Suggest full trait ref (with placeholders) on unresolved assoc tys

compiler/rustc_hir_analysis/src/errors.rs

@@ -128,19 +128,20 @@ pub enum AssocItemNotFoundSugg<'a> {
     },
     #[multipart_suggestion(
         hir_analysis_assoc_item_not_found_similar_in_other_trait_qpath_sugg,
-        style = "verbose",
+        style = "verbose"
-        applicability = "maybe-incorrect"
     )]
     SimilarInOtherTraitQPath {
         #[suggestion_part(code = "<")]
         lo: Span,
-        #[suggestion_part(code = " as {trait_}>")]
+        #[suggestion_part(code = " as {trait_ref}>")]
         mi: Span,
         #[suggestion_part(code = "{suggested_name}")]
         hi: Option<Span>,
-        trait_: &'a str,
+        trait_ref: String,
         suggested_name: Symbol,
         identically_named: bool,
+        #[applicability]
+        applicability: Applicability,
     },
     #[suggestion(
         hir_analysis_assoc_item_not_found_other_sugg,

compiler/rustc_hir_analysis/src/hir_ty_lowering/errors.rs

@@ -251,6 +251,18 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
                         return self.dcx().emit_err(err);
                     }
 
+                    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)
@@ -260,22 +272,15 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
                             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_ref,
-                            trait_: &trait_name,
                             identically_named,
                             suggested_name,
+                            applicability,
                         });
                     } else {
                         let mut err = self.dcx().create_err(err);
                         if suggest_constraining_type_param(
-                            tcx,
+                            tcx, generics, &mut err, &qself_str, &trait_ref, None, None,
-                            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

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

@@ -1,5 +1,5 @@
 error[E0220]: associated type `Assoc` not found for `T`
-  --> $DIR/unresolved-assoc-ty-suggest-trait.rs:11:22
+  --> $DIR/unresolved-assoc-ty-suggest-trait.rs:9:22
    |
 LL | type AssocOf<T> = T::Assoc;
    |                      ^^^^^ there is an associated type `Assoc` in the trait `Trait`
@@ -10,7 +10,7 @@ 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
+  --> $DIR/unresolved-assoc-ty-suggest-trait.rs:13:22
    |
 LL | type AssokOf<T> = T::Assok;
    |                      ^^^^^ there is a similarly named associated type `Assoc` in the trait `Trait`
@@ -20,6 +20,17 @@ help: consider fully qualifying and renaming the associated type
 LL | type AssokOf<T> = <T as Trait>::Assoc;
    |                   +  +++++++++  ~~~~~
 
-error: aborting due to 2 previous errors
+error[E0220]: associated type `Proj` not found for `T`
+  --> $DIR/unresolved-assoc-ty-suggest-trait.rs:22:21
+   |
+LL | type ProjOf<T> = T::Proj;
+   |                     ^^^^ there is an associated type `Proj` in the trait `Parametrized`
+   |
+help: consider fully qualifying the associated type
+   |
+LL | type ProjOf<T> = <T as Parametrized</* 'a, T, N */>>::Proj;
+   |                  +  ++++++++++++++++++++++++++++++++
+
+error: aborting due to 3 previous errors
 
 For more information about this error, try `rustc --explain E0220`.

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

@@ -1,5 +1,5 @@
 error[E0220]: associated type `Assoc` not found for `T`
-  --> $DIR/unresolved-assoc-ty-suggest-trait.rs:11:22
+  --> $DIR/unresolved-assoc-ty-suggest-trait.rs:9:22
    |
 LL | type AssocOf<T> = T::Assoc;
    |                      ^^^^^ there is an associated type `Assoc` in the trait `Trait`
@@ -10,7 +10,7 @@ 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
+  --> $DIR/unresolved-assoc-ty-suggest-trait.rs:13:22
    |
 LL | type AssokOf<T> = T::Assok;
    |                      ^^^^^ there is a similarly named associated type `Assoc` in the trait `Trait`
@@ -24,6 +24,17 @@ help: ...and changing the associated type name
 LL | type AssokOf<T> = T::Assoc;
    |                      ~~~~~
 
-error: aborting due to 2 previous errors
+error[E0220]: associated type `Proj` not found for `T`
+  --> $DIR/unresolved-assoc-ty-suggest-trait.rs:22:21
+   |
+LL | type ProjOf<T> = T::Proj;
+   |                     ^^^^ there is an associated type `Proj` in the trait `Parametrized`
+   |
+help: consider restricting type parameter `T`
+   |
+LL | type ProjOf<T: Parametrized</* 'a, T, N */>> = T::Proj;
+   |              ++++++++++++++++++++++++++++++
+
+error: aborting due to 3 previous errors
 
 For more information about this error, try `rustc --explain E0220`.

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

@@ -4,9 +4,7 @@
 //@ 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 { type Assoc; }
-
-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
@@ -17,4 +15,12 @@ type AssokOf<T> = T::Assok; //~ ERROR associated type `Assok` not found for `T`
 //[lazy]~| HELP consider restricting type parameter `T`
 //[lazy]~| HELP and changing the associated type name
 
+trait Parametrized<'a, T, const N: usize> {
+    type Proj;
+}
+
+type ProjOf<T> = T::Proj; //~ ERROR associated type `Proj` not found for `T`
+//[eager]~^ HELP consider fully qualifying the associated type
+//[lazy]~| HELP consider restricting type parameter `T`
+
 fn main() {}