Fix type alias being incorrectly marked as unused

Fixes rust-lang#42303

Author: est31

src/librustc/middle/dead.rs

@@ -424,10 +424,10 @@ struct DeadVisitor<'a, 'tcx: 'a> {
 impl<'a, 'tcx> DeadVisitor<'a, 'tcx> {
     fn should_warn_about_item(&mut self, item: &hir::Item) -> bool {
         let should_warn = match item.node {
+            hir::ItemTy(ref ty, _) => self.live_symbols.contains(&ty.id),
             hir::ItemStatic(..)
             | hir::ItemConst(..)
             | hir::ItemFn(..)
-            | hir::ItemTy(..)
             | hir::ItemEnum(..)
             | hir::ItemStruct(..)
             | hir::ItemUnion(..) => true,

src/test/run-pass/issue-42303.rs

@@ -0,0 +1,43 @@
+// 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.
+
+#![forbid(dead_code)]
+
+// The most simple case: support one layer
+// Issue #42303 was originally reported about this issue
+mod a {
+    pub struct Foo;
+
+    type Bar = Foo;
+
+    impl Bar {
+        pub fn baz(&self) {}
+    }
+}
+
+// Multiple layers of re-defining,
+// to make sure we don't regress here
+mod b {
+    pub struct Foo;
+
+    type Bar = Foo;
+    type FooBar = Bar;
+    type Baz = FooBar;
+    type FooBarBaz = Baz;
+
+    impl FooBarBaz {
+        pub fn baz(&self) {}
+    }
+}
+
+fn main() {
+    a::Foo.baz();
+    b::Foo.baz();
+}