Note if the user wants to use associated items of a trait directly

compiler/rustc_hir_analysis/src/astconv/mod.rs

@@ -3635,7 +3635,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
 
     fn maybe_lint_bare_trait(&self, self_ty: &hir::Ty<'_>, in_path: bool) {
         let tcx = self.tcx();
-        if let hir::TyKind::TraitObject([poly_trait_ref, ..], _, TraitObjectSyntax::None) =
+        if let hir::TyKind::TraitObject([poly_trait_ref, res @ ..], _, TraitObjectSyntax::None) =
             self_ty.kind
         {
             let needs_bracket = in_path
@@ -3680,6 +3680,13 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
                         Applicability::MachineApplicable,
                     );
                 }
+                // note if the user wants to use associated items of a trait directly
+                if in_path && res.len() == 0 {
+                    diag.span_label(
+                        self_ty.span,
+                        "no desired associated item found in this trait, if you do not want to use a bare trait object here",
+                    );
+                }
                 // check if the impl trait that we are considering is a impl of a local trait
                 self.maybe_lint_blanket_trait_impl(&self_ty, &mut diag);
                 diag.emit();

tests/ui/associated-item/issue-111312.rs

@@ -0,0 +1,12 @@
+// edition:2021
+
+trait Has {
+    fn has() {}
+}
+
+trait HasNot {}
+
+fn main() {
+    HasNot::has(); //~ ERROR E0782
+    //~^ ERROR E0599
+}

tests/ui/associated-item/issue-111312.stderr

@@ -0,0 +1,28 @@
+error[E0782]: trait objects must include the `dyn` keyword
+  --> $DIR/issue-111312.rs:10:5
+   |
+LL |     HasNot::has();
+   |     ^^^^^^ no desired associated item found in this trait, if you do not want to use a bare trait object here
+   |
+help: add `dyn` keyword before this trait
+   |
+LL |     <dyn HasNot>::has();
+   |     ++++       +
+
+error[E0599]: no function or associated item named `has` found for trait object `dyn HasNot` in the current scope
+  --> $DIR/issue-111312.rs:10:13
+   |
+LL |     HasNot::has();
+   |             ^^^ function or associated item not found in `dyn HasNot`
+   |
+   = help: items from traits can only be used if the trait is implemented and in scope
+note: `Has` defines an item `has`, perhaps you need to implement it
+  --> $DIR/issue-111312.rs:3:1
+   |
+LL | trait Has {
+   | ^^^^^^^^^
+
+error: aborting due to 2 previous errors
+
+Some errors have detailed explanations: E0599, E0782.
+For more information about an error, try `rustc --explain E0599`.

tests/ui/editions/dyn-trait-sugg-2021.stderr

@@ -2,7 +2,7 @@ error[E0782]: trait objects must include the `dyn` keyword
   --> $DIR/dyn-trait-sugg-2021.rs:10:5
    |
 LL |     Foo::hi(123);
-   |     ^^^
+   |     ^^^ no desired associated item found in this trait, if you do not want to use a bare trait object here
    |
 help: add `dyn` keyword before this trait
    |