ICE with opaquely typed constant used in pattern

[Centril]

#![feature(impl_trait_in_bindings)]

fn main() {
    const C: impl Copy = 0;
    match C {
        C | _ => {}
    }
}

ICE message:

error: internal compiler error: src/librustc_mir_build/build/matches/test.rs:417:
    non_scalar_compare called on non-reference type: impl std::marker::Copy`

Backtrace

error: internal compiler error: src/librustc_mir_build/build/matches/test.rs:417: non_scalar_compare called on non-reference type: impl std::marker::Copy

thread 'rustc' panicked at 'Box<Any>', src/librustc_errors/lib.rs:880:9
stack backtrace:
   0: backtrace::backtrace::libunwind::trace
             at /cargo/registry/src/git.luolix.top-1ecc6299db9ec823/backtrace-0.3.46/src/backtrace/libunwind.rs:86
   1: backtrace::backtrace::trace_unsynchronized
             at /cargo/registry/src/git.luolix.top-1ecc6299db9ec823/backtrace-0.3.46/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:1069
   5: std::io::Write::write_fmt
             at src/libstd/io/mod.rs:1504
   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:198
   9: std::panicking::default_hook
             at src/libstd/panicking.rs:218
  10: rustc_driver::report_ice
  11: std::panicking::rust_panic_with_hook
             at src/libstd/panicking.rs:515
  12: std::panicking::begin_panic
  13: rustc_errors::HandlerInner::bug
  14: rustc_errors::Handler::bug
  15: rustc_middle::util::bug::opt_span_bug_fmt::{{closure}}
  16: rustc_middle::ty::context::tls::with_opt::{{closure}}
  17: rustc_middle::ty::context::tls::with_opt
  18: rustc_middle::util::bug::opt_span_bug_fmt
  19: rustc_middle::util::bug::bug_fmt
  20: rustc_mir_build::build::matches::test::<impl rustc_mir_build::build::Builder>::non_scalar_compare
  21: rustc_mir_build::build::matches::<impl rustc_mir_build::build::Builder>::match_simplified_candidates
  22: rustc_mir_build::build::matches::<impl rustc_mir_build::build::Builder>::match_candidates
  23: rustc_mir_build::build::matches::<impl rustc_mir_build::build::Builder>::lower_match_tree
  24: rustc_mir_build::build::expr::into::<impl rustc_mir_build::build::Builder>::into_expr
  25: rustc_mir_build::build::expr::into::<impl rustc_mir_build::build::Builder>::into_expr
  26: rustc_mir_build::build::block::<impl rustc_mir_build::build::Builder>::ast_block_stmts
  27: rustc_mir_build::build::expr::into::<impl rustc_mir_build::build::Builder>::into_expr
  28: rustc_mir_build::build::expr::into::<impl rustc_mir_build::build::Builder>::into_expr
  29: rustc_mir_build::build::expr::into::<impl rustc_mir_build::build::Builder>::into_expr
  30: rustc_mir_build::build::construct_fn
  31: rustc_middle::ty::context::GlobalCtxt::enter_local
  32: rustc_mir_build::build::mir_built
  33: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::mir_built>::compute
  34: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  35: rustc_query_system::query::plumbing::get_query
  36: rustc_mir::transform::check_unsafety::unsafety_check_result
  37: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::unsafety_check_result>::compute
  38: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  39: rustc_query_system::query::plumbing::get_query
  40: rustc_mir::transform::mir_const
  41: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::mir_const>::compute
  42: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  43: rustc_query_system::query::plumbing::get_query
  44: rustc_mir::transform::mir_validated
  45: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::mir_validated>::compute
  46: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  47: rustc_query_system::query::plumbing::get_query
  48: rustc_mir::borrow_check::mir_borrowck
  49: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::mir_borrowck>::compute
  50: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  51: rustc_query_system::query::plumbing::get_query
  52: rustc_query_system::query::plumbing::ensure_query
  53: rustc_session::utils::<impl rustc_session::session::Session>::time
  54: rustc_interface::passes::analysis
  55: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::analysis>::compute
  56: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  57: rustc_query_system::query::plumbing::get_query
  58: rustc_middle::ty::context::tls::enter_global
  59: rustc_interface::interface::run_compiler_in_existing_thread_pool
  60: scoped_tls::ScopedKey<T>::set
  61: rustc_ast::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 1.44.0-nightly (94d346360 2020-04-09) 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 [mir_built] building MIR for
#1 [unsafety_check_result] unsafety-checking `main`
#2 [mir_const] processing `main`
#3 [mir_validated] processing `main`
#4 [mir_borrowck] borrow-checking `main`
#5 [analysis] running analysis passes on this crate
end of query stack
error: aborting due to previous error

error: could not compile `playground`.

To learn more, run the command again with --verbose.

[Centril]

This should be erroring in exhaustiveness checking; ty::Opaque shouldn't be allowed there.

cc @oli-obk @eddyb @pnkfelix @ecstatic-morse

[eddyb]

This should be erroring in exhaustiveness checking

It should be erroring earlier, when trying to create a pattern out of the constant.
And it should also not be considered structural-match type-wise.

[JohnTitor]

Triage: This is no longer ICE, seems that it's fixed by #72153, marking as E-needs-test.