internal compiler error: src/librustc_mir_build/hair/constant.rs:60: impossible case reached

[dwrensha]

I'm seeing an internal compiler error on the following input (found by fuzz-rustc):

const X: [(); 0.1] = [()];
fn main() {}

$ rustc main.rs
error: internal compiler error: src/librustc_mir_build/hair/constant.rs:60: impossible case reached

thread 'rustc' panicked at 'Box<Any>', src/librustc_errors/lib.rs:881:9
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

note: the compiler unexpectedly panicked. this is a bug.

note: we would appreciate a bug report: https://github.com/rust-lang/rust/blob/master/CONTRIBUTING.md#bug-reports

note: rustc 1.43.0-nightly (7760cd0fb 2020-02-19) running on x86_64-unknown-linux-gnu

error: aborting due to previous error

The error happens on nightly and beta, but not on stable.

Nightly backtrace

error: internal compiler error: src/librustc_mir_build/hair/constant.rs:60: impossible case reached

thread 'rustc' panicked at 'Box<Any>', src/librustc_errors/lib.rs:881:9
stack backtrace:
   0: backtrace::backtrace::libunwind::trace
             at /cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.44/src/backtrace/libunwind.rs:86
   1: backtrace::backtrace::trace_unsynchronized
             at /cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.44/src/backtrace/mod.rs:66
   2: std::sys_common::backtrace::_print_fmt
             at src/libstd/sys_common/backtrace.rs:78
   3: <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt
             at src/libstd/sys_common/backtrace.rs:59
   4: core::fmt::write
             at src/libcore/fmt/mod.rs:1052
   5: std::io::Write::write_fmt
             at src/libstd/io/mod.rs:1428
   6: std::sys_common::backtrace::_print
             at src/libstd/sys_common/backtrace.rs:62
   7: std::sys_common::backtrace::print
             at src/libstd/sys_common/backtrace.rs:49
   8: std::panicking::default_hook::{{closure}}
             at src/libstd/panicking.rs:204
   9: std::panicking::default_hook
             at src/libstd/panicking.rs:224
  10: rustc_driver::report_ice
  11: std::panicking::rust_panic_with_hook
             at src/libstd/panicking.rs:474
  12: std::panicking::begin_panic
  13: rustc_errors::HandlerInner::bug
  14: rustc_errors::Handler::bug
  15: rustc::util::bug::opt_span_bug_fmt::{{closure}}
  16: rustc::ty::context::tls::with_opt::{{closure}}
  17: rustc::ty::context::tls::with_opt
  18: rustc::util::bug::opt_span_bug_fmt
  19: rustc::util::bug::bug_fmt
  20: rustc_mir_build::hair::constant::lit_to_const
  21: rustc::ty::query::__query_compute::lit_to_const
  22: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::lit_to_const>::compute
  23: rustc::dep_graph::graph::DepGraph::with_task_impl
  24: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  25: <dyn rustc_typeck::astconv::AstConv>::ast_const_to_const
  26: <dyn rustc_typeck::astconv::AstConv>::ast_ty_to_ty
  27: rustc_typeck::collect::type_of::type_of
  28: rustc::ty::query::__query_compute::type_of
  29: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::type_of>::compute
  30: rustc::dep_graph::graph::DepGraph::with_task_impl
  31: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  32: <rustc_typeck::collect::CollectItemTypesVisitor as rustc_hir::intravisit::Visitor>::visit_item
  33: rustc::hir::map::Map::visit_item_likes_in_module
  34: rustc_typeck::collect::collect_mod_item_types
  35: rustc::ty::query::__query_compute::collect_mod_item_types
  36: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::collect_mod_item_types>::compute
  37: rustc::dep_graph::graph::DepGraph::with_task_impl
  38: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  39: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::ensure_query
  40: rustc_session::session::Session::track_errors
  41: rustc_typeck::check_crate
  42: rustc_interface::passes::analysis
  43: rustc::ty::query::__query_compute::analysis
  44: rustc::dep_graph::graph::DepGraph::with_task_impl
  45: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  46: rustc::ty::context::tls::enter_global
  47: rustc_interface::interface::run_compiler_in_existing_thread_pool
  48: scoped_tls::ScopedKey<T>::set
  49: syntax::attr::with_globals
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

note: the compiler unexpectedly panicked. this is a bug.

note: we would appreciate a bug report: https://github.com/rust-lang/rust/blob/master/CONTRIBUTING.md#bug-reports

note: rustc 1.43.0-nightly (7760cd0fb 2020-02-19) running on x86_64-unknown-linux-gnu

note: compiler flags: -C codegen-units=1 -C debuginfo=2 --crate-type bin

note: some of the compiler flags provided by cargo are hidden

query stack during panic:
#0 [lit_to_const] converting literal to const
#1 [type_of] processing `X`
#2 [collect_mod_item_types] collecting item types in top-level module
#3 [analysis] running analysis passes on this crate
end of query stack

[pnkfelix]

triage: P-high, assigning to self.

[pnkfelix]

(This ended up being a great motivation for me to tie off some cargo-bisect-rustc work I've been doing...)

[pnkfelix]

ICE was introduced between nightly 2020-01-15 and 2010-01-16.

Here are the merges from that time period:

3291ae33907f2a866ea6cea89113200555038d06 Auto merge of #68254 - Dylan-DPC:rollup-9vhc59u, r=Dylan-DPC
faf45c5dadc04e2ee2f2b772402321e621899641 Auto merge of #67603 - oli-obk:no_mut_static_ref_from_const, r=RalfJung
6d0bb91bcba33a70fae4b0c663fb4403ed78f071 Auto merge of #68248 - JohnTitor:rollup-x0kml5f, r=JohnTitor
c74353c7d2e61b111a9241490b9fbbd1ebe491fe Auto merge of #68221 - ehuss:update-cargo-rls, r=alexcrichton
632387f38dfbac0f2b8b8900c840fff7f1fb888e Auto merge of #66329 - ktrianta:mir-opt-unreachable-propagation, r=oli-obk
4b172cc73f5f1fb1e3d60527fb29605fa267985c Auto merge of #68118 - skinny121:eager_lit_eval, r=varkor

(cargo-bisect-rustc is independently working to identify which one is the culprit.)

[pnkfelix]

Okay cargo-bisect-rustc finished:

Regression found in the compiler

searched nightlies: from nightly-2020-01-15 to nightly-2020-01-22
regressed nightly: nightly-2020-01-16
searched commits: from 8a87b94 to 3291ae3
regressed commit: 4b172cc

[pnkfelix]

Looks like the problem was injected by PR #68118

[pnkfelix]

reassigning to @Centril

[Centril]

The following also causes an ICE:

const X: [(); b"a"] = [()];
fn main() {}

(Same reason, but different message.)

[Centril]

Fixed by #69330.