Don't warn on unused field on union

src/librustc/middle/dead.rs

@@ -13,7 +13,7 @@
 // from live codes are live, and everything else is dead.
 
 use hir::map as hir_map;
-use hir::{self, PatKind};
+use hir::{self, Item_, PatKind};
 use hir::intravisit::{self, Visitor, NestedVisitorMap};
 use hir::itemlikevisit::ItemLikeVisitor;
 
@@ -189,6 +189,24 @@ impl<'a, 'tcx> MarkSymbolVisitor<'a, 'tcx> {
         self.struct_has_extern_repr = had_extern_repr;
         self.inherited_pub_visibility = had_inherited_pub_visibility;
     }
+
+    fn mark_as_used_if_union(&mut self, did: DefId) {
+        if let Some(node_id) = self.tcx.hir.as_local_node_id(did) {
+            match self.tcx.hir.find(node_id) {
+                Some(hir_map::NodeItem(item)) => match item.node {
+                    Item_::ItemUnion(ref variant, _) => {
+                        if variant.fields().len() > 1 {
+                            for field in variant.fields() {
+                                self.live_symbols.insert(field.id);
+                            }
+                        }
+                    }
+                    _ => {}
+                },
+                _ => {}
+            }
+        }
+    }
 }
 
 impl<'a, 'tcx> Visitor<'tcx> for MarkSymbolVisitor<'a, 'tcx> {
@@ -221,6 +239,11 @@ impl<'a, 'tcx> Visitor<'tcx> for MarkSymbolVisitor<'a, 'tcx> {
             hir::ExprPath(ref qpath @ hir::QPath::TypeRelative(..)) => {
                 let def = self.tables.qpath_def(qpath, expr.id);
                 self.handle_definition(def);
+                self.mark_as_used_if_union(def.def_id());
+            }
+            hir::ExprPath(ref qpath @ hir::QPath::Resolved(..)) => {
+                let def = self.tables.qpath_def(qpath, expr.id);
+                self.mark_as_used_if_union(def.def_id());
             }
             hir::ExprMethodCall(..) => {
                 self.lookup_and_handle_method(expr.id);
@@ -561,7 +584,6 @@ impl<'a, 'tcx> Visitor<'tcx> for DeadVisitor<'a, 'tcx> {
             self.warn_dead_code(field.id, field.span,
                                 field.name, "field");
         }
-
         intravisit::walk_struct_field(self, field);
     }
 
@@ -603,6 +625,9 @@ pub fn check_crate<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>) {
     let access_levels = &tcx.privacy_access_levels(LOCAL_CRATE);
     let krate = tcx.hir.krate();
     let live_symbols = find_live(tcx, access_levels, krate);
-    let mut visitor = DeadVisitor { tcx: tcx, live_symbols: live_symbols };
+    let mut visitor = DeadVisitor {
+        tcx: tcx,
+        live_symbols: live_symbols,
+    };
     intravisit::walk_crate(&mut visitor, krate);
 }

src/test/ui/union-fields.rs

@@ -0,0 +1,33 @@
+// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#![deny(dead_code)]
+
+union U1 {
+    a: u8, // should not be reported
+    b: u8, // should not be reported
+    c: u8, // should be reported
+}
+union U2 {
+    a: u8, // should be reported
+    b: u8, // should not be reported
+    c: u8, // should not be reported
+}
+union NoDropLike { a: u8 } // should be reported as unused
+
+fn main() {
+    let u = U1 { a: 0 };
+    let _a = unsafe { u.b };
+
+    let u = U2 { c: 0 };
+    let _b = unsafe { u.b };
+
+    let _u = NoDropLike { a: 10 };
+}

src/test/ui/union-fields.stderr

@@ -0,0 +1,14 @@
+error: field is never used: `y`
+  --> $DIR/union-fields.rs:20:5
+   |
+20 |     y: u32,
+   |     ^^^^^^
+   |
+note: lint level defined here
+  --> $DIR/union-fields.rs:11:9
+   |
+11 | #![deny(dead_code)]
+   |         ^^^^^^^^^
+
+error: aborting due to previous error
+