ICE: lifetime substitution in struct fields when struct implements Drop

[pnkfelix]

This is an injection from sometime post 0.8 (probably very recent).

Some code:

#[deriving(Show)]
pub struct Context<'a> {
    field: Nested<'a>,
}

#[deriving(Show)]
pub struct Nested<'a>;

#[cfg(not(no_drop))]
impl<'a> Drop for Context<'a> {
    fn drop(&mut self) {
        println!("Dropping {}", *self);
    }
}

fn main() {
    let c = Context { field: Nested };
    println!("{}", c);
}

A transcript showing the ICE:

% rustc --version
rustc 0.11.0-pre (2c6caad 2014-06-14 04:46:46 +0000)
host: x86_64-apple-darwin
% rustc /tmp/foo.rs
error: internal compiler error: unexpected failure
note: the compiler hit an unexpected failure path. this is a bug.
note: we would appreciate a bug report: http://doc.rust-lang.org/complement-bugreport.html
note: run with `RUST_BACKTRACE=1` for a backtrace
task 'rustc' failed at 'index out of bounds: the len is 0 but the index is 0', /Users/pnkfelix/Dev/Mozilla/rust.git/src/librustc/lib.rs:1


% rustc --cfg no_drop /tmp/foo.rs
% 

The backtrace:

   1:        0x10be4c592 - rt::backtrace::imp::write::h4595e46a96942472aPp::v0.11.0.pre
   2:        0x10be5384a - failure::on_fail::h36bc5bc9e9066b83n5p::v0.11.0.pre
   3:        0x10c0dda29 - unwind::begin_unwind_inner::h7555e4d38a651235mOd::v0.11.0.pre
   4:        0x10c0dd5aa - unwind::begin_unwind_fmt::h0a25606ea9356b4ePLd::v0.11.0.pre
   5:        0x10c0dd341 - rust_begin_unwind
   6:        0x10c13717e - failure::begin_unwind::h2c605326302113e523v::v0.11.0.pre
   7:        0x10c13b709 - failure::fail_bounds_check::hce749997eee154e7e2v::v0.11.0.pre
   8:        0x108fbe691 - middle::subst::SubstFolder<'a>.TypeFolder::fold_region::h00e6419dabf970e3TqU::v0.11.0.pre
   9:        0x108fbe518 - middle::ty_fold::VecPerParamSpace<T>.TypeFoldable::fold_with::closure.62801
  10:        0x108fbd965 - middle::subst::VecPerParamSpace<T>::map::h10274010684604774229::v0.11.0.pre
  11:        0x108fbd70c - middle::ty_fold::TypeFolder::fold_substs::h9304712281823043231::v0.11.0.pre
  12:        0x108f673a4 - middle::subst::SubstFolder<'a>.TypeFolder::fold_ty::h462b8f3f57f4cc13srU::v0.11.0.pre
  13:        0x108f66b1e - middle::subst::T.Subst::subst_spanned::h15731439402557813363::v0.11.0.pre
  14:        0x1090d94f3 - middle::ty::lookup_field_type::h77f22892d9727d8a25R::v0.11.0.pre
  15:        0x108fa5856 - middle::ty::struct_fields::hea32f38181375c78jeS::v0.11.0.pre
  16:        0x109108152 - middle::ty::type_contents::tc_ty::he654f2f431a75d65OnP::v0.11.0.pre
  17:        0x1090a2191 - middle::ty::type_contents::h7ffc17733b627153KmP::v0.11.0.pre
  18:        0x109481b48 - middle::kind::check_item::h96b5a0fbc7aa7560tIV::v0.11.0.pre
  19:        0x109483eb1 - middle::kind::check_crate::h63e4bac94b391151SyV::v0.11.0.pre
  20:        0x10983ac1a - driver::driver::phase_3_run_analysis_passes::h4aab6aaf3689b4c1wfv::v0.11.0.pre
  21:        0x10983292a - driver::driver::compile_input::hdd225ecd32713dbbL3u::v0.11.0.pre
  22:        0x1098f455e - driver::run_compiler::hd1f9c8f1db63fe6djLx::v0.11.0.pre
  23:        0x1098f1ee6 - driver::main_args::closure.98487
  24:        0x10990c137 - driver::monitor::closure.99578
  25:        0x10990735b - task::TaskBuilder::try::closure.99341
  26:        0x108efabec - task::spawn_opts::closure.7249
  27:        0x10c0da5b8 - task::Task::run::closure.5244
  28:        0x10c150a0c - rust_try
  29:        0x10c0dceca - unwind::try::he15ed9069692b5dcLCd::v0.11.0.pre
  30:        0x10c0da485 - task::Task::run::hf8b632ad7174b7e9XSc::v0.11.0.pre
  31:        0x108efaa5b - task::spawn_opts::closure.7221
  32:        0x10c0dc509 - thread::thread_start::hc88622d48843830cmad::v0.11.0.pre
  33:     0x7fff8c110899 - _pthread_body
  34:     0x7fff8c11072a - _pthread_struct_init

---

Here is another version of the code that is closer to what originally led me to discover this ICE:

#[deriving(Show)]
pub struct Context<'a> {
    field: &'a (),
}

#[cfg(not(no_drop))]
impl<'a> Drop for Context<'a> {
    fn drop(&mut self) {
        println!("Dropping {}", *self);
    }
}

fn main() {
    let msg = ();
    let c = Context { field: &msg };
    println!("{}", c);
}

Note: Sometime between 0.8 and 0.10, we started requiring implementors of Drop to satisfy Send. So it is possible the latter code is just incorrect under the new rules.

[pnkfelix]

(If you add #[unsafe_destructor], which I had done to placate one version of this in 0.10, the problem goes away.)

[Ryman]

Possible dupe of either #13853 or #14377

[alexcrichton]

Another example:

fn main() {
}

struct OnExitSentinel<'a> {
    block: ||: 'a,
}

impl<'a> Drop for OnExitSentinel<'a> {
    fn drop(&mut self) {
    }
}

Seems fine now.

[alexcrichton]

Flagging as needstest

[pnkfelix]

@jakub- @alexcrichton wait, it looks to me like we now require unsafe_destructor even for structs that are just parameterized by lifetimes? Illustration on the playpen: http://is.gd/xnKJve

Note that I already said in a preceding comment that adding unsafe_destructor will fix this.

So what is our end-goal here? If the bug was that I was always supposed to be using unsafe_destructor, then this does not need a test, I think. (But I am not 100% clear why unsafe_destructor is necessary for structs that are solely lifetime parametric.)