Panic when piping --check to head

rustfmt 0.99.2-nightly (5c9a2b6 2018-08-07)

└─ env RUST_BACKTRACE=1 cargo +nightly fmt -- --check | head
Diff in /home/yalter/Source/rust/yalter-bot/src/bot.rs at line 21:
     pub fn new(discord: Discord, modules: Vec<Box<Module>>) -> Self {
         // Connect.
         let (connection, ready) = discord.connect().expect("Connect failed");
-        println!(
-            "[Ready] {} is serving {} servers.",
-            ready.user.username,
-            ready.servers.len()
-        );
+        println!("[Ready] {} is serving {} servers.",
thread 'main' panicked at 'failed printing to stdout: Broken pipe (os error 32)', libstd/io/stdio.rs:696:9
stack backtrace:
   0: std::sys::unix::backtrace::tracing::imp::unwind_backtrace
             at libstd/sys/unix/backtrace/tracing/gcc_s.rs:49
   1: std::sys_common::backtrace::print
             at libstd/sys_common/backtrace.rs:71
             at libstd/sys_common/backtrace.rs:59
   2: std::panicking::default_hook::{{closure}}
             at libstd/panicking.rs:211
   3: std::panicking::default_hook
             at libstd/panicking.rs:227
   4: std::panicking::rust_panic_with_hook
             at libstd/panicking.rs:475
   5: std::panicking::continue_panic_fmt
             at libstd/panicking.rs:390
   6: std::panicking::begin_panic_fmt
             at libstd/panicking.rs:345
   7: std::io::stdio::_print
             at libstd/io/stdio.rs:696
             at libstd/io/stdio.rs:705
   8: rustfmt_nightly::rustfmt_diff::OutputWriter::writeln
   9: rustfmt_nightly::filemap::write_file
  10: <rustfmt_nightly::Session<'b, T> as rustfmt_nightly::formatting::FormatHandler>::handle_formatted_file
  11: rustfmt_nightly::formatting::format_project
  12: syntax::with_globals
  13: <rustfmt_nightly::Session<'b, T>>::format
  14: rustfmt::format_and_emit_report
  15: <rustfmt_nightly::Session<'b, T>>::override_config
  16: rustfmt::execute
  17: rustfmt::main
  18: std::rt::lang_start::{{closure}}
  19: std::panicking::try::do_call
             at libstd/rt.rs:59
             at libstd/panicking.rs:310
  20: __rust_maybe_catch_panic
             at libpanic_unwind/lib.rs:105
  21: std::rt::lang_start_internal
             at libstd/panicking.rs:289
             at libstd/panic.rs:392
             at libstd/rt.rs:58
  22: main
  23: __libc_start_main
  24: <unknown>

Start with the rustfmt_diff::OutputWriter::writeln stack-frame entry and follow its use from filemap::write_file during cargo +nightly fmt -- --check | head. Reproduce the command with the reported nightly version and confirm that piping output to head no longer causes a broken-pipe panic.