ICE with #![feature(nll)] and elided lifetimes

[alekratz]

Hi, I'm not sure if this has been reported or is known at all. If it has feel free to close it out.

This example:

#![feature(nll)]

struct Bar;

struct Foo<'s> {
    bar: &'s mut Bar,
}

impl Foo<'_> {
    fn new(bar: &mut Bar) -> Self {
        Foo { bar }
    }
}

causes an ICE on rustc 1.31.0-nightly (4bd4e41 2018-10-25)

This is the error I'm getting, backtrace included:

error: internal compiler error: librustc/hir/map/mod.rs:815: expected expr, found method new in Foo::new (id=19)                                                                                                                                                                                                     [3/9508]

thread 'main' panicked at 'Box<Any>', librustc_errors/lib.rs:600:9                   
stack backtrace:
   0: std::sys::unix::backtrace::tracing::imp::unwind_backtrace
             at libstd/sys/unix/backtrace/tracing/gcc_s.rs:49
   1: std::sys_common::backtrace::print
             at libstd/sys_common/backtrace.rs:71
             at libstd/sys_common/backtrace.rs:59
   2: std::panicking::default_hook::{{closure}}
             at libstd/panicking.rs:211
   3: std::panicking::default_hook
             at libstd/panicking.rs:227
   4: rustc::util::common::panic_hook
   5: std::panicking::rust_panic_with_hook
             at libstd/panicking.rs:480
   6: std::panicking::begin_panic
   7: rustc_errors::Handler::bug
   8: rustc::util::bug::opt_span_bug_fmt::{{closure}}
   9: rustc::ty::context::tls::with_opt::{{closure}}
  10: rustc::ty::context::tls::with_context_opt
  11: rustc::ty::context::tls::with_opt
  12: rustc::util::bug::opt_span_bug_fmt
  13: rustc::util::bug::bug_fmt
  14: rustc::hir::map::Map::expect_expr
  15: rustc_mir::borrow_check::nll::region_infer::error_reporting::region_name::<impl rustc_mir::borrow_check::nll::region_infer::RegionInferenceContext<'tcx>>::give_region_a_name
  16: rustc_mir::borrow_check::nll::region_infer::error_reporting::<impl rustc_mir::borrow_check::nll::region_infer::RegionInferenceContext<'tcx>>::report_general_error
  17: rustc_mir::borrow_check::nll::region_infer::RegionInferenceContext::solve_inner
  18: rustc::util::common::time
  19: rustc_mir::borrow_check::nll::region_infer::RegionInferenceContext::solve
  20: rustc_mir::borrow_check::nll::compute_regions
  21: rustc_mir::borrow_check::do_mir_borrowck
  22: rustc::ty::context::tls::with_related_context
  23: rustc::infer::InferCtxtBuilder::enter
  24: rustc_mir::borrow_check::mir_borrowck
  25: rustc::ty::query::__query_compute::mir_borrowck
  26: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors<'tcx> for rustc::ty::query::queries::mir_borrowck<'tcx>>::compute
  27: rustc::dep_graph::graph::DepGraph::with_task_impl
  28: <rustc::ty::query::plumbing::JobOwner<'a, 'tcx, Q>>::start
  29: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::force_query_with_job
  30: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::get_query
  31: rustc::ty::query::<impl rustc::ty::context::TyCtxt<'a, 'tcx, 'lcx>>::mir_borrowck
  32: rustc::ty::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::par_body_owners
  33: rustc::util::common::time
  34: rustc::ty::context::tls::enter_context
  35: <std::thread::local::LocalKey<T>>::with
  36: rustc::ty::context::TyCtxt::create_and_enter
  37: rustc_driver::driver::compile_input
  38: rustc_driver::run_compiler_with_pool
  39: rustc_driver::driver::spawn_thread_pool
  40: rustc_driver::run_compiler
  41: <scoped_tls::ScopedKey<T>>::set
  42: <std::panic::AssertUnwindSafe<F> as core::ops::function::FnOnce<()>>::call_once
  43: __rust_maybe_catch_panic
             at libpanic_unwind/lib.rs:102
  44: rustc_driver::run
  45: rustc_driver::main
  46: std::rt::lang_start::{{closure}}
  47: std::panicking::try::do_call
             at libstd/rt.rs:59
             at libstd/panicking.rs:310
  48: __rust_maybe_catch_panic
             at libpanic_unwind/lib.rs:102
  49: std::rt::lang_start_internal
             at libstd/panicking.rs:289
             at libstd/panic.rs:392
             at libstd/rt.rs:58
  50: main
  51: __libc_start_main
  52: <unknown>
query stack during panic:
#0 [mir_borrowck] processing `Foo::new`
end of query stack

[pnkfelix]

Okay, the good news here is that I only see this ICE with #![feature(nll)]; it does not arise with the NLL migration mode being deployed as part of the 2018 edition. See e.g. this play.

[pnkfelix]

The code in question is rejected by AST-borrowck. It looks like it is being rejected by NLL as well, and the problem is that we hit an error when trying to generated our diagnostic.

Therefore, I'm going to tag this as NLL-sound (since this seems like a case where we want to reject the code in question) and NLL-diagnostics (since the ICE itself is stemming from diagnostics code).

And I think I'm going to make it P-high, because 1. ICE's are super annoying and 2. I bet its not hard to fix.

But I'm not going to tag it with the Release milestone, because its not something we need to worry about backporting to the beta channel (because it does not arise with NLL migration mode).

[pnkfelix]

actually I'll remove NLL-sound; this isn't a case where NLL is incorrectly accepting the code. Its just incorrectly behaving while generating the diagnostics.