).skip_binder();
+        record_defaulted_array!(self.tables.explicit_item_super_predicates[def_id] <- bounds);
+    }
+
     #[instrument(level = "debug", skip(self))]
     fn encode_info_for_assoc_item(&mut self, def_id: DefId) {
         let tcx = self.tcx;
@@ -1611,6 +1618,7 @@ impl<'a, 'tcx> EncodeContext<'a, 'tcx> {
             AssocItemContainer::TraitContainer => {
                 if let ty::AssocKind::Type = item.kind {
                     self.encode_explicit_item_bounds(def_id);
+                    self.encode_explicit_item_super_predicates(def_id);
                 }
             }
             AssocItemContainer::ImplContainer => {

compiler/rustc_metadata/src/rmeta/mod.rs

@@ -387,6 +387,7 @@ define_tables! {
     // corresponding DefPathHash.
     def_path_hashes: Table<DefIndex, u64>,
     explicit_item_bounds: Table<DefIndex, LazyArray<(ty::Clause<'static>, Span)>>,
+    explicit_item_super_predicates: Table<DefIndex, LazyArray<(ty::Clause<'static>, Span)>>,
     inferred_outlives_of: Table<DefIndex, LazyArray<(ty::Clause<'static>, Span)>>,
     inherent_impls: Table<DefIndex, LazyArray<DefIndex>>,
     associated_types_for_impl_traits_in_associated_fn: Table<DefIndex, LazyArray<DefId>>,

compiler/rustc_middle/src/query/mod.rs

@@ -343,11 +343,14 @@ rustc_queries! {
         }
     }
 
-    /// Returns the list of bounds that can be used for
+    /// Returns the list of bounds that are required to be satsified
-    /// `SelectionCandidate::ProjectionCandidate(_)` and
+    /// by a implementation or definition. For associated types, these
-    /// `ProjectionTyCandidate::TraitDef`.
+    /// must be satisfied for an implementation to be well-formed,
-    /// Specifically this is the bounds written on the trait's type
+    /// and for opaque types, these are required to be satisfied by
-    /// definition, or those after the `impl` keyword
+    /// the hidden-type of the opaque.
+    ///
+    /// Syntactially, these are the bounds written on the trait's type
+    /// definition, or those after the `impl` keyword for an opaque:
     ///
     /// ```ignore (incomplete)
     /// type X: Bound + 'lt
@@ -363,7 +366,16 @@ rustc_queries! {
         desc { |tcx| "finding item bounds for `{}`", tcx.def_path_str(key) }
         cache_on_disk_if { key.is_local() }
         separate_provide_extern
-        feedable
+    }
+
+    /// The set of item bounds (see [`TyCtxt::explicit_item_bounds`]) that
+    /// share the `Self` type of the item. These are a subset of the bounds
+    /// that may explicitly be used for things like closure signature
+    /// deduction.
+    query explicit_item_super_predicates(key: DefId) -> ty::EarlyBinder<&'tcx [(ty::Clause<'tcx>, Span)]> {
+        desc { |tcx| "finding item bounds for `{}`", tcx.def_path_str(key) }
+        cache_on_disk_if { key.is_local() }
+        separate_provide_extern
     }
 
     /// Elaborated version of the predicates from `explicit_item_bounds`.
@@ -390,6 +402,14 @@ rustc_queries! {
         desc { |tcx| "elaborating item bounds for `{}`", tcx.def_path_str(key) }
     }
 
+    query item_super_predicates(key: DefId) -> ty::EarlyBinder<&'tcx ty::List<ty::Clause<'tcx>>> {
+        desc { |tcx| "elaborating item assumptions for `{}`", tcx.def_path_str(key) }
+    }
+
+    query item_non_self_assumptions(key: DefId) -> ty::EarlyBinder<&'tcx ty::List<ty::Clause<'tcx>>> {
+        desc { |tcx| "elaborating item assumptions for `{}`", tcx.def_path_str(key) }
+    }
+
     /// Look up all native libraries this crate depends on.
     /// These are assembled from the following places:
     /// - `extern` blocks (depending on their `link` attributes)

compiler/rustc_middle/src/ty/context.rs

@@ -1828,7 +1828,7 @@ impl<'tcx> TyCtxt<'tcx> {
         let ty::Alias(ty::Opaque, ty::AliasTy { def_id, .. }) = ty.kind() else { return false };
         let future_trait = self.require_lang_item(LangItem::Future, None);
 
-        self.explicit_item_bounds(def_id).skip_binder().iter().any(|&(predicate, _)| {
+        self.explicit_item_super_predicates(def_id).skip_binder().iter().any(|&(predicate, _)| {
             let ty::ClauseKind::Trait(trait_predicate) = predicate.kind().skip_binder() else {
                 return false;
             };

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

@@ -1098,8 +1098,11 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
                         ))
                     }
                     ty::Alias(ty::Opaque, ty::AliasTy { def_id, args, .. }) => {
-                        self.tcx.item_bounds(def_id).instantiate(self.tcx, args).iter().find_map(
+                        self.tcx
-                            |pred| {
+                            .item_super_predicates(def_id)
+                            .instantiate(self.tcx, args)
+                            .iter()
+                            .find_map(|pred| {
                                 if let ty::ClauseKind::Projection(proj) = pred.kind().skip_binder()
                         && Some(proj.projection_ty.def_id) == self.tcx.lang_items().fn_once_output()
                         // args tuple will always be args[1]
@@ -1113,8 +1116,7 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
                                 } else {
                                     None
                                 }
-                            },
+                            })
-                        )
                     }
                     ty::Dynamic(data, _, ty::Dyn) => {
                         data.iter().find_map(|pred| {

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

@@ -1617,21 +1617,17 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 _ => return ControlFlow::Continue(()),
             };
 
-            for bound in
-                self.tcx().item_bounds(alias_ty.def_id).instantiate(self.tcx(), alias_ty.args)
-            {
             // HACK: On subsequent recursions, we only care about bounds that don't
             // share the same type as `self_ty`. This is because for truly rigid
             // projections, we will never be able to equate, e.g. `<T as Tr>::A`
             // with `<<T as Tr>::A as Tr>::A`.
-                if in_parent_alias_type {
+            let relevant_bounds = if in_parent_alias_type {
-                    match bound.kind().skip_binder() {
+                self.tcx().item_non_self_assumptions(alias_ty.def_id)
-                        ty::ClauseKind::Trait(tr) if tr.self_ty() == self_ty => continue,
+            } else {
-                        ty::ClauseKind::Projection(p) if p.self_ty() == self_ty => continue,
+                self.tcx().item_super_predicates(alias_ty.def_id)
-                        _ => {}
+            };
-                    }
-                }
 
+            for bound in relevant_bounds.instantiate(self.tcx(), alias_ty.args) {
                 for_each(self, bound, idx)?;
                 idx += 1;
             }

tests/ui/associated-type-bounds/dont-imply-atb-in-closure-inference.rs

@@ -0,0 +1,21 @@
+//@ check-pass
+
+#![feature(type_alias_impl_trait)]
+
+trait IsPtr {
+    type Assoc;
+}
+impl<T> IsPtr for T {
+    type Assoc = fn(i32);
+}
+
+type Tait = impl IsPtr<Assoc: Fn(i32)> + Fn(u32);
+
+fn hello()
+where
+    Tait:,
+{
+    let _: Tait = |x| {};
+}
+
+fn main() {}

tests/ui/lint/unused/unused-associated-item.rs

@@ -0,0 +1,21 @@
+//@ check-pass
+
+#![deny(unused_must_use)]
+
+use std::future::Future;
+use std::pin::Pin;
+
+trait Factory {
+    type Output;
+}
+
+impl Factory for () {
+    type Output = Pin<Box<dyn Future<Output = ()> + 'static>>;
+}
+
+// Make sure we don't get an `unused_must_use` error on the *associated type bound*.
+fn f() -> impl Factory<Output: Future> {}
+
+fn main() {
+    f();
+}