Merge from rustc

8 files changed, 176 insertions, 111 deletions

diff --git a/compiler/rustc_hir_analysis/src/check/wfcheck.rs b/compiler/rustc_hir_analysis/src/check/wfcheck.rs
index 0316ef69bf8..456dc4e4e07 100644
--- a/compiler/rustc_hir_analysis/src/check/wfcheck.rs
+++ b/compiler/rustc_hir_analysis/src/check/wfcheck.rs
@@ -831,7 +831,7 @@ impl<'tcx> TypeVisitor<TyCtxt<'tcx>> for GATArgsCollector<'tcx> {
 
 fn could_be_self(trait_def_id: LocalDefId, ty: &hir::Ty<'_>) -> bool {
     match ty.kind {
-        hir::TyKind::TraitObject([trait_ref], ..) => match trait_ref.trait_ref.path.segments {
+        hir::TyKind::TraitObject([(trait_ref, _)], ..) => match trait_ref.trait_ref.path.segments {
             [s] => s.res.opt_def_id() == Some(trait_def_id.to_def_id()),
             _ => false,
         },
diff --git a/compiler/rustc_hir_analysis/src/collect/resolve_bound_vars.rs b/compiler/rustc_hir_analysis/src/collect/resolve_bound_vars.rs
index 349dc9ad00e..02ea95852f0 100644
--- a/compiler/rustc_hir_analysis/src/collect/resolve_bound_vars.rs
+++ b/compiler/rustc_hir_analysis/src/collect/resolve_bound_vars.rs
@@ -652,7 +652,7 @@ impl<'a, 'tcx> Visitor<'tcx> for BoundVarContext<'a, 'tcx> {
                 debug!(?bounds, ?lifetime, "TraitObject");
                 let scope = Scope::TraitRefBoundary { s: self.scope };
                 self.with(scope, |this| {
-                    for bound in bounds {
+                    for (bound, _) in bounds {
                         this.visit_poly_trait_ref_inner(
                             bound,
                             NonLifetimeBinderAllowed::Deny("trait object types"),
diff --git a/compiler/rustc_hir_analysis/src/errors.rs b/compiler/rustc_hir_analysis/src/errors.rs
index c83788928a9..c364a561631 100644
--- a/compiler/rustc_hir_analysis/src/errors.rs
+++ b/compiler/rustc_hir_analysis/src/errors.rs
@@ -22,7 +22,7 @@ pub struct AmbiguousAssocItem<'a> {
     pub span: Span,
     pub assoc_kind: &'static str,
     pub assoc_name: Ident,
-    pub ty_param_name: &'a str,
+    pub qself: &'a str,
 }
 
 #[derive(Diagnostic)]
@@ -75,7 +75,7 @@ pub struct AssocItemNotFound<'a> {
     pub span: Span,
     pub assoc_name: Ident,
     pub assoc_kind: &'static str,
-    pub ty_param_name: &'a str,
+    pub qself: &'a str,
     #[subdiagnostic]
     pub label: Option<AssocItemNotFoundLabel<'a>>,
     #[subdiagnostic]
@@ -126,13 +126,32 @@ pub enum AssocItemNotFoundSugg<'a> {
         assoc_kind: &'static str,
         suggested_name: Symbol,
     },
-    #[suggestion(hir_analysis_assoc_item_not_found_other_sugg, code = "{suggested_name}")]
+    #[multipart_suggestion(
+        hir_analysis_assoc_item_not_found_similar_in_other_trait_qpath_sugg,
+        style = "verbose"
+    )]
+    SimilarInOtherTraitQPath {
+        #[suggestion_part(code = "<")]
+        lo: Span,
+        #[suggestion_part(code = " as {trait_ref}>")]
+        mi: Span,
+        #[suggestion_part(code = "{suggested_name}")]
+        hi: Option<Span>,
+        trait_ref: String,
+        suggested_name: Symbol,
+        identically_named: bool,
+        #[applicability]
+        applicability: Applicability,
+    },
+    #[suggestion(
+        hir_analysis_assoc_item_not_found_other_sugg,
+        code = "{suggested_name}",
+        applicability = "maybe-incorrect"
+    )]
     Other {
         #[primary_span]
         span: Span,
-        #[applicability]
-        applicability: Applicability,
-        ty_param_name: &'a str,
+        qself: &'a str,
         assoc_kind: &'static str,
         suggested_name: Symbol,
     },
diff --git a/compiler/rustc_hir_analysis/src/hir_ty_lowering/bounds.rs b/compiler/rustc_hir_analysis/src/hir_ty_lowering/bounds.rs
index 6f9c481650b..30c04aa47a3 100644
--- a/compiler/rustc_hir_analysis/src/hir_ty_lowering/bounds.rs
+++ b/compiler/rustc_hir_analysis/src/hir_ty_lowering/bounds.rs
@@ -6,19 +6,17 @@ use rustc_hir as hir;
 use rustc_hir::def::{DefKind, Res};
 use rustc_hir::def_id::{DefId, LocalDefId};
 use rustc_middle::bug;
-use rustc_middle::ty::print::PrintTraitRefExt as _;
 use rustc_middle::ty::{self as ty, IsSuggestable, Ty, TyCtxt};
 use rustc_span::symbol::Ident;
-use rustc_span::{ErrorGuaranteed, Span, Symbol};
+use rustc_span::{sym, ErrorGuaranteed, Span, Symbol};
 use rustc_trait_selection::traits;
 use rustc_type_ir::visit::{TypeSuperVisitable, TypeVisitable, TypeVisitableExt, TypeVisitor};
 use smallvec::SmallVec;
 
 use crate::bounds::Bounds;
 use crate::errors;
-use crate::hir_ty_lowering::{HirTyLowerer, OnlySelfBounds, PredicateFilter};
-
-use super::RegionInferReason;
+use crate::hir_ty_lowering::HirTyLowerer;
+use crate::hir_ty_lowering::{AssocItemQSelf, OnlySelfBounds, PredicateFilter, RegionInferReason};
 
 impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
     /// Add a `Sized` bound to the `bounds` if appropriate.
@@ -75,10 +73,22 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
             }
         }
 
+        let mut unique_bounds = FxIndexSet::default();
+        let mut seen_repeat = false;
+        for unbound in &unbounds {
+            if let Res::Def(DefKind::Trait, unbound_def_id) = unbound.trait_ref.path.res {
+                seen_repeat |= !unique_bounds.insert(unbound_def_id);
+            }
+        }
         if unbounds.len() > 1 {
-            self.dcx().emit_err(errors::MultipleRelaxedDefaultBounds {
+            let err = errors::MultipleRelaxedDefaultBounds {
                 spans: unbounds.iter().map(|ptr| ptr.span).collect(),
-            });
+            };
+            if seen_repeat {
+                self.dcx().emit_err(err);
+            } else if !tcx.features().more_maybe_bounds {
+                self.tcx().sess.create_feature_err(err, sym::more_maybe_bounds).emit();
+            };
         }
 
         let mut seen_sized_unbound = false;
@@ -288,8 +298,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
             // one that does define it.
             self.probe_single_bound_for_assoc_item(
                 || traits::supertraits(tcx, trait_ref),
-                trait_ref.skip_binder().print_only_trait_name(),
-                None,
+                AssocItemQSelf::Trait(trait_ref.def_id()),
                 assoc_kind,
                 constraint.ident,
                 path_span,
diff --git a/compiler/rustc_hir_analysis/src/hir_ty_lowering/errors.rs b/compiler/rustc_hir_analysis/src/hir_ty_lowering/errors.rs
index 8ff6ced8b39..20f06d77489 100644
--- a/compiler/rustc_hir_analysis/src/hir_ty_lowering/errors.rs
+++ b/compiler/rustc_hir_analysis/src/hir_ty_lowering/errors.rs
@@ -3,7 +3,7 @@ use crate::errors::{
     ParenthesizedFnTraitExpansion, TraitObjectDeclaredWithNoTraits,
 };
 use crate::fluent_generated as fluent;
-use crate::hir_ty_lowering::HirTyLowerer;
+use crate::hir_ty_lowering::{AssocItemQSelf, HirTyLowerer};
 use rustc_data_structures::fx::{FxIndexMap, FxIndexSet};
 use rustc_data_structures::sorted_map::SortedMap;
 use rustc_data_structures::unord::UnordMap;
@@ -11,9 +11,9 @@ use rustc_errors::MultiSpan;
 use rustc_errors::{
     codes::*, pluralize, struct_span_code_err, Applicability, Diag, ErrorGuaranteed,
 };
+use rustc_hir as hir;
 use rustc_hir::def::{DefKind, Res};
-use rustc_hir::def_id::{DefId, LocalDefId};
-use rustc_hir::{self as hir, Node};
+use rustc_hir::def_id::DefId;
 use rustc_middle::bug;
 use rustc_middle::query::Key;
 use rustc_middle::ty::print::{PrintPolyTraitRefExt as _, PrintTraitRefExt as _};
@@ -116,8 +116,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
     pub(super) fn complain_about_assoc_item_not_found<I>(
         &self,
         all_candidates: impl Fn() -> I,
-        ty_param_name: &str,
-        ty_param_def_id: Option<LocalDefId>,
+        qself: AssocItemQSelf,
         assoc_kind: ty::AssocKind,
         assoc_name: Ident,
         span: Span,
@@ -139,7 +138,8 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
             );
         }
 
-        let assoc_kind_str = super::assoc_kind_str(assoc_kind);
+        let assoc_kind_str = assoc_kind_str(assoc_kind);
+        let qself_str = qself.to_string(tcx);
 
         // The fallback span is needed because `assoc_name` might be an `Fn()`'s `Output` without a
         // valid span, so we point at the whole path segment instead.
@@ -149,7 +149,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
             span: if is_dummy { span } else { assoc_name.span },
             assoc_name,
             assoc_kind: assoc_kind_str,
-            ty_param_name,
+            qself: &qself_str,
             label: None,
             sugg: None,
         };
@@ -219,19 +219,28 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
                     suggested_name,
                     identically_named: suggested_name == assoc_name.name,
                 });
-                let hir = tcx.hir();
-                if let Some(def_id) = ty_param_def_id
-                    && let parent = hir.get_parent_item(tcx.local_def_id_to_hir_id(def_id))
-                    && let Some(generics) = hir.get_generics(parent.def_id)
+                if let AssocItemQSelf::TyParam(ty_param_def_id, ty_param_span) = qself
+                    // Not using `self.item_def_id()` here as that would yield the opaque type itself if we're
+                    // inside an opaque type while we're interested in the overarching type alias (TAIT).
+                    // FIXME: However, for trait aliases, this incorrectly returns the enclosing module...
+                    && let item_def_id =
+                        tcx.hir().get_parent_item(tcx.local_def_id_to_hir_id(ty_param_def_id))
+                    // FIXME: ...which obviously won't have any generics.
+                    && let Some(generics) = tcx.hir().get_generics(item_def_id.def_id)
                 {
-                    if generics.bounds_for_param(def_id).flat_map(|pred| pred.bounds.iter()).any(
-                        |b| match b {
+                    // FIXME: Suggest adding supertrait bounds if we have a `Self` type param.
+                    // FIXME(trait_alias): Suggest adding `Self: Trait` to
+                    // `trait Alias = where Self::Proj:;` with `trait Trait { type Proj; }`.
+                    if generics
+                        .bounds_for_param(ty_param_def_id)
+                        .flat_map(|pred| pred.bounds.iter())
+                        .any(|b| match b {
                             hir::GenericBound::Trait(t, ..) => {
                                 t.trait_ref.trait_def_id() == Some(best_trait)
                             }
                             _ => false,
-                        },
-                    ) {
+                        })
+                    {
                         // The type param already has a bound for `trait_name`, we just need to
                         // change the associated item.
                         err.sugg = Some(errors::AssocItemNotFoundSugg::SimilarInOtherTrait {
@@ -242,48 +251,60 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
                         return self.dcx().emit_err(err);
                     }
 
-                    let mut err = self.dcx().create_err(err);
-                    if suggest_constraining_type_param(
-                        tcx,
-                        generics,
-                        &mut err,
-                        &ty_param_name,
-                        &trait_name,
-                        None,
-                        None,
-                    ) && suggested_name != assoc_name.name
+                    let trait_args = &ty::GenericArgs::identity_for_item(tcx, best_trait)[1..];
+                    let mut trait_ref = trait_name.clone();
+                    let applicability = if let [arg, args @ ..] = trait_args {
+                        use std::fmt::Write;
+                        write!(trait_ref, "</* {arg}").unwrap();
+                        args.iter().try_for_each(|arg| write!(trait_ref, ", {arg}")).unwrap();
+                        trait_ref += " */>";
+                        Applicability::HasPlaceholders
+                    } else {
+                        Applicability::MaybeIncorrect
+                    };
+
+                    let identically_named = suggested_name == assoc_name.name;
+
+                    if let DefKind::TyAlias = tcx.def_kind(item_def_id)
+                        && !tcx.type_alias_is_lazy(item_def_id)
                     {
-                        // We suggested constraining a type parameter, but the associated item on it
-                        // was also not an exact match, so we also suggest changing it.
-                        err.span_suggestion_verbose(
-                            assoc_name.span,
-                            fluent::hir_analysis_assoc_item_not_found_similar_in_other_trait_with_bound_sugg,
+                        err.sugg = Some(errors::AssocItemNotFoundSugg::SimilarInOtherTraitQPath {
+                            lo: ty_param_span.shrink_to_lo(),
+                            mi: ty_param_span.shrink_to_hi(),
+                            hi: (!identically_named).then_some(assoc_name.span),
+                            trait_ref,
+                            identically_named,
                             suggested_name,
-                            Applicability::MaybeIncorrect,
-                        );
+                            applicability,
+                        });
+                    } else {
+                        let mut err = self.dcx().create_err(err);
+                        if suggest_constraining_type_param(
+                            tcx, generics, &mut err, &qself_str, &trait_ref, None, None,
+                        ) && !identically_named
+                        {
+                            // We suggested constraining a type parameter, but the associated item on it
+                            // was also not an exact match, so we also suggest changing it.
+                            err.span_suggestion_verbose(
+                                assoc_name.span,
+                                fluent::hir_analysis_assoc_item_not_found_similar_in_other_trait_with_bound_sugg,
+                                suggested_name,
+                                Applicability::MaybeIncorrect,
+                            );
+                        }
+                        return err.emit();
                     }
-                    return err.emit();
                 }
                 return self.dcx().emit_err(err);
             }
         }
 
         // If we still couldn't find any associated item, and only one associated item exists,
-        // suggests using it.
+        // suggest using it.
         if let [candidate_name] = all_candidate_names.as_slice() {
-            // This should still compile, except on `#![feature(associated_type_defaults)]`
-            // where it could suggests `type A = Self::A`, thus recursing infinitely.
-            let applicability =
-                if assoc_kind == ty::AssocKind::Type && tcx.features().associated_type_defaults {
-                    Applicability::Unspecified
-                } else {
-                    Applicability::MaybeIncorrect
-                };
-
             err.sugg = Some(errors::AssocItemNotFoundSugg::Other {
                 span: assoc_name.span,
-                applicability,
-                ty_param_name,
+                qself: &qself_str,
                 assoc_kind: assoc_kind_str,
                 suggested_name: *candidate_name,
             });
@@ -349,10 +370,10 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
 
         self.dcx().emit_err(errors::AssocKindMismatch {
             span,
-            expected: super::assoc_kind_str(expected),
-            got: super::assoc_kind_str(got),
+            expected: assoc_kind_str(expected),
+            got: assoc_kind_str(got),
             expected_because_label,
-            assoc_kind: super::assoc_kind_str(assoc_item.kind),
+            assoc_kind: assoc_kind_str(assoc_item.kind),
             def_span: tcx.def_span(assoc_item.def_id),
             bound_on_assoc_const_label,
             wrap_in_braces_sugg,
@@ -698,7 +719,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
         &self,
         associated_types: FxIndexMap<Span, FxIndexSet<DefId>>,
         potential_assoc_types: Vec<usize>,
-        trait_bounds: &[hir::PolyTraitRef<'_>],
+        trait_bounds: &[(hir::PolyTraitRef<'_>, hir::TraitBoundModifier)],
     ) {
         if associated_types.values().all(|v| v.is_empty()) {
             return;
@@ -744,12 +765,12 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
         // related to issue #91997, turbofishes added only when in an expr or pat
         let mut in_expr_or_pat = false;
         if let ([], [bound]) = (&potential_assoc_types[..], &trait_bounds) {
-            let grandparent = tcx.parent_hir_node(tcx.parent_hir_id(bound.trait_ref.hir_ref_id));
+            let grandparent = tcx.parent_hir_node(tcx.parent_hir_id(bound.0.trait_ref.hir_ref_id));
             in_expr_or_pat = match grandparent {
-                Node::Expr(_) | Node::Pat(_) => true,
+                hir::Node::Expr(_) | hir::Node::Pat(_) => true,
                 _ => false,
             };
-            match bound.trait_ref.path.segments {
+            match bound.0.trait_ref.path.segments {
                 // FIXME: `trait_ref.path.span` can point to a full path with multiple
                 // segments, even though `trait_ref.path.segments` is of length `1`. Work
                 // around that bug here, even though it should be fixed elsewhere.
@@ -790,7 +811,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
         // and we can then use their span to indicate this to the user.
         let bound_names = trait_bounds
             .iter()
-            .filter_map(|poly_trait_ref| {
+            .filter_map(|(poly_trait_ref, _)| {
                 let path = poly_trait_ref.trait_ref.path.segments.last()?;
                 let args = path.args?;
 
@@ -1612,3 +1633,11 @@ fn generics_args_err_extend<'a>(
         _ => {}
     }
 }
+
+pub(super) fn assoc_kind_str(kind: ty::AssocKind) -> &'static str {
+    match kind {
+        ty::AssocKind::Fn => "function",
+        ty::AssocKind::Const => "constant",
+        ty::AssocKind::Type => "type",
+    }
+}
diff --git a/compiler/rustc_hir_analysis/src/hir_ty_lowering/lint.rs b/compiler/rustc_hir_analysis/src/hir_ty_lowering/lint.rs
index 29c71c3fa50..e7aad0a29c5 100644
--- a/compiler/rustc_hir_analysis/src/hir_ty_lowering/lint.rs
+++ b/compiler/rustc_hir_analysis/src/hir_ty_lowering/lint.rs
@@ -34,7 +34,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
                 .ok()
                 .is_some_and(|s| s.trim_end().ends_with('<'));
 
-        let is_global = poly_trait_ref.trait_ref.path.is_global();
+        let is_global = poly_trait_ref.0.trait_ref.path.is_global();
 
         let mut sugg = vec![(
             self_ty.span.shrink_to_lo(),
@@ -176,7 +176,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
         let mut is_downgradable = true;
         let is_object_safe = match self_ty.kind {
             hir::TyKind::TraitObject(objects, ..) => {
-                objects.iter().all(|o| match o.trait_ref.path.res {
+                objects.iter().all(|(o, _)| match o.trait_ref.path.res {
                     Res::Def(DefKind::Trait, id) => {
                         if Some(id) == owner {
                             // For recursive traits, don't downgrade the error. (#119652)
diff --git a/compiler/rustc_hir_analysis/src/hir_ty_lowering/mod.rs b/compiler/rustc_hir_analysis/src/hir_ty_lowering/mod.rs
index d6eb1a66902..ce298641e60 100644
--- a/compiler/rustc_hir_analysis/src/hir_ty_lowering/mod.rs
+++ b/compiler/rustc_hir_analysis/src/hir_ty_lowering/mod.rs
@@ -55,7 +55,6 @@ use rustc_trait_selection::infer::InferCtxtExt;
 use rustc_trait_selection::traits::wf::object_region_bounds;
 use rustc_trait_selection::traits::{self, ObligationCtxt};
 
-use std::fmt::Display;
 use std::slice;
 
 /// A path segment that is semantically allowed to have generic arguments.
@@ -193,6 +192,25 @@ pub trait HirTyLowerer<'tcx> {
     }
 }
 
+/// The "qualified self" of an associated item path.
+///
+/// For diagnostic purposes only.
+enum AssocItemQSelf {
+    Trait(DefId),
+    TyParam(LocalDefId, Span),
+    SelfTyAlias,
+}
+
+impl AssocItemQSelf {
+    fn to_string(&self, tcx: TyCtxt<'_>) -> String {
+        match *self {
+            Self::Trait(def_id) => tcx.def_path_str(def_id),
+            Self::TyParam(def_id, _) => tcx.hir().ty_param_name(def_id).to_string(),
+            Self::SelfTyAlias => kw::SelfUpper.to_string(),
+        }
+    }
+}
+
 /// New-typed boolean indicating whether explicit late-bound lifetimes
 /// are present in a set of generic arguments.
 ///
@@ -802,6 +820,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
     fn probe_single_ty_param_bound_for_assoc_ty(
         &self,
         ty_param_def_id: LocalDefId,
+        ty_param_span: Span,
         assoc_name: Ident,
         span: Span,
     ) -> Result<ty::PolyTraitRef<'tcx>, ErrorGuaranteed> {
@@ -811,19 +830,14 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
         let predicates = &self.probe_ty_param_bounds(span, ty_param_def_id, assoc_name).predicates;
         debug!("predicates={:#?}", predicates);
 
-        let param_name = tcx.hir().ty_param_name(ty_param_def_id);
         self.probe_single_bound_for_assoc_item(
             || {
-                traits::transitive_bounds_that_define_assoc_item(
-                    tcx,
-                    predicates
-                        .iter()
-                        .filter_map(|(p, _)| Some(p.as_trait_clause()?.map_bound(|t| t.trait_ref))),
-                    assoc_name,
-                )
+                let trait_refs = predicates
+                    .iter()
+                    .filter_map(|(p, _)| Some(p.as_trait_clause()?.map_bound(|t| t.trait_ref)));
+                traits::transitive_bounds_that_define_assoc_item(tcx, trait_refs, assoc_name)
             },
-            param_name,
-            Some(ty_param_def_id),
+            AssocItemQSelf::TyParam(ty_param_def_id, ty_param_span),
             ty::AssocKind::Type,
             assoc_name,
             span,
@@ -835,12 +849,11 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
     ///
     /// This fails if there is no such bound in the list of candidates or if there are multiple
     /// candidates in which case it reports ambiguity.
-    #[instrument(level = "debug", skip(self, all_candidates, ty_param_name, constraint), ret)]
+    #[instrument(level = "debug", skip(self, all_candidates, qself, constraint), ret)]
     fn probe_single_bound_for_assoc_item<I>(
         &self,
         all_candidates: impl Fn() -> I,
-        ty_param_name: impl Display,
-        ty_param_def_id: Option<LocalDefId>,
+        qself: AssocItemQSelf,
         assoc_kind: ty::AssocKind,
         assoc_name: Ident,
         span: Span,
@@ -858,8 +871,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
         let Some(bound) = matching_candidates.next() else {
             let reported = self.complain_about_assoc_item_not_found(
                 all_candidates,
-                &ty_param_name.to_string(),
-                ty_param_def_id,
+                qself,
                 assoc_kind,
                 assoc_name,
                 span,
@@ -872,13 +884,13 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
         if let Some(bound2) = matching_candidates.next() {
             debug!(?bound2);
 
-            let assoc_kind_str = assoc_kind_str(assoc_kind);
-            let ty_param_name = &ty_param_name.to_string();
+            let assoc_kind_str = errors::assoc_kind_str(assoc_kind);
+            let qself_str = qself.to_string(tcx);
             let mut err = self.dcx().create_err(crate::errors::AmbiguousAssocItem {
                 span,
                 assoc_kind: assoc_kind_str,
                 assoc_name,
-                ty_param_name,
+                qself: &qself_str,
             });
             // Provide a more specific error code index entry for equality bindings.
             err.code(
@@ -929,7 +941,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
                         err.span_suggestion_verbose(
                             span.with_hi(assoc_name.span.lo()),
                             "use fully-qualified syntax to disambiguate",
-                            format!("<{ty_param_name} as {}>::", bound.print_only_trait_path()),
+                            format!("<{qself_str} as {}>::", bound.print_only_trait_path()),
                             Applicability::MaybeIncorrect,
                         );
                     }
@@ -943,7 +955,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
             if !where_bounds.is_empty() {
                 err.help(format!(
                     "consider introducing a new type parameter `T` and adding `where` constraints:\
-                     \n    where\n        T: {ty_param_name},\n{}",
+                     \n    where\n        T: {qself_str},\n{}",
                     where_bounds.join(",\n"),
                 ));
             }
@@ -997,11 +1009,6 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
         let tcx = self.tcx();
 
         let assoc_ident = assoc_segment.ident;
-        let qself_res = if let hir::TyKind::Path(hir::QPath::Resolved(_, path)) = &qself.kind {
-            path.res
-        } else {
-            Res::Err
-        };
 
         // Check if we have an enum variant or an inherent associated type.
         let mut variant_resolution = None;
@@ -1038,6 +1045,12 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
             }
         }
 
+        let qself_res = if let hir::TyKind::Path(hir::QPath::Resolved(_, path)) = &qself.kind {
+            path.res
+        } else {
+            Res::Err
+        };
+
         // Find the type of the associated item, and the trait where the associated
         // item is declared.
         let bound = match (&qself_ty.kind(), qself_res) {
@@ -1056,8 +1069,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
                             ty::Binder::dummy(trait_ref.instantiate_identity()),
                         )
                     },
-                    kw::SelfUpper,
-                    None,
+                    AssocItemQSelf::SelfTyAlias,
                     ty::AssocKind::Type,
                     assoc_ident,
                     span,
@@ -1069,6 +1081,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
                 Res::SelfTyParam { trait_: param_did } | Res::Def(DefKind::TyParam, param_did),
             ) => self.probe_single_ty_param_bound_for_assoc_ty(
                 param_did.expect_local(),
+                qself.span,
                 assoc_ident,
                 span,
             )?,
@@ -2522,11 +2535,3 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
         Some(r)
     }
 }
-
-fn assoc_kind_str(kind: ty::AssocKind) -> &'static str {
-    match kind {
-        ty::AssocKind::Fn => "function",
-        ty::AssocKind::Const => "constant",
-        ty::AssocKind::Type => "type",
-    }
-}
diff --git a/compiler/rustc_hir_analysis/src/hir_ty_lowering/object_safety.rs b/compiler/rustc_hir_analysis/src/hir_ty_lowering/object_safety.rs
index aafadc7f9cb..b3c7a1ff8a8 100644
--- a/compiler/rustc_hir_analysis/src/hir_ty_lowering/object_safety.rs
+++ b/compiler/rustc_hir_analysis/src/hir_ty_lowering/object_safety.rs
@@ -27,7 +27,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
         &self,
         span: Span,
         hir_id: hir::HirId,
-        hir_trait_bounds: &[hir::PolyTraitRef<'tcx>],
+        hir_trait_bounds: &[(hir::PolyTraitRef<'tcx>, hir::TraitBoundModifier)],
         lifetime: &hir::Lifetime,
         borrowed: bool,
         representation: DynKind,
@@ -37,7 +37,10 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
         let mut bounds = Bounds::default();
         let mut potential_assoc_types = Vec::new();
         let dummy_self = self.tcx().types.trait_object_dummy_self;
-        for trait_bound in hir_trait_bounds.iter().rev() {
+        for (trait_bound, modifier) in hir_trait_bounds.iter().rev() {
+            if *modifier == hir::TraitBoundModifier::Maybe {
+                continue;
+            }
             if let GenericArgCountResult {
                 correct:
                     Err(GenericArgCountMismatch { invalid_args: cur_potential_assoc_types, .. }),
@@ -249,7 +252,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
                 let args = tcx.mk_args(&args);
 
                 let span = i.bottom().1;
-                let empty_generic_args = hir_trait_bounds.iter().any(|hir_bound| {
+                let empty_generic_args = hir_trait_bounds.iter().any(|(hir_bound, _)| {
                     hir_bound.trait_ref.path.res == Res::Def(DefKind::Trait, trait_ref.def_id)
                         && hir_bound.span.contains(span)
                 });