Add syntax for negative implementations of traits

This commit introduces the syntax for negative implmenetations of traits
as shown below:

`impl !Trait for Type {}`

cc #13231
Part of RFC #3

1 files changed, 8 insertions, 0 deletions

diff --git a/src/libsyntax/print/pprust.rs b/src/libsyntax/print/pprust.rs
index 9702c79719c..61b7aa408a8 100644
--- a/src/libsyntax/print/pprust.rs
+++ b/src/libsyntax/print/pprust.rs
@@ -917,6 +917,7 @@ impl<'a> State<'a> {
             }
 
             ast::ItemImpl(unsafety,
+                          polarity,
                           ref generics,
                           ref opt_trait,
                           ref ty,
@@ -931,6 +932,13 @@ impl<'a> State<'a> {
                     try!(space(&mut self.s));
                 }
 
+                match polarity {
+                    ast::ImplPolarity::Negative => {
+                        try!(word(&mut self.s, "!"));
+                    },
+                    _ => {}
+                }
+
                 match opt_trait {
                     &Some(ref t) => {
                         try!(self.print_trait_ref(t));