ICE: no name for expr |a|

[jsartisohn]

The compiler panics when referencing a captured variable inside a nested closure.

This happens on the playground nightly build "rustc 1.42.0-nightly (ed33453 2019-12-25) running on x86_64-unknown-linux-gnu" as well as MSVC stable "stable-x86_64-pc-windows-msvc updated - rustc 1.40.0 (73528e3 2019-12-16)".

This issue feels awfully related to #67252. But from what I gathered, its fix (#67289) went live a couple of days ago and should be in the playground version 1.42.0-nightly (ed33453 2019-12-25).

I tried this code (minimal-ish example):
https://gist.github.com/rust-play/362fc228ed152a41e5b216603e7c236f

fn main() {
    let _ = vec![0]
        .iter()
        .flat_map(|a| vec![0].iter().map(|_| &a))
        .collect::<Vec<_>>();
}

I expected to see this happen: No compiler errors or warnings and especially no crash.

Meta

rustc --version --verbose:
rustc 1.40.0 (73528e3 2019-12-16)
binary: rustc
commit-hash: 73528e3
commit-date: 2019-12-16
host: x86_64-pc-windows-msvc
release: 1.40.0
LLVM version: 9.0

Backtrace (from the playground):

Standard Error

   Compiling playground v0.0.1 (/playground)
error: internal compiler error: src/librustc/hir/map/mod.rs:955: no name for expr |a| <[_]>::into_vec(box [0]).iter().map(|_| &a) (hir_id=HirId { owner: DefIndex(3), local_id: 40 })

thread 'rustc' panicked at 'Box<Any>', src/librustc_errors/lib.rs:895:9
stack backtrace:
   0: backtrace::backtrace::libunwind::trace
             at /cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.40/src/backtrace/libunwind.rs:88
   1: backtrace::backtrace::trace_unsynchronized
             at /cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.40/src/backtrace/mod.rs:66
   2: std::sys_common::backtrace::_print_fmt
             at src/libstd/sys_common/backtrace.rs:77
   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:1057
   5: std::io::Write::write_fmt
             at src/libstd/io/mod.rs:1426
   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:195
   9: std::panicking::default_hook
             at src/libstd/panicking.rs:215
  10: rustc_driver::report_ice
  11: std::panicking::rust_panic_with_hook
             at src/libstd/panicking.rs:476
  12: std::panicking::begin_panic
  13: rustc_errors::HandlerInner::bug
  14: rustc_errors::Handler::bug
  15: rustc::util::bug::opt_span_bug_fmt::{{closure}}
  16: rustc::ty::context::tls::with_opt::{{closure}}
  17: rustc::ty::context::tls::with_opt
  18: rustc::util::bug::opt_span_bug_fmt
  19: rustc::util::bug::bug_fmt
  20: rustc::hir::map::Map::name
  21: rustc_mir::borrow_check::diagnostics::conflict_errors::<impl rustc_mir::borrow_check::MirBorrowckCtxt>::report_borrowed_value_does_not_live_long_enough
  22: rustc_mir::borrow_check::MirBorrowckCtxt::check_for_invalidation_at_exit
  23: rustc_mir::borrow_check::flows::Flows::with_outgoing_borrows
  24: <rustc_mir::borrow_check::MirBorrowckCtxt as rustc_mir::dataflow::DataflowResultsConsumer>::visit_terminator_entry
  25: rustc_mir::borrow_check::do_mir_borrowck
  26: rustc::ty::context::GlobalCtxt::enter_local
  27: rustc_mir::borrow_check::mir_borrowck
  28: rustc::ty::query::__query_compute::mir_borrowck
  29: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::mir_borrowck>::compute
  30: rustc::dep_graph::graph::DepGraph::with_task_impl
  31: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  32: rustc_mir::borrow_check::type_check::TypeChecker::check_rvalue
  33: rustc_mir::borrow_check::type_check::TypeChecker::typeck_mir
  34: rustc_mir::borrow_check::type_check::type_check
  35: rustc_mir::borrow_check::nll::compute_regions
  36: rustc_mir::borrow_check::do_mir_borrowck
  37: rustc::ty::context::GlobalCtxt::enter_local
  38: rustc_mir::borrow_check::mir_borrowck
  39: rustc::ty::query::__query_compute::mir_borrowck
  40: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::mir_borrowck>::compute
  41: rustc::dep_graph::graph::DepGraph::with_task_impl
  42: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  43: rustc::ty::<impl rustc::ty::context::TyCtxt>::par_body_owners
  44: rustc_interface::passes::analysis
  45: rustc::ty::query::__query_compute::analysis
  46: rustc::dep_graph::graph::DepGraph::with_task_impl
  47: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  48: rustc::ty::context::tls::enter_global
  49: rustc_interface::interface::run_compiler_in_existing_thread_pool
  50: std::thread::local::LocalKey<T>::with
  51: scoped_tls::ScopedKey<T>::set
  52: syntax::with_globals
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

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 1.42.0-nightly (ed33453a3 2019-12-25) running on x86_64-unknown-linux-gnu

note: compiler flags: -C codegen-units=1 -C debuginfo=2 --crate-type bin

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

query stack during panic:
#0 [mir_borrowck] processing `main::{{closure}}#0`
#1 [mir_borrowck] processing `main`
#2 [analysis] running analysis passes on this crate
end of query stack
error: aborting due to previous error

error: could not compile `playground`.

To learn more, run the command again with --verbose.

[Centril]

Reduced:

fn main() {
    [0].iter().flat_map(|a| [0].iter().map(|_| &a));
}

[matthewjasper]

Assertion is from name when called here:

rust/src/librustc_mir/borrow_check/diagnostics/conflict_errors.rs

Line 887 in a06baa5

self.infcx.tcx.hir().name(fn_hir_id),