privacy: normalize associated types before visiting

This permits associated types to reference private types and
traits, so long as the normalized type does not itself violate
type privacy.

Fixes rust-lang#45713

compiler/rustc_privacy/src/lib.rs

@@ -1302,7 +1302,19 @@ impl SearchInterfaceForPrivateItemsVisitor<'_> {
 
     fn ty(&mut self) -> &mut Self {
         self.in_primary_interface = true;
-        self.visit(self.tcx.type_of(self.item_def_id).instantiate_identity());
+        let ty = self.tcx.type_of(self.item_def_id).instantiate_identity();
+
+        // If `in_assoc_ty`, attempt to normalize `ty`.
+        // Ideally, we would normalize in all circumstances, but doing so
+        // currently causes some unexpected type errors.
+        let maybe_normalized_ty = if self.in_assoc_ty {
+            let param_env = self.tcx.param_env(self.item_def_id);
+            self.tcx.try_normalize_erasing_regions(param_env, ty).ok()
+        } else {
+            None
+        };
+
+        self.visit(maybe_normalized_ty.unwrap_or(ty));
         self
     }
 

tests/ui/privacy/projections2.rs

@@ -17,8 +17,7 @@ mod m {
 
     impl Trait4 for u8 {
         type A<T: Trait> = <u8 as Trait3>::A<T>;
-        //~^ ERROR: private associated type `Trait3::A` in public interface
-        //~| ERROR: private trait `Trait3` in public interface
+        //~^ ERROR: private type `Priv` in public interface
     }
 }
 

tests/ui/privacy/projections2.stderr

@@ -11,24 +11,15 @@ LL |     struct Priv;
    |     ^^^^^^^^^^^
    = note: `#[warn(private_interfaces)]` on by default
 
-error[E0446]: private associated type `Trait3::A` in public interface
+error[E0446]: private type `Priv` in public interface
   --> $DIR/projections2.rs:19:9
    |
-LL |         type A<T: Trait>;
-   |         ---------------- `Trait3::A` declared as private
-...
-LL |         type A<T: Trait> = <u8 as Trait3>::A<T>;
-   |         ^^^^^^^^^^^^^^^^ can't leak private associated type
-
-error[E0446]: private trait `Trait3` in public interface
-  --> $DIR/projections2.rs:19:9
-   |
-LL |     trait Trait3 {
-   |     ------------ `Trait3` declared as private
+LL |     struct Priv;
+   |     ----------- `Priv` declared as private
 ...
 LL |         type A<T: Trait> = <u8 as Trait3>::A<T>;
-   |         ^^^^^^^^^^^^^^^^ can't leak private trait
+   |         ^^^^^^^^^^^^^^^^ can't leak private type
 
-error: aborting due to 2 previous errors; 1 warning emitted
+error: aborting due to 1 previous error; 1 warning emitted
 
 For more information about this error, try `rustc --explain E0446`.