Unboxed closure ICE: cannot relate bound region: ReInfer(16) <= ReLateBound

[tomjakubowski]

#![feature(overloaded_calls, unboxed_closure_sugar, unboxed_closures)]

fn main() {
    struct Symbol<'a, F: Fn(Vec<&'a str>) -> &'a str> { function: F }
    let f = |&: x: Vec<&str>| -> &str "foobar";
    let sym = Symbol { function: f };
    (sym.function)(vec![]);
}

$ RUST_BACKTRACE=1 rustc  foo.rs 
foo.rs:7:5: 7:27 error: internal compiler error: cannot relate bound region: ReInfer(16) <= ReLateBound(33, BrAnon(0))
foo.rs:7     (sym.function)(vec![]);
             ^~~~~~~~~~~~~~~~~~~~~~
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 'Box<Any>', /home/tom/src/rust/rust/src/libsyntax/diagnostic.rs:113

stack backtrace:
   1:     0x7f85d03d5800 - rt::backtrace::imp::write::hbbd9bf4ceb840fc0KAq
   2:     0x7f85d03be190 - failure::on_fail::hc28b027ccb4467c1cWq
   3:     0x7f85d0bc3c90 - unwind::begin_unwind_inner::h110735be165faa3fIxd
   4:     0x7f85cedcddc0 - unwind::begin_unwind::h293010665865661344
   5:     0x7f85cedcdd50 - diagnostic::SpanHandler::span_bug::h2ea40ca72e774bb3GMF
   6:     0x7f85d16249b0 - driver::session::Session::span_bug::hdab774e2397de37fPtx
   7:     0x7f85d1202bd0 - middle::typeck::infer::region_inference::RegionVarBindings<'a, 'tcx>::make_subregion::hdc4568b6616a2a24L47
   8:     0x7f85d11df440 - middle::typeck::infer::region_inference::RegionVarBindings<'a, 'tcx>::make_eqregion::h4aa77277dfe179e5a47
   9:     0x7f85d11df040 - middle::typeck::infer::equate::Equate<'f, 'tcx>.Combine<'tcx>::regions::h21a863ae7fa5ec80KX3
  10:     0x7f85d11d9f30 - middle::typeck::infer::equate::Equate<'f, 'tcx>.Combine<'tcx>::tys::h35f4a7237a23cc5bX43
  11:     0x7f85d11e22d0 - middle::typeck::infer::combine::Combine::substs::h324391049216757114
  12:     0x7f85d11d9f30 - middle::typeck::infer::equate::Equate<'f, 'tcx>.Combine<'tcx>::tys::h35f4a7237a23cc5bX43
  13:     0x7f85d11e4a00 - middle::typeck::infer::combine::super_tys::closure.158023
  14:     0x7f85d11e4730 - iter::Iterator::collect::h9565269870161006725
  15:     0x7f85d11d9f30 - middle::typeck::infer::equate::Equate<'f, 'tcx>.Combine<'tcx>::tys::h35f4a7237a23cc5bX43
  16:     0x7f85d1760d40 - middle::typeck::infer::combine::Combine::substs::h15550467844381804152
  17:     0x7f85d176b550 - middle::typeck::infer::InferCtxt<'a, 'tcx>::sub_trait_refs::hce3459e301875ec2Drd
  18:     0x7f85d13a63c0 - middle::traits::select::SelectionContext<'cx, 'tcx>::confirm::h67d954f9925112deqZW
  19:     0x7f85d1398700 - middle::traits::select::SelectionContext<'cx, 'tcx>::select::h1e9ce11f39286d16CtV
  20:     0x7f85d1397120 - middle::traits::fulfill::FulfillmentContext::select_where_possible::h301e00b483f5538598U
  21:     0x7f85d11b6ff0 - middle::typeck::check::vtable2::select_fcx_obligations_where_possible::h32e182d4cde2e439l9K
  22:     0x7f85d1a1aaf0 - middle::typeck::check::check_argument_types::h5d391020df5cb75fEeV
  23:     0x7f85d1a1cf30 - middle::typeck::check::check_expr_with_unifier::he6beb0ddba7fb81elNV
  24:     0x7f85d1a6a430 - middle::typeck::check::check_decl_local::h3dbf38cc6fab7488QRX
  25:     0x7f85d1a6a620 - middle::typeck::check::check_stmt::h56381332213a7caeYTX
  26:     0x7f85d19e6c30 - middle::typeck::check::check_block_with_expected::h4c36c485fa96748dbYX
  27:     0x7f85d1a1cf30 - middle::typeck::check::check_expr_with_unifier::he6beb0ddba7fb81elNV
  28:     0x7f85d1a1aaf0 - middle::typeck::check::check_argument_types::h5d391020df5cb75fEeV
  29:     0x7f85d1a1a060 - middle::typeck::check::check_method_argument_types::h58ec3324eba34260ScV
  30:     0x7f85d1a1cf30 - middle::typeck::check::check_expr_with_unifier::he6beb0ddba7fb81elNV
  31:     0x7f85d1a6a620 - middle::typeck::check::check_stmt::h56381332213a7caeYTX
  32:     0x7f85d19e6c30 - middle::typeck::check::check_block_with_expected::h4c36c485fa96748dbYX
  33:     0x7f85d19e2480 - middle::typeck::check::check_fn::hee24634b21a2885a2sS
  34:     0x7f85d19e2280 - middle::typeck::check::check_bare_fn::h6e806ee1bf2c7308aiS
  35:     0x7f85d19dd8b0 - middle::typeck::check::check_item::h51dfba04fcdcac33dKS
  36:     0x7f85d19e0cd0 - middle::typeck::check::check_item_types::hd3964d97cc4fc778khS
  37:     0x7f85d10a9190 - util::common::time::h11171893184677193938
  38:     0x7f85d1a70480 - middle::typeck::check_crate::ha819c660b3fa1ce7Xdk
  39:     0x7f85d1261d40 - driver::driver::phase_3_run_analysis_passes::ha3f2a6dccee8b431XKw
  40:     0x7f85d125ab60 - driver::driver::compile_input::hffa1c362c29e45e9Jrw
  41:     0x7f85d12bceb0 - driver::run_compiler::h72fc31697a8853f50hA
  42:     0x7f85d12bcd80 - driver::run::closure.180762
  43:     0x7f85d12ca670 - task::TaskBuilder<S>::try_future::closure.181584
  44:     0x7f85d12ca4e0 - task::TaskBuilder<S>::spawn_internal::closure.181576
  45:     0x7f85d23b0f20 - task::spawn_opts::closure.8955
  46:     0x7f85d0c32820 - rust_try_inner
  47:     0x7f85d0c32810 - rust_try
  48:     0x7f85d0bc3440 - unwind::try::he0499702d1c7d9dfqmd
  49:     0x7f85d0bbe150 - task::Task::run::h620c5fb327a4e3c14Bc
  50:     0x7f85d23b0ce0 - task::spawn_opts::closure.8893
  51:     0x7f85d0bc31e0 - thread::thread_start::haaa628b6babd0f08fWc
  52:     0x7f85cfe9e250 - start_thread
  53:     0x7f85d08813b9 - clone
  54:                0x0 - <unknown>

Seems related to #17545 but not sure if a dupe since the ReInfer and ReLateBound are on opposite sides of the <=.

[bkoropoff]

Smaller test case:

#![feature(overloaded_calls, unboxed_closures)]

fn main() {
    let f = |:| -> &str "";
    f();
}

[bkoropoff]

I think the problem is that the &str return type is generating an anonymous ReLateBound region since none is specified explicitly, and this is leaking into a non-function-scope context where it doesn't belong. Specifying 'static manually fixes it, as does removing the return type annotation entirely so that a region inference variable is used instead.

I suspect this will be fixed as a side effect of the work @nikomatsakis is doing with higher-rank trait lifetimes.

[japaric]

Both examples compile now that HRTB (#18993) has landed.