Replace visibility test with reachability test in dead code detection

Fixes #119545

Author: krtab

compiler/rustc_passes/src/dead.rs

@@ -529,15 +529,16 @@ impl<'tcx> Visitor<'tcx> for MarkSymbolVisitor<'tcx> {
         let tcx = self.tcx;
         let unconditionally_treat_fields_as_live = self.repr_unconditionally_treats_fields_as_live;
         let has_repr_simd = self.repr_has_repr_simd;
+        let effective_visibilities = &tcx.effective_visibilities(());
         let live_fields = def.fields().iter().filter_map(|f| {
             let def_id = f.def_id;
             if unconditionally_treat_fields_as_live || (f.is_positional() && has_repr_simd) {
                 return Some(def_id);
             }
-            if !tcx.visibility(f.hir_id.owner.def_id).is_public() {
+            if !effective_visibilities.is_reachable(f.hir_id.owner.def_id) {
                 return None;
             }
-            if tcx.visibility(def_id).is_public() { Some(def_id) } else { None }
+            if effective_visibilities.is_reachable(def_id) { Some(def_id) } else { None }
         });
         self.live_symbols.extend(live_fields);
 

tests/ui/lint/dead-code/pub-field-in-priv-mod.rs

@@ -0,0 +1,11 @@
+#![deny(dead_code)]
+
+fn main() {
+    let _ = foo::S{f: false};
+}
+
+mod foo {
+    pub struct S {
+        pub f: bool, //~ ERROR field `f` is never read
+    }
+}

tests/ui/lint/dead-code/pub-field-in-priv-mod.stderr

@@ -0,0 +1,16 @@
+error: field `f` is never read
+  --> $DIR/pub-field-in-priv-mod.rs:9:13
+   |
+LL |     pub struct S {
+   |                - field in this struct
+LL |         pub f: bool,
+   |             ^
+   |
+note: the lint level is defined here
+  --> $DIR/pub-field-in-priv-mod.rs:1:9
+   |
+LL | #![deny(dead_code)]
+   |         ^^^^^^^^^
+
+error: aborting due to 1 previous error
+