internal compiler error: encountered errors lowering pattern:

[madmalik]

I encountered an internal compiler error (on stable, beta and nightly) in my code and i could reduce it to this: playground link.

const NUM: u8 = std::mem::size_of::<u8>() as u8;

fn main() {
    match 1 {
        NUM => unimplemented!(),
        _ => unimplemented!(),
    }
}

I would expect that this program won't compile because of the non-const fn in a const declaration.

Nightly rightly gives an error message to that effect and panics, stable just panics.

Stacktrace:

rustc 1.20.0-nightly (3610a70ce 2017-07-05)
error[E0015]: calls in constants are limited to constant functions, struct and enum constructors
 --> <anon>:2:17
  |
2 | const NUM: u8 = std::mem::size_of::<u8>() as u8;
  |                 ^^^^^^^^^^^^^^^^^^^^^^^^^

error: internal compiler error: /checkout/src/librustc_const_eval/pattern.rs:282: encountered errors lowering pattern: [ConstEval(ConstEvalErr { span: <anon>:2:17: 2:42, kind: TypeckError })]
 --> <anon>:6:9
  |
6 |         NUM => unimplemented!(),
  |         ^^^

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 1.20.0-nightly (3610a70ce 2017-07-05) running on x86_64-unknown-linux-gnu

note: run with `RUST_BACKTRACE=1` for a backtrace

thread 'rustc' panicked at 'Box<Any>', /checkout/src/librustc_errors/lib.rs:426:8
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.
stack backtrace:
   0: std::sys::imp::backtrace::tracing::imp::unwind_backtrace
             at /checkout/src/libstd/sys/unix/backtrace/tracing/gcc_s.rs:49
   1: std::sys_common::backtrace::_print
             at /checkout/src/libstd/sys_common/backtrace.rs:71
   2: std::panicking::default_hook::{{closure}}
             at /checkout/src/libstd/sys_common/backtrace.rs:60
             at /checkout/src/libstd/panicking.rs:380
   3: std::panicking::default_hook
             at /checkout/src/libstd/panicking.rs:390
   4: std::panicking::rust_panic_with_hook
             at /checkout/src/libstd/panicking.rs:611
   5: std::panicking::begin_panic_new
   6: rustc::session::opt_span_bug_fmt::{{closure}}
   7: rustc::session::span_bug_fmt
   8: rustc_const_eval::pattern::Pattern::from_hir
   9: rustc_mir::hair::cx::expr::make_mirror_unadjusted
  10: rustc_mir::hair::cx::expr::<impl rustc_mir::hair::Mirror<'tcx> for &'tcx rustc::hir::Expr>::make_mirror
  11: <rustc_mir::hair::ExprRef<'tcx> as rustc_mir::build::into::EvalInto<'tcx>>::eval_into
  12: rustc_mir::build::block::<impl rustc_mir::build::Builder<'a, 'gcx, 'tcx>>::ast_block_stmts
  13: rustc_mir::build::scope::<impl rustc_mir::build::Builder<'a, 'gcx, 'tcx>>::in_scope
  14: rustc_mir::build::scope::<impl rustc_mir::build::Builder<'a, 'gcx, 'tcx>>::in_opt_scope
  15: rustc_mir::build::expr::into::<impl rustc_mir::build::Builder<'a, 'gcx, 'tcx>>::into_expr
  16: <rustc_mir::hair::ExprRef<'tcx> as rustc_mir::build::into::EvalInto<'tcx>>::eval_into
  17: rustc_mir::build::expr::into::<impl rustc_mir::build::Builder<'a, 'gcx, 'tcx>>::into_expr
  18: <rustc_mir::hair::ExprRef<'tcx> as rustc_mir::build::into::EvalInto<'tcx>>::eval_into
  19: rustc_mir::build::expr::into::<impl rustc_mir::build::Builder<'a, 'gcx, 'tcx>>::into_expr
  20: rustc_mir::transform::mir_const
  21: rustc::ty::maps::<impl rustc::ty::maps::queries::mir_const<'tcx>>::try_get
  22: rustc::ty::maps::TyCtxtAt::mir_const
  23: rustc::ty::maps::<impl rustc::ty::context::TyCtxt<'a, 'tcx, 'lcx>>::mir_const
  24: rustc_mir::transform::mir_validated
  25: rustc::ty::maps::<impl rustc::ty::maps::queries::mir_validated<'tcx>>::try_get
  26: rustc::ty::maps::TyCtxtAt::mir_validated
  27: rustc::ty::maps::<impl rustc::ty::context::TyCtxt<'a, 'tcx, 'lcx>>::mir_validated
  28: rustc_borrowck::borrowck::borrowck
  29: rustc::ty::maps::<impl rustc::ty::maps::queries::borrowck<'tcx>>::try_get
  30: rustc::ty::maps::<impl rustc::ty::context::TyCtxt<'a, 'tcx, 'lcx>>::borrowck
  31: rustc_borrowck::borrowck::check_crate
  32: rustc_driver::driver::phase_3_run_analysis_passes::{{closure}}
  33: rustc_driver::driver::phase_3_run_analysis_passes
  34: rustc_driver::driver::compile_input
  35: rustc_driver::run_compiler

[arielb1]

No ICE in current stable

[arielb1]

But using a random non-const-fn ICEs:

fn xyz() -> u8 { 42 }

const NUM: u8 = xyz();

fn main() {
    match 1 {
        NUM => unimplemented!(),
        _ => unimplemented!(),
    }
}

cc @eddyb

[eddyb]

Ah, TypeckError CTFE errors are supposed to be silently ignored.