Adjust dead code lint to account for fields that implement Drop, see #21775

Author: dagit

src/librustc/middle/dead.rs

@@ -460,10 +460,16 @@ impl<'a, 'tcx> DeadVisitor<'a, 'tcx> {
             Some(def_id) => self.tcx.lang_items.items().any(|(_, item)| *item == Some(def_id)),
             _ => false
         };
+        let field_has_dtor = match field_type.ty_adt_def() {
+            Some(def) => def.has_dtor(),
+            _ => false
+        };
+
         is_named
             && !self.symbol_is_live(node.id, None)
             && !is_marker_field
             && !has_allow_dead_code_or_lang_attr(&node.attrs)
+            && !field_has_dtor
     }
 
     fn should_warn_about_variant(&mut self, variant: &hir::Variant_) -> bool {

src/test/compile-fail/lint-dead-code-1.rs

@@ -115,3 +115,27 @@ fn bar() { //~ ERROR: function is never used
 #[allow(dead_code)]
 fn g() { h(); }
 fn h() {}
+
+// This implements Drop, so we should not see a warning on
+// 'unused' field 's'.
+struct S(u32);
+
+impl Drop for S {
+    fn drop(&mut self) { }
+}
+
+struct Container {
+    s: S,
+}
+
+impl Container {
+    fn new() -> Container {
+        let s = S(4);
+
+        Container { s: s }
+    }
+}
+
+pub fn baz() {
+    Container::new();
+}