GC sometimes SEGVs while collecting dtables

[liquidev]

This only happens in the REPL, and only sometimes.

> "a"
> "a"
> Gc.collect

Here's some fish code that reproduces the issue reliably:

#!/usr/bin/fish

while true
  printf \"a\"\n\"a\"\nGc.collect\n | ./target/debug/mica
  if test $status -ne 0
    exit 1
  end
end

[liquidev]

Here's a backtrace captured by a coredump:

* thread #1, name = 'mica', stop reason = signal SIGSEGV
  * frame #0: 0x000055f7ce6df0cc mica`core::option::Option$LT$$RF$T$GT$::copied::he3c1d7bfe74ca5ac(self=Option<&core::option::Option<mica_language::gc::GcRaw<mica_language::value::Closure>>> @ 0x00007ffcf3c8db70) at option.rs:1755:19
    frame #1: 0x000055f7ce6a01cf mica`_$LT$core..iter..adapters..copied..Copied$LT$I$GT$$u20$as$u20$core..iter..traits..iterator..Iterator$GT$::next::hca07a674231b87ae(self=0x00007ffcf3c8dde8) at copied.rs:44:9
    frame #2: 0x000055f7ce6a13de mica`_$LT$core..iter..adapters..fuse..Fuse$LT$I$GT$$u20$as$u20$core..iter..adapters..fuse..FuseImpl$LT$I$GT$$GT$::next::hd97124b0a65addd8(self=0x00007ffcf3c8dde8) at fuse.rs:365:9
    frame #3: 0x000055f7ce6a12ff mica`_$LT$core..iter..adapters..fuse..Fuse$LT$I$GT$$u20$as$u20$core..iter..traits..iterator..Iterator$GT$::next::h29e058535559f7cb(self=0x00007ffcf3c8dde8) at fuse.rs:70:9
    frame #4: 0x000055f7ce6a0752 mica`_$LT$core..iter..adapters..flatten..FlattenCompat$LT$I$C$U$GT$$u20$as$u20$core..iter..traits..iterator..Iterator$GT$::next::hf128e0b9e164aa4d(self=0x00007ffcf3c8dde8) at flatten.rs:299:19
    frame #5: 0x000055f7ce6a047f mica`_$LT$core..iter..adapters..flatten..Flatten$LT$I$GT$$u20$as$u20$core..iter..traits..iterator..Iterator$GT$::next::h23fc05be1bdfd259(self=0x00007ffcf3c8dde8) at flatten.rs:192:9
    frame #6: 0x000055f7ce6fed1a mica`mica_language::gc::Memory::mark_dtable_reachable_rec::h918f332f5ee6cd49(self=0x00007ffcf3c92648, mem=GcRaw<mica_language::bytecode::DispatchTable> @ 0x00007ffcf3c8dd80) at gc.rs:147:24
    frame #7: 0x000055f7ce6fecd3 mica`mica_language::gc::Memory::mark_dtable_reachable_rec::h918f332f5ee6cd49(self=0x00007ffcf3c92648, mem=GcRaw<mica_language::bytecode::DispatchTable> @ 0x00007ffcf3c8de80) at gc.rs:145:13
    frame #8: 0x000055f7ce6fef48 mica`mica_language::gc::Memory::mark_all_gray_reachable::h4acbf482a47ae0ec(self=0x00007ffcf3c92648) at gc.rs:205:16
    frame #9: 0x000055f7ce68961a mica`mica_language::gc::Memory::collect::hb771515776d407aa(self=0x00007ffcf3c92648, roots=(a = core::option::Option<> @ 0x00007ffcf3c8e380, b = core::option::Option<> @ 0x00007ffcf3c8e3a8)) at gc.rs:132:10
    frame #10: 0x000055f7ce699752 mica`mica_language::vm::Fiber::call_control::hf4715bfdd8777ad7(self=0x00007ffcf3c937b8, env=0x00007ffcf3c92540, globals=0x00007ffcf3c92630, gc=0x00007ffcf3c92648, ctl=Control @ 0x00007ffcf3c8e3d8, argument_count=1) at vm.rs:308:22
    frame #11: 0x000055f7ce699204 mica`mica_language::vm::Fiber::enter_function::hca39c8fc321dc86f(self=0x00007ffcf3c937b8, env=0x00007ffcf3c92540, globals=0x00007ffcf3c92630, gc=0x00007ffcf3c92648, closure=GcRaw<mica_language::value::Closure> @ 0x00007ffcf3c8e4c0, argument_count=1) at vm.rs:284:13
    frame #12: 0x000055f7ce69cbf5 mica`mica_language::vm::Fiber::interpret::hc1b33c638664c231(self=0x00007ffcf3c937b8, env=0x00007ffcf3c92540, globals=0x00007ffcf3c92630, gc=0x00007ffcf3c92648) at vm.rs:641:19
    frame #13: 0x000055f7ce47b401 mica`mica_hl::fiber::Fiber::resume::h2abf83396be38057(self=0x00007ffcf3c937b0) at fiber.rs:23:23
    frame #14: 0x000055f7ce49f243 mica`mica::interpret::_$u7b$$u7b$closure$u7d$$u7d$::h0fb5c34c0a0cf525 at main.rs:72:45
    frame #15: 0x000055f7ce49b7b6 mica`_$LT$core..iter..sources..from_fn..FromFn$LT$F$GT$$u20$as$u20$core..iter..traits..iterator..Iterator$GT$::next::hfb669165750e6d19(self=0x00007ffcf3c937b0) at from_fn.rs:69:9
    frame #16: 0x000055f7ce4a3c69 mica`_$LT$core..iter..adapters..flatten..FlattenCompat$LT$I$C$U$GT$$u20$as$u20$core..iter..traits..iterator..Iterator$GT$::next::h320cba3ec9126945(self=0x00007ffcf3c93718) at flatten.rs:294:23
    frame #17: 0x000055f7ce4a3bd6 mica`_$LT$core..iter..adapters..flatten..Flatten$LT$I$GT$$u20$as$u20$core..iter..traits..iterator..Iterator$GT$::next::h15780c3d2d0dde10(self=0x00007ffcf3c93718) at flatten.rs:192:9
    frame #18: 0x000055f7ce47b841 mica`_$LT$core..iter..adapters..fuse..Fuse$LT$I$GT$$u20$as$u20$core..iter..adapters..fuse..FuseImpl$LT$I$GT$$GT$::next::hf35964c994710594(self=0x00007ffcf3c93718) at fuse.rs:365:9
    frame #19: 0x000055f7ce47b786 mica`_$LT$core..iter..adapters..fuse..Fuse$LT$I$GT$$u20$as$u20$core..iter..traits..iterator..Iterator$GT$::next::ha1777f3540071b07(self=0x00007ffcf3c93718) at fuse.rs:70:9
    frame #20: 0x000055f7ce4a408a mica`_$LT$core..iter..adapters..flatten..FlattenCompat$LT$I$C$U$GT$$u20$as$u20$core..iter..traits..iterator..Iterator$GT$::next::hec38e7866fc01db1(self=0x00007ffcf3c93718) at flatten.rs:299:19
    frame #21: 0x000055f7ce4a3bf6 mica`_$LT$core..iter..adapters..flatten..Flatten$LT$I$GT$$u20$as$u20$core..iter..traits..iterator..Iterator$GT$::next::h80d3ad2296334163(self=0x00007ffcf3c93718) at flatten.rs:192:9
    frame #22: 0x000055f7ce49495b mica`mica::repl::h157ff84a9761513f(engine_options=0x00007ffcf3c93a88) at main.rs:111:21
    frame #23: 0x000055f7ce494bd6 mica`mica::main::h96b711c30d0cdb43 at main.rs:135:7
    frame #24: 0x000055f7ce475a1b mica`core::ops::function::FnOnce::call_once::hd06b0bdb256b676c((null)=(mica`mica::main::h96b711c30d0cdb43 at main.rs:120), (null)=<unavailable>) at function.rs:227:5
    frame #25: 0x000055f7ce49f75e mica`std::sys_common::backtrace::__rust_begin_short_backtrace::h36ced528fa18bc53(f=(mica`mica::main::h96b711c30d0cdb43 at main.rs:120)) at backtrace.rs:122:18
    frame #26: 0x000055f7ce474e91 mica`std::rt::lang_start::_$u7b$$u7b$closure$u7d$$u7d$::h80edc60958cf3d45 at rt.rs:145:18
    frame #27: 0x000055f7ce71f0fe mica`std::rt::lang_start_internal::h72f1a7b2c2989a79 [inlined] core::ops::function::impls::_$LT$impl$u20$core..ops..function..FnOnce$LT$A$GT$$u20$for$u20$$RF$F$GT$::call_once::hdb5c472c8933c85f at function.rs:259:13
    frame #28: 0x000055f7ce71f0f8 mica`std::rt::lang_start_internal::h72f1a7b2c2989a79 [inlined] std::panicking::try::do_call::h7655f94a1d21bccc at panicking.rs:492:40
    frame #29: 0x000055f7ce71f0f8 mica`std::rt::lang_start_internal::h72f1a7b2c2989a79 [inlined] std::panicking::try::h5dd2ba2ac3a43bfe at panicking.rs:456:19
    frame #30: 0x000055f7ce71f0f8 mica`std::rt::lang_start_internal::h72f1a7b2c2989a79 [inlined] std::panic::catch_unwind::h5d7e31322f6d63dd at panic.rs:137:14
    frame #31: 0x000055f7ce71f0f8 mica`std::rt::lang_start_internal::h72f1a7b2c2989a79 [inlined] std::rt::lang_start_internal::_$u7b$$u7b$closure$u7d$$u7d$::h41a646921a7b21f6 at rt.rs:128:48
    frame #32: 0x000055f7ce71f0f8 mica`std::rt::lang_start_internal::h72f1a7b2c2989a79 [inlined] std::panicking::try::do_call::h18284440578558f1 at panicking.rs:492:40
    frame #33: 0x000055f7ce71f0f8 mica`std::rt::lang_start_internal::h72f1a7b2c2989a79 [inlined] std::panicking::try::h768d8b30463938d9 at panicking.rs:456:19
    frame #34: 0x000055f7ce71f0f8 mica`std::rt::lang_start_internal::h72f1a7b2c2989a79 [inlined] std::panic::catch_unwind::hfdd0843d7d8b5025 at panic.rs:137:14
    frame #35: 0x000055f7ce71f0f8 mica`std::rt::lang_start_internal::h72f1a7b2c2989a79 at rt.rs:128:20
    frame #36: 0x000055f7ce474e60 mica`std::rt::lang_start::h90a512d947521cc8(main=(mica`mica::main::h96b711c30d0cdb43 at main.rs:120), argc=1, argv=0x00007ffcf3c952f8) at rt.rs:144:17
    frame #37: 0x000055f7ce495c3c mica`main + 28
    frame #38: 0x00007f7646fd2310 libc.so.6`__libc_start_call_main + 128
    frame #39: 0x00007f7646fd23c1 libc.so.6`__libc_start_main@@GLIBC_2.34 + 129
    frame #40: 0x000055f7ce474855 mica`_start + 37

I generated a few coredumps with the script above and the crash always seems to happen there.

[liquidev]

I identified that this seems to only happen with the new lists branch. master is unaffected.