[debug assertion] thread 'rustc' panicked at 'assertion failed: !value.has_escaping_bound_vars()'

[dwrensha]

Using a rustc built from commit 853c477 with debug assertions enabled (debug=true in config.toml), I'm seeing an internal compiler error on the following input (found by fuzz-rustc):

pub trait Foo {}
pub trait FooExt where for<'b> &'b &mut Self: Foo {}
impl<T> FooExt for T where for<'a> &'a mut T: Foo {}

fn main() {}

$ RUST_BACKTRACE=1 build/x86_64-unknown-linux-gnu/stage1/bin/rustc main.rs
error[E0637]: `&` without an explicit lifetime name cannot be used here
 --> main.rs:2:36
  |
2 | pub trait FooExt where for<'b> &'b &mut Self: Foo {}
  |                                    ^ explicit lifetime name needed here

thread 'rustc' panicked at 'assertion failed: !value.has_escaping_bound_vars()', src/librustc_middle/ty/sty.rs:877:9
stack backtrace:
   0: backtrace::backtrace::libunwind::trace
             at /home/dwrensha/.cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.46/src/backtrace/libunwind.rs:86
   1: backtrace::backtrace::trace_unsynchronized
             at /home/dwrensha/.cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.46/src/backtrace/mod.rs:66
   2: std::sys_common::backtrace::_print_fmt
             at src/libstd/sys_common/backtrace.rs:78
   3: <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt
             at src/libstd/sys_common/backtrace.rs:59
   4: core::fmt::write
             at src/libcore/fmt/mod.rs:1069
   5: std::io::Write::write_fmt
             at src/libstd/io/mod.rs:1439
   6: std::sys_common::backtrace::_print
             at src/libstd/sys_common/backtrace.rs:62
   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:198
   9: std::panicking::default_hook
             at src/libstd/panicking.rs:218
  10: <alloc::boxed::Box<F> as core::ops::function::Fn<A>>::call
             at ./src/liballoc/boxed.rs:1022
  11: rustc_driver::report_ice
             at src/librustc_driver/lib.rs:1175
  12: std::panicking::rust_panic_with_hook
             at src/libstd/panicking.rs:515
  13: std::panicking::begin_panic
             at ./src/libstd/panicking.rs:438
  14: rustc_middle::ty::sty::Binder<T>::dummy
             at src/librustc_middle/ty/sty.rs:877
  15: <rustc_middle::ty::ConstnessAnd<rustc_middle::ty::sty::TraitRef> as rustc_middle::ty::ToPredicate>::to_predicate
             at src/librustc_middle/ty/mod.rs:1478
  16: <rustc_infer::infer::InferCtxt as rustc_trait_selection::traits::error_reporting::InferCtxtPrivExt>::mk_obligation_for_def_id
             at src/librustc_trait_selection/traits/error_reporting/mod.rs:1309
  17: <rustc_infer::infer::InferCtxt as rustc_trait_selection::traits::error_reporting::suggestions::InferCtxtExt>::suggest_change_mut
             at src/librustc_trait_selection/traits/error_reporting/suggestions.rs:574
  18: <rustc_infer::infer::InferCtxt as rustc_trait_selection::traits::error_reporting::InferCtxtExt>::report_selection_error
             at src/librustc_trait_selection/traits/error_reporting/mod.rs:399
  19: <rustc_infer::infer::InferCtxt as rustc_trait_selection::traits::error_reporting::InferCtxtPrivExt>::report_fulfillment_error
             at src/librustc_trait_selection/traits/error_reporting/mod.rs:1006
  20: <rustc_infer::infer::InferCtxt as rustc_trait_selection::traits::error_reporting::InferCtxtExt>::report_fulfillment_errors
             at src/librustc_trait_selection/traits/error_reporting/mod.rs:165
  21: rustc_typeck::check::FnCtxt::select_all_obligations_or_error
             at src/librustc_typeck/check/mod.rs:3512
  22: rustc_typeck::check::wfcheck::CheckWfFcxBuilder::with_fcx::{{closure}}
             at src/librustc_typeck/check/wfcheck.rs:54
  23: rustc_typeck::check::InheritedBuilder::enter::{{closure}}
             at src/librustc_typeck/check/mod.rs:659
  24: rustc_infer::infer::InferCtxtBuilder::enter::{{closure}}
             at ./src/librustc_infer/infer/mod.rs:595
  25: rustc_middle::ty::context::GlobalCtxt::enter_local::{{closure}}::{{closure}}
             at ./src/librustc_middle/ty/context.rs:1527
  26: rustc_middle::ty::context::tls::enter_context::{{closure}}
             at ./src/librustc_middle/ty/context.rs:1695
  27: rustc_middle::ty::context::tls::set_tlv
             at ./src/librustc_middle/ty/context.rs:1679
  28: rustc_middle::ty::context::tls::enter_context
             at ./src/librustc_middle/ty/context.rs:1695
  29: rustc_middle::ty::context::GlobalCtxt::enter_local::{{closure}}
             at ./src/librustc_middle/ty/context.rs:1527
  30: rustc_middle::ty::context::tls::with_related_context::{{closure}}
             at ./src/librustc_middle/ty/context.rs:1783
  31: rustc_middle::ty::context::tls::with_context::{{closure}}
             at ./src/librustc_middle/ty/context.rs:1767
  32: rustc_middle::ty::context::tls::with_context_opt
             at ./src/librustc_middle/ty/context.rs:1756
  33: rustc_middle::ty::context::tls::with_context
             at ./src/librustc_middle/ty/context.rs:1767
  34: rustc_middle::ty::context::tls::with_related_context
             at ./src/librustc_middle/ty/context.rs:1780
  35: rustc_middle::ty::context::GlobalCtxt::enter_local
             at ./src/librustc_middle/ty/context.rs:1519
  36: rustc_infer::infer::InferCtxtBuilder::enter
             at ./src/librustc_infer/infer/mod.rs:594
  37: rustc_typeck::check::InheritedBuilder::enter
             at src/librustc_typeck/check/mod.rs:659
  38: rustc_typeck::check::wfcheck::CheckWfFcxBuilder::with_fcx
             at src/librustc_typeck/check/wfcheck.rs:45
  39: rustc_typeck::check::wfcheck::check_impl
             at src/librustc_typeck/check/wfcheck.rs:600
  40: rustc_typeck::check::wfcheck::check_item_well_formed
             at src/librustc_typeck/check/wfcheck.rs:123
  41: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::check_item_well_formed>::compute
             at ./src/librustc_middle/ty/query/plumbing.rs:357
  42: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
             at ./src/librustc_query_system/dep_graph/graph.rs:303
  43: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task
             at ./src/librustc_query_system/dep_graph/graph.rs:200
  44: rustc_query_system::query::plumbing::force_query_with_job::{{closure}}::{{closure}}
             at ./src/librustc_query_system/query/plumbing.rs:592
  45: rustc_middle::ty::query::plumbing::<impl rustc_query_system::query::QueryContext for rustc_middle::ty::context::TyCtxt>::start_query::{{closure}}::{{closure}}
             at ./src/librustc_middle/ty/query/plumbing.rs:71
  46: rustc_middle::ty::context::tls::enter_context::{{closure}}
             at ./src/librustc_middle/ty/context.rs:1695
  47: rustc_middle::ty::context::tls::set_tlv
             at ./src/librustc_middle/ty/context.rs:1679
  48: rustc_middle::ty::context::tls::enter_context
             at ./src/librustc_middle/ty/context.rs:1695
  49: rustc_middle::ty::query::plumbing::<impl rustc_query_system::query::QueryContext for rustc_middle::ty::context::TyCtxt>::start_query::{{closure}}
             at ./src/librustc_middle/ty/query/plumbing.rs:71
  50: rustc_middle::ty::context::tls::with_related_context::{{closure}}
             at ./src/librustc_middle/ty/context.rs:1783
  51: rustc_middle::ty::context::tls::with_context::{{closure}}
             at ./src/librustc_middle/ty/context.rs:1767
  52: rustc_middle::ty::context::tls::with_context_opt
             at ./src/librustc_middle/ty/context.rs:1756
  53: rustc_middle::ty::context::tls::with_context
             at ./src/librustc_middle/ty/context.rs:1767
  54: rustc_middle::ty::context::tls::with_related_context
             at ./src/librustc_middle/ty/context.rs:1780
  55: rustc_middle::ty::query::plumbing::<impl rustc_query_system::query::QueryContext for rustc_middle::ty::context::TyCtxt>::start_query
             at ./src/librustc_middle/ty/query/plumbing.rs:60
  56: rustc_query_system::query::plumbing::force_query_with_job::{{closure}}
             at ./src/librustc_query_system/query/plumbing.rs:582
  57: rustc_query_system::query::plumbing::with_diagnostics
             at ./src/librustc_query_system/query/plumbing.rs:292
  58: rustc_query_system::query::plumbing::force_query_with_job
             at ./src/librustc_query_system/query/plumbing.rs:581
  59: rustc_query_system::query::plumbing::try_execute_query
             at ./src/librustc_query_system/query/plumbing.rs:409
  60: rustc_query_system::query::plumbing::get_query::{{closure}}
             at ./src/librustc_query_system/query/plumbing.rs:626
  61: <rustc_query_system::query::caches::DefaultCache<K,V> as rustc_query_system::query::caches::QueryCache>::lookup
             at ./src/librustc_query_system/query/caches.rs:91
  62: rustc_query_system::query::plumbing::try_get_cached
             at ./src/librustc_query_system/query/plumbing.rs:367
  63: rustc_query_system::query::plumbing::get_query
             at ./src/librustc_query_system/query/plumbing.rs:618
  64: rustc_query_system::query::plumbing::ensure_query
             at ./src/librustc_query_system/query/plumbing.rs:660
  65: rustc_middle::ty::query::TyCtxtEnsure::check_item_well_formed
             at ./src/librustc_middle/ty/query/plumbing.rs:384
  66: <rustc_typeck::check::wfcheck::CheckTypeWellFormedVisitor as rustc_hir::itemlikevisit::ParItemLikeVisitor>::visit_item
             at src/librustc_typeck/check/wfcheck.rs:1259
  67: rustc_hir::hir::Crate::par_visit_all_item_likes::{{closure}}::{{closure}}
             at ./src/librustc_hir/hir.rs:687
  68: rustc_data_structures::sync::par_for_each_in::{{closure}}::{{closure}}
             at ./src/librustc_data_structures/sync.rs:211
  69: <std::panic::AssertUnwindSafe<F> as core::ops::function::FnOnce<()>>::call_once
             at ./src/libstd/panic.rs:318
  70: std::panicking::try::do_call
             at ./src/libstd/panicking.rs:331
  71: std::panicking::try::do_try
             at src/libstd/panicking.rs:298
  72: std::panicking::try
             at ./src/libstd/panicking.rs:274
  73: std::panic::catch_unwind
             at ./src/libstd/panic.rs:394
  74: rustc_data_structures::sync::par_for_each_in::{{closure}}
             at ./src/librustc_data_structures/sync.rs:211
  75: core::iter::traits::iterator::Iterator::for_each::call::{{closure}}
             at ./src/libcore/iter/traits/iterator.rs:655
  76: core::iter::traits::iterator::Iterator::fold::ok::{{closure}}
             at ./src/libcore/iter/traits/iterator.rs:2002
  77: core::iter::traits::iterator::Iterator::try_fold
             at ./src/libcore/iter/traits/iterator.rs:1878
  78: core::iter::traits::iterator::Iterator::fold
             at ./src/libcore/iter/traits/iterator.rs:2005
  79: core::iter::traits::iterator::Iterator::for_each
             at ./src/libcore/iter/traits/iterator.rs:658
  80: rustc_data_structures::sync::par_for_each_in
             at ./src/librustc_data_structures/sync.rs:210
  81: rustc_hir::hir::Crate::par_visit_all_item_likes::{{closure}}
             at ./src/librustc_hir/hir.rs:686
  82: core::ops::function::FnOnce::call_once
             at ./src/libcore/ops/function.rs:232
  83: <std::panic::AssertUnwindSafe<F> as core::ops::function::FnOnce<()>>::call_once
             at ./src/libstd/panic.rs:318
  84: std::panicking::try::do_call
             at ./src/libstd/panicking.rs:331
  85: std::panicking::try::do_try
             at src/libstd/panicking.rs:298
  86: std::panicking::try
             at ./src/libstd/panicking.rs:274
  87: std::panic::catch_unwind
             at ./src/libstd/panic.rs:394
  88: rustc_hir::hir::Crate::par_visit_all_item_likes
             at ./<::rustc_data_structures::sync::parallel macros>:4
  89: rustc_typeck::check::check_wf_new
             at src/librustc_typeck/check/mod.rs:743
  90: rustc_typeck::check_crate::{{closure}}::{{closure}}
             at src/librustc_typeck/lib.rs:337
  91: rustc_data_structures::profiling::VerboseTimingGuard::run
             at ./src/librustc_data_structures/profiling.rs:569
  92: rustc_session::utils::<impl rustc_session::session::Session>::time
             at ./src/librustc_session/utils.rs:9
  93: rustc_typeck::check_crate::{{closure}}
             at src/librustc_typeck/lib.rs:337
  94: rustc_session::session::Session::track_errors
             at ./src/librustc_session/session.rs:299
  95: rustc_typeck::check_crate
             at src/librustc_typeck/lib.rs:336
  96: rustc_interface::passes::analysis
             at src/librustc_interface/passes.rs:808
  97: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::analysis>::compute
             at ./src/librustc_middle/ty/query/plumbing.rs:357
  98: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
             at ./src/librustc_query_system/dep_graph/graph.rs:303
  99: rustc_query_system::dep_graph::graph::DepGraph<K>::with_eval_always_task
             at ./src/librustc_query_system/dep_graph/graph.rs:336
 100: rustc_query_system::query::plumbing::force_query_with_job::{{closure}}::{{closure}}
             at ./src/librustc_query_system/query/plumbing.rs:584
 101: rustc_middle::ty::query::plumbing::<impl rustc_query_system::query::QueryContext for rustc_middle::ty::context::TyCtxt>::start_query::{{closure}}::{{closure}}
             at ./src/librustc_middle/ty/query/plumbing.rs:71
 102: rustc_middle::ty::context::tls::enter_context::{{closure}}
             at ./src/librustc_middle/ty/context.rs:1695
 103: rustc_middle::ty::context::tls::set_tlv
             at ./src/librustc_middle/ty/context.rs:1679
 104: rustc_middle::ty::context::tls::enter_context
             at ./src/librustc_middle/ty/context.rs:1695
 105: rustc_middle::ty::query::plumbing::<impl rustc_query_system::query::QueryContext for rustc_middle::ty::context::TyCtxt>::start_query::{{closure}}
             at ./src/librustc_middle/ty/query/plumbing.rs:71
 106: rustc_middle::ty::context::tls::with_related_context::{{closure}}
             at ./src/librustc_middle/ty/context.rs:1783
 107: rustc_middle::ty::context::tls::with_context::{{closure}}
             at ./src/librustc_middle/ty/context.rs:1767
 108: rustc_middle::ty::context::tls::with_context_opt
             at ./src/librustc_middle/ty/context.rs:1756
 109: rustc_middle::ty::context::tls::with_context
             at ./src/librustc_middle/ty/context.rs:1767
 110: rustc_middle::ty::context::tls::with_related_context
             at ./src/librustc_middle/ty/context.rs:1780
 111: rustc_middle::ty::query::plumbing::<impl rustc_query_system::query::QueryContext for rustc_middle::ty::context::TyCtxt>::start_query
             at ./src/librustc_middle/ty/query/plumbing.rs:60
 112: rustc_query_system::query::plumbing::force_query_with_job::{{closure}}
             at ./src/librustc_query_system/query/plumbing.rs:582
 113: rustc_query_system::query::plumbing::with_diagnostics
             at ./src/librustc_query_system/query/plumbing.rs:292
 114: rustc_query_system::query::plumbing::force_query_with_job
             at ./src/librustc_query_system/query/plumbing.rs:581
 115: rustc_query_system::query::plumbing::try_execute_query
             at ./src/librustc_query_system/query/plumbing.rs:409
 116: rustc_query_system::query::plumbing::get_query::{{closure}}
             at ./src/librustc_query_system/query/plumbing.rs:626
 117: <rustc_query_system::query::caches::DefaultCache<K,V> as rustc_query_system::query::caches::QueryCache>::lookup
             at ./src/librustc_query_system/query/caches.rs:91
 118: rustc_query_system::query::plumbing::try_get_cached
             at ./src/librustc_query_system/query/plumbing.rs:367
 119: rustc_query_system::query::plumbing::get_query
             at ./src/librustc_query_system/query/plumbing.rs:618
 120: rustc_middle::ty::query::TyCtxtAt::analysis
             at ./src/librustc_middle/ty/query/plumbing.rs:462
 121: rustc_middle::ty::query::<impl rustc_middle::ty::context::TyCtxt>::analysis
             at ./src/librustc_middle/ty/query/plumbing.rs:425
 122: rustc_driver::run_compiler::{{closure}}::{{closure}}::{{closure}}
             at src/librustc_driver/lib.rs:386
 123: rustc_middle::ty::context::tls::enter_global::{{closure}}
             at ./src/librustc_middle/ty/context.rs:1718
 124: rustc_middle::ty::context::tls::enter_context::{{closure}}
             at ./src/librustc_middle/ty/context.rs:1695
 125: rustc_middle::ty::context::tls::set_tlv
             at ./src/librustc_middle/ty/context.rs:1679
 126: rustc_middle::ty::context::tls::enter_context
             at ./src/librustc_middle/ty/context.rs:1695
 127: rustc_middle::ty::context::tls::enter_global
             at ./src/librustc_middle/ty/context.rs:1718
 128: rustc_interface::passes::QueryContext::enter
             at ./src/librustc_interface/passes.rs:709
 129: rustc_driver::run_compiler::{{closure}}::{{closure}}
             at src/librustc_driver/lib.rs:386
 130: rustc_interface::queries::<impl rustc_interface::interface::Compiler>::enter
             at ./src/librustc_interface/queries.rs:385
 131: rustc_driver::run_compiler::{{closure}}
             at src/librustc_driver/lib.rs:286
 132: rustc_interface::interface::run_compiler_in_existing_thread_pool
             at ./src/librustc_interface/interface.rs:199
 133: rustc_interface::interface::run_compiler::{{closure}}
             at ./src/librustc_interface/interface.rs:213
 134: rustc_interface::util::spawn_thread_pool::{{closure}}::{{closure}}::{{closure}}
             at ./src/librustc_interface/util.rs:152
 135: scoped_tls::ScopedKey<T>::set
             at /home/dwrensha/.cargo/registry/src/github.com-1ecc6299db9ec823/scoped-tls-1.0.0/src/lib.rs:137
 136: rustc_interface::util::spawn_thread_pool::{{closure}}::{{closure}}
             at ./src/librustc_interface/util.rs:148
 137: scoped_tls::ScopedKey<T>::set
             at /home/dwrensha/.cargo/registry/src/github.com-1ecc6299db9ec823/scoped-tls-1.0.0/src/lib.rs:137
 138: rustc_ast::attr::with_globals::{{closure}}
             at ./src/librustc_ast/attr/mod.rs:44
 139: scoped_tls::ScopedKey<T>::set
             at /home/dwrensha/.cargo/registry/src/github.com-1ecc6299db9ec823/scoped-tls-1.0.0/src/lib.rs:137
 140: rustc_ast::attr::with_globals
             at ./src/librustc_ast/attr/mod.rs:44
 141: rustc_interface::util::spawn_thread_pool::{{closure}}
             at ./src/librustc_interface/util.rs:147
 142: rustc_interface::util::scoped_thread::{{closure}}
             at ./src/librustc_interface/util.rs:122
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 1.44.0-dev running on x86_64-unknown-linux-gnu

query stack during panic:
#0 [check_item_well_formed] processing `<T as FooExt>`
#1 [analysis] running analysis passes on this crate
end of query stack
error: aborting due to previous error

[estebank]

Fixed in current stable, beta and nightly:

error[E0637]: `&` without an explicit lifetime name cannot be used here
 --> file55.rs:2:36
  |
2 | pub trait FooExt where for<'b> &'b &mut Self: Foo {}
  |                                    ^ explicit lifetime name needed here

error[E0277]: the trait bound `for<'b> &'b &'static mut T: Foo` is not satisfied
 --> file55.rs:3:9
  |
2 | pub trait FooExt where for<'b> &'b &mut Self: Foo {}
  |                                               --- required by this bound in `FooExt`
3 | impl<T> FooExt for T where for<'a> &'a mut T: Foo {}
  |         ^^^^^^ the trait `for<'b> Foo` is not implemented for `&'b &'static mut T`

error: aborting due to 2 previous errors

The second error should probably not be emitted as I believe it is caused by a placeholder 'static lifetime used during recovery. We should introduce (or use if it is already there) a lifetime equivalent to TyErr.

[matthewjasper]

This only happens in builds with debug assertions. It's not fixed in master.

[spastorino]

Assigning P-low as discussed as part of the Prioritization Working Group process and removing I-prioritize.