ICE 'assertion failed: !value.needs_subst()', librustc/traits/query/normalize_erasing_regions.rs:69:9

[SimonSapin]

I don’t know how related this is to #55265, the assertion is different.

struct VTable<DST: ?Sized> {
    _to_dst_ptr: fn(*mut ()) -> *mut DST,
}

trait HasVTableFor<DST: ?Sized + 'static> {
    const VTABLE: &'static VTable<DST>;
}

impl<T, DST: ?Sized + 'static> HasVTableFor<DST> for T {
    const VTABLE: &'static VTable<DST> = &VTable {
        _to_dst_ptr: |_: *mut ()| unsafe { std::mem::zeroed() },
    };
}

pub fn push<DST: ?Sized + 'static, T>() {
    <T as HasVTableFor<DST>>::VTABLE;
}

(Playground)

Errors:

thread 'main' panicked at 'assertion failed: !value.needs_subst()', librustc/traits/query/normalize_erasing_regions.rs:69:9
stack backtrace:
   0: std::sys::unix::backtrace::tracing::imp::unwind_backtrace
             at libstd/sys/unix/backtrace/tracing/gcc_s.rs:49
   1: std::sys_common::backtrace::print
             at libstd/sys_common/backtrace.rs:71
             at libstd/sys_common/backtrace.rs:59
   2: std::panicking::default_hook::{{closure}}
             at libstd/panicking.rs:211
   3: std::panicking::default_hook
             at libstd/panicking.rs:227
   4: rustc::util::common::panic_hook
   5: std::panicking::rust_panic_with_hook
             at libstd/panicking.rs:480
   6: std::panicking::begin_panic
   7: rustc::traits::query::normalize_erasing_regions::<impl rustc::ty::context::TyCtxt<'cx, 'tcx, 'tcx>>::normalize_erasing_late_bound_regions
   8: rustc::ty::instance::Instance::resolve_closure
   9: rustc_mir::interpret::cast::<impl rustc_mir::interpret::eval_context::EvalContext<'a, 'mir, 'tcx, M>>::cast
  10: rustc_mir::interpret::step::<impl rustc_mir::interpret::eval_context::EvalContext<'a, 'mir, 'tcx, M>>::run
  11: rustc_mir::const_eval::eval_body_using_ecx
  12: rustc_mir::const_eval::const_eval_provider
  13: rustc::ty::query::__query_compute::const_eval
  14: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors<'tcx> for rustc::ty::query::queries::const_eval<'tcx>>::compute
  15: rustc::dep_graph::graph::DepGraph::with_task_impl
  16: rustc::ty::context::tls::with_related_context
  17: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::force_query_with_job
  18: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::get_query
  19: rustc::ty::query::TyCtxtAt::const_eval
  20: rustc_mir::interpret::operand::<impl rustc_mir::interpret::eval_context::EvalContext<'a, 'mir, 'tcx, M>>::global_to_op
  21: rustc_mir::interpret::operand::<impl rustc_mir::interpret::eval_context::EvalContext<'a, 'mir, 'tcx, M>>::const_value_to_op
  22: rustc_mir::interpret::operand::<impl rustc_mir::interpret::eval_context::EvalContext<'a, 'mir, 'tcx, M>>::const_to_op
  23: rustc_mir::transform::const_prop::ConstPropagator::eval_constant
  24: rustc_mir::transform::const_prop::ConstPropagator::const_prop
  25: <rustc_mir::transform::const_prop::ConstPropagator<'b, 'a, 'tcx> as rustc::mir::visit::Visitor<'tcx>>::visit_statement
  26: <rustc_mir::transform::const_prop::ConstProp as rustc_mir::transform::MirPass>::run_pass
  27: rustc_mir::transform::optimized_mir::{{closure}}
  28: rustc_mir::transform::optimized_mir
  29: rustc::ty::query::__query_compute::optimized_mir
  30: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors<'tcx> for rustc::ty::query::queries::optimized_mir<'tcx>>::compute
  31: rustc::dep_graph::graph::DepGraph::with_task_impl
  32: rustc::ty::context::tls::with_related_context
  33: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::force_query_with_job
  34: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::try_get_query
  35: rustc::ty::query::<impl rustc::ty::context::TyCtxt<'a, 'tcx, 'lcx>>::optimized_mir
  36: rustc_metadata::encoder::<impl rustc_metadata::isolated_encoder::IsolatedEncoder<'a, 'b, 'tcx>>::encode_optimized_mir
  37: rustc_metadata::encoder::<impl rustc_metadata::isolated_encoder::IsolatedEncoder<'a, 'b, 'tcx>>::encode_info_for_item
  38: rustc::ty::context::tls::with_context
  39: <rustc_metadata::encoder::EncodeVisitor<'a, 'b, 'tcx> as rustc::hir::intravisit::Visitor<'tcx>>::visit_item
  40: rustc::hir::Crate::visit_all_item_likes
  41: rustc_metadata::encoder::encode_metadata
  42: rustc_metadata::cstore_impl::<impl rustc::middle::cstore::CrateStore for rustc_metadata::cstore::CStore>::encode_metadata
  43: rustc::ty::context::TyCtxt::encode_metadata
  44: rustc_codegen_llvm::base::write_metadata
  45: rustc::util::common::time
  46: rustc_codegen_llvm::base::codegen_crate
  47: <rustc_codegen_llvm::LlvmCodegenBackend as rustc_codegen_utils::codegen_backend::CodegenBackend>::codegen_crate
  48: rustc::util::common::time
  49: rustc_driver::driver::phase_4_codegen
  50: rustc_driver::driver::compile_input::{{closure}}
  51: rustc::ty::context::tls::enter_context
  52: <std::thread::local::LocalKey<T>>::with
  53: rustc::ty::context::TyCtxt::create_and_enter
  54: rustc_driver::driver::compile_input
  55: rustc_driver::run_compiler_with_pool
  56: rustc_driver::driver::spawn_thread_pool
  57: rustc_driver::run_compiler
  58: <scoped_tls::ScopedKey<T>>::set
  59: <std::panic::AssertUnwindSafe<F> as core::ops::function::FnOnce<()>>::call_once
  60: __rust_maybe_catch_panic
             at libpanic_unwind/lib.rs:102
  61: rustc_driver::run
  62: rustc_driver::main
  63: std::rt::lang_start::{{closure}}
  64: std::panicking::try::do_call
             at libstd/rt.rs:59
             at libstd/panicking.rs:310
  65: __rust_maybe_catch_panic
             at libpanic_unwind/lib.rs:102
  66: std::rt::lang_start_internal
             at libstd/panicking.rs:289
             at libstd/panic.rs:392
             at libstd/rt.rs:58
  67: main
  68: __libc_start_main
  69: <unknown>
query stack during panic:
#0 [const_eval] const-evaluating `<T as HasVTableFor<DST>>::VTABLE`
  --> src/lib.rs:16:5
   |
16 |     <T as HasVTableFor<DST>>::VTABLE;
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
#1 [optimized_mir] processing `push`
end of query stack

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 1.31.0-nightly (424a749a0 2018-10-21) 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

error: Could not compile `playground`.

[SimonSapin]

Adding a field to the struct works around this:

https://play.rust-lang.org/?version=nightly&mode=debug&edition=2015&gist=67b1d78546132d843c7652437c70406f

struct VTable<DST: ?Sized> {
    _size: usize,
    _to_dst_ptr: fn(*mut ()) -> *mut DST,
}

trait HasVTableFor<DST: ?Sized + 'static> {
    const VTABLE: &'static VTable<DST>;
}

impl<T, DST: ?Sized + 'static> HasVTableFor<DST> for T {
    const VTABLE: &'static VTable<DST> = &VTable {
        _size: std::mem::size_of::<T>(),
        _to_dst_ptr: |_: *mut ()| unsafe { std::mem::zeroed() },
    };
}

pub fn push<DST: ?Sized + 'static, T>() {
    <T as HasVTableFor<DST>>::VTABLE;
}

[estebank]

No longer reproduces.