ICE when structurally matching a struct containing an associated type on latest Nightly.

[Lymia]

One of the tests in my enumset crate has started throwing an ICE on the latest nightly.

The problem seems to be that I use an associated type as a field in a struct I'm structurally matching, and this has been broken on the latest nightly.

Code

Playground Link

trait EnumSetType: Copy + Eq {
    type Repr;
}

#[derive(Copy, Clone, PartialEq, Eq)]
enum Enum8 { }
impl EnumSetType for Enum8 {
    type Repr = u8;
}

#[derive(Copy, Clone, PartialEq, Eq)]
struct EnumSet<T: EnumSetType> {
    __enumset_underlying: T::Repr,
}

const CONST_SET: EnumSet<Enum8> = EnumSet { __enumset_underlying: 3 };

#[test]
fn match_const_test() {
    match CONST_SET {
        CONST_SET => { /* ok */ }
        _ => panic!("match fell through?"),
    }
}

Error output

   Compiling test_crate v0.1.0 (/home/[...]/enumset_test)
error: internal compiler error: src/librustc_mir_build/hair/pattern/const_to_pat.rs:137: use of a constant whose type is a projection inside a pattern

thread 'rustc' panicked at 'Box<Any>', src/librustc_errors/lib.rs:907:9
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 1.45.0-nightly (5fd2f06e9 2020-05-31) running on x86_64-unknown-linux-gnu

note: compiler flags: -C embed-bitcode=no -C debuginfo=2 -C incremental

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

error: aborting due to previous error

error: could not compile `test_crate`.

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

Backtrace

   Compiling test_crate v0.1.0 (/home/[...]/enumset_test)
error: internal compiler error: src/librustc_mir_build/hair/pattern/const_to_pat.rs:137: use of a constant whose type is a projection inside a pattern

thread 'rustc' panicked at 'Box<Any>', src/librustc_errors/lib.rs:907:9
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:1076
   5: std::io::Write::write_fmt
             at src/libstd/io/mod.rs:1537
   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:490
  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::hair::pattern::const_to_pat::ConstToPat::to_pat
  21: rustc_middle::ty::context::GlobalCtxt::enter_local
  22: rustc_mir_build::hair::pattern::PatCtxt::lower_path
  23: rustc_mir_build::hair::pattern::PatCtxt::lower_pattern
  24: rustc_mir_build::hair::pattern::check_match::MatchVisitor::lower_pattern
  25: <core::iter::adapters::Map<I,F> as core::iter::traits::iterator::Iterator>::fold
  26: <rustc_mir_build::hair::pattern::check_match::MatchVisitor as rustc_hir::intravisit::Visitor>::visit_expr
  27: <rustc_mir_build::hair::pattern::check_match::MatchVisitor as rustc_hir::intravisit::Visitor>::visit_expr
  28: rustc_mir_build::hair::pattern::check_match::check_match
  29: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::check_match>::compute
  30: rustc_middle::dep_graph::<impl rustc_query_system::dep_graph::DepKind for rustc_middle::dep_graph::dep_node::DepKind>::with_deps
  31: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  32: rustc_data_structures::stack::ensure_sufficient_stack
  33: rustc_query_system::query::plumbing::get_query_impl
  34: rustc_query_system::query::plumbing::ensure_query_impl
  35: rustc_session::utils::<impl rustc_session::session::Session>::time
  36: rustc_session::utils::<impl rustc_session::session::Session>::time
  37: rustc_interface::passes::analysis
  38: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::analysis>::compute
  39: rustc_middle::dep_graph::<impl rustc_query_system::dep_graph::DepKind for rustc_middle::dep_graph::dep_node::DepKind>::with_deps
  40: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  41: rustc_query_system::query::plumbing::get_query_impl
  42: rustc_middle::ty::context::tls::enter_global
  43: rustc_interface::interface::run_compiler_in_existing_thread_pool
  44: 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 1.45.0-nightly (5fd2f06e9 2020-05-31) running on x86_64-unknown-linux-gnu

note: compiler flags: -C embed-bitcode=no -C debuginfo=2 -C incremental

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

query stack during panic:
#0 [check_match] processing `match_const_test`
#1 [analysis] running analysis passes on this crate
end of query stack
error: aborting due to previous error

error: could not compile `test_crate`.

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

[eddyb]

cc @pnkfelix @oli-obk

[eddyb]

cc @lcnr looks like it's likely #72153

EDIT: oh CONST_SET's type is fully concrete, so the field type (<Enum8 as EnumSetType>::Repr) could be normalized and then it would be structurally matcheable. This is a reason why using a trait might be better, it wouldn't have edge cases like this (although it has other complications).
At the very least we should normalize.

[lcnr]

I also believe we are forgetting to normalize here, will look into it.

@rustbot claim

[LeSeulArtichaut]

cargo-bisect-rustc points a regression in f93bb2a which may confirm that #72153 is the culprit.

[LeSeulArtichaut]

Assigning P-critical as discussed as part of the Prioritization WG procedure.