support `default impl` for specialization

    rebase after support for llvm-3.7

src/librustc/hir/lowering.rs

@@ -1346,7 +1346,7 @@ fn lower_item_kind(&mut self,
 
                 hir::ItemImpl(self.lower_unsafety(unsafety),
                               self.lower_impl_polarity(polarity),
-                              self.lower_defaultness(defaultness),
+                              self.lower_defaultness(defaultness, true /* [1] */),
                               self.lower_generics(generics),
                               ifce,
                               self.lower_ty(ty),

src/librustc/hir/print.rs

@@ -823,8 +823,9 @@ pub fn print_visibility(&mut self, vis: &hir::Visibility) -> io::Result<()> {
     }
 
     pub fn print_defaultness(&mut self, defaultness: hir::Defaultness) -> io::Result<()> {
-        if let hir::Defaultness::Default = defaultness {
-            self.word_nbsp("default")?;
+        match defaultness {
+            hir::Defaultness::Default { .. } => self.word_nbsp("default")?,
+            hir::Defaultness::Final => (),
         }
         Ok(())
     }