feature(drop_types_in_const): type with drop flag in a const causes ICE/trap

[durka]

Code:

#![feature(drop_types_in_const)]

struct Dropper;

impl Drop for Dropper { fn drop(&mut self) { println!("dropping"); } }

static mut D: Dropper = Dropper;

fn main() {
    unsafe { D = Dropper; }
}

With MIR on:

thread 'rustc' panicked at 'assertion failed: `(left == right)` (left: `0`, right: `1`)', ../src/librustc_trans/adt.rs:1424
stack backtrace:
   1:        0x1121b57fe - std::sys::backtrace::tracing::imp::write::h29f5fdb9fc0a7395
   2:        0x1121c48ef - std::panicking::default_hook::_{{closure}}::h2cc84f0378700526
   3:        0x1121c2dfd - std::panicking::default_hook::hbbe7fa36a995aca0
   4:        0x1121c3497 - std::panicking::rust_panic_with_hook::h105c3d42fcd2fb5e
   5:        0x1121c3354 - std::panicking::begin_panic::hbf62ea4a5ff3f9de
   6:        0x1121c3262 - std::panicking::begin_panic_fmt::h20f5943904e5791d
   7:        0x10dbaea73 - rustc_trans::adt::build_const_struct::h553d38025453e4ea
   8:        0x10dbad8d5 - rustc_trans::adt::trans_const::h31bc6eab0284c8f0
   9:        0x10dc7188d - rustc_trans::mir::constant::MirConstContext::trans::hf3d43154f4de3a91
  10:        0x10dc6f80e - rustc_trans::mir::constant::MirConstContext::trans_def::hc17841f2936ef79b
  11:        0x10dc7565e - rustc_trans::mir::constant::trans_static_initializer::h98e4a45ec90977e8
  12:        0x10dc875d1 - rustc_trans::trans_item::TransItem::define::h1e5a74077917a073
  13:        0x10dbc4f7b - rustc_trans::base::trans_crate::h9baf3a0389546061
  14:        0x10d6cad8a - rustc_driver::driver::phase_4_translate_to_llvm::h54e99578fb1bb696
  15:        0x10d707fcc - rustc_driver::driver::compile_input::_{{closure}}::h7236bd0d96fe8ce9
  16:        0x10d6fac3a - rustc_driver::driver::phase_3_run_analysis_passes::_{{closure}}::h7f2cec505064b4bf
  17:        0x10d656565 - rustc::ty::context::TyCtxt::create_and_enter::hab8fd8369376d0f6
  18:        0x10d6b6721 - rustc_driver::driver::compile_input::hb4cc34cf85dc1edf
  19:        0x10d6dd5b0 - rustc_driver::run_compiler::h50f95674bd902ab5
  20:        0x10d62d29e - std::panicking::try::call::h4577500a5284c6ff
  21:        0x1121cb76a - __rust_maybe_catch_panic
  22:        0x10d645791 - _<F as alloc..boxed..FnBox<A>>::call_box::h24f3eb0b42327962
  23:        0x1121c1ba5 - std::sys::thread::Thread::new::thread_start::h8f3bd45211e9f5ea
  24:     0x7fff84dd9898 - _pthread_body
  25:     0x7fff84dd9729 - _pthread_start

With MIR off, the code compiles but traps (Trace/BPT trap: 5) at runtime.

[Aatch]

I assume this is related to the drop flag (which I believe still exists, even if it is on it's last legs).

/cc @rust-lang/compiler

[eddyb]

Indeed, the drop_types_in_const feature doesn't work with drop flags yet. Hopefully we'll sort this out after the beta branches off.

[durka]

@eddyb you mean, doesn't work with stack drop flags? Note that it's broken (in different ways) with and without MIR, so I'm not sure this is new...

[eddyb]

@durka No, I mean the drop flags, the old ones embedded in every type implementing Drop.

[durka]

OK, so the drop_types_in_const feature has never actually worked so far?

On Mon, Aug 15, 2016 at 1:22 AM, Eduard-Mihai Burtescu <
[email protected]> wrote:

@durka https://github.com/durka No, I mean the drop flags, the old
ones embedded in every type implementing Drop.

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[Aatch]

@durka presumably types with #[unsafe_no_drop_flag] work.

[eddyb]

@durka presumably types with #[unsafe_no_drop_flag] worked.

Indeed, Option<Box<T>> and Option<Vec<T>> included.

[durka]

OK. Should I leave this open then?

[durka]

Looks like maybe a dupe of #34053. Anyway, works fine with nightly since in-struct drop flags are gone.