Allow trailing commas in format!

This is more consistent with other parts of the language and it also makes it
easier to use in situations where format string is massive.

Author: alexcrichton

src/libsyntax/ext/format.rs

@@ -76,6 +76,7 @@ impl Context {
                 self.ecx.span_err(sp, "expected token: `,`");
                 return (extra, None);
             }
+            if *p.token == token::EOF { break } // accept trailing commas
             if named || (token::is_ident(p.token) &&
                          p.look_ahead(1, |t| *t == token::EQ)) {
                 named = true;

src/test/run-pass/ifmt.rs

@@ -243,6 +243,10 @@ pub fn main() {
     }
 
     test_format_args();
+
+    // test that trailing commas are acceptable
+    format!("{}", "test",);
+    format!("{foo}", foo="test",);
 }
 
 // Basic test to make sure that we can invoke the `write!` macro with an