autocompleting tilde + forward slash (~/) causes a crash

Reproduction

press tab to initiate the autocomplete and the crash on the last step:

user:~# export RUST_BACKTRACE=1
user:~# ion ^> /path/to/log
user:~# ~/

Expected behaviour

ion lists the files/directories in $HOME/

Actual behaviour

ion crashes, STDERR is printed into /path/to/log:

thread 'main' panicked at 'byte index 47 is out of bounds of `~/`', libcore/str/mod.rs:2094:9
stack backtrace:
   0: std::sys::unix::backtrace::tracing::imp::unwind_backtrace
             at libstd/sys/unix/backtrace/tracing/gcc_s.rs:49
   1: std::panicking::default_hook::{{closure}}
             at libstd/sys_common/backtrace.rs:71
   2: std::panicking::rust_panic_with_hook
             at libstd/panicking.rs:227
             at libstd/panicking.rs:476
   3: std::panicking::continue_panic_fmt
             at libstd/panicking.rs:390
   4: rust_begin_unwind
             at libstd/panicking.rs:325
   5: core::panicking::panic_fmt
             at libcore/panicking.rs:77
   6: core::str::slice_error_fail
             at libcore/str/mod.rs:0
   7: <ion_shell::shell::completer::IonFileCompleter as liner::complete::Completer>::completions
   8: <ion_shell::shell::completer::MultiCompleter<A, B> as liner::complete::Completer>::completions
   9: <liner::editor::Editor<'a, W>>::complete
  10: liner::context::Context::read_line
  11: ion_shell::shell::binary::readln::readln
  12: <&mut I as core::iter::iterator::Iterator>::next
  13: <ion_shell::parser::quotes::Terminator<I> as core::iter::iterator::Iterator>::next
  14: <ion_shell::shell::Shell as ion_shell::shell::binary::Binary>::execute_interactive
  15: ion::main
  16: std::rt::lang_start::{{closure}}
  17: main
  18: __libc_start_main
             at ../csu/libc-start.c:308
  19: _start
             at ../sysdeps/x86_64/start.S:120

Build information

user:~# uname -a
Linux a 4.19.30_1 #1 SMP PREEMPT Wed Mar 20 09:17:21 UTC 2019 x86_64 GNU/Linux
user:~# ion --version
ion 1.0.0-alpha (x86_64-unknown-linux-gnu)
rev fde4aff113a0c7690a158d55ff1798b6d1e91369

ion was built with rustc 1.31.0