Panic in nightly rustc with type interference for const_generics

[Zettroke]

Code

#![feature(const_generics)]
#![feature(const_evaluatable_checked)]
#![allow(incomplete_features)]
pub trait IsTrue {}
pub trait IsFalse {}

pub struct Assert<const CHECK: bool> {}

impl IsTrue for Assert<true> {}
impl IsFalse for Assert<false> {}

pub struct SliceConstWriter<'a, const N: usize> {
    ptr: &'a mut [u8]
}
impl<'a, const N: usize> SliceConstWriter<'a, {N}> {
    pub fn from_slice(vec: &'a mut [u8]) -> Self {
        Self {
            ptr: vec
        }
    }

    pub fn convert<const NN: usize>(mut self) -> SliceConstWriter<'a, {NN}> {
        SliceConstWriter {
            ptr: self.ptr
        }
    }
}

impl<'a, const N: usize> SliceConstWriter<'a, {N}> where Assert::<{N >= 2}>: IsTrue {
    // broken
    pub fn write_u8(mut self) -> SliceConstWriter<'a, {N-2}> {
        self.convert()
    }
    
    //working
    // pub fn write_u8(mut self) -> SliceConstWriter<'a, {N-2}> {
    //     SliceConstWriter {
    //         ptr: self.ptr
    //     }
    // }
}


#[cfg(test)]
mod tests {
    use crate::SliceConstWriter;

    #[test]
    fn it_works() {
        let mut buff = [0u8; 128];
        let mut a = SliceConstWriter::<10>::from_slice(&mut buff);

        let mut a = a.write_u8().write_u8().write_u8().write_u8().write_u8();
    }
}

Reproduces in playground

Meta

rustc --version --verbose:

rustc 1.52.0-nightly (e9920ef77 2021-02-11)
binary: rustc
commit-hash: e9920ef7749d11fc71cc32ca4ba055bcfeaab945
commit-date: 2021-02-11
host: x86_64-pc-windows-gnu
release: 1.52.0-nightly
LLVM version: 11.0.1

Error output

error: internal compiler error: compiler\rustc_middle\src\ich\impls_ty.rs:94:17: StableHasher: unexpected region '_#2r

thread 'rustc' panicked at 'Box<Any>', /rustc/e9920ef7749d11fc71cc32ca4ba055bcfeaab945\library\std\src\panic.rs:59:5
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

note: the compiler unexpectedly panicked. this is a bug.

note: we would appreciate a bug report: https://github.com/rust-lang/rust/issues/new?labels=C-bug%2C+I-ICE%2C+T-compiler&template=ice.md

note: rustc 1.52.0-nightly (e9920ef77 2021-02-11) running on x86_64-pc-windows-gnu

note: compiler flags: -C embed-bitcode=no -C debuginfo=2 -C incremental --crate-type lib

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

query stack during panic:
#0 [type_op_ascribe_user_type] evaluating `type_op_ascribe_user_type` `Canonical { max_universe: U0, variables: [CanonicalVarInfo { kind: Region(U0) }, CanonicalVarInfo { kind: Region(U0) }, CanonicalVarInfo { kind: Region(U0) }, CanonicalVarInfo { kind: Region(U0) }, CanonicalVarInfo { kind: Const(U0) }, Canon
icalVarInfo { kind: Ty(General(U0)) }], value: ParamEnvAnd { param_env: ParamEnv { caller_bounds: [Binder(ConstEvaluatable(WithOptConstParam { did: DefId(0:29 ~ const_writer[f037]::{impl#3}::write_u8::{constant#0}), const_param_did: None }, [ReLateBound(DebruijnIndex(1), BoundRegion { kind: BrAnon(0) }), Const
{ ty: usize, val: Param(N/#1) }])), Binder(ConstEvaluatable(WithOptConstParam { did: DefId(0:26 ~ const_writer[f037]::{impl#3}::{constant#0}), const_param_did: None }, [ReLateBound(DebruijnIndex(1), BoundRegion { kind: BrAnon(0) }), Const { ty: usize, val: Param(N/#1) }])), Binder(TraitPredicate(<Assert<{N >= 2
}> as IsTrue>))], reveal: UserFacing }, value: AscribeUserType { mir_ty: fn(SliceConstWriter<'_, N>) -> SliceConstWriter<'_, {N-2}> {SliceConstWriter::<'_, N>::convert::<{N-2}>}, def_id: DefId(0:20 ~ const_writer[f037]::{impl#2}::convert), user_substs: UserSubsts { substs: [ReLateBound(DebruijnIndex(0), BoundRe
gion { kind: BrAnon(3) }), Const { ty: ^5, val: Bound(DebruijnIndex(0), 4) }, Const { ty: usize, val: Unevaluated(WithOptConstParam { did: DefId(0:30 ~ const_writer[f037]::{impl#3}::write_u8::{constant#1}), const_param_did: Some(DefId(0:21 ~ const_writer[f037]::{impl#2}::convert::NN)) }, [ReLateBound(DebruijnIn
dex(0), BoundRegion { kind: BrAnon(0) }), Const { ty: usize, val: Param(N/#1) }], None) }], user_self_ty: None } } } }`
#1 [mir_borrowck] borrow-checking `SliceConstWriter::<'a, N>::write_u8`
end of query stack
error: aborting due to previous error; 1 warning emitted

Backtrace

error: internal compiler error: compiler\rustc_middle\src\ich\impls_ty.rs:94:17: StableHasher: unexpected region '_#2r

thread 'rustc' panicked at 'Box<Any>', /rustc/e9920ef7749d11fc71cc32ca4ba055bcfeaab945\library\std\src\panic.rs:59:5
stack backtrace:
   0: std::panicking::begin_panic
   1: std::panic::panic_any
   2: rustc_errors::HandlerInner::bug
   3: rustc_errors::Handler::bug
   4: rustc_middle::util::bug::opt_span_bug_fmt::{{closure}}
   5: rustc_middle::ty::context::tls::with_opt::{{closure}}
   6: rustc_middle::ty::context::tls::with_opt
   7: rustc_middle::util::bug::opt_span_bug_fmt
   8: rustc_middle::util::bug::bug_fmt
   9: rustc_middle::ich::impls_ty::<impl rustc_data_structures::stable_hasher::HashStable<rustc_middle::ich::hcx::StableHashingContext> for rustc_middle::ty::sty::RegionKind>::hash_stable
  10: std::thread::local::LocalKey<T>::with
  11: <T as rustc_query_system::dep_graph::dep_node::DepNodeParams<Ctxt>>::to_fingerprint
  12: rustc_query_system::query::plumbing::get_query_impl
  13: rustc_trait_selection::traits::fulfill::FulfillProcessor::progress_changed_obligations
  14: rustc_data_structures::obligation_forest::ObligationForest<O>::process_obligations
  15: <rustc_trait_selection::traits::fulfill::FulfillmentContext as rustc_infer::traits::engine::TraitEngine>::select_where_possible
  16: rustc_infer::infer::canonical::query_response::<impl rustc_infer::infer::InferCtxt>::make_canonicalized_query_response
  17: <rustc_infer::infer::InferCtxtBuilder as rustc_trait_selection::infer::InferCtxtBuilderExt>::enter_canonical_trait_query
  18: rustc_traits::type_op::type_op_ascribe_user_type
  19: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::type_op_ascribe_user_type>::compute
  20: rustc_middle::dep_graph::<impl rustc_query_system::dep_graph::DepKind for rustc_middle::dep_graph::dep_node::DepKind>::with_deps
  21: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  22: rustc_data_structures::stack::ensure_sufficient_stack
  23: rustc_query_system::query::plumbing::force_query_with_job
  24: rustc_query_system::query::plumbing::get_query_impl
  25: rustc_trait_selection::traits::query::type_op::ascribe_user_type::<impl rustc_trait_selection::traits::query::type_op::QueryTypeOp for rustc_middle::traits::query::type_op::AscribeUserType>::perform_query
  26: rustc_trait_selection::traits::query::type_op::QueryTypeOp::fully_perform_into
  27: <rustc_middle::ty::ParamEnvAnd<Q> as rustc_trait_selection::traits::query::type_op::TypeOp>::fully_perform
  28: rustc_mir::borrow_check::type_check::type_check
  29: rustc_mir::borrow_check::nll::compute_regions
  30: rustc_mir::borrow_check::do_mir_borrowck
  31: rustc_infer::infer::InferCtxtBuilder::enter
  32: rustc_mir::borrow_check::mir_borrowck
  33: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::mir_borrowck>::compute
  34: rustc_middle::dep_graph::<impl rustc_query_system::dep_graph::DepKind for rustc_middle::dep_graph::dep_node::DepKind>::with_deps
  35: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  36: rustc_data_structures::stack::ensure_sufficient_stack
  37: rustc_query_system::query::plumbing::force_query_with_job
  38: rustc_query_system::query::plumbing::get_query_impl
  39: rustc_query_system::query::plumbing::ensure_query_impl
  40: rustc_interface::passes::analysis
  41: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::analysis>::compute
  42: rustc_middle::dep_graph::<impl rustc_query_system::dep_graph::DepKind for rustc_middle::dep_graph::dep_node::DepKind>::with_deps
  43: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  44: rustc_data_structures::stack::ensure_sufficient_stack
  45: rustc_query_system::query::plumbing::force_query_with_job
  46: rustc_query_system::query::plumbing::get_query_impl
  47: rustc_interface::passes::QueryContext::enter
  48: rustc_interface::queries::<impl rustc_interface::interface::Compiler>::enter
  49: rustc_span::with_source_map
  50: rustc_interface::interface::create_compiler_and_run
  51: scoped_tls::ScopedKey<T>::set

Description

Caught this error while playing with const generics.

After some experiments I concluded error caused by type inference for SliceConstWriter::convert method

Error very elusive. Even replacing test block with main function "fixed" it

[fanninpm]

Confusingly, I haven't (yet) been able to reproduce this without using cargo. @Alexendoo @JohnTitor mind taking a look at this?

[Alexendoo]

Sure, if you pass --verbose to cargo it'll tell you what rustc command it's running, then you can whittle away flags

Here rustc -C incremental=foo gets it to fail (if you add a main)

[fanninpm]

@Alexendoo thanks! rustc --test -C incremental=foo did the trick.

[hellow554]

Is this a dup of #77708 ?