ICE: Relating argument and result lifetimes causes 'cannot relate bound' panic

[pierzchalski]

I tried using higher-order lifetimes to bound the lifetimes of the returned value, got an ICE.

I tried this code:

trait T {}
impl T for () {}

pub fn closure<A>()
    -> Box<for<'a> Fn(&'a A) -> Box<T + 'a>>
{
    Box::new(|a: &A| {
        Box::new(())
    })
}

I expected to get, at worst, an error about the return box outliving a, and so requiring an explicit lifetime on a.

Instead, when compiled with rustc --crate-type=lib, I got an ICE:

test.rs:7:5: 9:7 error: internal compiler error: cannot relate bound region: ReScope(DestructionScope(24)) <= ReLateBound(DebruijnIndex { depth: 1 }, BrNamed(DefId { krate: 0, node: 13 }, 'a))
test.rs:7     Box::new(|a: &A| {
test.rs:8         Box::new(())
test.rs:9     })

Meta

rustc --version --verbose:

rustc 1.2.0-nightly (0cc99f9cc 2015-05-17) (built 2015-05-18)
binary: rustc
commit-hash: 0cc99f9cc9b59518f618a2c3f5011000e60f922e
commit-date: 2015-05-17
build-date: 2015-05-18
host: x86_64-unknown-linux-gnu
release: 1.2.0-nightly

Backtrace:

thread 'rustc' panicked at 'Box<Any>', /home/rustbuild/src/rust-buildbot/slave/nightly-dist-rustc-linux/build/src/libsyntax/diagnostic.rs:176

stack backtrace:
   1:     0x7f3db886f169 - sys::backtrace::write::h4052d6a51ba10f8105r
   2:     0x7f3db88770b9 - panicking::on_panic::h90cadee2464dd3b41Kw
   3:     0x7f3db8838192 - rt::unwind::begin_unwind_inner::ha0307b6be50a7723bqw
   4:     0x7f3db5c1405d - rt::unwind::begin_unwind::h1117890260308880149
   5:     0x7f3db5c13ff2 - diagnostic::SpanHandler::span_bug::h549aac4617510b93f6B
   6:     0x7f3db68f632a - middle::infer::region_inference::RegionVarBindings<'a, 'tcx>::make_subregion::h151182c416f4068dkfv
   7:     0x7f3db690ec2f - middle::infer::mk_subr::h45ed748f70b8a8516ty
   8:     0x7f3db7fb4f5f - check::regionck::type_must_outlive::h6330ac58585719f9Lue
   9:     0x7f3db7fb310a - check::dropck::iterate_over_potentially_unsafe_regions_in_type::ha5ed1d4709738abfMDa
  10:     0x7f3db7fb1c98 - check::dropck::check_safety_of_destructor_if_necessary::h5a4d6b86a46f915dWxa
  11:     0x7f3db7fe2ead - check::regionck::check_safety_of_rvalue_destructor_if_necessary::hb017fa90b92435b9bQd
  12:     0x7f3db7fdcc01 - check::regionck::visit_expr::h8e3f06644616962c7ad
  13:     0x7f3db7fdacfa - check::regionck::Rcx<'a, 'tcx>::visit_fn_body::h31bf9eff19779ab3zNc
  14:     0x7f3db805365d - check::check_bare_fn::h6168788eb123df8aTun
  15:     0x7f3db8051442 - check::CheckItemBodiesVisitor<'a, 'tcx>.Visitor<'tcx>::visit_item::hc2da6145fe079e8eWrn
  16:     0x7f3db811103a - check_crate::closure.38914
  17:     0x7f3db810aaac - check_crate::hf6e0aa588aa35f58hIC
  18:     0x7f3db8dbf29f - driver::phase_3_run_analysis_passes::h83b5112ae07f494ftGa
  19:     0x7f3db8da30cc - driver::compile_input::h0a7842b43c159d70Qba
  20:     0x7f3db8e59011 - run_compiler::hb2dfa1770188c5e175b
  21:     0x7f3db8e56862 - boxed::F.FnBox<A>::call_box::h10739628233630710986
  22:     0x7f3db8e55e29 - rt::unwind::try::try_fn::h12004866116241301408
  23:     0x7f3db88ef7c8 - rust_try_inner
  24:     0x7f3db88ef7b5 - rust_try
  25:     0x7f3db8e560c4 - boxed::F.FnBox<A>::call_box::h9242018289459349633
  26:     0x7f3db8875e51 - sys::thread::Thread::new::thread_start::he4baa243e39f5dd6ixv
  27:     0x7f3db2914373 - start_thread
  28:     0x7f3db84c827c - clone
  29:                0x0 - <unknown>

Other issues for the same panic:

I'm comparing against the first 4 results for searching for 'cannot relate bound', ordered from most recent to least:

• ICE with manual implementation of Fn Trait #20225: the example code produces a (non-ICE) error, and does not involve relating return lifetimes: associated type bindings are not allowed here).
• ICE cannot relate bound region  #20186: the example code isn't complete, and does not involve relating return lifetimes.
• ICE: Cannot relate bound region with for and unboxed closures. #19098: the example code now compiles.
• ICE (cannot relate bound region) with universally-quantified closure lifetime bound #19009: the example code is old enough that it does not compile. The example does relate output lifetimes to quantified ones, however I don't know the equivalent current syntax.

I'm hoping that since #19009 was closed on 2014-12-19, it's safe to assume all the issues before that date aren't going to be relevant.