effective visibility: Fix private visibility calculation for modules

Optimizations removed in the previous commit required this function to behave incorrectly, but now those optimizations are gone so we can fix the bug.

Fixes rust-lang#104249

compiler/rustc_resolve/src/effective_visibilities.rs

@@ -9,7 +9,7 @@ use rustc_hir::def_id::LocalDefId;
 use rustc_hir::def_id::CRATE_DEF_ID;
 use rustc_middle::middle::privacy::{EffectiveVisibilities, EffectiveVisibility};
 use rustc_middle::middle::privacy::{IntoDefIdTree, Level};
-use rustc_middle::ty::Visibility;
+use rustc_middle::ty::{DefIdTree, Visibility};
 
 type ImportId<'a> = Interned<'a, NameBinding<'a>>;
 
@@ -54,10 +54,12 @@ impl Resolver<'_> {
     }
 
     fn private_vis_def(&mut self, def_id: LocalDefId) -> Visibility {
-        if def_id == CRATE_DEF_ID {
-            Visibility::Public
+        // For mod items `nearest_normal_mod` returns its argument, but we actually need its parent.
+        let normal_mod_id = self.nearest_normal_mod(def_id);
+        if normal_mod_id == def_id {
+            self.opt_local_parent(def_id).map_or(Visibility::Public, Visibility::Restricted)
         } else {
-            Visibility::Restricted(self.nearest_normal_mod(def_id))
+            Visibility::Restricted(normal_mod_id)
         }
     }
 }

src/test/ui/privacy/effective_visibilities_invariants.rs

@@ -5,7 +5,7 @@
 pub mod m {} //~ ERROR module has missing stability attribute
 
 pub mod m { //~ ERROR the name `m` is defined multiple times
-    // mod inner {} - ICE
+    mod inner {}
     type Inner = u8;
 }