ICE when using write!() without a mutable ref

[seanmonstar]

If you do write!(w, ..) instead of write!(&mut w, ..).

task 'rustc' panicked at 'index out of bounds: the len is 6 but the index is 6', /home/rustbuild/src/rust-buildbot/slave/nightly-linux/build/src/libsyntax/lib.rs:1

stack backtrace:
   1:     0x7fabba9c69c0 - rt::backtrace::imp::write::h7fe8534f465d0e09y9s
   2:     0x7fabba9c9b60 - failure::on_fail::h3fe2ecde6aab0bb8rwt
   3:     0x7fabbb18fc80 - unwind::begin_unwind_inner::he6bc2073d4eb582enbd
   4:     0x7fabbb18f900 - unwind::begin_unwind_fmt::h305b4b28825355b2y8c
   5:     0x7fabbb18f8c0 - rust_begin_unwind
   6:     0x7fabbb1d5a70 - panicking::panic_fmt::h9d03fcf6b4c3f8e10Nl
   7:     0x7fabbb1dae60 - panicking::panic_bounds_check::h5c6357fea6530f04yMl
   8:     0x7fabb883a130 - codemap::FileMap::get_line::h8bd7cfe56e6cfd5e1aF
   9:     0x7fabb88689b0 - diagnostic::emit::h2322127ddc0c36debCG
  10:     0x7fabb88653d0 - diagnostic::EmitterWriter.Emitter::emit::hb9a4d5c26c04e1df0xG
  11:     0x7fabb8863c50 - diagnostic::Handler::emit::ha390aea82acc4940kfG
  12:     0x7fabb8836910 - diagnostic::SpanHandler::span_err::h83b27707d23c7bfej3F
  13:     0x7fabb9c34720 - middle::typeck::infer::InferCtxt<'a, 'tcx>::type_error_message_str_with_expected::h72941615d6be270289E
  14:     0x7fabb9b40840 - middle::typeck::infer::InferCtxt<'a, 'tcx>::type_error_message::h3e78576a7aeba36csfF
  15:     0x7fabb9b49f70 - middle::typeck::check::check_expr_with_unifier::h23742768f253d831DLl
  16:     0x7fabb9b49f70 - middle::typeck::check::check_expr_with_unifier::h23742768f253d831DLl
  17:     0x7fabb9bbec40 - middle::typeck::check::check_decl_local::h24893462a5fe4409xSn
  18:     0x7fabb9bbee00 - middle::typeck::check::check_stmt::h1ca15bfe8a7d5a87xUn
  19:     0x7fabb9af8d90 - middle::typeck::check::check_block_with_expected::heebc1a2b04a42760MYn
  20:     0x7fabb9b49f70 - middle::typeck::check::check_expr_with_unifier::h23742768f253d831DLl
  21:     0x7fabb9a75100 - middle::typeck::check::_match::check_match::h2c871678897b201eeA8
  22:     0x7fabb9b49f70 - middle::typeck::check::check_expr_with_unifier::h23742768f253d831DLl
  23:     0x7fabb9af8d90 - middle::typeck::check::check_block_with_expected::heebc1a2b04a42760MYn
  24:     0x7fabb9b49f70 - middle::typeck::check::check_expr_with_unifier::h23742768f253d831DLl
  25:     0x7fabb9bbee00 - middle::typeck::check::check_stmt::h1ca15bfe8a7d5a87xUn
  26:     0x7fabb9af8d90 - middle::typeck::check::check_block_with_expected::heebc1a2b04a42760MYn
  27:     0x7fabb9af4810 - middle::typeck::check::check_fn::hd39b1b6b62f22c3fici
  28:     0x7fabb9af4530 - middle::typeck::check::check_bare_fn::h779ff7b2db70ca5bn1h
  29:     0x7fabb9afbda0 - middle::typeck::check::check_method_body::h6418d5a11969c3d7nwi
  30:     0x7fabb9af0310 - middle::typeck::check::check_item::hde22067b8b575546vli
  31:     0x7fabb9af3e60 - visit::walk_mod::h974639367282189498
  32:     0x7fabb9af3e60 - visit::walk_mod::h974639367282189498
  33:     0x7fabb9af41c0 - middle::typeck::check::check_item_types::h7118271db2ab8ea8x0h
  34:     0x7fabb9ee0e10 - util::common::time::h4997555828079517418
  35:     0x7fabb9ee0020 - middle::typeck::check_crate::h0949dffd97487b5ejVL
  36:     0x7fabbb693a10 - driver::driver::phase_3_run_analysis_passes::h7f3610cf195b4c3abfS
  37:     0x7fabbb688830 - driver::driver::compile_input::h13734d451479d6e7VVR
  38:     0x7fabbb70d7b0 - driver::run_compiler::h0a1cb9376c105f68pUT
  39:     0x7fabbb70d6a0 - driver::run::closure.59745
  40:     0x7fabbb519ee0 - task::TaskBuilder<S>::try_future::closure.39051
  41:     0x7fabbb519cd0 - task::TaskBuilder<S>::spawn_internal::closure.39022
  42:     0x7fabbba2db90 - task::NativeSpawner.Spawner::spawn::closure.2472
  43:     0x7fabbb1e84c0 - rust_try_inner
  44:     0x7fabbb1e84b0 - rust_try
  45:     0x7fabbb18d5e0 - unwind::try::h7c27f4e149aed3f2IZc
  46:     0x7fabbb18d470 - task::Task::run::h75863ac7ddab20c0y5b
  47:     0x7fabbba2d8d0 - task::NativeSpawner.Spawner::spawn::closure.2398
  48:     0x7fabbb18ec90 - thread::thread_start::h7822af80b22f729bEqc
  49:     0x7fabb5a4e0c0 - start_thread
  50:     0x7fabbae54f89 - __clone
  51:                0x0 - <unknown>

[japaric]

I reported this earlier in #19142, and it got closed as a duplicate of #14091.

cc @jakub-

[seanmonstar]

woops. i thought it was introduced because of the fmt changes, and I didn't notice an issue recently filed.