Panic in List of length > 4035

[dwightguth]

Consider the following program:

extern crate im_rc;

fn main() {
    let mut l = im_rc::Vector::singleton(0);
    for _ in 0..4036 {
      let mut tmp = im_rc::Vector::singleton(0);
      tmp.append(l);
      l = tmp;
    }
    let len = l.len();
    l.slice(1..len);
  }

I would expect this program to terminate without panicking, but instead I get the following panic:

thread 'main' panicked at 'attempt to subtract with overflow', /home/dwightguth/.cargo/registry/src/gh.neting.cc-1ecc6299db9ec823/im-rc-12.2.0/./src/nodes/rrb.rs:754:44
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:511
   5: std::panicking::continue_panic_fmt
             at libstd/panicking.rs:426
   6: rust_begin_unwind
             at libstd/panicking.rs:337
   7: core::panicking::panic_fmt
             at libcore/panicking.rs:92
   8: core::panicking::panic
             at libcore/panicking.rs:53
   9: <im_rc::nodes::rrb::Node<A>>::split
             at /home/dwightguth/.cargo/registry/src/gh.neting.cc-1ecc6299db9ec823/im-rc-12.2.0/./src/nodes/rrb.rs:754
  10: <im_rc::vector::Vector<A>>::split_off
             at /home/dwightguth/.cargo/registry/src/gh.neting.cc-1ecc6299db9ec823/im-rc-12.2.0/./src/vector/mod.rs:968
  11: <im_rc::vector::Vector<A>>::slice
             at /home/dwightguth/.cargo/registry/src/gh.neting.cc-1ecc6299db9ec823/im-rc-12.2.0/./src/vector/mod.rs:1082
  12: rust::main
             at src/main.rs:11
  13: std::rt::lang_start::{{closure}}
             at /checkout/src/libstd/rt.rs:74
  14: std::panicking::try::do_call
             at libstd/rt.rs:59
             at libstd/panicking.rs:310
  15: __rust_maybe_catch_panic
             at libpanic_unwind/lib.rs:105
  16: std::rt::lang_start_internal
             at libstd/panicking.rs:289
             at libstd/panic.rs:392
             at libstd/rt.rs:58
  17: std::rt::lang_start
             at /checkout/src/libstd/rt.rs:74
  18: main
  19: __libc_start_main
  20: _start

Note that the program works if I reduce the length of the loop to 4035.

[bodil]

Wow, this turned out to be a monster bug. Thanks for the tricky spot!