ICE: The type checker should prevent reading from a never-written local

[DutchGhost]

I tried this code:

#![feature(const_trait_impl)]
#![feature(fn_traits)]
#![feature(unboxed_closures)]

struct Closure;

impl const FnOnce<&usize> for Closure {
    type Output = usize;
    
    extern "rust-call" fn call_once(self, arg: &usize) -> Self::Output {
        *arg
    }
}

enum Bug<T = [(); Closure.call_once(&0) ]> {
    V(T),
}

I expected to see this happen: Maybe it should've compiled? Maybe it should've just emitted an error explaining I did something wrong?

Instead, this happened: It crashed!

Meta

rustc --version --verbose:

1.43.0-nightly (6fd8798f4 2020-02-25)

Backtrace

error: internal compiler error: src/librustc_mir/interpret/eval_context.rs:143: The type checker should prevent reading from a never-written local

thread 'rustc' panicked at 'Box<Any>', src/librustc_errors/lib.rs:881:9
stack backtrace:
   0: backtrace::backtrace::libunwind::trace
             at /cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.44/src/backtrace/libunwind.rs:86
   1: backtrace::backtrace::trace_unsynchronized
             at /cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.44/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:1052
   5: std::io::Write::write_fmt
             at src/libstd/io/mod.rs:1428
   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:204
   9: std::panicking::default_hook
             at src/libstd/panicking.rs:224
  10: rustc_driver::report_ice
  11: std::panicking::rust_panic_with_hook
             at src/libstd/panicking.rs:474
  12: std::panicking::begin_panic
  13: rustc_errors::HandlerInner::bug
  14: rustc_errors::Handler::bug
  15: rustc::util::bug::opt_span_bug_fmt::{{closure}}
  16: rustc::ty::context::tls::with_opt::{{closure}}
  17: rustc::ty::context::tls::with_opt
  18: rustc::util::bug::opt_span_bug_fmt
  19: rustc::util::bug::bug_fmt
  20: rustc_mir::interpret::operand::<impl rustc_mir::interpret::eval_context::InterpCx<M>>::access_local
  21: rustc_mir::interpret::operand::<impl rustc_mir::interpret::eval_context::InterpCx<M>>::eval_place_to_op
  22: rustc_mir::interpret::operand::<impl rustc_mir::interpret::eval_context::InterpCx<M>>::eval_operand
  23: rustc_mir::interpret::step::<impl rustc_mir::interpret::eval_context::InterpCx<M>>::eval_rvalue_into_place
  24: rustc_mir::interpret::step::<impl rustc_mir::interpret::eval_context::InterpCx<M>>::run
  25: rustc_mir::const_eval::eval_queries::const_eval_raw_provider
  26: rustc::ty::query::__query_compute::const_eval_raw
  27: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::const_eval_raw>::compute
  28: rustc::dep_graph::graph::DepGraph::with_task_impl
  29: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  30: rustc_mir::const_eval::eval_queries::const_eval_validated_provider
  31: rustc::ty::query::__query_compute::const_eval_validated
  32: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::const_eval_validated>::compute
  33: rustc::dep_graph::graph::DepGraph::with_task_impl
  34: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  35: rustc_mir::const_eval::eval_queries::const_eval_validated_provider
  36: rustc::ty::query::__query_compute::const_eval_validated
  37: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::const_eval_validated>::compute
  38: rustc::dep_graph::graph::DepGraph::with_task_impl
  39: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  40: rustc::mir::interpret::queries::<impl rustc::ty::context::TyCtxt>::const_eval_resolve
  41: rustc::ty::sty::Const::eval::{{closure}}
  42: <rustc_infer::traits::project::AssocTypeNormalizer as rustc::ty::fold::TypeFolder>::fold_const
  43: <rustc_infer::traits::project::AssocTypeNormalizer as rustc::ty::fold::TypeFolder>::fold_ty
  44: rustc::ty::fold::TypeFoldable::fold_with
  45: rustc::ty::fold::TypeFoldable::fold_with
  46: rustc_infer::traits::project::normalize
  47: rustc_infer::infer::InferCtxt::partially_normalize_associated_types_in
  48: <core::iter::adapters::Map<I,F> as core::iter::traits::iterator::Iterator>::next
  49: rustc_typeck::check::wfcheck::check_where_clauses
  50: rustc::ty::context::GlobalCtxt::enter_local
  51: rustc_typeck::check::wfcheck::check_item_well_formed
  52: rustc::ty::query::__query_compute::check_item_well_formed
  53: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::check_item_well_formed>::compute
  54: rustc::dep_graph::graph::DepGraph::with_task_impl
  55: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  56: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::ensure_query
  57: __rust_maybe_catch_panic
             at src/libpanic_unwind/lib.rs:86
  58: rustc_data_structures::sync::par_for_each_in
  59: __rust_maybe_catch_panic
             at src/libpanic_unwind/lib.rs:86
  60: rustc_hir::hir::Crate::par_visit_all_item_likes
  61: rustc_typeck::check_crate
  62: rustc_interface::passes::analysis
  63: rustc::ty::query::__query_compute::analysis
  64: rustc::dep_graph::graph::DepGraph::with_task_impl
  65: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  66: rustc::ty::context::tls::enter_global
  67: rustc_interface::interface::run_compiler_in_existing_thread_pool
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 1.43.0-nightly (6fd8798f4 2020-02-25) running on x86_64-unknown-linux-gnu

note: compiler flags: -C opt-level=3 -C codegen-units=1 --crate-type lib

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

query stack during panic:
#0 [const_eval_raw] const-evaluating `Bug::{{constant}}#0`
#1 [const_eval_validated] const-evaluating + checking `Bug::{{constant}}#0`
#2 [const_eval_validated] const-evaluating + checking `Bug::{{constant}}#0`
#3 [check_item_well_formed] processing `Bug`
#4 [analysis] running analysis passes on this crate
end of query stack
error: aborting due to previous error

[oli-obk]

this is a general issue with const eval. Typeck needs to const eval certain constants, for which we may not have invoked typeck yet. This ordering issue has existed since 1.0 afaik.

[oli-obk]

A simpler repro and one that works on stable is

struct Bug {
    A: [(); {
        let x: usize;
        x
    }],
}

taken from #72115

[Alexendoo]

Original no longer ICEs since #78961

#69487 (comment) still ICEs