Internal compiler error: pattern.rs:572: impossible case reached

[nortiero]

Hi! I was mixing function pointers with match patterns (they don't mix at all, as far as I understand -- please correct me), obtained the following error:

error: internal compiler error: /Users/rustbuild/src/rust-buildbot/slave/stable-dist-rustc-mac/build/src/librustc_const_eval/pattern.rs:572: impossible case reached

Happens in playground too.

Code:

use std::fmt;

fn du() {    println!("booo");  }
fn du2() {    println!("booo2");  }


struct Baonzo(fn());
const PUM: Baonzo = Baonzo(du);

fn main() {
    let fpx = Baonzo(du);
   
    match fpx  {
        PUM => { println!("Here we are"); },
        _ => {},
    } 
}

##Meta
rustc 1.16.0 (30cf806 2017-03-10)
binary: rustc
commit-hash: 30cf806
commit-date: 2017-03-10
host: x86_64-apple-darwin
release: 1.16.0
LLVM version: 3.9

Backtrace:

thread 'rustc' panicked at 'Box<Any>', /Users/rustbuild/src/rust-buildbot/slave/stable-dist-rustc-mac/build/src/librustc_errors/lib.rs:416
stack backtrace:
   1:        0x107788a1c - std::sys::imp::backtrace::tracing::imp::write::h21ca2762819c7ae8
   2:        0x10779537e - std::panicking::default_hook::{{closure}}::h38f99a37d00bb19b
   3:        0x107794f23 - std::panicking::default_hook::ha2186ee24b50729c
   4:        0x107795837 - std::panicking::rust_panic_with_hook::h979db19ee91d2a53
   5:        0x1052b667a - std::panicking::begin_panic::h8030d48880744908
   6:        0x1052cb43c - rustc_errors::Handler::bug::h760c488cd55d4fdd
   7:        0x10470c82c - rustc::session::opt_span_bug_fmt::{{closure}}::hcb2fbcb88915d137
   8:        0x10470c2e9 - rustc::session::opt_span_bug_fmt::ha610cb4388904d76
   9:        0x10470be6a - rustc::session::bug_fmt::ha5b9adbd8728566e
  10:        0x1043c189d - rustc_const_eval::pattern::PatternContext::lower_variant_or_leaf::h2213017936435fa1
  11:        0x1043c1aa1 - rustc_const_eval::pattern::PatternContext::lower_path::h6738fa3412313318
  12:        0x1043c24b3 - rustc_const_eval::pattern::PatternContext::lower_const_expr::h6183b4b4e74d1517
  13:        0x1043a0a10 - <collections::vec::Vec<T> as collections::vec::SpecExtend<T, I>>::from_iter::h4aa641585b72a90f
  14:        0x1043c271d - rustc_const_eval::pattern::PatternContext::lower_const_expr::h6183b4b4e74d1517
  15:        0x1043c1b4b - rustc_const_eval::pattern::PatternContext::lower_path::h6738fa3412313318
  16:        0x1043bfab8 - rustc_const_eval::pattern::PatternContext::lower_pattern::h3af23c0975bb15dd
  17:        0x1043b929f - rustc_const_eval::check_match::MatchVisitor::check_match::{{closure}}::hacf69a1f1c3994c1
  18:        0x1043b8382 - <rustc_const_eval::check_match::MatchVisitor<'a, 'tcx> as rustc::hir::intravisit::Visitor<'tcx>>::visit_expr::h5cc0afdd47df9e0c
  19:        0x1043b6fd0 - <rustc_const_eval::check_match::MatchVisitor<'a, 'tcx> as rustc::hir::intravisit::Visitor<'tcx>>::visit_expr::h5cc0afdd47df9e0c
  20:        0x1043b8f90 - <rustc_const_eval::check_match::MatchVisitor<'a, 'tcx> as rustc::hir::intravisit::Visitor<'tcx>>::visit_body::hacb2f5fa0cd27a97
  21:        0x1043b5a37 - <rustc_const_eval::check_match::OuterVisitor<'a, 'tcx> as rustc::hir::intravisit::Visitor<'tcx>>::visit_fn::h0e4df7e312384e9b
  22:        0x1043987dc - rustc::hir::intravisit::walk_item::h62266a77147d9707
  23:        0x1043b5f73 - rustc_const_eval::check_match::check_crate::hbb131b3de2ff0e4c
  24:        0x10366fe87 - rustc_driver::driver::phase_3_run_analysis_passes::{{closure}}::h9e118a56f5957bd6
  25:        0x103651367 - rustc_driver::driver::phase_3_run_analysis_passes::h0569f99df4eb53d4
  26:        0x103640a90 - rustc_driver::driver::compile_input::hb50de2aa3bf90e59
  27:        0x10368848e - rustc_driver::run_compiler::h4b71cb0b28ba1b4a
  28:        0x10359fff8 - std::panicking::try::do_call::h7cd9045c0206869e
  29:        0x10779844a - __rust_maybe_catch_panic
  30:        0x1035c8913 - <F as alloc::boxed::FnBox<A>>::call_box::hca1d0347a9530a7f
  31:        0x1077944a4 - std::sys::imp::thread::Thread::new::thread_start::hca57ad22492f366f
  32:     0x7fff854ef898 - _pthread_body
  33:     0x7fff854ef729 - _pthread_start

Edit to avoid emojis in text.. silly as it is to enable emojis in technical text

[estebank]

Triggered at src/librustc_const_eval/pattern.rs:572.

@nikomatsakis am I correct to understand that the accepted way to do this would be to raise and error and recommend using pattern guard instead?

fn main() {
    let fpx = Baonzo(du);
    match &fpx {
        x if *x == PUM => {
            println!("Here we are");
        }
        _ => {}
    }
}

[nikomatsakis]

This does seem like it should be an error, since we only intend to support constants that derive Eq at the moment.

[oli-obk]

The error is now

error: to use a constant of type `Baonzo` in a pattern, `Baonzo` must be annotated with `#[derive(PartialEq, Eq)]`
  --> src/main.rs:14:9
   |
14 |         PUM => { println!("Here we are"); },
   |         ^^^

warning: unreachable pattern
  --> src/main.rs:15:9
   |
15 |         _ => {},
   |         ^
   |
   = note: #[warn(unreachable_patterns)] on by default

error: aborting due to previous error

and no more ICE