When encountering sealed traits, point types that implement it

```
error[E0277]: the trait bound `S: d::Hidden` is not satisfied
  --> $DIR/sealed-trait-local.rs:53:20
   |
LL | impl c::Sealed for S {}
   |                    ^ the trait `d::Hidden` is not implemented for `S`
   |
note: required by a bound in `c::Sealed`
  --> $DIR/sealed-trait-local.rs:17:23
   |
LL |     pub trait Sealed: self::d::Hidden {
   |                       ^^^^^^^^^^^^^^^ required by this bound in `Sealed`
   = note: `Sealed` is a "sealed trait", because to implement it you also need to implement `c::d::Hidden`, which is not accessible; this is usually done to force you to use one of the provided types that already implement it
   = help: the following types implement the trait:
            - c::X
            - c::Y
```

The last `help` is new.

Author: estebank

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

@@ -2668,15 +2668,49 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
                         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 implement `{}`, 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)),
                             ));
+                            let impls_of = tcx.trait_impls_of(def_id);
+                            let impls = impls_of
+                                .non_blanket_impls()
+                                .values()
+                                .flatten()
+                                .chain(impls_of.blanket_impls().iter())
+                                .collect::<Vec<_>>();
+                            match &impls[..] {
+                                [] => {}
+                                [only] => {
+                                    err.help(with_no_trimmed_paths!(format!(
+                                        "type `{}` implements the trait",
+                                        tcx.type_of(*only).instantiate_identity(),
+                                    )));
+                                }
+                                impls => {
+                                    let mut types = impls.iter()
+                                        .map(|t| with_no_trimmed_paths!(format!(
+                                            " - {}\n",
+                                            tcx.type_of(*t).instantiate_identity(),
+                                        )))
+                                        .collect::<Vec<_>>();
+                                    let post = if types.len() > 9 {
+                                        let len = types.len();
+                                        types.truncate(8);
+                                        format!("and {} others", len - 8)
+                                    } else {
+                                        String::new()
+                                    };
+                                    err.help(format!(
+                                        "the following types implement the trait:\n{}{post}",
+                                        types.join(""),
+                                    ));
+                                }
+                            }
+
                         }
                     }
                 } else {

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

@@ -13,7 +13,43 @@ pub mod a {
     }
 }
 
-struct S;
-impl a::Sealed for S {} //~ ERROR the trait bound `S: Hidden` is not satisfied
+pub mod c {
+    pub trait Sealed: self::d::Hidden {
+        fn foo() {}
+    }
+
+    struct X;
+    impl Sealed for X {}
+    impl self::d::Hidden for X {}
+
+    struct Y;
+    impl Sealed for Y {}
+    impl self::d::Hidden for Y {}
+
+    mod d {
+        pub trait Hidden {}
+    }
+}
 
+pub mod e {
+    pub trait Sealed: self::f::Hidden {
+        fn foo() {}
+    }
+
+    struct X;
+    impl self::f::Hidden for X {}
+
+    struct Y;
+    impl self::f::Hidden for Y {}
+    impl<T: self::f::Hidden> Sealed for T {}
+
+    mod f {
+        pub trait Hidden {}
+    }
+}
+
+struct S;
+impl a::Sealed for S {} //~ ERROR the trait bound
+impl c::Sealed for S {} //~ ERROR the trait bound
+impl e::Sealed for S {} //~ ERROR the trait bound
 fn main() {}

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

@@ -1,16 +1,51 @@
-error[E0277]: the trait bound `S: Hidden` is not satisfied
-  --> $DIR/sealed-trait-local.rs:17:20
+error[E0277]: the trait bound `S: b::Hidden` is not satisfied
+  --> $DIR/sealed-trait-local.rs:52:20
    |
 LL | impl a::Sealed for S {}
-   |                    ^ the trait `Hidden` is not implemented for `S`
+   |                    ^ the trait `b::Hidden` is not implemented for `S`
    |
-note: required by a bound in `Sealed`
+note: required by a bound in `a::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 implement `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
+   = help: type `a::X` implements the trait
 
-error: aborting due to previous error
+error[E0277]: the trait bound `S: d::Hidden` is not satisfied
+  --> $DIR/sealed-trait-local.rs:53:20
+   |
+LL | impl c::Sealed for S {}
+   |                    ^ the trait `d::Hidden` is not implemented for `S`
+   |
+note: required by a bound in `c::Sealed`
+  --> $DIR/sealed-trait-local.rs:17:23
+   |
+LL |     pub trait Sealed: self::d::Hidden {
+   |                       ^^^^^^^^^^^^^^^ required by this bound in `Sealed`
+   = note: `Sealed` is a "sealed trait", because to implement it you also need to implement `c::d::Hidden`, which is not accessible; this is usually done to force you to use one of the provided types that already implement it
+   = help: the following types implement the trait:
+            - c::X
+            - c::Y
+           
+
+error[E0277]: the trait bound `S: f::Hidden` is not satisfied
+  --> $DIR/sealed-trait-local.rs:54:20
+   |
+LL | impl e::Sealed for S {}
+   |                    ^ the trait `f::Hidden` is not implemented for `S`
+   |
+note: required by a bound in `e::Sealed`
+  --> $DIR/sealed-trait-local.rs:35:23
+   |
+LL |     pub trait Sealed: self::f::Hidden {
+   |                       ^^^^^^^^^^^^^^^ required by this bound in `Sealed`
+   = note: `Sealed` is a "sealed trait", because to implement it you also need to implement `e::f::Hidden`, which is not accessible; this is usually done to force you to use one of the provided types that already implement it
+   = help: the following types implement the trait:
+            - e::X
+            - e::Y
+           
+
+error: aborting due to 3 previous errors
 
 For more information about this error, try `rustc --explain E0277`.