Macro invocations inside impl bodies handled incorrectly

[crumblingstatue]

In one of my projects, rustfmt removed a macro invocation inside an impl block:

         try!(writer.write_all(&self.unknown));⏎
         Ok(())⏎
     }⏎
-    chunk_write_impl!();⏎
 }⏎

When I tried creating a test case, rustfmt panicked instead.

struct Foo;

macro_rules! add_fun {
    () => { fn fun() {} }
}

impl Foo { add_fun!(); }

rustfmt backtrace:

thread '<main>' panicked at 'index 0 and/or 1 in `` do not lie on character boundary', ../src/libcore/str/mod.rs:1447
stack backtrace:
   1:     0x5602721f42c0 - sys::backtrace::tracing::imp::write::h064b2475ee0d772cxXt
   2:     0x5602721f6825 - panicking::log_panic::_<closure>::closure.40939
   3:     0x5602721f62a0 - panicking::log_panic::h29f8d24759545d7862x
   4:     0x5602721e87c3 - sys_common::unwind::begin_unwind_inner::hdadcc96a1ecb24974Ps
   5:     0x5602721e8b58 - sys_common::unwind::begin_unwind_fmt::h9442bd88fc02b35caPs
   6:     0x5602721f3921 - rust_begin_unwind
   7:     0x56027222297f - panicking::panic_fmt::h03bdc82f7b816525VFK
   8:     0x560272229a8a - str::slice_error_fail::hc00b53e4559d1b43eWS
   9:     0x56027206034f - visitor::_<impl>::visit_item::hef83e33b67452e74X6d
  10:     0x560272004eca - visitor::_<impl>::format_separate_mod::h55e73f776f9071a6Nme
  11:     0x560272010301 - format::h59de26ef8075f4dec8l
  12:     0x5602720108a3 - run::hbe2cf45382bbd7a7k9l
  13:     0x560271ffb41d - main::h4e74fdf9b0b12376iha
  14:     0x5602721f6044 - sys_common::unwind::try::try_fn::h7625493052369861041
  15:     0x5602721f3768 - __rust_try
  16:     0x5602721f5ce6 - rt::lang_start::h47c94448bf012481e0x
  17:     0x7f9c072e660f - __libc_start_main
  18:     0x560271ff3de8 - _start
  19:                0x0 - <unknown>

[nrc]

This might be fixed by #655

[crumblingstatue]

I can confirm that this issue has been fixed by #655.