When suggesting writing a fully qualified path probe for appropriate types

Fix #46585.

18 files changed, 295 insertions, 55 deletions

diff --git a/compiler/rustc_hir_analysis/src/astconv/mod.rs b/compiler/rustc_hir_analysis/src/astconv/mod.rs
index 5a7957be318..7127dadc1e1 100644
--- a/compiler/rustc_hir_analysis/src/astconv/mod.rs
+++ b/compiler/rustc_hir_analysis/src/astconv/mod.rs
@@ -24,6 +24,7 @@ use rustc_hir::def::{CtorOf, DefKind, Namespace, Res};
 use rustc_hir::def_id::{DefId, LocalDefId};
 use rustc_hir::intravisit::{walk_generics, Visitor as _};
 use rustc_hir::{GenericArg, GenericArgs, OpaqueTyOrigin};
+use rustc_infer::infer::TyCtxtInferExt;
 use rustc_middle::middle::stability::AllowUnstable;
 use rustc_middle::ty::subst::{self, GenericArgKind, InternalSubsts, SubstsRef};
 use rustc_middle::ty::GenericParamDefKind;
@@ -1633,8 +1634,8 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
     fn report_ambiguous_associated_type(
         &self,
         span: Span,
-        type_str: &str,
-        trait_str: &str,
+        types: &[String],
+        traits: &[String],
         name: Symbol,
     ) -> ErrorGuaranteed {
         let mut err = struct_span_err!(self.tcx().sess, span, E0223, "ambiguous associated type");
@@ -1645,19 +1646,92 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
             .keys()
             .any(|full_span| full_span.contains(span))
         {
-            err.span_suggestion(
+            err.span_suggestion_verbose(
                 span.shrink_to_lo(),
                 "you are looking for the module in `std`, not the primitive type",
                 "std::",
                 Applicability::MachineApplicable,
             );
         } else {
-            err.span_suggestion(
-                span,
-                "use fully-qualified syntax",
-                format!("<{} as {}>::{}", type_str, trait_str, name),
-                Applicability::HasPlaceholders,
-            );
+            match (types, traits) {
+                ([], []) => {
+                    err.span_suggestion_verbose(
+                        span,
+                        &format!(
+                            "if there were a type named `Type` that implements a trait named \
+                             `Trait` with associated type `{name}`, you could use the \
+                             fully-qualified path",
+                        ),
+                        format!("<Type as Trait>::{name}"),
+                        Applicability::HasPlaceholders,
+                    );
+                }
+                ([], [trait_str]) => {
+                    err.span_suggestion_verbose(
+                        span,
+                        &format!(
+                            "if there were a type named `Example` that implemented `{trait_str}`, \
+                             you could use the fully-qualified path",
+                        ),
+                        format!("<Example as {trait_str}>::{name}"),
+                        Applicability::HasPlaceholders,
+                    );
+                }
+                ([], traits) => {
+                    err.span_suggestions(
+                        span,
+                        &format!(
+                            "if there were a type named `Example` that implemented one of the \
+                             traits with associated type `{name}`, you could use the \
+                             fully-qualified path",
+                        ),
+                        traits
+                            .iter()
+                            .map(|trait_str| format!("<Example as {trait_str}>::{name}"))
+                            .collect::<Vec<_>>(),
+                        Applicability::HasPlaceholders,
+                    );
+                }
+                ([type_str], []) => {
+                    err.span_suggestion_verbose(
+                        span,
+                        &format!(
+                            "if there were a trait named `Example` with associated type `{name}` \
+                             implemented for `{type_str}`, you could use the fully-qualified path",
+                        ),
+                        format!("<{type_str} as Example>::{name}"),
+                        Applicability::HasPlaceholders,
+                    );
+                }
+                (types, []) => {
+                    err.span_suggestions(
+                        span,
+                        &format!(
+                            "if there were a trait named `Example` with associated type `{name}` \
+                             implemented for one of the types, you could use the fully-qualified \
+                             path",
+                        ),
+                        types
+                            .into_iter()
+                            .map(|type_str| format!("<{type_str} as Example>::{name}")),
+                        Applicability::HasPlaceholders,
+                    );
+                }
+                (types, traits) => {
+                    let mut suggestions = vec![];
+                    for type_str in types {
+                        for trait_str in traits {
+                            suggestions.push(format!("<{type_str} as {trait_str}>::{name}"));
+                        }
+                    }
+                    err.span_suggestions(
+                        span,
+                        "use the fully-qualified path",
+                        suggestions,
+                        Applicability::MachineApplicable,
+                    );
+                }
+            }
         }
         err.emit()
     }
@@ -2040,12 +2114,67 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
                     err.emit()
                 } else if let Err(reported) = qself_ty.error_reported() {
                     reported
+                } else if let ty::Alias(ty::Opaque, alias_ty) = qself_ty.kind() {
+                    // `<impl Trait as OtherTrait>::Assoc` makes no sense.
+                    struct_span_err!(
+                        tcx.sess,
+                        tcx.def_span(alias_ty.def_id),
+                        E0667,
+                        "`impl Trait` is not allowed in path parameters"
+                    )
+                    .emit() // Already reported in an earlier stage.
                 } else {
+                    // Find all the `impl`s that `qself_ty` has for any trait that has the
+                    // associated type, so that we suggest the right one.
+                    let infcx = tcx.infer_ctxt().build();
+                    // We create a fresh `ty::ParamEnv` instead of the one for `self.item_def_id()`
+                    // to avoid a cycle error in `src/test/ui/resolve/issue-102946.rs`.
+                    let param_env = ty::ParamEnv::new(
+                        ty::List::empty(),
+                        traits::Reveal::All,
+                        hir::Constness::NotConst,
+                    );
+                    let traits: Vec<_> = self
+                        .tcx()
+                        .all_traits()
+                        .filter(|trait_def_id| {
+                            // Consider only traits with the associated type
+                            tcx.associated_items(*trait_def_id)
+                                .in_definition_order()
+                                .find(|i| {
+                                    i.kind.namespace() == Namespace::TypeNS
+                                        && i.ident(tcx).normalize_to_macros_2_0() == assoc_ident
+                                        && matches!(i.kind, ty::AssocKind::Type)
+                                })
+                                .is_some()
+                            // Consider only accessible traits
+                            && tcx.visibility(*trait_def_id)
+                                .is_accessible_from(self.item_def_id(), tcx)
+                            && tcx.all_impls(*trait_def_id)
+                                .filter(|impl_def_id| {
+                                    let trait_ref = tcx.impl_trait_ref(impl_def_id);
+                                    trait_ref.map_or(false, |impl_| {
+                                        infcx
+                                            .can_eq(
+                                                param_env,
+                                                tcx.erase_regions(impl_.self_ty()),
+                                                tcx.erase_regions(qself_ty),
+                                            )
+                                            .is_ok()
+                                    })
+                                    && tcx.impl_polarity(impl_def_id) != ty::ImplPolarity::Negative
+                                })
+                                .next()
+                                .is_some()
+                        })
+                        .map(|trait_def_id| tcx.def_path_str(trait_def_id))
+                        .collect();
+
                     // Don't print `TyErr` to the user.
                     self.report_ambiguous_associated_type(
                         span,
-                        &qself_ty.to_string(),
-                        "Trait",
+                        &[qself_ty.to_string()],
+                        &traits,
                         assoc_ident.name,
                     )
                 };
@@ -2163,16 +2292,30 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
             let is_part_of_self_trait_constraints = def_id == trait_def_id;
             let is_part_of_fn_in_self_trait = parent_def_id == Some(trait_def_id);
 
-            let type_name = if is_part_of_self_trait_constraints || is_part_of_fn_in_self_trait {
-                "Self"
+            let type_names = if is_part_of_self_trait_constraints || is_part_of_fn_in_self_trait {
+                vec!["Self".to_string()]
             } else {
-                "Type"
+                // Find all the types that have an `impl` for the trait.
+                tcx.all_impls(trait_def_id)
+                    .filter(|impl_def_id| {
+                        // Consider only accessible traits
+                        tcx.visibility(*impl_def_id).is_accessible_from(self.item_def_id(), tcx)
+                            && tcx.impl_polarity(impl_def_id) != ty::ImplPolarity::Negative
+                    })
+                    .filter_map(|impl_def_id| tcx.impl_trait_ref(impl_def_id))
+                    .map(|impl_| impl_.self_ty())
+                    // We don't care about blanket impls.
+                    .filter(|self_ty| !self_ty.has_non_region_infer())
+                    .map(|self_ty| tcx.erase_regions(self_ty).to_string())
+                    .collect()
             };
-
+            // FIXME: also look at `tcx.generics_of(self.item_def_id()).params` any that
+            // references the trait. Relevant for the first case in
+            // `src/test/ui/associated-types/associated-types-in-ambiguous-context.rs`
             let reported = self.report_ambiguous_associated_type(
                 span,
-                type_name,
-                &path_str,
+                &type_names,
+                &[path_str],
                 item_segment.ident.name,
             );
             return tcx.ty_error_with_guaranteed(reported)
diff --git a/tests/ui/associated-item/associated-item-duplicate-names-3.stderr b/tests/ui/associated-item/associated-item-duplicate-names-3.stderr
index bf4bd634cf1..d0c17062076 100644
--- a/tests/ui/associated-item/associated-item-duplicate-names-3.stderr
+++ b/tests/ui/associated-item/associated-item-duplicate-names-3.stderr
@@ -13,7 +13,7 @@ error[E0223]: ambiguous associated type
   --> $DIR/associated-item-duplicate-names-3.rs:18:12
    |
 LL |     let x: Baz::Bar = 5;
-   |            ^^^^^^^^ help: use fully-qualified syntax: `<Baz as Trait>::Bar`
+   |            ^^^^^^^^ help: use the fully-qualified path: `<Baz as Foo>::Bar`
 
 error: aborting due to 2 previous errors
 
diff --git a/tests/ui/associated-types/associated-types-in-ambiguous-context.stderr b/tests/ui/associated-types/associated-types-in-ambiguous-context.stderr
index 289911779ff..00856b55df5 100644
--- a/tests/ui/associated-types/associated-types-in-ambiguous-context.stderr
+++ b/tests/ui/associated-types/associated-types-in-ambiguous-context.stderr
@@ -2,31 +2,46 @@ error[E0223]: ambiguous associated type
   --> $DIR/associated-types-in-ambiguous-context.rs:6:36
    |
 LL | fn get<T:Get,U:Get>(x: T, y: U) -> Get::Value {}
-   |                                    ^^^^^^^^^^ help: use fully-qualified syntax: `<Type as Get>::Value`
+   |                                    ^^^^^^^^^^
+   |
+help: if there were a type named `Example` that implemented `Get`, you could use the fully-qualified path
+   |
+LL | fn get<T:Get,U:Get>(x: T, y: U) -> <Example as Get>::Value {}
+   |                                    ~~~~~~~~~~~~~~~~~~~~~~~
 
 error[E0223]: ambiguous associated type
   --> $DIR/associated-types-in-ambiguous-context.rs:20:17
    |
 LL | trait Foo where Foo::Assoc: Bar {
-   |                 ^^^^^^^^^^ help: use fully-qualified syntax: `<Self as Foo>::Assoc`
+   |                 ^^^^^^^^^^ help: use the fully-qualified path: `<Self as Foo>::Assoc`
 
 error[E0223]: ambiguous associated type
   --> $DIR/associated-types-in-ambiguous-context.rs:25:10
    |
 LL | type X = std::ops::Deref::Target;
-   |          ^^^^^^^^^^^^^^^^^^^^^^^ help: use fully-qualified syntax: `<Type as Deref>::Target`
+   |          ^^^^^^^^^^^^^^^^^^^^^^^
+   |
+help: if there were a type named `Example` that implemented `Deref`, you could use the fully-qualified path
+   |
+LL | type X = <Example as Deref>::Target;
+   |          ~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 error[E0223]: ambiguous associated type
   --> $DIR/associated-types-in-ambiguous-context.rs:11:23
    |
 LL |     fn grab(&self) -> Grab::Value;
-   |                       ^^^^^^^^^^^ help: use fully-qualified syntax: `<Self as Grab>::Value`
+   |                       ^^^^^^^^^^^ help: use the fully-qualified path: `<Self as Grab>::Value`
 
 error[E0223]: ambiguous associated type
   --> $DIR/associated-types-in-ambiguous-context.rs:14:22
    |
 LL |     fn get(&self) -> Get::Value;
-   |                      ^^^^^^^^^^ help: use fully-qualified syntax: `<Type as Get>::Value`
+   |                      ^^^^^^^^^^
+   |
+help: if there were a type named `Example` that implemented `Get`, you could use the fully-qualified path
+   |
+LL |     fn get(&self) -> <Example as Get>::Value;
+   |                      ~~~~~~~~~~~~~~~~~~~~~~~
 
 error: aborting due to 5 previous errors
 
diff --git a/tests/ui/did_you_mean/bad-assoc-ty.stderr b/tests/ui/did_you_mean/bad-assoc-ty.stderr
index 21f957ab549..7cd349c7507 100644
--- a/tests/ui/did_you_mean/bad-assoc-ty.stderr
+++ b/tests/ui/did_you_mean/bad-assoc-ty.stderr
@@ -61,25 +61,45 @@ error[E0223]: ambiguous associated type
   --> $DIR/bad-assoc-ty.rs:1:10
    |
 LL | type A = [u8; 4]::AssocTy;
-   |          ^^^^^^^^^^^^^^^^ help: use fully-qualified syntax: `<[u8; 4] as Trait>::AssocTy`
+   |          ^^^^^^^^^^^^^^^^
+   |
+help: if there were a trait named `Example` with associated type `AssocTy` implemented for `[u8; 4]`, you could use the fully-qualified path
+   |
+LL | type A = <[u8; 4] as Example>::AssocTy;
+   |          ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 error[E0223]: ambiguous associated type
   --> $DIR/bad-assoc-ty.rs:5:10
    |
 LL | type B = [u8]::AssocTy;
-   |          ^^^^^^^^^^^^^ help: use fully-qualified syntax: `<[u8] as Trait>::AssocTy`
+   |          ^^^^^^^^^^^^^
+   |
+help: if there were a trait named `Example` with associated type `AssocTy` implemented for `[u8]`, you could use the fully-qualified path
+   |
+LL | type B = <[u8] as Example>::AssocTy;
+   |          ~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 error[E0223]: ambiguous associated type
   --> $DIR/bad-assoc-ty.rs:9:10
    |
 LL | type C = (u8)::AssocTy;
-   |          ^^^^^^^^^^^^^ help: use fully-qualified syntax: `<u8 as Trait>::AssocTy`
+   |          ^^^^^^^^^^^^^
+   |
+help: if there were a trait named `Example` with associated type `AssocTy` implemented for `u8`, you could use the fully-qualified path
+   |
+LL | type C = <u8 as Example>::AssocTy;
+   |          ~~~~~~~~~~~~~~~~~~~~~~~~
 
 error[E0223]: ambiguous associated type
   --> $DIR/bad-assoc-ty.rs:13:10
    |
 LL | type D = (u8, u8)::AssocTy;
-   |          ^^^^^^^^^^^^^^^^^ help: use fully-qualified syntax: `<(u8, u8) as Trait>::AssocTy`
+   |          ^^^^^^^^^^^^^^^^^
+   |
+help: if there were a trait named `Example` with associated type `AssocTy` implemented for `(u8, u8)`, you could use the fully-qualified path
+   |
+LL | type D = <(u8, u8) as Example>::AssocTy;
+   |          ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 error[E0121]: the placeholder `_` is not allowed within types on item signatures for type aliases
   --> $DIR/bad-assoc-ty.rs:17:10
@@ -91,13 +111,23 @@ error[E0223]: ambiguous associated type
   --> $DIR/bad-assoc-ty.rs:21:19
    |
 LL | type F = &'static (u8)::AssocTy;
-   |                   ^^^^^^^^^^^^^ help: use fully-qualified syntax: `<u8 as Trait>::AssocTy`
+   |                   ^^^^^^^^^^^^^
+   |
+help: if there were a trait named `Example` with associated type `AssocTy` implemented for `u8`, you could use the fully-qualified path
+   |
+LL | type F = &'static <u8 as Example>::AssocTy;
+   |                   ~~~~~~~~~~~~~~~~~~~~~~~~
 
 error[E0223]: ambiguous associated type
   --> $DIR/bad-assoc-ty.rs:27:10
    |
 LL | type G = dyn 'static + (Send)::AssocTy;
-   |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use fully-qualified syntax: `<(dyn Send + 'static) as Trait>::AssocTy`
+   |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+help: if there were a trait named `Example` with associated type `AssocTy` implemented for `(dyn Send + 'static)`, you could use the fully-qualified path
+   |
+LL | type G = <(dyn Send + 'static) as Example>::AssocTy;
+   |          ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 warning: trait objects without an explicit `dyn` are deprecated
   --> $DIR/bad-assoc-ty.rs:33:10
@@ -117,24 +147,38 @@ error[E0223]: ambiguous associated type
   --> $DIR/bad-assoc-ty.rs:33:10
    |
 LL | type H = Fn(u8) -> (u8)::Output;
-   |          ^^^^^^^^^^^^^^^^^^^^^^ help: use fully-qualified syntax: `<(dyn Fn(u8) -> u8 + 'static) as Trait>::Output`
+   |          ^^^^^^^^^^^^^^^^^^^^^^
+   |
+help: if there were a trait named `Example` with associated type `Output` implemented for `(dyn Fn(u8) -> u8 + 'static)`, you could use the fully-qualified path
+   |
+LL | type H = <(dyn Fn(u8) -> u8 + 'static) as Example>::Output;
+   |          ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 error[E0223]: ambiguous associated type
   --> $DIR/bad-assoc-ty.rs:39:19
    |
 LL |     ($ty: ty) => ($ty::AssocTy);
-   |                   ^^^^^^^^^^^^ help: use fully-qualified syntax: `<u8 as Trait>::AssocTy`
+   |                   ^^^^^^^^^^^^
 ...
 LL | type J = ty!(u8);
    |          ------- in this macro invocation
    |
    = note: this error originates in the macro `ty` (in Nightly builds, run with -Z macro-backtrace for more info)
+help: if there were a trait named `Example` with associated type `AssocTy` implemented for `u8`, you could use the fully-qualified path
+   |
+LL |     ($ty: ty) => (<u8 as Example>::AssocTy);
+   |                   ~~~~~~~~~~~~~~~~~~~~~~~~
 
 error[E0223]: ambiguous associated type
   --> $DIR/bad-assoc-ty.rs:46:10
    |
 LL | type I = ty!()::AssocTy;
-   |          ^^^^^^^^^^^^^^ help: use fully-qualified syntax: `<u8 as Trait>::AssocTy`
+   |          ^^^^^^^^^^^^^^
+   |
+help: if there were a trait named `Example` with associated type `AssocTy` implemented for `u8`, you could use the fully-qualified path
+   |
+LL | type I = <u8 as Example>::AssocTy;
+   |          ~~~~~~~~~~~~~~~~~~~~~~~~
 
 error[E0121]: the placeholder `_` is not allowed within types on item signatures for functions
   --> $DIR/bad-assoc-ty.rs:51:13
diff --git a/tests/ui/error-codes/E0223.rs b/tests/ui/error-codes/E0223.rs
index 6031b682d72..2fe252de256 100644
--- a/tests/ui/error-codes/E0223.rs
+++ b/tests/ui/error-codes/E0223.rs
@@ -1,4 +1,8 @@
 trait MyTrait { type X; }
+struct MyStruct;
+impl MyTrait for MyStruct {
+    type X = ();
+}
 
 fn main() {
     let foo: MyTrait::X;
diff --git a/tests/ui/error-codes/E0223.stderr b/tests/ui/error-codes/E0223.stderr
index 726f39e11f1..42945e42f6e 100644
--- a/tests/ui/error-codes/E0223.stderr
+++ b/tests/ui/error-codes/E0223.stderr
@@ -1,8 +1,8 @@
 error[E0223]: ambiguous associated type
-  --> $DIR/E0223.rs:4:14
+  --> $DIR/E0223.rs:8:14
    |
 LL |     let foo: MyTrait::X;
-   |              ^^^^^^^^^^ help: use fully-qualified syntax: `<Type as MyTrait>::X`
+   |              ^^^^^^^^^^ help: use the fully-qualified path: `<MyStruct as MyTrait>::X`
 
 error: aborting due to previous error
 
diff --git a/tests/ui/impl-trait/impl_trait_projections.rs b/tests/ui/impl-trait/impl_trait_projections.rs
index fd0986d7c0a..b3ff2ce5a7b 100644
--- a/tests/ui/impl-trait/impl_trait_projections.rs
+++ b/tests/ui/impl-trait/impl_trait_projections.rs
@@ -11,7 +11,7 @@ fn path_parametrized_type_is_allowed() -> option::Option<impl Debug> {
 
 fn projection_is_disallowed(x: impl Iterator) -> <impl Iterator>::Item {
 //~^ ERROR `impl Trait` is not allowed in path parameters
-//~^^ ERROR ambiguous associated type
+//~| ERROR `impl Trait` is not allowed in path parameters
     x.next().unwrap()
 }
 
diff --git a/tests/ui/impl-trait/impl_trait_projections.stderr b/tests/ui/impl-trait/impl_trait_projections.stderr
index 82d2422c407..4deb24731bc 100644
--- a/tests/ui/impl-trait/impl_trait_projections.stderr
+++ b/tests/ui/impl-trait/impl_trait_projections.stderr
@@ -22,13 +22,12 @@ error[E0667]: `impl Trait` is not allowed in path parameters
 LL |     -> <dyn Iterator<Item = impl Debug> as Iterator>::Item
    |                             ^^^^^^^^^^
 
-error[E0223]: ambiguous associated type
-  --> $DIR/impl_trait_projections.rs:12:50
+error[E0667]: `impl Trait` is not allowed in path parameters
+  --> $DIR/impl_trait_projections.rs:12:51
    |
 LL | fn projection_is_disallowed(x: impl Iterator) -> <impl Iterator>::Item {
-   |                                                  ^^^^^^^^^^^^^^^^^^^^^ help: use fully-qualified syntax: `<impl Iterator as Trait>::Item`
+   |                                                   ^^^^^^^^^^^^^
 
 error: aborting due to 5 previous errors
 
-Some errors have detailed explanations: E0223, E0667.
-For more information about an error, try `rustc --explain E0223`.
+For more information about this error, try `rustc --explain E0667`.
diff --git a/tests/ui/issues/issue-23073.stderr b/tests/ui/issues/issue-23073.stderr
index 3a10a1ab11a..3a9f49ef167 100644
--- a/tests/ui/issues/issue-23073.stderr
+++ b/tests/ui/issues/issue-23073.stderr
@@ -2,7 +2,12 @@ error[E0223]: ambiguous associated type
   --> $DIR/issue-23073.rs:6:17
    |
 LL |     type FooT = <<Self as Bar>::Foo>::T;
-   |                 ^^^^^^^^^^^^^^^^^^^^^^^ help: use fully-qualified syntax: `<<Self as Bar>::Foo as Trait>::T`
+   |                 ^^^^^^^^^^^^^^^^^^^^^^^
+   |
+help: if there were a trait named `Example` with associated type `T` implemented for `<Self as Bar>::Foo`, you could use the fully-qualified path
+   |
+LL |     type FooT = <<Self as Bar>::Foo as Example>::T;
+   |                 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 error: aborting due to previous error
 
diff --git a/tests/ui/issues/issue-78622.stderr b/tests/ui/issues/issue-78622.stderr
index f7d44f21d3b..70daf8a2f1a 100644
--- a/tests/ui/issues/issue-78622.stderr
+++ b/tests/ui/issues/issue-78622.stderr
@@ -2,7 +2,12 @@ error[E0223]: ambiguous associated type
   --> $DIR/issue-78622.rs:5:5
    |
 LL |     S::A::<f> {}
-   |     ^^^^ help: use fully-qualified syntax: `<S as Trait>::A`
+   |     ^^^^
+   |
+help: if there were a trait named `Example` with associated type `A` implemented for `S`, you could use the fully-qualified path
+   |
+LL |     <S as Example>::A::<f> {}
+   |     ~~~~~~~~~~~~~~~~~
 
 error: aborting due to previous error
 
diff --git a/tests/ui/lint/bare-trait-objects-path.stderr b/tests/ui/lint/bare-trait-objects-path.stderr
index 8ed303ca606..a19f4963c23 100644
--- a/tests/ui/lint/bare-trait-objects-path.stderr
+++ b/tests/ui/lint/bare-trait-objects-path.stderr
@@ -16,7 +16,7 @@ error[E0223]: ambiguous associated type
   --> $DIR/bare-trait-objects-path.rs:23:12
    |
 LL |     let _: Dyn::Ty;
-   |            ^^^^^^^ help: use fully-qualified syntax: `<dyn Dyn as Trait>::Ty`
+   |            ^^^^^^^ help: use the fully-qualified path: `<dyn Dyn as Assoc>::Ty`
 
 warning: trait objects without an explicit `dyn` are deprecated
   --> $DIR/bare-trait-objects-path.rs:14:5
diff --git a/tests/ui/qualified/qualified-path-params-2.stderr b/tests/ui/qualified/qualified-path-params-2.stderr
index 948f21fce4b..b6cf19b8286 100644
--- a/tests/ui/qualified/qualified-path-params-2.stderr
+++ b/tests/ui/qualified/qualified-path-params-2.stderr
@@ -2,7 +2,12 @@ error[E0223]: ambiguous associated type
   --> $DIR/qualified-path-params-2.rs:18:10
    |
 LL | type A = <S as Tr>::A::f<u8>;
-   |          ^^^^^^^^^^^^^^^^^^^ help: use fully-qualified syntax: `<<S as Tr>::A as Trait>::f`
+   |          ^^^^^^^^^^^^^^^^^^^
+   |
+help: if there were a trait named `Example` with associated type `f` implemented for `<S as Tr>::A`, you could use the fully-qualified path
+   |
+LL | type A = <<S as Tr>::A as Example>::f;
+   |          ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 error: aborting due to previous error
 
diff --git a/tests/ui/resolve/issue-103202.stderr b/tests/ui/resolve/issue-103202.stderr
index 880389371ef..d4d141fb06f 100644
--- a/tests/ui/resolve/issue-103202.stderr
+++ b/tests/ui/resolve/issue-103202.stderr
@@ -2,7 +2,12 @@ error[E0223]: ambiguous associated type
   --> $DIR/issue-103202.rs:4:17
    |
 LL |     fn f(self: &S::x) {}
-   |                 ^^^^ help: use fully-qualified syntax: `<S as Trait>::x`
+   |                 ^^^^
+   |
+help: if there were a trait named `Example` with associated type `x` implemented for `S`, you could use the fully-qualified path
+   |
+LL |     fn f(self: &<S as Example>::x) {}
+   |                 ~~~~~~~~~~~~~~~~~
 
 error: aborting due to previous error
 
diff --git a/tests/ui/self/self-impl.stderr b/tests/ui/self/self-impl.stderr
index fb47f27e022..36372b644d6 100644
--- a/tests/ui/self/self-impl.stderr
+++ b/tests/ui/self/self-impl.stderr
@@ -2,13 +2,13 @@ error[E0223]: ambiguous associated type
   --> $DIR/self-impl.rs:23:16
    |
 LL |         let _: <Self>::Baz = true;
-   |                ^^^^^^^^^^^ help: use fully-qualified syntax: `<Bar as Trait>::Baz`
+   |                ^^^^^^^^^^^ help: use the fully-qualified path: `<Bar as Foo>::Baz`
 
 error[E0223]: ambiguous associated type
   --> $DIR/self-impl.rs:25:16
    |
 LL |         let _: Self::Baz = true;
-   |                ^^^^^^^^^ help: use fully-qualified syntax: `<Bar as Trait>::Baz`
+   |                ^^^^^^^^^ help: use the fully-qualified path: `<Bar as Foo>::Baz`
 
 error: aborting due to 2 previous errors
 
diff --git a/tests/ui/structs/struct-path-associated-type.stderr b/tests/ui/structs/struct-path-associated-type.stderr
index abb445214f3..ca5f0b7e21e 100644
--- a/tests/ui/structs/struct-path-associated-type.stderr
+++ b/tests/ui/structs/struct-path-associated-type.stderr
@@ -48,19 +48,19 @@ error[E0223]: ambiguous associated type
   --> $DIR/struct-path-associated-type.rs:32:13
    |
 LL |     let s = S::A {};
-   |             ^^^^ help: use fully-qualified syntax: `<S as Trait>::A`
+   |             ^^^^ help: use the fully-qualified path: `<S as Tr>::A`
 
 error[E0223]: ambiguous associated type
   --> $DIR/struct-path-associated-type.rs:33:13
    |
 LL |     let z = S::A::<u8> {};
-   |             ^^^^ help: use fully-qualified syntax: `<S as Trait>::A`
+   |             ^^^^ help: use the fully-qualified path: `<S as Tr>::A`
 
 error[E0223]: ambiguous associated type
   --> $DIR/struct-path-associated-type.rs:35:9
    |
 LL |         S::A {} => {}
-   |         ^^^^ help: use fully-qualified syntax: `<S as Trait>::A`
+   |         ^^^^ help: use the fully-qualified path: `<S as Tr>::A`
 
 error: aborting due to 8 previous errors
 
diff --git a/tests/ui/suggestions/let-binding-init-expr-as-ty.stderr b/tests/ui/suggestions/let-binding-init-expr-as-ty.stderr
index 2bf072ef521..b90ae051fb7 100644
--- a/tests/ui/suggestions/let-binding-init-expr-as-ty.stderr
+++ b/tests/ui/suggestions/let-binding-init-expr-as-ty.stderr
@@ -21,7 +21,12 @@ error[E0223]: ambiguous associated type
   --> $DIR/let-binding-init-expr-as-ty.rs:2:14
    |
 LL |     let foo: i32::from_be(num);
-   |              ^^^^^^^^^^^^^^^^^ help: use fully-qualified syntax: `<i32 as Trait>::from_be`
+   |              ^^^^^^^^^^^^^^^^^
+   |
+help: if there were a trait named `Example` with associated type `from_be` implemented for `i32`, you could use the fully-qualified path
+   |
+LL |     let foo: <i32 as Example>::from_be;
+   |              ~~~~~~~~~~~~~~~~~~~~~~~~~
 
 error: aborting due to 3 previous errors
 
diff --git a/tests/ui/traits/item-privacy.stderr b/tests/ui/traits/item-privacy.stderr
index f137a298a7f..293cfbda86c 100644
--- a/tests/ui/traits/item-privacy.stderr
+++ b/tests/ui/traits/item-privacy.stderr
@@ -148,19 +148,24 @@ error[E0223]: ambiguous associated type
   --> $DIR/item-privacy.rs:115:12
    |
 LL |     let _: S::A;
-   |            ^^^^ help: use fully-qualified syntax: `<S as Trait>::A`
+   |            ^^^^
+   |
+help: if there were a trait named `Example` with associated type `A` implemented for `S`, you could use the fully-qualified path
+   |
+LL |     let _: <S as Example>::A;
+   |            ~~~~~~~~~~~~~~~~~
 
 error[E0223]: ambiguous associated type
   --> $DIR/item-privacy.rs:116:12
    |
 LL |     let _: S::B;
-   |            ^^^^ help: use fully-qualified syntax: `<S as Trait>::B`
+   |            ^^^^ help: use the fully-qualified path: `<S as assoc_ty::B>::B`
 
 error[E0223]: ambiguous associated type
   --> $DIR/item-privacy.rs:117:12
    |
 LL |     let _: S::C;
-   |            ^^^^ help: use fully-qualified syntax: `<S as Trait>::C`
+   |            ^^^^ help: use the fully-qualified path: `<S as assoc_ty::C>::C`
 
 error[E0624]: associated type `A` is private
   --> $DIR/item-privacy.rs:119:12
diff --git a/tests/ui/ufcs/ufcs-partially-resolved.stderr b/tests/ui/ufcs/ufcs-partially-resolved.stderr
index 5f7f6aa9f6e..72fccea8ae3 100644
--- a/tests/ui/ufcs/ufcs-partially-resolved.stderr
+++ b/tests/ui/ufcs/ufcs-partially-resolved.stderr
@@ -205,7 +205,12 @@ error[E0223]: ambiguous associated type
   --> $DIR/ufcs-partially-resolved.rs:36:12
    |
 LL |     let _: <u8 as Tr>::Y::NN;
-   |            ^^^^^^^^^^^^^^^^^ help: use fully-qualified syntax: `<<u8 as Tr>::Y as Trait>::NN`
+   |            ^^^^^^^^^^^^^^^^^
+   |
+help: if there were a trait named `Example` with associated type `NN` implemented for `<u8 as Tr>::Y`, you could use the fully-qualified path
+   |
+LL |     let _: <<u8 as Tr>::Y as Example>::NN;
+   |            ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 error[E0599]: no associated item named `NN` found for type `u16` in the current scope
   --> $DIR/ufcs-partially-resolved.rs:38:20