Handle type aliases as well

src/librustc/middle/dead.rs

@@ -247,9 +247,12 @@ impl<'a, 'tcx> Visitor<'tcx> for MarkSymbolVisitor<'a, 'tcx> {
             hir::ExprTupField(ref lhs, idx) => {
                 self.handle_tup_field_access(&lhs, idx.node);
             }
-            hir::ExprStruct(ref qpath, ref fields, _) => {
-                let def = self.tables.qpath_def(qpath, expr.id);
-                self.mark_as_used_if_union(def.def_id(), fields);
+            hir::ExprStruct(_, ref fields, _) => {
+                if let ty::TypeVariants::TyAdt(ref def, _) = self.tables.expr_ty(expr).sty {
+                    if def.is_union() {
+                        self.mark_as_used_if_union(def.did, fields);
+                    }
+                }
             }
             _ => ()
         }

src/test/ui/union-fields.rs

@@ -22,6 +22,13 @@ union U2 {
 }
 union NoDropLike { a: u8 } // should be reported as unused
 
+union U {
+    a: u8, // should not be reported
+    b: u8, // should not be reported
+    c: u8, // should be reported
+}
+type A = U;
+
 fn main() {
     let u = U1 { a: 0 };
     let _a = unsafe { u.b };
@@ -30,4 +37,6 @@ fn main() {
     let _b = unsafe { u.b };
 
     let _u = NoDropLike { a: 10 };
+    let u = A { a: 0 };
+    let _b = unsafe { u.b };
 }

src/test/ui/union-fields.stderr

@@ -22,5 +22,11 @@ error: field is never used: `a`
 23 | union NoDropLike { a: u8 } // should be reported as unused
    |                    ^^^^^
 
-error: aborting due to 3 previous errors
+error: field is never used: `c`
+  --> $DIR/union-fields.rs:28:5
+   |
+28 |     c: u8, // should be reported
+   |     ^^^^^
+
+error: aborting due to 4 previous errors