Implement type ascription.

src/libsyntax/print/pprust.rs

@@ -445,7 +445,7 @@ fn needs_parentheses(expr: &ast::Expr) -> bool {
         ast::ExprAssign(..) | ast::ExprBinary(..) |
         ast::ExprClosure(..) |
         ast::ExprAssignOp(..) | ast::ExprCast(..) |
-        ast::ExprInPlace(..) => true,
+        ast::ExprInPlace(..) | ast::ExprType(..) => true,
         _ => false,
     }
 }
@@ -2036,6 +2036,11 @@ impl<'a> State<'a> {
                 try!(self.word_space("as"));
                 try!(self.print_type(&**ty));
             }
+            ast::ExprType(ref expr, ref ty) => {
+                try!(self.print_expr(&**expr));
+                try!(self.word_space(":"));
+                try!(self.print_type(&**ty));
+            }
             ast::ExprIf(ref test, ref blk, ref elseopt) => {
                 try!(self.print_if(&**test, &**blk, elseopt.as_ref().map(|e| &**e)));
             }