ICE: "can't hash a TyVid" when failing to infer a const generic

[carado]

A clean rust project on the latest nightly rustc with just this src/main.rs:

#![feature(const_generics)]

struct S<T, const N: usize>([T; N]);

fn f<T, const N: usize>(x: T) -> S<T, {N}> { panic!() }

fn main() {
	f(0u8);
}

produces the following error on cargo check/build:

carado@ram:~/tmp/rust13 $ RUST_BACKTRACE=full cargo check
    Checking rust13 v0.1.0 (/home/carado/tmp/rust13)
warning: the feature `const_generics` is incomplete and may cause the compiler to crash
 --> src/main.rs:1:12
  |
1 | #![feature(const_generics)]
  |            ^^^^^^^^^^^^^^

error: internal compiler error: src/librustc/ich/impls_ty.rs:213: ty::TyKind::hash_stable() - can't hash a TyVid _#0t.

thread 'rustc' panicked at 'Box<Any>', src/librustc_errors/lib.rs:649:9
stack backtrace:
   0:     0x7fe25657923b - backtrace::backtrace::libunwind::trace::h4f04af92f39f2b78
                               at /cargo/registry/src/github.jparrowsec.cn-1ecc6299db9ec823/backtrace-0.3.29/src/backtrace/libunwind.rs:88
   1:     0x7fe25657923b - backtrace::backtrace::trace_unsynchronized::hb28ce8d2b7fb3e31
                               at /cargo/registry/src/github.jparrowsec.cn-1ecc6299db9ec823/backtrace-0.3.29/src/backtrace/mod.rs:66
   2:     0x7fe25657923b - std::sys_common::backtrace::_print::he3f49d2d13a51ff1
                               at src/libstd/sys_common/backtrace.rs:47
   3:     0x7fe25657923b - std::sys_common::backtrace::print::h46c2f3b13519e9bf
                               at src/libstd/sys_common/backtrace.rs:36
   4:     0x7fe25657923b - std::panicking::default_hook::{{closure}}::h6694acddafc67583
                               at src/libstd/panicking.rs:200
   5:     0x7fe256578f17 - std::panicking::default_hook::hb4761d2feeb6d93c
                               at src/libstd/panicking.rs:214
   6:     0x7fe25833eab1 - rustc::util::common::panic_hook::h048f1ff4bc2b561b
   7:     0x7fe256579a99 - std::panicking::rust_panic_with_hook::h7a5f9620b0a832bc
                               at src/libstd/panicking.rs:481
   8:     0x7fe2587d3f5d - std::panicking::begin_panic::hb73227afb9857db6
   9:     0x7fe2587f8f7f - rustc_errors::Handler::bug::h2054eb06936e67b5
  10:     0x7fe257f6d4e3 - rustc::util::bug::opt_span_bug_fmt::{{closure}}::h3c0421ab2fdc9ea6
  11:     0x7fe257f6ca73 - rustc::ty::context::tls::with_opt::{{closure}}::hd2fbc2b0ebae2095
  12:     0x7fe257f6c4e8 - rustc::ty::context::tls::with_context_opt::h941a3adb8b126e2b
  13:     0x7fe257f6c527 - rustc::ty::context::tls::with_opt::h3eb1995ec6f2c295
  14:     0x7fe257f6d3f8 - rustc::util::bug::opt_span_bug_fmt::h5a2a260784b50bee
  15:     0x7fe257f6d362 - rustc::util::bug::bug_fmt::h7e4e2350b3fe730c
  16:     0x7fe25840f4b2 - rustc::ich::impls_ty::<impl rustc_data_structures::stable_hasher::HashStable<rustc::ich::hcx::StableHashingContext> for rustc::ty::sty::TyVid>::hash_stable::h1463ce28b76e87fc
  17:     0x7fe258470fa7 - rustc::ty::sty::_DERIVE_rustc_data_structures_stable_hasher_HashStable_rustc_ich_StableHashingContext_ctx_FOR_TyKind::<impl rustc_data_structures::stable_hasher::HashStable<rustc::ich::hcx::StableHashingContext> for rustc::ty::sty::TyKind>::hash_stable::h7dcfc11871930cfb
  18:     0x7fe257eb7be2 - rustc::ich::impls_ty::<impl rustc_data_structures::stable_hasher::HashStable<rustc::ich::hcx::StableHashingContext> for rustc::ty::subst::Kind>::hash_stable::h5c3796ae412cb81f
  19:     0x7fe2584eb052 - rustc::infer::canonical::_DERIVE_rustc_data_structures_stable_hasher_HashStable_rustc_ich_StableHashingContext_ctx_FOR_Canonical::<impl rustc_data_structures::stable_hasher::HashStable<rustc::ich::hcx::StableHashingContext> for rustc::infer::canonical::Canonical<V>>::hash_stable::h8e0f5f69f3ed9b3c
  20:     0x7fe257e5c85b - rustc::dep_graph::graph::DepGraph::with_task_impl::hebe93126a1c46ef0
  21:     0x7fe2580f06e3 - rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query::h668eeb3ee62e8a11
  22:     0x7fe257ddde3c - rustc::traits::query::dropck_outlives::<impl rustc::infer::at::At>::dropck_outlives::hb3441b0093584465
  23:     0x7fe25700a8f9 - rustc_typeck::check::dropck::check_safety_of_destructor_if_necessary::h39fe2f333c370e50
  24:     0x7fe256f839d7 - rustc_typeck::check::regionck::RegionCtxt::check_safety_of_rvalue_destructor_if_necessary::h5dee61f898d3b300
  25:     0x7fe256f82296 - <rustc_typeck::check::regionck::RegionCtxt as rustc::hir::intravisit::Visitor>::visit_expr::h170804856875af5e
  26:     0x7fe2570c872f - rustc::hir::intravisit::walk_expr::h7747ebb77c7fd9e3
  27:     0x7fe256f8277b - <rustc_typeck::check::regionck::RegionCtxt as rustc::hir::intravisit::Visitor>::visit_expr::h170804856875af5e
  28:     0x7fe256f80f5e - rustc_typeck::check::regionck::RegionCtxt::visit_fn_body::hf253f947ee1a5219
  29:     0x7fe256f171a9 - rustc_typeck::check::regionck::<impl rustc_typeck::check::FnCtxt>::regionck_fn::h0fc45e22409227c9
  30:     0x7fe2570a325a - rustc::ty::context::GlobalCtxt::enter_local::h7659f32b1ea661a0
  31:     0x7fe256f4a755 - rustc_typeck::check::typeck_tables_of::hfb1f10d39fb5fa69
  32:     0x7fe256f6bbbe - rustc::ty::query::__query_compute::typeck_tables_of::h037cb95db35bf44f
  33:     0x7fe25701f5db - rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::typeck_tables_of>::compute::h535de4befaa63b09
  34:     0x7fe256fa0c72 - rustc::dep_graph::graph::DepGraph::with_task_impl::hbcc48ac4d5a17f83
  35:     0x7fe25705b74d - rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query::h7a171614700603c7
  36:     0x7fe25701c3ef - rustc::ty::<impl rustc::ty::context::TyCtxt>::par_body_owners::h14b7ec1cc219548f
  37:     0x7fe256f4a25d - rustc_typeck::check::typeck_item_bodies::h09210f3d89e8e316
  38:     0x7fe256f6c019 - rustc::ty::query::__query_compute::typeck_item_bodies::h2b59bdafb83bcacf
  39:     0x7fe256fa18a4 - rustc::dep_graph::graph::DepGraph::with_task_impl::hc80a68025f840170
  40:     0x7fe257039316 - rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query::h2d01be60148cc515
  41:     0x7fe256f70b5e - rustc::util::common::time::h3ea0d1c7348e741b
  42:     0x7fe257140557 - rustc_typeck::check_crate::h2dccc5ff35a7fab7
  43:     0x7fe256b31795 - rustc_interface::passes::analysis::hcc2364d20e32d836
  44:     0x7fe256ab5f05 - rustc::ty::query::__query_compute::analysis::hb6494d9db9853dcd
  45:     0x7fe256ab8439 - rustc::dep_graph::graph::DepGraph::with_task_impl::hc526faaa29e0549a
  46:     0x7fe256a24e4e - rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query::hc2ff9ebdf5b52ec2
  47:     0x7fe256a4299a - rustc_interface::passes::BoxedGlobalCtxt::access::{{closure}}::h14f5a411e02f867a
  48:     0x7fe256b7932a - rustc_interface::passes::create_global_ctxt::{{closure}}::h373e0b54add389be
  49:     0x7fe256a454a8 - rustc_interface::interface::run_compiler_in_existing_thread_pool::he615c75133ce557e
  50:     0x7fe256a589e2 - std::thread::local::LocalKey<T>::with::h01d8730b937bafe6
  51:     0x7fe256a6bc61 - scoped_tls::ScopedKey<T>::set::h93202ef68249584c
  52:     0x7fe256a837e4 - syntax::with_globals::hec7bd40bfbb002de
  53:     0x7fe256aad39d - std::sys_common::backtrace::__rust_begin_short_backtrace::h1b4bf6169be40163
  54:     0x7fe25658a4fa - __rust_maybe_catch_panic
                               at src/libpanic_unwind/lib.rs:82
  55:     0x7fe256a2bfd9 - core::ops::function::FnOnce::call_once{{vtable.shim}}::hb217ce5152f7b02c
  56:     0x7fe25655ce7f - <alloc::boxed::Box<F> as core::ops::function::FnOnce<A>>::call_once::h352281c34878505f
                               at /rustc/78ca1bda3522b14bc0336bc01dd1d49fdba2cda7/src/liballoc/boxed.rs:766
  57:     0x7fe2565891e0 - <alloc::boxed::Box<F> as core::ops::function::FnOnce<A>>::call_once::h77507a97b5d0b6ca
                               at /rustc/78ca1bda3522b14bc0336bc01dd1d49fdba2cda7/src/liballoc/boxed.rs:766
  58:     0x7fe2565891e0 - std::sys_common::thread::start_thread::h8e874688d93a6d97
                               at src/libstd/sys_common/thread.rs:13
  59:     0x7fe2565891e0 - std::sys::unix::thread::Thread::new::thread_start::hbfd3b1248521427e
                               at src/libstd/sys/unix/thread.rs:79
  60:     0x7fe2564bffa8 - start_thread
                               at /builddir/glibc-2.29/nptl/pthread_create.c:486
  61:     0x7fe2563e1b9f - __GI___clone
  62:                0x0 - <unknown>
query stack during panic:
#0 [dropck_outlives] computing dropck types for `Canonical { max_universe: U0, variables: [CanonicalVarInfo { kind: Const(U0) }], value: ParamEnvAnd { param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, def_id: None }, value: S<u8, _: usize> } }`
#1 [typeck_tables_of] processing `main`
#2 [typeck_item_bodies] type-checking all item bodies
#3 [analysis] running analysis passes on this crate
end of query stack
error: aborting due to previous error


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 1.38.0-nightly (78ca1bda3 2019-07-08) running on x86_64-unknown-linux-gnu

note: compiler flags: -C debuginfo=2 -C incremental --crate-type bin

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

error: Could not compile `rust13`.

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

The error is most likely caused by the inability to deduce the value of the const generic parameter N; when it is made clear (with, say, let x: S<_, {3}> = f(0u8);) the ICE no longer occurs.
Thus the expected behavior is probably still a failure to build (just not an ICE).

[novacrazy]

EDIT: Apparently const-generics in array repeat positions isn't a thing yet, at all. I apologize for the not-even-currently-valid example.

I get the same ICE for this code:

#![feature(const_generics, const_fn)]

pub struct TinyStack<T, const N: usize> {
    pub index: usize,
    pub stack: [T; N],
}

impl<T, const N: usize> TinyStack<T, {N}>
where
    T: Copy,
{
    #[inline(always)]
    pub const fn new(value: T) -> TinyStack<T, {N}> {
        TinyStack { index: 0, stack: [value; N] }
    }

    #[inline(always)]
    pub fn push(&mut self, x: T) {
        if self.index < N {
            self.stack[self.index] = x;
            self.index += 1;
        }
    }

    #[inline(always)]
    pub fn pop(&mut self) -> Option<T> {
        if self.index > 0 {
            let res = Some(self.stack[self.index]);
            self.index -= 1;
            res
        } else {
            None
        }
    }
}

fn main() {
    let stack: TinyStack<usize, {64}> = TinyStack::new(0);
}

which does give the explicit type, so it's not (entirely) a deduction thing.

Entire build log:

PS F:\code\projects\bugs\const_generic_62536> cargo build
   Compiling const_generic_62536 v0.1.0 (F:\code\projects\bugs\const_generic_62536)
warning: the feature `const_generics` is incomplete and may cause the compiler to crash
 --> src\main.rs:1:12
  |
1 | #![feature(const_generics, const_fn)]
  |            ^^^^^^^^^^^^^^

warning: unused variable: `stack`
  --> src\main.rs:38:9
   |
38 |     let stack: TinyStack<usize, {64}> = TinyStack::new(0);
   |         ^^^^^ help: consider prefixing with an underscore: `_stack`
   |
   = note: #[warn(unused_variables)] on by default

error: array lengths can't depend on generic parameters
  --> src\main.rs:14:46
   |
14 |         TinyStack { index: 0, stack: [value; N] }
   |                                              ^

error: internal compiler error: src\librustc\ich\impls_ty.rs:213: ty::TyKind::hash_stable() - can't hash a TyVid _#1t.

thread 'rustc' panicked at 'Box<Any>', src\librustc_errors\lib.rs:649:9
stack backtrace:
   0: std::sys_common::alloc::realloc_fallback
   1: std::panicking::take_hook
   2: std::panicking::take_hook
   3: <rustc::ty::layout::LayoutCx<rustc::ty::query::TyCtxtAt> as rustc_target::abi::LayoutOf>::layout_of
   4: std::panicking::rust_panic_with_hook
   5: <rustc_errors::lock::acquire_global_lock::Guard as core::ops::drop::Drop>::drop
   6: rustc_errors::Handler::bug
   7: rustc::util::bug::bug_fmt
   8: rustc::ty::wf::object_region_bounds
   9: rustc::ty::wf::object_region_bounds
  10: rustc::ty::wf::object_region_bounds
  11: rustc::util::bug::bug_fmt
  12: rustc::util::bug::bug_fmt
  13: rustc::hir::def::NonMacroAttrKind::descr
  14: <rustc::ty::sty::TyKind as core::fmt::Debug>::fmt
  15: rustc::hir::print::State::print_is_auto
  16: <alloc::vec::Vec<(alloc::string::String, u64)> as rustc::session::config::dep_tracking::DepTrackingHash>::hash
  17: <rustc::ty::subst::UnpackedKind as core::fmt::Debug>::fmt
  18: <rustc::ty::sty::InferConst as core::fmt::Debug>::fmt
  19: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::try_print_query_stack
  20: <rustc::traits::query::type_op::ascribe_user_type::AscribeUserType as rustc::traits::query::type_op::QueryTypeOp>::perform_query
  21: <rustc_mir::dataflow::move_paths::abs_domain::AbstractOperand as core::fmt::Debug>::fmt
  22: <rustc_mir::interpret::intern::InternMode as core::fmt::Debug>::fmt
  23: <rustc_mir::borrow_check::nll::type_check::TypeVerifier as rustc::mir::visit::Visitor>::visit_body
  24: <rustc_mir::borrow_check::nll::constraint_generation::ConstraintGeneration as rustc::mir::visit::Visitor>::visit_terminator
  25: <rustc_mir::borrow_check::nll::member_constraints::MemberConstraintSet<rustc::ty::sty::RegionVid> as core::default::Default>::default
  26: rustc_mir::borrow_check::nll::universal_regions::UniversalRegionIndices::insert_late_bound_region
  27: <rustc_mir::hair::pattern::PatternRange as core::fmt::Debug>::fmt
  28: rustc_mir::borrow_check::nll::universal_regions::UniversalRegionIndices::insert_late_bound_region
  29: <rustc_interface::util::ReplaceBodyWithLoop as syntax::mut_visit::MutVisitor>::visit_mac
  30: rustc_interface::passes::BoxedGlobalCtxt::complete
  31: rustc_interface::passes::BoxedGlobalCtxt::complete
  32: rustc_interface::passes::BoxedGlobalCtxt::complete
  33: rustc_interface::passes::BoxedGlobalCtxt::complete
  34: rustc_interface::passes::BoxedResolver::to_expansion_result
  35: <rustc_privacy::ObsoleteCheckTypeForPrivatenessVisitor as rustc::hir::intravisit::Visitor>::nested_visit_map
  36: <rustc_privacy::ObsoleteCheckTypeForPrivatenessVisitor as rustc::hir::intravisit::Visitor>::nested_visit_map
  37: <rustc_lint::BuiltinCombinedPreExpansionLintPass as rustc::lint::EarlyLintPass>::check_expr_post
  38: <rustc_lint::BuiltinCombinedPreExpansionLintPass as rustc::lint::EarlyLintPass>::check_expr_post
  39: <humantime::date::Rfc3339Timestamp as core::fmt::Debug>::fmt
  40: <rustc_lint::BuiltinCombinedPreExpansionLintPass as rustc::lint::EarlyLintPass>::check_expr_post
  41: <env_logger::filter::inner::Filter as core::fmt::Display>::fmt
  42: <env_logger::filter::inner::Filter as core::fmt::Display>::fmt
  43: <rustc_driver::Compilation as core::fmt::Debug>::fmt
  44: <rustc_driver::Compilation as core::fmt::Debug>::fmt
  45: _rust_maybe_catch_panic
  46: <rustc_lint::BuiltinCombinedPreExpansionLintPass as rustc::lint::EarlyLintPass>::check_expr_post
  47: ZN244_$LT$std..error..$LT$impl$u20$core..convert..From$LT$alloc..string..String$GT$$u20$for$u20$alloc..boxed..Box$LT$dyn$u20$std..error..Error$u2b$core..marker..Send$u2b$core..marker..Sync$GT$$GT$..from..StringError$u20$as$u20$core..fmt..Display$GT$3fmt17
  48: std::sys::windows::thread::Thread::new
  49: BaseThreadInitThunk
  50: RtlUserThreadStart
query stack during panic:
#0 [mir_borrowck] processing `main`
#1 [analysis] running analysis passes on this crate
end of query stack
error: aborting due to 2 previous errors


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 1.38.0-nightly (6e310f2ab 2019-07-07) running on x86_64-pc-windows-msvc

note: compiler flags: -C debuginfo=2 -C incremental -C target-cpu=native --crate-type bin

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

error: Could not compile `const_generic_62536`.

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

[JohnTitor]

Could you check the ICE still appears? I cannot reproduce it with latest nightly on playground.

https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=3504980d8445be3ee76087f26bf37bc8
https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=271dd5b69ac5d8c5cd6f206ccf921f8b

[varkor]

Although this isn't throwing an error on the playground, I get an assertion failure when adding a test:

thread 'rustc' panicked at 'assertion failed: !out_value.has_type_flags(TypeFlags::KEEP_IN_LOCAL_TCX)', src/librustc/infer/canonical/canonicalizer.rs:548:9

Let's leave this open until we have a test for it.