libsyntax/ext: trailing commas in builtin macros

Most notably this changes 'syntax::ext::base::get_single_str_from_tts'
to accept a trailing comma, and revises the documentation so that this
aspect is not surprising. I made this change under the understanding
that this crate is private rustc implementation detail (I hope this is
correct!). After reviewing all call sites, I believe the revised
semantics are closer to the intended spirit of the function.

1 files changed, 2 insertions, 0 deletions

diff --git a/src/libsyntax_ext/cfg.rs b/src/libsyntax_ext/cfg.rs
index 1d8dc406468..1eeba9b30b8 100644
--- a/src/libsyntax_ext/cfg.rs
+++ b/src/libsyntax_ext/cfg.rs
@@ -28,6 +28,8 @@ pub fn expand_cfg<'cx>(cx: &mut ExtCtxt,
     let mut p = cx.new_parser_from_tts(tts);
     let cfg = panictry!(p.parse_meta_item());
 
+    let _ = p.eat(&token::Comma);
+
     if !p.eat(&token::Eof) {
         cx.span_err(sp, "expected 1 cfg-pattern");
         return DummyResult::expr(sp);