privacy: Add publically-reexported foreign item to exported item set

Close #15740

Author: ktt3ja

src/librustc/middle/privacy.rs

@@ -325,7 +325,7 @@ impl<'a> Visitor<()> for EmbargoVisitor<'a> {
     }
 
     fn visit_foreign_item(&mut self, a: &ast::ForeignItem, _: ()) {
-        if self.prev_exported && a.vis == ast::Public {
+        if (self.prev_exported && a.vis == ast::Public) || self.reexports.contains(&a.id) {
             self.exported_items.insert(a.id);
         }
     }

src/test/compile-fail/lint-dead-code-3.rs

@@ -16,6 +16,11 @@
 
 extern crate libc;
 
+pub use x = extern_foo;
+extern {
+    fn extern_foo();
+}
+
 struct Foo; //~ ERROR: code is never used
 impl Foo {
     fn foo(&self) { //~ ERROR: code is never used