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.

src/libsyntax/ext/base.rs

@@ -890,8 +890,8 @@ pub fn check_zero_tts(cx: &ExtCtxt,
     }
 }
 
-/// Extract the string literal from the first token of `tts`. If this
-/// is not a string literal, emit an error and return None.
+/// Interpreting `tts` as a comma-separated sequence of expressions,
+/// expect exactly one string literal, or emit an error and return None.
 pub fn get_single_str_from_tts(cx: &mut ExtCtxt,
                                sp: Span,
                                tts: &[tokenstream::TokenTree],
@@ -903,6 +903,8 @@ pub fn get_single_str_from_tts(cx: &mut ExtCtxt,
         return None
     }
     let ret = panictry!(p.parse_expr());
+    let _ = p.eat(&token::Comma);
+
     if p.token != token::Eof {
         cx.span_err(sp, &format!("{} takes 1 argument", name));
     }

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);