ICE 'index out of bounds: the len is 0 but the index is 0', src/librustc_typeck/check/pat.rs:729:23

[najamelan]

commit that introduced ICE: najamelan/pharos@fa9b467

The ICE happens on cargo check.
The most fundamental change here I imagine is introducing continue inside the match...

Backtrace

thread 'rustc' panicked at 'index out of bounds: the len is 0 but the index is 0', src/librustc_typeck/check/pat.rs:729:23
stack backtrace:
   0: backtrace::backtrace::libunwind::trace
             at /cargo/registry/src/github.aaakk.us.kg-1ecc6299db9ec823/backtrace-0.3.37/src/backtrace/libunwind.rs:88
   1: backtrace::backtrace::trace_unsynchronized
             at /cargo/registry/src/github.aaakk.us.kg-1ecc6299db9ec823/backtrace-0.3.37/src/backtrace/mod.rs:66
   2: std::sys_common::backtrace::_print_fmt
             at src/libstd/sys_common/backtrace.rs:76
   3: <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt
             at src/libstd/sys_common/backtrace.rs:60
   4: core::fmt::write
             at src/libcore/fmt/mod.rs:1030
   5: std::io::Write::write_fmt
             at src/libstd/io/mod.rs:1412
   6: std::sys_common::backtrace::_print
             at src/libstd/sys_common/backtrace.rs:64
   7: std::sys_common::backtrace::print
             at src/libstd/sys_common/backtrace.rs:49
   8: std::panicking::default_hook::{{closure}}
             at src/libstd/panicking.rs:196
   9: std::panicking::default_hook
             at src/libstd/panicking.rs:210
  10: rustc_driver::report_ice
  11: std::panicking::rust_panic_with_hook
             at src/libstd/panicking.rs:477
  12: std::panicking::continue_panic_fmt
             at src/libstd/panicking.rs:380
  13: rust_begin_unwind
             at src/libstd/panicking.rs:307
  14: core::panicking::panic_fmt
             at src/libcore/panicking.rs:85
  15: core::panicking::panic_bounds_check
             at src/libcore/panicking.rs:61
  16: rustc_typeck::check::pat::<impl rustc_typeck::check::FnCtxt>::check_pat_tuple_struct
  17: rustc_typeck::check::pat::<impl rustc_typeck::check::FnCtxt>::check_pat
  18: rustc_typeck::check::pat::<impl rustc_typeck::check::FnCtxt>::check_pat_tuple_struct
  19: rustc_typeck::check::pat::<impl rustc_typeck::check::FnCtxt>::check_pat
  20: <core::iter::adapters::Map<I,F> as core::iter::traits::iterator::Iterator>::fold
  21: rustc_typeck::check::_match::<impl rustc_typeck::check::FnCtxt>::check_match
  22: rustc_typeck::check::expr::<impl rustc_typeck::check::FnCtxt>::check_expr_kind
  23: rustc_typeck::check::expr::<impl rustc_typeck::check::FnCtxt>::check_expr_with_expectation_and_needs
  24: rustc_typeck::check::FnCtxt::check_block_with_expected
  25: rustc_typeck::check::expr::<impl rustc_typeck::check::FnCtxt>::check_expr_kind
  26: rustc_typeck::check::expr::<impl rustc_typeck::check::FnCtxt>::check_expr_with_expectation_and_needs
  27: rustc_typeck::check::_match::<impl rustc_typeck::check::FnCtxt>::check_match
  28: rustc_typeck::check::expr::<impl rustc_typeck::check::FnCtxt>::check_expr_kind
  29: rustc_typeck::check::expr::<impl rustc_typeck::check::FnCtxt>::check_expr_with_expectation_and_needs
  30: rustc_typeck::check::FnCtxt::check_block_with_expected
  31: rustc_typeck::check::expr::<impl rustc_typeck::check::FnCtxt>::check_expr_kind
  32: rustc_typeck::check::expr::<impl rustc_typeck::check::FnCtxt>::check_expr_with_expectation_and_needs
  33: rustc_typeck::check::expr::<impl rustc_typeck::check::FnCtxt>::check_expr_meets_expectation_or_error
  34: rustc_typeck::check::FnCtxt::check_stmt
  35: rustc_typeck::check::FnCtxt::check_block_with_expected
  36: rustc_typeck::check::expr::<impl rustc_typeck::check::FnCtxt>::check_expr_kind
  37: rustc_typeck::check::expr::<impl rustc_typeck::check::FnCtxt>::check_expr_with_expectation_and_needs
  38: rustc_typeck::check::_match::<impl rustc_typeck::check::FnCtxt>::check_match
  39: rustc_typeck::check::expr::<impl rustc_typeck::check::FnCtxt>::check_expr_kind
  40: rustc_typeck::check::expr::<impl rustc_typeck::check::FnCtxt>::check_expr_with_expectation_and_needs
  41: rustc_typeck::check::expr::<impl rustc_typeck::check::FnCtxt>::check_expr_kind
  42: rustc_typeck::check::expr::<impl rustc_typeck::check::FnCtxt>::check_expr_with_expectation_and_needs
  43: rustc_typeck::check::expr::<impl rustc_typeck::check::FnCtxt>::check_expr_meets_expectation_or_error
  44: rustc_typeck::check::FnCtxt::check_stmt
  45: rustc_typeck::check::FnCtxt::check_block_with_expected
  46: rustc_typeck::check::expr::<impl rustc_typeck::check::FnCtxt>::check_expr_kind
  47: rustc_typeck::check::expr::<impl rustc_typeck::check::FnCtxt>::check_expr_with_expectation_and_needs
  48: rustc_typeck::check::expr::<impl rustc_typeck::check::FnCtxt>::check_return_expr
  49: rustc_typeck::check::check_fn
  50: rustc::ty::context::GlobalCtxt::enter_local
  51: rustc_typeck::check::typeck_tables_of
  52: rustc::ty::query::__query_compute::typeck_tables_of
  53: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::typeck_tables_of>::compute
  54: rustc::dep_graph::graph::DepGraph::with_task_impl
  55: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  56: rustc::ty::<impl rustc::ty::context::TyCtxt>::par_body_owners
  57: rustc_typeck::check::typeck_item_bodies
  58: rustc::ty::query::__query_compute::typeck_item_bodies
  59: rustc::dep_graph::graph::DepGraph::with_task_impl
  60: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  61: rustc::util::common::time
  62: rustc_typeck::check_crate
  63: rustc_interface::passes::analysis
  64: rustc::ty::query::__query_compute::analysis
  65: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  66: rustc_interface::passes::BoxedGlobalCtxt::access::{{closure}}
  67: rustc_interface::passes::create_global_ctxt::{{closure}}
  68: rustc_interface::interface::run_compiler_in_existing_thread_pool
  69: std::thread::local::LocalKey<T>::with
  70: scoped_tls::ScopedKey<T>::set
  71: syntax::with_globals
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/blob/master/CONTRIBUTING.md#bug-reports

note: rustc 1.39.0-nightly (66bf391c3 2019-09-23) running on x86_64-unknown-linux-gnu

note: compiler flags: -C debuginfo=2 -C incremental --crate-type lib

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

query stack during panic:
#0 [typeck_tables_of] processing `<pharos::Pharos<Event> as futures_sink::Sink<Event>>::poll_flush`
#1 [typeck_item_bodies] type-checking all item bodies
#2 [analysis] running analysis passes on this crate
end of query stack

[jonas-schievink]

Can reproduce on the current nightly (rustc 1.39.0-nightly (66bf391 2019-09-23)), but not on rustc 1.39.0-nightly (9b91b9c 2019-08-26), so it's a regression.

[najamelan]

Ok, this tries to eliminate stuff, but it doesn't fix the ICE: najamelan/pharos@8b1db4c

Removing the match fixes the ICE.

[Centril]

Minimal reproducer:

struct A(());

fn main() {
    let A() = A(());
}

Problem was introduced in #64619.

Specifically the line:

rust/src/librustc_typeck/check/pat.rs

Line 729 in 6ef275e

vec![(subpats[0].span.shrink_to_lo(), "(".to_string()),

is the problem... Seems we neglected to account for the nullary tuple case (()). The fix is to account for the lack of any elements in deciding what spans to use.

[Centril]

cc @estebank @sam09

[nikomatsakis]

Can we use cargo-bisect-rustc to narrow this down to a specific PR?

NM

[estebank]

@Centril I see you self-assigned, just add && !subpats.is_empty() to the if expr.

[sam09]

@Centril In case you haven't started, I can help with this.

[Centril]

Investigating a possibly better solution than @estebank's for the zero case, I'll let you know how it pans out.

[Centril]

Fixed in #64748.