ICE: thread 'rustc' panicked at 'region constraints already solved'

[hkmatsumoto]

Code

#![feature(rustc_private)]

extern crate rustc_middle;

use rustc_middle::ty::TyCtxt;

fn foo(tcx: TyCtxt<'_>) {
    let krate = tcx.hir().krate();
    for &item in krate.items() {
        todo!()
    }
}

Compiles with &item replaced for item

Meta

rustc --version --verbose:

rustc 1.56.0-nightly (1f0a591b3 2021-07-30)
binary: rustc
commit-hash: 1f0a591b3a5963a0ab11a35dc525ad9d46f612e4
commit-date: 2021-07-30
host: x86_64-unknown-linux-gnu
release: 1.56.0-nightly
LLVM version: 12.0.1

Error output

    Checking ice v0.1.0 (/home/xxx/ice)
warning: unused variable: `item`
 --> src/lib.rs:9:10
  |
9 |     for &item in krate.items() {
  |          ^^^^ help: if this is intentional, prefix it with an underscore: `_item`
  |
  = note: `#[warn(unused_variables)]` on by default

thread 'rustc' panicked at 'region constraints already solved', compiler/rustc_infer/src/infer/mod.rs:268:14
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 1.56.0-nightly (1f0a591b3 2021-07-30) running on x86_64-unknown-linux-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 [mir_borrowck] borrow-checking `foo`
#1 [analysis] running analysis passes on this crate
end of query stack
warning: `ice` (lib) generated 1 warning
error: could not compile `ice`; 1 warning emitted

Backtrace

stack backtrace:
   0: rust_begin_unwind
             at /rustc/1f0a591b3a5963a0ab11a35dc525ad9d46f612e4/library/std/src/panicking.rs:516:5
   1: core::panicking::panic_fmt
             at /rustc/1f0a591b3a5963a0ab11a35dc525ad9d46f612e4/library/core/src/panicking.rs:93:14
   2: core::option::expect_failed
             at /rustc/1f0a591b3a5963a0ab11a35dc525ad9d46f612e4/library/core/src/option.rs:1578:5
   3: <rustc_infer::infer::canonical::canonicalizer::Canonicalizer as rustc_middle::ty::fold::TypeFolder>::fold_region
   4: rustc_middle::ty::fold::TypeFoldable::fold_with
   5: rustc_middle::ty::structural_impls::<impl rustc_middle::ty::fold::TypeFoldable for &rustc_middle::ty::TyS>::super_fold_with
   6: rustc_middle::ty::structural_impls::<impl rustc_middle::ty::fold::TypeFoldable for &rustc_middle::ty::TyS>::super_fold_with
   7: rustc_middle::ty::fold::TypeFoldable::fold_with
   8: rustc_middle::ty::fold::TypeFoldable::fold_with
   9: rustc_infer::infer::canonical::canonicalizer::Canonicalizer::canonicalize
  10: <rustc_infer::infer::InferCtxt as rustc_trait_selection::traits::query::evaluate_obligation::InferCtxtExt>::evaluate_obligation
  11: <rustc_infer::infer::InferCtxt as rustc_trait_selection::traits::query::evaluate_obligation::InferCtxtExt>::evaluate_obligation_no_overflow
  12: rustc_trait_selection::traits::type_known_to_meet_bound_modulo_regions
  13: rustc_mir::borrow_check::diagnostics::move_errors::<impl rustc_mir::borrow_check::MirBorrowckCtxt>::report_move_errors
  14: rustc_mir::borrow_check::do_mir_borrowck
  15: rustc_infer::infer::InferCtxtBuilder::enter
  16: rustc_mir::borrow_check::mir_borrowck
  17: core::ops::function::FnOnce::call_once
  18: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  19: rustc_data_structures::stack::ensure_sufficient_stack
  20: rustc_query_system::query::plumbing::force_query_with_job
  21: rustc_query_system::query::plumbing::get_query_impl
  22: <rustc_query_impl::Queries as rustc_middle::ty::query::QueryEngine>::mir_borrowck
  23: rustc_middle::ty::<impl rustc_middle::ty::context::TyCtxt>::par_body_owners
  24: rustc_interface::passes::analysis
  25: rustc_middle::dep_graph::<impl rustc_query_system::dep_graph::DepKind for rustc_middle::dep_graph::dep_node::DepKind>::with_deps
  26: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  27: rustc_query_system::dep_graph::graph::DepGraph<K>::with_eval_always_task
  28: rustc_data_structures::stack::ensure_sufficient_stack
  29: rustc_query_system::query::plumbing::force_query_with_job
  30: rustc_query_system::query::plumbing::get_query_impl
  31: <rustc_query_impl::Queries as rustc_middle::ty::query::QueryEngine>::analysis
  32: rustc_interface::passes::QueryContext::enter
  33: rustc_interface::queries::<impl rustc_interface::interface::Compiler>::enter
  34: rustc_span::with_source_map
  35: rustc_interface::interface::create_compiler_and_run
  36: scoped_tls::ScopedKey<T>::set
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

[chorman0773]

I got the same issue on the latest nightly and on play.rust-lang.org, without using rustc-private

// lazy_static::lazy_static! {
//     static ref LOADERS: Vec<&'static u8> = Vec::new();
// }

const LOADERS: &Vec<&'static u8> = &Vec::new();

pub fn break_code() -> Option<&'static u8> {
    for loader in *LOADERS {
        return Some(loader);
    }
    None
}

The commented out lazy_static! block was used originally and triggers the ICE as well.

[SkiFire13]

searched nightlies: from nightly-2021-07-29 to nightly-2021-08-02
regressed nightly: nightly-2021-07-31
searched commits: from 4927238 to 1f0a591
regressed commit: 1f0a591

bisected with cargo-bisect-rustc v0.6.0

Host triple: x86_64-pc-windows-msvc
Reproduce with:

cargo bisect-rustc --regress=ice

I think this might have been introduced by #87559, in particular this piece of code:

rust/compiler/rustc_mir/src/borrow_check/diagnostics/move_errors.rs

Lines 412 to 418 in d08460e

	Some(def_id) => type_known_to_meet_bound_modulo_regions(
	&self.infcx,
	self.param_env,
	self.infcx.tcx.mk_imm_ref(self.infcx.tcx.lifetimes.re_erased, ty),
	def_id,
	DUMMY_SP,
	),

[lqd]

If that's indeed caused by #87559 then we should let its author know: @estebank.

[estebank]

Just checked and thankfully it didn't land in beta.

[estebank]

@rustbot claim

[apiraino]

Assigning priority as discussed in the Zulip thread of the Prioritization Working Group.

@rustbot label -I-prioritize +P-high