rollup merge of #24487: erickt/syntax

This removes the usage of `#[feature(into_cow, slice_patterns, box_syntax, box_patterns, quote, unsafe_destructor)]` from being used in libsyntax. My main desire for this is that it brings me one step closer to letting [syntex](https://github.com/erickt/rust-syntex) compile with stable rust. Hopefully this doesn't inconvenience rust development.

1 files changed, 3 insertions, 4 deletions

diff --git a/src/libsyntax/ext/trace_macros.rs b/src/libsyntax/ext/trace_macros.rs
index 3fcc6a8d692..646e6fec405 100644
--- a/src/libsyntax/ext/trace_macros.rs
+++ b/src/libsyntax/ext/trace_macros.rs
@@ -28,12 +28,11 @@ pub fn expand_trace_macros(cx: &mut ExtCtxt,
         return base::DummyResult::any(sp);
     }
 
-
-    match tt {
-        [ast::TtToken(_, ref tok)] if tok.is_keyword(keywords::True) => {
+    match (tt.len(), tt.first()) {
+        (1, Some(&ast::TtToken(_, ref tok))) if tok.is_keyword(keywords::True) => {
             cx.set_trace_macros(true);
         }
-        [ast::TtToken(_, ref tok)] if tok.is_keyword(keywords::False) => {
+        (1, Some(&ast::TtToken(_, ref tok))) if tok.is_keyword(keywords::False) => {
             cx.set_trace_macros(false);
         }
         _ => cx.span_err(sp, "trace_macros! accepts only `true` or `false`"),