Panic in `region_scope_tree`: `assertion failed: prev.is_none()`

[danielhenrymantilla]

Context: "regression" caught by future-proofing CI from https://github.com/danielhenrymantilla/stackbox.rs/runs/3278204045?check_suite_focus=true

After working on reducing the code, I've identified the root cause of the ICE.

Code

macro_rules! two_items {() => (
    extern {}
    extern {}
)}

macro_rules! single_item_funneler {( $item:item ) => ( $item )}

fn inside_some_function() {
    single_item_funneler! { two_items! {} }
}

• For context: this patterns stem from "old" Rust code wanting to support proc-macros emitting item definitions in "statement" position for <1.45.0 Rust: the trick is to use the single_item_funneler to wrap an item-defs-emitting proc-macro call inside a function so that Rust doesn't interpret it as a statement, refusing the then-unstable stmt-emitting proc-macro call (minimal example).

Meta

rustc --version --verbose:

note: rustc 1.56.0-nightly (ad981d58e 2021-08-08) running on x86_64-apple-darwin

• Bonus: the ICE seems to have been introduced in between today's (👆) and yesterday's nightly.

Error output

thread 'rustc' panicked at 'assertion failed: prev.is_none()', compiler/rustc_middle/src/middle/region.rs:334:13

Backtrace

stack backtrace:
   0: _rust_begin_unwind
   1: core::panicking::panic_fmt
   2: core::panicking::panic
   3: rustc_middle::middle::region::ScopeTree::record_scope_parent
   4: <rustc_passes::region::RegionResolutionVisitor as rustc_hir::intravisit::Visitor>::visit_stmt
   5: <rustc_passes::region::RegionResolutionVisitor as rustc_hir::intravisit::Visitor>::visit_block
   6: rustc_passes::region::resolve_expr
   7: <rustc_passes::region::RegionResolutionVisitor as rustc_hir::intravisit::Visitor>::visit_body
   8: rustc_passes::region::region_scope_tree
   9: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  10: rustc_data_structures::stack::ensure_sufficient_stack
  11: rustc_query_system::query::plumbing::force_query_with_job
  12: rustc_query_system::query::plumbing::get_query_impl
  13: <rustc_query_impl::Queries as rustc_middle::ty::query::QueryEngine>::region_scope_tree
  14: rustc_typeck::check::regionck::RegionCtxt::new
  15: rustc_typeck::check::regionck::<impl rustc_typeck::check::fn_ctxt::FnCtxt>::regionck_item
  16: rustc_infer::infer::InferCtxtBuilder::enter
  17: rustc_typeck::check::wfcheck::check_item_well_formed
  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>::check_item_well_formed
  23: <rustc_typeck::check::wfcheck::CheckTypeWellFormedVisitor as rustc_hir::intravisit::Visitor>::visit_item
  24: <core::panic::unwind_safe::AssertUnwindSafe<F> as core::ops::function::FnOnce<()>>::call_once
  25: rustc_data_structures::sync::par_for_each_in
  26: rustc_session::session::Session::track_errors
  27: rustc_typeck::check_crate
  28: rustc_interface::passes::analysis
  29: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  30: rustc_data_structures::stack::ensure_sufficient_stack
  31: rustc_query_system::query::plumbing::force_query_with_job
  32: rustc_query_system::query::plumbing::get_query_impl
  33: <rustc_query_impl::Queries as rustc_middle::ty::query::QueryEngine>::analysis
  34: rustc_interface::queries::<impl rustc_interface::interface::Compiler>::enter
  35: rustc_span::with_source_map
  36: rustc_interface::interface::create_compiler_and_run
  37: scoped_tls::ScopedKey<T>::set
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 1.56.0-nightly (ad981d58e 2021-08-08) running on x86_64-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 [region_scope_tree] computing drop scopes for `main`
#1 [check_item_well_formed] checking that `main` is well-formed
#2 [analysis] running analysis passes on this crate
end of query stack

cc @Aaron1011 I suspect this is up your alley 😉

[hellow554]

searched nightlies: from nightly-2021-08-01 to nightly-2021-08-10
regressed nightly: nightly-2021-08-07
searched commits: from 2f07ae4 to 5ad7389
regressed commit: 5ad7389

bisected with cargo-bisect-rustc v0.6.0

Host triple: x86_64-unknown-linux-gnu
Reproduce with:

cargo bisect-rustc --end=2021-08-10 --regress=ice --access=github --without-cargo -- check

[apiraino]

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

@rustbot label -I-prioritize +P-high +A-macros