ICE: error with const in 2021 closure capture

[ehuss]

The following ICE was found migrating rusb 0.8.1 to 2021. I believe it (or something similar) was found in object 0.26.0 and curl 0.4.38.

cc #87426 which was an error on the same line, though I think the cause is slightly different.

Note that I think this is also triggering an incorrect unsued lint for the event parameter (the actual code uses the parameter).

Code

const LIBUSB_HOTPLUG_EVENT_DEVICE_ARRIVED: i32 = 0x01;
const LIBUSB_HOTPLUG_EVENT_DEVICE_LEFT: i32 = 0x02;

pub fn hotplug_callback(event: i32) {
    let _ = || {
        match event {
            LIBUSB_HOTPLUG_EVENT_DEVICE_ARRIVED => (),
            LIBUSB_HOTPLUG_EVENT_DEVICE_LEFT => (),
            _ => (),
        };
    };
}

Meta

rustc --version --verbose:

rustc 1.56.0-nightly (ccffcafd5 2021-08-11)
binary: rustc
commit-hash: ccffcafd55e58f769d4b0efc0064bf65e76998e4
commit-date: 2021-08-11
host: x86_64-apple-darwin
release: 1.56.0-nightly
LLVM version: 12.0.1

Error output

warning: unused variable: `event`
 --> src/lib.rs:4:25
  |
4 | pub fn hotplug_callback(event: i32) {
  |                         ^^^^^ help: if this is intentional, prefix it with an underscore: `_event`
  |
  = note: `#[warn(unused_variables)]` on by default

thread 'rustc' panicked at 'called `Option::unwrap()` on a `None` value', compiler/rustc_mir_build/src/build/matches/mod.rs:1685:53

Backtrace

stack backtrace:
   0: _rust_begin_unwind
   1: core::panicking::panic_fmt
   2: core::panicking::panic
   3: rustc_mir_build::build::matches::<impl rustc_mir_build::build::Builder>::bind_and_guard_matched_candidate
   4: rustc_mir_build::build::matches::<impl rustc_mir_build::build::Builder>::bind_pattern
   5: rustc_mir_build::build::scope::<impl rustc_mir_build::build::Builder>::in_scope
   6: <core::iter::adapters::map::Map<I,F> as core::iter::traits::iterator::Iterator>::fold
   7: alloc::vec::source_iter_marker::<impl alloc::vec::spec_from_iter::SpecFromIter<T,I> for alloc::vec::Vec<T>>::from_iter
   8: rustc_mir_build::build::expr::into::<impl rustc_mir_build::build::Builder>::expr_into_dest
   9: rustc_mir_build::build::expr::as_temp::<impl rustc_mir_build::build::Builder>::as_temp_inner
  10: rustc_mir_build::build::expr::stmt::<impl rustc_mir_build::build::Builder>::stmt_expr
  11: rustc_mir_build::build::expr::stmt::<impl rustc_mir_build::build::Builder>::stmt_expr
  12: rustc_mir_build::build::block::<impl rustc_mir_build::build::Builder>::ast_block_stmts
  13: rustc_mir_build::build::expr::into::<impl rustc_mir_build::build::Builder>::expr_into_dest
  14: rustc_mir_build::build::scope::<impl rustc_mir_build::build::Builder>::in_scope
  15: rustc_data_structures::stack::ensure_sufficient_stack
  16: rustc_mir_build::build::expr::into::<impl rustc_mir_build::build::Builder>::expr_into_dest
  17: rustc_mir_build::build::scope::<impl rustc_mir_build::build::Builder>::in_scope
  18: rustc_data_structures::stack::ensure_sufficient_stack
  19: rustc_mir_build::build::expr::into::<impl rustc_mir_build::build::Builder>::expr_into_dest
  20: rustc_mir_build::build::construct_fn
  21: rustc_infer::infer::InferCtxtBuilder::enter
  22: rustc_mir_build::build::mir_built
  23: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  24: rustc_data_structures::stack::ensure_sufficient_stack
  25: rustc_query_system::query::plumbing::get_query_impl
  26: <rustc_query_impl::Queries as rustc_middle::ty::query::QueryEngine>::mir_built
  27: rustc_mir::transform::check_unsafety::unsafety_check_result
  28: core::ops::function::FnOnce::call_once
  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>::unsafety_check_result
  34: <rustc_mir::transform::check_unsafety::UnsafetyChecker as rustc_middle::mir::visit::Visitor>::visit_rvalue
  35: rustc_mir::transform::check_unsafety::unsafety_check_result
  36: core::ops::function::FnOnce::call_once
  37: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  38: rustc_data_structures::stack::ensure_sufficient_stack
  39: rustc_query_system::query::plumbing::force_query_with_job
  40: rustc_query_system::query::plumbing::get_query_impl
  41: <rustc_query_impl::Queries as rustc_middle::ty::query::QueryEngine>::unsafety_check_result
  42: rustc_mir::transform::mir_const
  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::get_query_impl
  46: <rustc_query_impl::Queries as rustc_middle::ty::query::QueryEngine>::mir_const
  47: rustc_mir::transform::mir_promoted
  48: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  49: rustc_data_structures::stack::ensure_sufficient_stack
  50: rustc_query_system::query::plumbing::get_query_impl
  51: <rustc_query_impl::Queries as rustc_middle::ty::query::QueryEngine>::mir_promoted
  52: rustc_mir::borrow_check::mir_borrowck
  53: core::ops::function::FnOnce::call_once
  54: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  55: rustc_data_structures::stack::ensure_sufficient_stack
  56: rustc_query_system::query::plumbing::force_query_with_job
  57: rustc_query_system::query::plumbing::get_query_impl
  58: <rustc_query_impl::Queries as rustc_middle::ty::query::QueryEngine>::mir_borrowck
  59: rustc_middle::ty::<impl rustc_middle::ty::context::TyCtxt>::par_body_owners
  60: rustc_interface::passes::analysis
  61: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  62: rustc_data_structures::stack::ensure_sufficient_stack
  63: rustc_query_system::query::plumbing::force_query_with_job
  64: rustc_query_system::query::plumbing::get_query_impl
  65: <rustc_query_impl::Queries as rustc_middle::ty::query::QueryEngine>::analysis
  66: rustc_interface::queries::<impl rustc_interface::interface::Compiler>::enter
  67: rustc_span::with_source_map
  68: rustc_interface::interface::create_compiler_and_run
  69: 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 (ccffcafd5 2021-08-11) running on x86_64-apple-darwin

note: compiler flags: -Z unstable-options -C embed-bitcode=no -C split-debuginfo=unpacked -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_built] building MIR for `hotplug_callback::{closure#0}`
#1 [unsafety_check_result] unsafety-checking `hotplug_callback::{closure#0}`
#2 [unsafety_check_result] unsafety-checking `hotplug_callback`
#3 [mir_const] processing MIR for `hotplug_callback`
#4 [mir_promoted] processing `hotplug_callback`
#5 [mir_borrowck] borrow-checking `hotplug_callback`
#6 [analysis] running analysis passes on this crate
end of query stack

cc @rust-lang/wg-rfc-2229

[roxelo]

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