ICE when debug-printing an instance

[RalfJung]

Even with #55199 landed, we still get ICEs when printing some instances. This is witnessed by applying this patch. Subsequently, building libcore fails.

Show backtrace

Building stage1 std artifacts (x86_64-unknown-linux-gnu -> x86_64-unknown-linux-gnu)
   Compiling core v0.0.0 (/home/r/src/rust/rustc.2/src/libcore)                                                                                                                                                    
thread 'main' panicked at 'index out of bounds: the len is 0 but the index is 0', librustc/ty/subst.rs:268:41                                                                                                      
stack backtrace:
   0: std::sys::unix::backtrace::tracing::imp::unwind_backtrace
             at libstd/sys/unix/backtrace/tracing/gcc_s.rs:49
   1: std::sys_common::backtrace::print
             at libstd/sys_common/backtrace.rs:71
             at libstd/sys_common/backtrace.rs:59
   2: std::panicking::default_hook::{{closure}}
             at libstd/panicking.rs:211
   3: std::panicking::default_hook
             at libstd/panicking.rs:227
   4: rustc::util::common::panic_hook
             at librustc/util/common.rs:51
   5: std::panicking::rust_panic_with_hook
             at libstd/panicking.rs:480
   6: std::panicking::continue_panic_fmt
             at libstd/panicking.rs:390
   7: rust_begin_unwind
             at libstd/panicking.rs:325
   8: core::panicking::panic_fmt
             at libcore/panicking.rs:77
   9: core::panicking::panic_bounds_check
             at libcore/panicking.rs:59
  10: rustc::ty::context::tls::with
             at librustc/ty/subst.rs:268
             at librustc/util/ppaux.rs:322
             at ./libcore/result.rs:647
             at librustc/util/ppaux.rs:322
             at librustc/ty/context.rs:2168
             at librustc/ty/context.rs:2122
             at librustc/ty/context.rs:2113
             at librustc/ty/context.rs:2122
             at librustc/ty/context.rs:2168
  11: rustc::util::ppaux::PrintContext::parameterized
             at librustc/util/ppaux.rs:265
  12: rustc::util::ppaux::parameterized
             at librustc/util/ppaux.rs:563
  13: <rustc::ty::instance::Instance<'tcx> as core::fmt::Display>::fmt
             at librustc/ty/instance.rs:185
  14: core::fmt::write
             at libcore/fmt/mod.rs:1102
             at libcore/fmt/mod.rs:1049
  15: rustc_mir::const_eval::const_eval_raw_provider
             at ./libcore/fmt/mod.rs:226
             at ./liballoc/string.rs:2150
             at librustc_mir/const_eval.rs:617
  16: rustc::ty::query::__query_compute::const_eval_raw
             at librustc/ty/query/plumbing.rs:834
             at librustc/ty/query/plumbing.rs:796
  17: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors<'tcx> for rustc::ty::query::queries::const_eval_raw<'tcx>>::compute
             at librustc/ty/query/plumbing.rs:826
  18: rustc::ty::context::tls::with_context
             at librustc/dep_graph/graph.rs:275
             at librustc/ty/context.rs:2038
             at librustc/ty/context.rs:1977
             at librustc/ty/context.rs:2037
             at librustc/dep_graph/graph.rs:274
             at librustc/ty/context.rs:2122
             at librustc/ty/context.rs:2113
             at librustc/ty/context.rs:2122
  19: rustc::dep_graph::graph::DepGraph::with_task_impl
             at librustc/dep_graph/graph.rs:268
  20: rustc::ty::context::tls::with_related_context
             at librustc/dep_graph/graph.rs:208
             at librustc/ty/query/plumbing.rs:550
             at librustc/ty/query/plumbing.rs:208
             at librustc/ty/context.rs:2038
             at librustc/ty/context.rs:1977
             at librustc/ty/context.rs:2037
             at librustc/ty/query/plumbing.rs:207
             at librustc/ty/context.rs:2138
             at librustc/ty/context.rs:2122
             at librustc/ty/context.rs:2113
             at librustc/ty/context.rs:2122
             at librustc/ty/context.rs:2133
  21: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::force_query_with_job
             at librustc/ty/query/plumbing.rs:197
             at librustc/ty/query/plumbing.rs:543
  22: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::get_query
             at librustc/ty/query/plumbing.rs:424
             at librustc/ty/query/plumbing.rs:629
             at librustc/ty/query/plumbing.rs:640
  23: rustc::ty::query::<impl rustc::ty::context::TyCtxt<'a, 'tcx, 'lcx>>::const_eval_raw
             at librustc/ty/query/plumbing.rs:888
             at librustc/ty/query/plumbing.rs:881
  24: rustc_mir::const_eval::const_eval_provider
             at librustc_mir/const_eval.rs:584
  25: rustc::ty::query::__query_compute::const_eval
             at librustc/ty/query/plumbing.rs:834
             at librustc/ty/query/plumbing.rs:796
  26: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors<'tcx> for rustc::ty::query::queries::const_eval<'tcx>>::compute
             at librustc/ty/query/plumbing.rs:826
  27: rustc::ty::context::tls::with_context
             at librustc/dep_graph/graph.rs:275
             at librustc/ty/context.rs:2038
             at librustc/ty/context.rs:1977
             at librustc/ty/context.rs:2037
             at librustc/dep_graph/graph.rs:274
             at librustc/ty/context.rs:2122
             at librustc/ty/context.rs:2113
             at librustc/ty/context.rs:2122
  28: rustc::dep_graph::graph::DepGraph::with_task_impl
             at librustc/dep_graph/graph.rs:268
  29: rustc::ty::context::tls::with_related_context
             at librustc/dep_graph/graph.rs:208
             at librustc/ty/query/plumbing.rs:550
             at librustc/ty/query/plumbing.rs:208
             at librustc/ty/context.rs:2038
             at librustc/ty/context.rs:1977
             at librustc/ty/context.rs:2037
             at librustc/ty/query/plumbing.rs:207
             at librustc/ty/context.rs:2138
             at librustc/ty/context.rs:2122
             at librustc/ty/context.rs:2113
             at librustc/ty/context.rs:2122
             at librustc/ty/context.rs:2133
  30: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::force_query_with_job
             at librustc/ty/query/plumbing.rs:197
             at librustc/ty/query/plumbing.rs:543
  31: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::get_query
             at librustc/ty/query/plumbing.rs:424
             at librustc/ty/query/plumbing.rs:629
             at librustc/ty/query/plumbing.rs:640
  32: <rustc::traits::project::AssociatedTypeNormalizer<'a, 'b, 'gcx, 'tcx> as rustc::ty::fold::TypeFolder<'gcx, 'tcx>>::fold_const
             at librustc/ty/query/plumbing.rs:888
             at librustc/ty/query/plumbing.rs:881
             at librustc/traits/project.rs:451
  33: rustc::ty::structural_impls::<impl rustc::ty::fold::TypeFoldable<'tcx> for &'tcx rustc::ty::TyS<'tcx>>::super_fold_with
             at librustc/ty/structural_impls.rs:1043
             at librustc/ty/structural_impls.rs:712
  34: <rustc::traits::project::AssociatedTypeNormalizer<'a, 'b, 'gcx, 'tcx> as rustc::ty::fold::TypeFolder<'gcx, 'tcx>>::fold_ty
             at librustc/traits/project.rs:367
  35: rustc::traits::project::normalize
             at ./librustc/ty/structural_impls.rs:751
             at ./librustc/traits/project.rs:345
             at ./librustc/traits/project.rs:304
             at ./librustc/traits/project.rs:288
  36: rustc::infer::InferCtxt::partially_normalize_associated_types_in
             at ./librustc/infer/mod.rs:1466
  37: <rustc_typeck::check::FnCtxt<'a, 'gcx, 'tcx> as rustc_typeck::astconv::AstConv<'gcx, 'tcx>>::normalize_ty
             at librustc_typeck/check/mod.rs:685
             at librustc_typeck/check/mod.rs:2311
             at librustc_typeck/check/mod.rs:1934
  38: <(dyn rustc_typeck::astconv::AstConv<'gcx, 'tcx> + 'o)>::ast_ty_to_ty
             at librustc_typeck/astconv.rs:1506
  39: <(dyn rustc_typeck::astconv::AstConv<'gcx, 'tcx> + 'o)>::create_substs_for_generic_args
             at librustc_typeck/check/mod.rs:2355
             at librustc_typeck/check/mod.rs:5162
             at librustc_typeck/astconv.rs:502
  40: rustc_typeck::check::FnCtxt::instantiate_value_path
             at librustc_typeck/check/mod.rs:5131
  41: rustc_typeck::check::FnCtxt::check_expr_kind
             at librustc_typeck/check/mod.rs:3934
  42: rustc_typeck::check::FnCtxt::check_expr_with_expectation_and_needs
             at librustc_typeck/check/mod.rs:3755
  43: rustc_typeck::check::FnCtxt::check_expr_kind
             at librustc_typeck/check/mod.rs:3063
             at librustc_typeck/check/mod.rs:3067
             at librustc_typeck/check/callee.rs:52
             at librustc_typeck/check/mod.rs:4166
  44: rustc_typeck::check::FnCtxt::check_expr_with_expectation_and_needs
             at librustc_typeck/check/mod.rs:3755
  45: rustc_typeck::check::FnCtxt::with_breakable_ctxt
             at librustc_typeck/check/mod.rs:3063
             at librustc_typeck/check/mod.rs:4612
             at ./libcore/option.rs:424
             at librustc_typeck/check/mod.rs:4612
             at librustc_typeck/check/mod.rs:5293
  46: rustc_typeck::check::FnCtxt::check_block_with_expected
             at librustc_typeck/check/mod.rs:4605
  47: rustc_typeck::check::FnCtxt::check_expr_kind
             at librustc_typeck/check/mod.rs:4163
  48: rustc_typeck::check::FnCtxt::check_expr_with_expectation_and_needs
             at librustc_typeck/check/mod.rs:3755
  49: rustc_typeck::check::FnCtxt::check_expr_kind
             at librustc_typeck/check/mod.rs:3908
  50: rustc_typeck::check::FnCtxt::check_expr_with_expectation_and_needs
             at librustc_typeck/check/mod.rs:3755
  51: rustc_typeck::check::FnCtxt::check_argument_types
             at librustc_typeck/check/mod.rs:3063
             at librustc_typeck/check/mod.rs:2888
  52: rustc_typeck::check::FnCtxt::check_method_argument_types
             at librustc_typeck/check/mod.rs:2711
  53: rustc_typeck::check::FnCtxt::check_expr_kind
             at librustc_typeck/check/mod.rs:3179
             at librustc_typeck/check/mod.rs:4169
  54: rustc_typeck::check::FnCtxt::check_expr_with_expectation_and_needs
             at librustc_typeck/check/mod.rs:3755
  55: rustc_typeck::check::FnCtxt::with_breakable_ctxt
             at librustc_typeck/check/mod.rs:3063
             at librustc_typeck/check/mod.rs:4612
             at ./libcore/option.rs:424
             at librustc_typeck/check/mod.rs:4612
             at librustc_typeck/check/mod.rs:5293
  56: rustc_typeck::check::FnCtxt::check_block_with_expected
             at librustc_typeck/check/mod.rs:4605
  57: rustc_typeck::check::FnCtxt::check_expr_kind
             at librustc_typeck/check/mod.rs:4163
  58: rustc_typeck::check::FnCtxt::check_expr_with_expectation_and_needs
             at librustc_typeck/check/mod.rs:3755
  59: rustc_typeck::check::FnCtxt::check_return_expr
             at librustc_typeck/check/mod.rs:3063
             at librustc_typeck/check/mod.rs:3057
             at librustc_typeck/check/mod.rs:3195
  60: rustc_typeck::check::check_fn
             at librustc_typeck/check/mod.rs:1103
  61: rustc::ty::context::tls::with_context::{{closure}}
             at librustc_typeck/check/mod.rs:866
             at librustc_typeck/check/mod.rs:621
             at ./librustc/infer/mod.rs:526
             at ./librustc/ty/context.rs:1674
             at ./librustc/ty/context.rs:2038
             at ./librustc/ty/context.rs:1977
             at ./librustc/ty/context.rs:2037
             at ./librustc/ty/context.rs:1673
             at ./librustc/ty/context.rs:2138
             at ./librustc/ty/context.rs:2122
  62: rustc::ty::context::GlobalCtxt::enter_local
             at ./librustc/ty/context.rs:2113
             at ./librustc/ty/context.rs:2122
             at ./librustc/ty/context.rs:2133
             at ./librustc/ty/context.rs:1666
  63: rustc::infer::InferCtxtBuilder::enter
             at ./librustc/infer/mod.rs:525
  64: rustc_typeck::check::typeck_tables_of
             at librustc_typeck/check/mod.rs:621
             at librustc_typeck/check/mod.rs:850
  65: rustc::ty::query::__query_compute::typeck_tables_of
             at librustc/ty/query/plumbing.rs:834
             at librustc/ty/query/plumbing.rs:796
  66: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors<'tcx> for rustc::ty::query::queries::typeck_tables_of<'tcx>>::compute
             at librustc/ty/query/plumbing.rs:826
  67: rustc::ty::context::tls::with_context
             at librustc/dep_graph/graph.rs:275
             at librustc/ty/context.rs:2038
             at librustc/ty/context.rs:1977
             at librustc/ty/context.rs:2037
             at librustc/dep_graph/graph.rs:274
             at librustc/ty/context.rs:2122
             at librustc/ty/context.rs:2113
             at librustc/ty/context.rs:2122
  68: rustc::dep_graph::graph::DepGraph::with_task_impl
             at librustc/dep_graph/graph.rs:268
  69: rustc::ty::context::tls::with_related_context
             at librustc/dep_graph/graph.rs:208
             at librustc/ty/query/plumbing.rs:550
             at librustc/ty/query/plumbing.rs:208
             at librustc/ty/context.rs:2038
             at librustc/ty/context.rs:1977
             at librustc/ty/context.rs:2037
             at librustc/ty/query/plumbing.rs:207
             at librustc/ty/context.rs:2138
             at librustc/ty/context.rs:2122
             at librustc/ty/context.rs:2113
             at librustc/ty/context.rs:2122
             at librustc/ty/context.rs:2133
  70: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::force_query_with_job
             at librustc/ty/query/plumbing.rs:197
             at librustc/ty/query/plumbing.rs:543
  71: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::get_query
             at librustc/ty/query/plumbing.rs:424
             at librustc/ty/query/plumbing.rs:629
             at librustc/ty/query/plumbing.rs:640
  72: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::ensure_query
             at librustc/ty/query/plumbing.rs:602
  73: rustc::ty::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::par_body_owners
             at librustc_typeck/check/mod.rs:723
             at ./librustc/ty/mod.rs:2553
             at ./libcore/iter/iterator.rs:614
             at ./libcore/slice/mod.rs:2906
             at ./libcore/iter/iterator.rs:614
             at ./librustc/ty/mod.rs:2552
  74: rustc_typeck::check::typeck_item_bodies
             at librustc_typeck/check/mod.rs:722
             at ./librustc/session/mod.rs:323
             at librustc_typeck/check/mod.rs:721
  75: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors<'tcx> for rustc::ty::query::queries::typeck_item_bodies<'tcx>>::compute
             at librustc/ty/query/plumbing.rs:826
  76: rustc::ty::context::tls::with_context
             at librustc/dep_graph/graph.rs:275
             at librustc/ty/context.rs:2038
             at librustc/ty/context.rs:1977
             at librustc/ty/context.rs:2037
             at librustc/dep_graph/graph.rs:274
             at librustc/ty/context.rs:2122
             at librustc/ty/context.rs:2113
             at librustc/ty/context.rs:2122
  77: rustc::dep_graph::graph::DepGraph::with_task_impl
             at librustc/dep_graph/graph.rs:268
  78: rustc::ty::context::tls::with_related_context
             at librustc/dep_graph/graph.rs:208
             at librustc/ty/query/plumbing.rs:550
             at librustc/ty/query/plumbing.rs:208
             at librustc/ty/context.rs:2038
             at librustc/ty/context.rs:1977
             at librustc/ty/context.rs:2037
             at librustc/ty/query/plumbing.rs:207
             at librustc/ty/context.rs:2138
             at librustc/ty/context.rs:2122
             at librustc/ty/context.rs:2113
             at librustc/ty/context.rs:2122
             at librustc/ty/context.rs:2133
  79: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::force_query_with_job
             at librustc/ty/query/plumbing.rs:197
             at librustc/ty/query/plumbing.rs:543
  80: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::get_query
             at librustc/ty/query/plumbing.rs:424
             at librustc/ty/query/plumbing.rs:629
             at librustc/ty/query/plumbing.rs:640
  81: rustc::util::common::time
             at librustc_typeck/check/mod.rs:714
             at librustc_typeck/lib.rs:360
             at ./librustc/util/common.rs:163
             at ./librustc/util/common.rs:157
  82: rustc_typeck::check_crate
             at librustc_typeck/lib.rs:360
  83: rustc::ty::context::tls::enter_context
             at librustc_driver/driver.rs:1289
             at ./librustc/ty/context.rs:2070
             at ./librustc/ty/context.rs:2038
             at ./librustc/ty/context.rs:1977
             at ./librustc/ty/context.rs:2037
  84: <std::thread::local::LocalKey<T>>::with
             at ./librustc/ty/context.rs:2069
             at ./librustc/ty/context.rs:2027
             at ./libstd/thread/local.rs:300
             at ./libstd/thread/local.rs:254
             at ./librustc/ty/context.rs:2019
             at ./libstd/thread/local.rs:300
             at ./libstd/thread/local.rs:254
  85: rustc::ty::context::TyCtxt::create_and_enter
             at ./librustc/ty/context.rs:2011
             at ./librustc/ty/context.rs:2049
             at ./librustc/ty/context.rs:1247
  86: rustc_driver::driver::phase_3_run_analysis_passes
             at librustc_driver/driver.rs:1263
  87: rustc_driver::driver::compile_input
             at librustc_driver/driver.rs:287
  88: rustc_driver::run_compiler_with_pool
             at librustc_driver/lib.rs:544
  89: <scoped_tls::ScopedKey<T>>::set
             at librustc_driver/lib.rs:466
             at librustc_driver/driver.rs:76
             at /home/r/.cargo/registry/src/github.com-1ecc6299db9ec823/scoped-tls-0.1.2/src/lib.rs:155
  90: rustc_driver::run_compiler
             at librustc_driver/driver.rs:75
             at librustc_driver/lib.rs:465
  91: <scoped_tls::ScopedKey<T>>::set
             at librustc_driver/lib.rs:1717
             at librustc_driver/lib.rs:181
             at /home/r/.cargo/registry/src/github.com-1ecc6299db9ec823/scoped-tls-0.1.2/src/lib.rs:155
             at ./libsyntax/lib.rs:106
             at /home/r/.cargo/registry/src/github.com-1ecc6299db9ec823/scoped-tls-0.1.2/src/lib.rs:155
  92: std::panicking::try::do_call
             at ./libsyntax/lib.rs:105
             at librustc_driver/lib.rs:180
             at librustc_driver/lib.rs:1632
             at ./libstd/panic.rs:313
             at ./libstd/panicking.rs:310
  93: __rust_maybe_catch_panic
             at libpanic_unwind/lib.rs:102
  94: std::panicking::try
             at ./libstd/panicking.rs:289
  95: rustc_driver::run
             at ./libstd/panic.rs:392
             at librustc_driver/lib.rs:1546
             at librustc_driver/lib.rs:1557
             at librustc_driver/lib.rs:1631
             at librustc_driver/lib.rs:179
  96: rustc_driver::main
             at librustc_driver/lib.rs:1710
  97: std::rt::lang_start::{{closure}}
             at ./libstd/rt.rs:74
  98: std::panicking::try::do_call
             at libstd/rt.rs:59
             at libstd/panicking.rs:310
  99: __rust_maybe_catch_panic
             at libpanic_unwind/lib.rs:102
query stack during panic:
#0 [const_eval_raw] const-evaluating `hash::Hasher::write_u16::{{constant}}`
#1 [const_eval] const-evaluating + checking `hash::Hasher::write_u16::{{constant}}`
#2 [typeck_tables_of] processing `hash::Hasher::write_u16`
#3 [typeck_item_bodies] type-checking all item bodies
end of query stack

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 1.31.0-dev running on x86_64-unknown-linux-gnu

note: compiler flags: -Z always-encode-mir -Z mir-emit-validate=1 -Z force-unstable-if-unmarked -C opt-level=2 -C incremental -C prefer-dynamic -C debuginfo=1 -C debug-assertions=y -C link-args=-Wl,-rpath,$ORIGIN/../lib --crate-type lib

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

error: Could not compile `core`.                                                                                                                                                                                   

Cc @oli-obk

[RalfJung]

The offending out-of-bounds access is at

rust/src/librustc/util/ppaux.rs

Lines 321 to 323 in 4e88b73

	if has_self {
	print!(f, self, write("<"), print_display(substs.type_at(0)), write(" as "))?;
	}

[RalfJung]

@oli-obk Any idea what this could be caused by? Any hints where I could start debugging this?

[oli-obk]

Nope, sorry. It's been in my queue to address, but low in it.

From the snippet you posted I'm assuming that the has_self computation is wrong. The query path is hash::Hasher::write_u16::{{constant}}, I'd start looking at which constants are available. I'm assuming the code believes it has Self since we are in a method impl. But constants other than associated constants don't have this.

[RalfJung]

I heard that @eddyb is working on refactoring this printing code, maybe even fixing this issue along the way. Is there any place to follow that progress?

@eddyb it'd be great if you could inline something like this patch if/when you did that refactoring, that'll ensure (in a debug build) that we don't ICE any more when printing constants' instances.

[RalfJung]

This will be fixed by #58140