Make a distinction between public and sealed traits

Author: estebank

compiler/rustc_infer/src/traits/error_reporting/mod.rs

@@ -123,9 +123,24 @@ pub fn report_object_safety_error<'tcx>(
     } else {
         vec![]
     };
+    let externally_visible = if !impls.is_empty()
+        && let Some(def_id) = trait_def_id.as_local()
+        && tcx.effective_visibilities(()).is_exported(def_id)
+    {
+        true
+    } else {
+        false
+    };
     match &impls[..] {
         [] => {}
         _ if impls.len() > 9 => {}
+        [only] if externally_visible => {
+            err.help(with_no_trimmed_paths!(format!(
+                "only type `{}` is seen to implement the trait in this crate, consider using it \
+                 directly instead",
+                tcx.type_of(*only).instantiate_identity(),
+            )));
+        }
         [only] => {
             err.help(with_no_trimmed_paths!(format!(
                 "only type `{}` implements the trait, consider using it directly instead",
@@ -148,6 +163,12 @@ pub fn report_object_safety_error<'tcx>(
             ));
         }
     }
+    if externally_visible {
+        err.note(format!(
+            "`{trait_str}` can be implemented in other crates; if you want to support your users \
+             passing their own types here, you can't refer to a specific type",
+        ));
+    }
 
     err
 }

tests/ui/generic-associated-types/issue-76535.base.stderr

@@ -28,7 +28,8 @@ LL | pub trait SuperTrait {
 LL |     type SubType<'a>: SubTrait where Self: 'a;
    |          ^^^^^^^ ...because it contains the generic associated type `SubType`
    = help: consider moving `SubType` to another trait
-   = help: only type `SuperStruct` implements the trait, consider using it directly instead
+   = help: only type `SuperStruct` is seen to implement the trait in this crate, consider using it directly instead
+   = note: `SuperTrait` can be implemented in other crates; if you want to support your users passing their own types here, you can't refer to a specific type
 
 error[E0038]: the trait `SuperTrait` cannot be made into an object
   --> $DIR/issue-76535.rs:39:57
@@ -44,7 +45,8 @@ LL | pub trait SuperTrait {
 LL |     type SubType<'a>: SubTrait where Self: 'a;
    |          ^^^^^^^ ...because it contains the generic associated type `SubType`
    = help: consider moving `SubType` to another trait
-   = help: only type `SuperStruct` implements the trait, consider using it directly instead
+   = help: only type `SuperStruct` is seen to implement the trait in this crate, consider using it directly instead
+   = note: `SuperTrait` can be implemented in other crates; if you want to support your users passing their own types here, you can't refer to a specific type
    = note: required for the cast from `Box<SuperStruct>` to `Box<dyn SuperTrait<SubType = SubStruct<'_>>>`
 
 error: aborting due to 3 previous errors