Account for sealed traits in trait bound errors

When implementing a public trait with a private super-trait, we now emit
a note that the missing bound is not going to be able to be satisfied,
and we explain the concept of a sealed trait.

compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs

@@ -2724,6 +2724,32 @@ fn note_obligation_cause_code<T>(
                     let msg = format!("required by this bound in `{short_item_name}`");
                     multispan.push_span_label(span, msg);
                     err.span_note(multispan, descr);
+                    if let ty::PredicateKind::Clause(clause) = predicate.kind().skip_binder()
+                        && let ty::ClauseKind::Trait(trait_pred) = clause
+                    {
+                        let def_id = trait_pred.def_id();
+                        let visible_item = if let Some(local) = def_id.as_local() {
+                            // Check for local traits being reachable.
+                            let vis = &self.tcx.resolutions(()).effective_visibilities;
+                            // Account for non-`pub` traits in the root of the local crate.
+                            let is_locally_reachable = self.tcx.parent(def_id).is_crate_root();
+                            vis.is_reachable(local) || is_locally_reachable
+                        } else {
+                            // Check for foreign traits being reachable.
+                            self.tcx.visible_parent_map(()).get(&def_id).is_some()
+                        };
+                        if let DefKind::Trait = tcx.def_kind(item_def_id) && !visible_item {
+                            // FIXME(estebank): extend this to search for all the types that do
+                            // implement this trait and list them.
+                            err.note(format!(
+                                "`{short_item_name}` is a \"sealed trait\", because to implement \
+                                 it you also need to implelement `{}`, which is not accessible; \
+                                 this is usually done to force you to use one of the provided \
+                                 types that already implement it",
+                                with_no_trimmed_paths!(tcx.def_path_str(def_id)),
+                            ));
+                        }
+                    }
                 } else {
                     err.span_note(tcx.def_span(item_def_id), descr);
                 }

tests/ui/privacy/sealed-traits/sealed-trait-local.rs

@@ -0,0 +1,19 @@
+// provide custom privacy error for sealed traits
+pub mod a {
+    pub trait Sealed: self::b::Hidden {
+        fn foo() {}
+    }
+
+    struct X;
+    impl Sealed for X {}
+    impl self::b::Hidden for X {}
+
+    mod b {
+        pub trait Hidden {}
+    }
+}
+
+struct S;
+impl a::Sealed for S {} //~ ERROR the trait bound `S: Hidden` is not satisfied
+
+fn main() {}

tests/ui/privacy/sealed-traits/sealed-trait-local.stderr

@@ -0,0 +1,16 @@
+error[E0277]: the trait bound `S: Hidden` is not satisfied
+  --> $DIR/sealed-trait-local.rs:17:20
+   |
+LL | impl a::Sealed for S {}
+   |                    ^ the trait `Hidden` is not implemented for `S`
+   |
+note: required by a bound in `Sealed`
+  --> $DIR/sealed-trait-local.rs:3:23
+   |
+LL |     pub trait Sealed: self::b::Hidden {
+   |                       ^^^^^^^^^^^^^^^ required by this bound in `Sealed`
+   = note: `Sealed` is a "sealed trait", because to implement it you also need to implelement `a::b::Hidden`, which is not accessible; this is usually done to force you to use one of the provided types that already implement it
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0277`.