syntax: Pretty-print 'do' correctly

src/libsyntax/print/pprust.rs

@@ -879,6 +879,7 @@ fn print_field(s: ps, field: ast::field) {
             let blk_arg = vec::pop(base_args);
             alt blk_arg.node {
               ast::expr_loop_body(_) { word_nbsp(s, "for"); }
+              ast::expr_do_body(_) { word_nbsp(s, "do"); }
               _ {}
             }
             some(blk_arg)

src/test/pretty/do1.rs

@@ -0,0 +1,7 @@
+// pretty-exact
+
+fn f(f: fn@(int)) { f(10) }
+
+fn main() {
+    do f { |i| assert i == 10 }
+}