Inside eager ty aliases on unresolved assoc tys suggest fully qualifying instead of restricting their self ty

compiler/rustc_hir_analysis/messages.ftl

@@ -20,8 +20,13 @@ hir_analysis_assoc_item_not_found_found_in_other_trait_label = there is {$identi
     } associated {$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 = `{$qself}` has the following associated {$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

@@ -126,6 +126,22 @@ pub enum AssocItemNotFoundSugg<'a> {
         assoc_kind: &'static str,
         suggested_name: Symbol,
     },
+    #[multipart_suggestion(
+        hir_analysis_assoc_item_not_found_similar_in_other_trait_qpath_sugg,
+        style = "verbose",
+        applicability = "maybe-incorrect"
+    )]
+    SimilarInOtherTraitQPath {
+        #[suggestion_part(code = "<")]
+        lo: Span,
+        #[suggestion_part(code = " as {trait_}>")]
+        mi: Span,
+        #[suggestion_part(code = "{suggested_name}")]
+        hi: Option<Span>,
+        trait_: &'a str,
+        suggested_name: Symbol,
+        identically_named: bool,
+    },
     #[suggestion(
         hir_analysis_assoc_item_not_found_other_sugg,
         code = "{suggested_name}",

compiler/rustc_hir_analysis/src/hir_ty_lowering/errors.rs

@@ -219,7 +219,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
                     suggested_name,
                     identically_named: suggested_name == assoc_name.name,
                 });
-                if let AssocItemQSelf::TyParam(ty_param_def_id) = qself
+                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...
@@ -251,27 +251,44 @@ 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,
-                        &qself_str,
-                        &trait_name,
-                        None,
-                        None,
-                    ) && suggested_name != assoc_name.name
+                    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),
+                            // FIXME(fmease): Use a full trait ref here (with placeholders).
+                            trait_: &trait_name,
+                            identically_named,
                             suggested_name,
-                            Applicability::MaybeIncorrect,
-                        );
+                        });
+                    } else {
+                        let mut err = self.dcx().create_err(err);
+                        if suggest_constraining_type_param(
+                            tcx,
+                            generics,
+                            &mut err,
+                            &qself_str,
+                            // FIXME(fmease): Use a full trait ref here (with placeholders).
+                            &trait_name,
+                            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);
             }

compiler/rustc_hir_analysis/src/hir_ty_lowering/mod.rs

@@ -197,7 +197,7 @@ pub trait HirTyLowerer<'tcx> {
 /// For diagnostic purposes only.
 enum AssocItemQSelf {
     Trait(DefId),
-    TyParam(LocalDefId),
+    TyParam(LocalDefId, Span),
     SelfTyAlias,
 }
 
@@ -205,7 +205,7 @@ 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::TyParam(def_id, _) => tcx.hir().ty_param_name(def_id).to_string(),
             Self::SelfTyAlias => kw::SelfUpper.to_string(),
         }
     }
@@ -820,6 +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> {
@@ -836,7 +837,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
                     .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)
             },
-            AssocItemQSelf::TyParam(ty_param_def_id),
+            AssocItemQSelf::TyParam(ty_param_def_id, ty_param_span),
             ty::AssocKind::Type,
             assoc_name,
             span,
@@ -1080,6 +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,
             )?,

tests/ui/resolve/issue-55673.stderr

@@ -19,7 +19,7 @@ help: consider further restricting type parameter `T`
    |
 LL |     T::Baa: std::fmt::Debug, T: Foo
    |                            ~~~~~~~~
-help: and also change the associated type name
+help: ...and changing the associated type name
    |
 LL |     T::Bar: std::fmt::Debug,
    |        ~~~

tests/ui/type-alias/unresolved-assoc-ty-suggest-trait.eager.stderr

@@ -0,0 +1,25 @@
+error[E0220]: associated type `Assoc` not found for `T`
+  --> $DIR/unresolved-assoc-ty-suggest-trait.rs:11:22
+   |
+LL | type AssocOf<T> = T::Assoc;
+   |                      ^^^^^ there is an associated type `Assoc` in the trait `Trait`
+   |
+help: consider fully qualifying the associated type
+   |
+LL | type AssocOf<T> = <T as Trait>::Assoc;
+   |                   +  +++++++++
+
+error[E0220]: associated type `Assok` not found for `T`
+  --> $DIR/unresolved-assoc-ty-suggest-trait.rs:15:22
+   |
+LL | type AssokOf<T> = T::Assok;
+   |                      ^^^^^ there is a similarly named associated type `Assoc` in the trait `Trait`
+   |
+help: consider fully qualifying and renaming the associated type
+   |
+LL | type AssokOf<T> = <T as Trait>::Assoc;
+   |                   +  +++++++++  ~~~~~
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0220`.

tests/ui/type-alias/unresolved-assoc-ty-suggest-trait.lazy.stderr

@@ -0,0 +1,29 @@
+error[E0220]: associated type `Assoc` not found for `T`
+  --> $DIR/unresolved-assoc-ty-suggest-trait.rs:11:22
+   |
+LL | type AssocOf<T> = T::Assoc;
+   |                      ^^^^^ there is an associated type `Assoc` in the trait `Trait`
+   |
+help: consider restricting type parameter `T`
+   |
+LL | type AssocOf<T: Trait> = T::Assoc;
+   |               +++++++
+
+error[E0220]: associated type `Assok` not found for `T`
+  --> $DIR/unresolved-assoc-ty-suggest-trait.rs:15:22
+   |
+LL | type AssokOf<T> = T::Assok;
+   |                      ^^^^^ there is a similarly named associated type `Assoc` in the trait `Trait`
+   |
+help: consider restricting type parameter `T`
+   |
+LL | type AssokOf<T: Trait> = T::Assok;
+   |               +++++++
+help: ...and changing the associated type name
+   |
+LL | type AssokOf<T> = T::Assoc;
+   |                      ~~~~~
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0220`.

tests/ui/type-alias/unresolved-assoc-ty-suggest-trait.rs

@@ -0,0 +1,20 @@
+// Ensure that we don't suggest *type alias bounds* for **eager** type aliases.
+// issue: rust-lang/rust#125789
+
+//@ revisions: eager lazy
+#![cfg_attr(lazy, feature(lazy_type_alias), allow(incomplete_features))]
+
+// FIXME(fmease): Suggest a full trait ref (with placeholders) instead of just a trait name.
+
+trait Trait<T> { type Assoc; }
+
+type AssocOf<T> = T::Assoc; //~ ERROR associated type `Assoc` not found for `T`
+//[eager]~^ HELP consider fully qualifying the associated type
+//[lazy]~| HELP consider restricting type parameter `T`
+
+type AssokOf<T> = T::Assok; //~ ERROR associated type `Assok` not found for `T`
+//[eager]~^ HELP consider fully qualifying and renaming the associated type
+//[lazy]~| HELP consider restricting type parameter `T`
+//[lazy]~| HELP and changing the associated type name
+
+fn main() {}