Crash with `or_patterns` in irrefutable match

[Patryk27]

Hi,

#![feature(or_patterns)]

fn foo((Some(_) | None): Option<u32>) {
    //
}

(playground)

... crashes on nightly with:

error: internal compiler error: match pairs [MatchPair { place: _1, pattern: Pat { ty: std::option::Option<u32>, span: src/lib.rs:3:9: 3:23, kind: Or { pats: [Pat { ty: std::option::Option<u32>, span: src/lib.rs:3:9: 3:16, kind: Variant { adt_def: std::option::Option, substs: [u32], variant_index: 1, subpatterns: [FieldPat { field: field[0], pattern: Pat { ty: u32, span: src/lib.rs:3:14: 3:15, kind: Wild } }] } }, Pat { ty: std::option::Option<u32>, span: src/lib.rs:3:19: 3:23, kind: Variant { adt_def: std::option::Option, substs: [u32], variant_index: 0, subpatterns: [] } }] } } }] remaining after simplifying irrefutable pattern

Backtrace

warning: the feature `or_patterns` is incomplete and may cause the compiler to crash
 --> src/lib.rs:1:12
  |
1 | #![feature(or_patterns)]
  |            ^^^^^^^^^^^
  |
  = note: `#[warn(incomplete_features)]` on by default

warning: function is never used: `foo`
 --> src/lib.rs:3:4
  |
3 | fn foo((Some(_) | None): Option<u32>) {
  |    ^^^
  |
  = note: `#[warn(dead_code)]` on by default

error: internal compiler error: match pairs [MatchPair { place: _1, pattern: Pat { ty: std::option::Option<u32>, span: src/lib.rs:3:9: 3:23, kind: Or { pats: [Pat { ty: std::option::Option<u32>, span: src/lib.rs:3:9: 3:16, kind: Variant { adt_def: std::option::Option, substs: [u32], variant_index: 1, subpatterns: [FieldPat { field: field[0], pattern: Pat { ty: u32, span: src/lib.rs:3:14: 3:15, kind: Wild } }] } }, Pat { ty: std::option::Option<u32>, span: src/lib.rs:3:19: 3:23, kind: Variant { adt_def: std::option::Option, substs: [u32], variant_index: 0, subpatterns: [] } }] } } }] remaining after simplifying irrefutable pattern
 --> src/lib.rs:3:9
  |
3 | fn foo((Some(_) | None): Option<u32>) {
  |         ^^^^^^^^^^^^^^

thread 'rustc' panicked at 'no errors encountered even though `delay_span_bug` issued', src/librustc_errors/lib.rs:347:17
stack backtrace:
   0: backtrace::backtrace::libunwind::trace
             at /cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.40/src/backtrace/libunwind.rs:88
   1: backtrace::backtrace::trace_unsynchronized
             at /cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.40/src/backtrace/mod.rs:66
   2: std::sys_common::backtrace::_print_fmt
             at src/libstd/sys_common/backtrace.rs:84
   3: <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt
             at src/libstd/sys_common/backtrace.rs:61
   4: core::fmt::write
             at src/libcore/fmt/mod.rs:1057
   5: std::io::Write::write_fmt
             at src/libstd/io/mod.rs:1426
   6: std::sys_common::backtrace::_print
             at src/libstd/sys_common/backtrace.rs:65
   7: std::sys_common::backtrace::print
             at src/libstd/sys_common/backtrace.rs:50
   8: std::panicking::default_hook::{{closure}}
             at src/libstd/panicking.rs:193
   9: std::panicking::default_hook
             at src/libstd/panicking.rs:210
  10: rustc_driver::report_ice
  11: std::panicking::rust_panic_with_hook
             at src/libstd/panicking.rs:475
  12: std::panicking::begin_panic
  13: <rustc_errors::HandlerInner as core::ops::drop::Drop>::drop
  14: core::ptr::real_drop_in_place
  15: core::ptr::real_drop_in_place
  16: core::ptr::real_drop_in_place
  17: rustc_interface::interface::run_compiler_in_existing_thread_pool
  18: std::thread::local::LocalKey<T>::with
  19: scoped_tls::ScopedKey<T>::set
  20: syntax::with_globals
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

error: internal compiler error: unexpected panic

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 1.42.0-nightly (fc5deca21 2019-12-21) running on x86_64-unknown-linux-gnu

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

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

query stack during panic:
end of query stack
error: could not compile `playground`.

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

It doesn't crash on current stable and beta, but it may be because they ignore the feature flag.

[varkor]

This affects any irrefutable match, e.g.

#![feature(or_patterns)]

fn main() {
    let x: Option<u32> = Some(0);
    let Some(_) | None = x;
}

[varkor]

The problem is:

rust/src/librustc_mir/build/matches/simplify.rs

Lines 162 to 163 in 9b98af8

	let irrefutable = adt_def.variants.iter_enumerated().all(|(i, v)| {
	i == variant_index || {

Specifically, whenever an enum variant is encountered that is not visibly a singleton enum, it counts as refutable in the MIR simplifier. Without or-patterns, this is always true. However, or-patterns allow us to specify multiple variants. It'll require a little refactoring to make it take this into account.