ICE when working with infinitely nesting types and ?-operator

[dastbe]

Code

I've hacked it down from an advent of code solution. When

    let version = extend_number(0, itr, 3)?;
    let type_id = extend_number(0, itr, 3)?;

is replaced with u32 literals the compiler emits a (correct) compilation error due to infinitely nesting types.

use std::vec::Vec;
use std::iter::Peekable;

pub fn main() {
    let packet = decode(vec![1,0,1,0]);
}

pub fn decode(bitstream: Vec<u64>) -> Packet {
    let mut bitstream_itr = bitstream.into_iter().peekable();
    return match decode_packet(&mut bitstream_itr) {
        Some(p) => p,
        None    => panic!("expected outer packet"),
    }
}

pub fn decode_packets<I: Iterator<Item = u64>>(itr: &mut Peekable<I>) -> Vec<Packet> {
    let mut res = Vec::new();
    loop {
        match decode_packet(itr) {
            Some(p) => { res.push(p); },
            None    => break
        }
    }

    return res;
}

pub fn decode_packet<I: Iterator<Item = u64>>(itr: &mut Peekable<I>) -> Option<Packet> {
    // get version digits
    let version = extend_number(0, itr, 3)?;
    let type_id = extend_number(0, itr, 3)?;
    return operator_packet(version, type_id, itr);
}

pub fn operator_packet<I: Iterator<Item = u64>>(version: u64, type_id: u64, itr: &mut Peekable<I>) -> Option<Packet> {
    let p = OperatorPacket {
        version: version,
        type_id: type_id,
        packets: decode_packets(&mut itr.take(0).peekable()),
    };

    return Some(Packet::Operator(p));
}

pub fn extend_number<I: Iterator<Item = u64>>(num: u64, itr: &mut Peekable<I>, take: u64) -> Option<u64> {
    let mut value = num;
    for _ in 0..take {
        value *= 2;
        value += itr.next()?;
    }

    return Some(value);
}

#[derive(Debug)]
pub enum Packet {
    Operator(OperatorPacket),
}

#[derive(Debug)]
pub struct OperatorPacket {
    version: u64,
    type_id: u64,
    packets: Vec<Packet>
}

Meta

rustc --version --verbose:

rustc 1.59.0-nightly (db9d361a4 2021-11-28)
binary: rustc
commit-hash: db9d361a4731ca0bb48533fab6297a8fea75696f
commit-date: 2021-11-28
host: aarch64-apple-darwin
release: 1.59.0-nightly
LLVM version: 13.0.0

Error output

thread 'rustc' panicked at 'index out of bounds: the len is 0 but the index is 0', /Users/runner/.cargo/registry/src/github.com-1ecc6299db9ec823/ena-0.14.0/src/snapshot_vec.rs:199:10
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

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/issues/new?labels=C-bug%2C+I-ICE%2C+T-compiler&template=ice.md

note: rustc 1.59.0-nightly (db9d361a4 2021-11-28) running on aarch64-apple-darwin

note: compiler flags: -C embed-bitcode=no -C split-debuginfo=unpacked -C debuginfo=2 -C incremental --crate-type bin

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

query stack during panic:
#0 [is_copy_raw] computing whether `core::option::Option<core::option::Option<_>>` is `Copy`
#1 [needs_drop_raw] computing whether `core::option::Option<core::option::Option<_>>` needs drop
end of query stack
warning: `aoc-uh-oh` (bin "aoc-uh-oh") generated 4 warnings
error: could not compile `aoc-uh-oh`; 4 warnings emitted

Backtrace

stack backtrace:
   0: _rust_begin_unwind
   1: core::panicking::panic_fmt
   2: core::panicking::panic_bounds_check
   3: <rustc_infer::infer::type_variable::TypeVariableTable>::probe
   4: <rustc_infer::infer::InferCtxt>::probe_ty_var
   5: <rustc_infer::infer::canonical::canonicalizer::Canonicalizer as rustc_middle::ty::fold::TypeFolder>::fold_ty
   6: <&rustc_middle::ty::list::List<rustc_middle::ty::subst::GenericArg> as rustc_middle::ty::fold::TypeFoldable>::fold_with::<rustc_infer::infer::canonical::canonicalizer::Canonicalizer>
   7: <&rustc_middle::ty::TyS as rustc_middle::ty::fold::TypeFoldable>::super_fold_with::<rustc_infer::infer::canonical::canonicalizer::Canonicalizer>
   8: <&rustc_middle::ty::list::List<rustc_middle::ty::subst::GenericArg> as rustc_middle::ty::fold::TypeFoldable>::fold_with::<rustc_infer::infer::canonical::canonicalizer::Canonicalizer>
   9: <&rustc_middle::ty::TyS as rustc_middle::ty::fold::TypeFoldable>::super_fold_with::<rustc_infer::infer::canonical::canonicalizer::Canonicalizer>
  10: <&rustc_middle::ty::list::List<rustc_middle::ty::subst::GenericArg> as rustc_middle::ty::fold::TypeFoldable>::fold_with::<rustc_infer::infer::canonical::canonicalizer::Canonicalizer>
  11: <rustc_middle::ty::PredicateKind as rustc_middle::ty::fold::TypeFoldable>::fold_with::<rustc_infer::infer::canonical::canonicalizer::Canonicalizer>
  12: <rustc_infer::infer::canonical::canonicalizer::Canonicalizer>::canonicalize::<rustc_middle::ty::ParamEnvAnd<rustc_middle::ty::Predicate>>
  13: <rustc_infer::infer::InferCtxt as rustc_trait_selection::traits::query::evaluate_obligation::InferCtxtExt>::evaluate_obligation
  14: <rustc_infer::infer::InferCtxt as rustc_trait_selection::traits::query::evaluate_obligation::InferCtxtExt>::evaluate_obligation_no_overflow
  15: rustc_trait_selection::traits::type_known_to_meet_bound_modulo_regions
  16: <rustc_infer::infer::InferCtxtBuilder>::enter::<bool, rustc_ty_utils::common_traits::is_item_raw::{closure#0}>
  17: rustc_ty_utils::common_traits::is_copy_raw
  18: <rustc_query_system::dep_graph::graph::DepGraph<rustc_middle::dep_graph::dep_node::DepKind>>::with_task::<rustc_middle::ty::context::TyCtxt, rustc_middle::ty::ParamEnvAnd<&rustc_middle::ty::TyS>, bool>
  19: rustc_data_structures::stack::ensure_sufficient_stack::<(bool, rustc_query_system::dep_graph::graph::DepNodeIndex), rustc_query_system::query::plumbing::execute_job<rustc_query_impl::plumbing::QueryCtxt, rustc_middle::ty::ParamEnvAnd<&rustc_middle::ty::TyS>, bool>::{closure#3}>
  20: rustc_query_system::query::plumbing::try_execute_query::<rustc_query_impl::plumbing::QueryCtxt, rustc_query_system::query::caches::DefaultCache<rustc_middle::ty::ParamEnvAnd<&rustc_middle::ty::TyS>, bool>>
  21: rustc_query_system::query::plumbing::get_query::<rustc_query_impl::queries::is_copy_raw, rustc_query_impl::plumbing::QueryCtxt>
  22: <rustc_middle::ty::TyS>::is_copy_modulo_regions
  23: rustc_ty_utils::needs_drop::needs_drop_raw
  24: <rustc_query_system::dep_graph::graph::DepGraph<rustc_middle::dep_graph::dep_node::DepKind>>::with_task::<rustc_middle::ty::context::TyCtxt, rustc_middle::ty::ParamEnvAnd<&rustc_middle::ty::TyS>, bool>
  25: rustc_data_structures::stack::ensure_sufficient_stack::<(bool, rustc_query_system::dep_graph::graph::DepNodeIndex), rustc_query_system::query::plumbing::execute_job<rustc_query_impl::plumbing::QueryCtxt, rustc_middle::ty::ParamEnvAnd<&rustc_middle::ty::TyS>, bool>::{closure#3}>
  26: rustc_query_system::query::plumbing::try_execute_query::<rustc_query_impl::plumbing::QueryCtxt, rustc_query_system::query::caches::DefaultCache<rustc_middle::ty::ParamEnvAnd<&rustc_middle::ty::TyS>, bool>>
  27: rustc_query_system::query::plumbing::get_query::<rustc_query_impl::queries::needs_drop_raw, rustc_query_impl::plumbing::QueryCtxt>
  28: rustc_ty_utils::instance::inner_resolve_instance
  29: rustc_ty_utils::instance::resolve_instance
  30: <rustc_middle::dep_graph::dep_node::DepKind as rustc_query_system::dep_graph::DepKind>::with_deps::<<rustc_query_system::dep_graph::graph::DepGraph<rustc_middle::dep_graph::dep_node::DepKind>>::with_task_impl<rustc_middle::ty::context::TyCtxt, rustc_middle::ty::ParamEnvAnd<(rustc_span::def_id::DefId, &rustc_middle::ty::list::List<rustc_middle::ty::subst::GenericArg>)>, core::result::Result<core::option::Option<rustc_middle::ty::instance::Instance>, rustc_errors::ErrorReported>>::{closure#0}, core::result::Result<core::option::Option<rustc_middle::ty::instance::Instance>, rustc_errors::ErrorReported>>
  31: <rustc_query_system::dep_graph::graph::DepGraph<rustc_middle::dep_graph::dep_node::DepKind>>::with_task::<rustc_middle::ty::context::TyCtxt, rustc_middle::ty::ParamEnvAnd<(rustc_span::def_id::DefId, &rustc_middle::ty::list::List<rustc_middle::ty::subst::GenericArg>)>, core::result::Result<core::option::Option<rustc_middle::ty::instance::Instance>, rustc_errors::ErrorReported>>
  32: rustc_data_structures::stack::ensure_sufficient_stack::<(core::result::Result<core::option::Option<rustc_middle::ty::instance::Instance>, rustc_errors::ErrorReported>, rustc_query_system::dep_graph::graph::DepNodeIndex), rustc_query_system::query::plumbing::execute_job<rustc_query_impl::plumbing::QueryCtxt, rustc_middle::ty::ParamEnvAnd<(rustc_span::def_id::DefId, &rustc_middle::ty::list::List<rustc_middle::ty::subst::GenericArg>)>, core::result::Result<core::option::Option<rustc_middle::ty::instance::Instance>, rustc_errors::ErrorReported>>::{closure#3}>
  33: rustc_query_system::query::plumbing::get_query::<rustc_query_impl::queries::resolve_instance, rustc_query_impl::plumbing::QueryCtxt>
  34: <rustc_query_impl::Queries as rustc_middle::ty::query::QueryEngine>::resolve_instance
  35: <rustc_middle::ty::instance::Instance>::resolve_opt_const_arg
  36: <rustc_middle::ty::instance::Instance>::resolve_drop_in_place
  37: <rustc_monomorphize::collector::MirNeighborCollector as rustc_middle::mir::visit::Visitor>::visit_terminator
  38: rustc_monomorphize::collector::collect_neighbours
  39: rustc_monomorphize::collector::collect_items_rec
  40: rustc_monomorphize::collector::collect_items_rec
  41: rustc_monomorphize::collector::collect_items_rec
  42: rustc_monomorphize::collector::collect_items_rec
  43: rustc_monomorphize::collector::collect_items_rec
  44: rustc_monomorphize::collector::collect_items_rec
  45: rustc_monomorphize::collector::collect_items_rec
  46: rustc_monomorphize::collector::collect_items_rec
  47: rustc_monomorphize::collector::collect_items_rec
  48: rustc_monomorphize::collector::collect_items_rec
  49: rustc_monomorphize::collector::collect_items_rec
  50: rustc_monomorphize::collector::collect_items_rec
  51: rustc_monomorphize::collector::collect_items_rec
  52: rustc_monomorphize::collector::collect_items_rec
  53: rustc_monomorphize::collector::collect_items_rec
  54: rustc_monomorphize::collector::collect_items_rec
  55: rustc_monomorphize::collector::collect_items_rec
  56: rustc_monomorphize::collector::collect_items_rec
  57: rustc_monomorphize::collector::collect_items_rec
  58: rustc_monomorphize::collector::collect_items_rec
  59: rustc_monomorphize::collector::collect_items_rec
  60: rustc_monomorphize::collector::collect_items_rec
  61: rustc_monomorphize::collector::collect_items_rec
  62: rustc_monomorphize::collector::collect_items_rec
  63: rustc_monomorphize::collector::collect_items_rec
  64: rustc_monomorphize::collector::collect_items_rec
  65: rustc_monomorphize::collector::collect_items_rec
  66: rustc_monomorphize::collector::collect_items_rec
  67: rustc_monomorphize::collector::collect_items_rec
  68: rustc_monomorphize::collector::collect_items_rec
  69: rustc_monomorphize::collector::collect_items_rec
  70: rustc_monomorphize::collector::collect_items_rec
  71: rustc_monomorphize::collector::collect_items_rec
  72: rustc_monomorphize::collector::collect_items_rec
  73: rustc_monomorphize::collector::collect_items_rec
  74: rustc_monomorphize::collector::collect_items_rec
  75: rustc_monomorphize::collector::collect_items_rec
  76: rustc_monomorphize::collector::collect_items_rec
  77: rustc_monomorphize::collector::collect_items_rec
  78: rustc_monomorphize::collector::collect_items_rec
  79: rustc_monomorphize::collector::collect_items_rec
  80: rustc_monomorphize::collector::collect_items_rec
  81: rustc_monomorphize::collector::collect_items_rec
  82: rustc_monomorphize::collector::collect_items_rec
  83: rustc_monomorphize::collector::collect_items_rec
  84: rustc_monomorphize::collector::collect_items_rec
  85: rustc_monomorphize::collector::collect_items_rec
  86: rustc_monomorphize::collector::collect_items_rec
  87: rustc_monomorphize::collector::collect_items_rec
  88: rustc_monomorphize::collector::collect_items_rec
  89: rustc_monomorphize::collector::collect_items_rec
  90: rustc_monomorphize::collector::collect_items_rec
  91: rustc_monomorphize::collector::collect_items_rec
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

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/issues/new?labels=C-bug%2C+I-ICE%2C+T-compiler&template=ice.md

note: rustc 1.59.0-nightly (db9d361a4 2021-11-28) running on aarch64-apple-darwin

note: compiler flags: -C embed-bitcode=no -C split-debuginfo=unpacked -C debuginfo=2 -C incremental --crate-type bin

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

query stack during panic:
#0 [is_copy_raw] computing whether `core::option::Option<core::option::Option<_>>` is `Copy`
#1 [needs_drop_raw] computing whether `core::option::Option<core::option::Option<_>>` needs drop
#2 [resolve_instance] resolving instance `core::ptr::drop_in_place::<core::option::Option<core::option::Option<_>>>`
#3 [collect_and_partition_mono_items] collect_and_partition_mono_items
end of query stack

[Aaron1011]

We're incorrectly invoking resolve_instance with an inference car - this should never happen, but we seem to still have inference cars present during monomorphization.

I suspect that this is related to the overflow, and could be solved by skipping Instance::resolve and calling delay_span_bug when we encounter an unresolved inference variable.

[JohnTitor]

Triage: Fixed by #100757, I think it has a regression test that should cover this case, closing.

[matthiaskrgr]

Reopened by #104610

[compiler-errors]

This now overflows with an error.

[matthiaskrgr]

This crashes again 🤣