Make it crystal clear what lint type_alias_bounds actually signifies

compiler/rustc_hir_analysis/messages.ftl

@@ -1,4 +1,4 @@
-hir_analysis_ambiguous_assoc_item = ambiguous associated {$assoc_kind} `{$assoc_name}` in bounds of `{$ty_param_name}`
+hir_analysis_ambiguous_assoc_item = ambiguous associated {$assoc_kind} `{$assoc_name}` in bounds of `{$qself}`
     .label = ambiguous associated {$assoc_kind} `{$assoc_name}`
 
 hir_analysis_ambiguous_lifetime_bound =
@@ -12,16 +12,21 @@ hir_analysis_assoc_item_is_private = {$kind} `{$name}` is private
     .label = private {$kind}
     .defined_here_label = the {$kind} is defined here
 
-hir_analysis_assoc_item_not_found = associated {$assoc_kind} `{$assoc_name}` not found for `{$ty_param_name}`
+hir_analysis_assoc_item_not_found = associated {$assoc_kind} `{$assoc_name}` not found for `{$qself}`
 
 hir_analysis_assoc_item_not_found_found_in_other_trait_label = there is {$identically_named ->
         [true] an
         *[false] a similarly named
     } associated {$assoc_kind} `{$suggested_name}` in the trait `{$trait_name}`
 hir_analysis_assoc_item_not_found_label = associated {$assoc_kind} `{$assoc_name}` not found
-hir_analysis_assoc_item_not_found_other_sugg = `{$ty_param_name}` has the following associated {$assoc_kind}
+hir_analysis_assoc_item_not_found_other_sugg = `{$qself}` has the following associated {$assoc_kind}
+hir_analysis_assoc_item_not_found_similar_in_other_trait_qpath_sugg =
+    consider fully qualifying{$identically_named ->
+        [true] {""}
+        *[false] {" "}and renaming
+    } the associated {$assoc_kind}
 hir_analysis_assoc_item_not_found_similar_in_other_trait_sugg = change the associated {$assoc_kind} name to use `{$suggested_name}` from `{$trait_name}`
-hir_analysis_assoc_item_not_found_similar_in_other_trait_with_bound_sugg = and also change the associated {$assoc_kind} name
+hir_analysis_assoc_item_not_found_similar_in_other_trait_with_bound_sugg = ...and changing the associated {$assoc_kind} name
 hir_analysis_assoc_item_not_found_similar_sugg = there is an associated {$assoc_kind} with a similar name
 
 hir_analysis_assoc_kind_mismatch = expected {$expected}, found {$got}

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,
     },

compiler/rustc_hir_analysis/src/hir_ty_lowering/bounds.rs

@@ -6,7 +6,6 @@ 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};
@@ -16,9 +15,8 @@ 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.
@@ -288,8 +286,7 @@ 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,

compiler/rustc_hir_analysis/src/hir_ty_lowering/errors.rs

@@ -3,17 +3,17 @@ 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;
 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,
@@ -746,7 +767,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
         if let ([], [bound]) = (&potential_assoc_types[..], &trait_bounds) {
             let grandparent = tcx.parent_hir_node(tcx.parent_hir_id(bound.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 {
@@ -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",
+    }
+}