Fix compiling empty fmt! strings

src/libsyntax/ext/fmt.rs

@@ -249,6 +249,11 @@ fn pieces_to_expr(cx: @ExtCtxt, sp: span,
         }
     }
 
+    /* Short circuit an easy case up front (won't work otherwise) */
+    if pieces.len() == 0 {
+        return cx.expr_str_uniq(args[0].span, @"");
+    }
+
     let fmt_sp = args[0].span;
     let mut n = 0u;
     let nargs = args.len();

src/test/run-pass/syntax-extension-fmt.rs

@@ -58,6 +58,9 @@ fn part1() {
     test(fmt!("%x", 0xffffffff_u), ~"ffffffff");
     test(fmt!("%o", 0xffffffff_u), ~"37777777777");
     test(fmt!("%t", 0xffffffff_u), ~"11111111111111111111111111111111");
+
+    // Don't result in a compilation error
+    test(fmt!(""), ~"");
 }
 fn part2() {
     // Widths