Rustdoc panics while documenting typenum on beta and nightly

[ecstatic-morse]

Summary

After adding some functionality to typenum, the compiler panicked while generating documentation on both the nightly and beta channels. The code compiles and runs successfully on all compiler versions, but docs can only be generated on stable.

I'm not sure how to go about generating a reduced test case, so here's a link to the PR which caused the ICE.

Output

This is the output with RUST_BACKTRACE=1 enabled:

error: internal compiler error: librustc/traits/structural_impls.rs:178: impossible case reached

thread '<unnamed>' panicked at 'Box<Any>', librustc_errors/lib.rs:587:9
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: std::panicking::rust_panic_with_hook
             at libstd/panicking.rs:477
   5: std::panicking::begin_panic
   6: rustc_errors::Handler::bug
   7: rustc::util::bug::opt_span_bug_fmt::{{closure}}
   8: rustc::ty::context::tls::with_opt::{{closure}}
   9: rustc::ty::context::tls::with_context_opt
  10: rustc::ty::context::tls::with_opt
  11: rustc::util::bug::opt_span_bug_fmt
  12: rustc::util::bug::bug_fmt
  13: rustc::traits::structural_impls::<impl rustc::ty::context::Lift<'tcx> for rustc::traits::SelectionError<'a>>::lift_to_tcx
  14: rustc::ty::context::TyCtxt::lift_to_global
  15: rustc::traits::select::SelectionContext::candidate_from_obligation
  16: rustc::traits::select::SelectionContext::evaluate_stack
  17: rustc::dep_graph::graph::DepGraph::with_anon_task
  18: rustc::traits::select::SelectionContext::evaluate_predicate_recursively
  19: rustc::infer::InferCtxt::probe
  20: <&'a mut I as core::iter::iterator::Iterator>::next
  21: <alloc::vec::Vec<T> as alloc::vec::SpecExtend<T, I>>::from_iter
  22: rustc::traits::select::SelectionContext::candidate_from_obligation_no_cache
  23: rustc::dep_graph::graph::DepGraph::with_anon_task
  24: rustc::traits::select::SelectionContext::candidate_from_obligation
  25: rustc::traits::select::SelectionContext::evaluate_stack
  26: rustc::dep_graph::graph::DepGraph::with_anon_task
  27: rustc::traits::select::SelectionContext::evaluate_predicate_recursively
  28: rustc::infer::InferCtxt::probe
  29: <&'a mut I as core::iter::iterator::Iterator>::next
  30: <alloc::vec::Vec<T> as alloc::vec::SpecExtend<T, I>>::from_iter
  31: rustc::traits::select::SelectionContext::candidate_from_obligation_no_cache
  32: rustc::dep_graph::graph::DepGraph::with_anon_task
  33: rustc::traits::select::SelectionContext::candidate_from_obligation
  34: rustc::traits::select::SelectionContext::evaluate_stack
  35: rustc::dep_graph::graph::DepGraph::with_anon_task
  36: rustc::traits::select::SelectionContext::evaluate_predicate_recursively
  37: rustc::infer::InferCtxt::probe
  38: <&'a mut I as core::iter::iterator::Iterator>::next
  39: <alloc::vec::Vec<T> as alloc::vec::SpecExtend<T, I>>::from_iter
  40: rustc::traits::select::SelectionContext::candidate_from_obligation_no_cache
  41: rustc::dep_graph::graph::DepGraph::with_anon_task
  42: rustc::traits::select::SelectionContext::candidate_from_obligation
  43: rustc::traits::select::SelectionContext::evaluate_stack
  44: rustc::dep_graph::graph::DepGraph::with_anon_task
  45: rustc::traits::select::SelectionContext::evaluate_predicate_recursively
  46: rustc::infer::InferCtxt::probe
  47: <&'a mut I as core::iter::iterator::Iterator>::next
  48: <alloc::vec::Vec<T> as alloc::vec::SpecExtend<T, I>>::from_iter
  49: rustc::traits::select::SelectionContext::candidate_from_obligation_no_cache
  50: rustc::dep_graph::graph::DepGraph::with_anon_task
  51: rustc::traits::select::SelectionContext::candidate_from_obligation
  52: rustc::traits::select::SelectionContext::evaluate_stack
  53: rustc::dep_graph::graph::DepGraph::with_anon_task
  54: rustc::traits::select::SelectionContext::evaluate_predicate_recursively
  55: rustc::infer::InferCtxt::probe
  56: <&'a mut I as core::iter::iterator::Iterator>::next
  57: <alloc::vec::Vec<T> as alloc::vec::SpecExtend<T, I>>::from_iter
  58: rustc::traits::select::SelectionContext::candidate_from_obligation_no_cache
  59: rustc::dep_graph::graph::DepGraph::with_anon_task
  60: rustc::traits::select::SelectionContext::candidate_from_obligation
  61: rustc::traits::select::SelectionContext::evaluate_stack
  62: rustc::dep_graph::graph::DepGraph::with_anon_task
  63: rustc::traits::select::SelectionContext::evaluate_predicate_recursively
  64: rustc::infer::InferCtxt::probe
  65: <&'a mut I as core::iter::iterator::Iterator>::next
  66: <alloc::vec::Vec<T> as alloc::vec::SpecExtend<T, I>>::from_iter
  67: rustc::traits::select::SelectionContext::candidate_from_obligation_no_cache
  68: rustc::dep_graph::graph::DepGraph::with_anon_task
  69: rustc::traits::select::SelectionContext::candidate_from_obligation
  70: rustc::traits::select::SelectionContext::evaluate_stack
  71: rustc::dep_graph::graph::DepGraph::with_anon_task
  72: rustc::traits::select::SelectionContext::evaluate_predicate_recursively
  73: rustc::infer::InferCtxt::probe
  74: <&'a mut I as core::iter::iterator::Iterator>::next
  75: <alloc::vec::Vec<T> as alloc::vec::SpecExtend<T, I>>::from_iter
  76: rustc::traits::select::SelectionContext::candidate_from_obligation_no_cache
  77: rustc::dep_graph::graph::DepGraph::with_anon_task
  78: rustc::traits::select::SelectionContext::candidate_from_obligation
  79: rustc::traits::select::SelectionContext::evaluate_stack
  80: rustc::dep_graph::graph::DepGraph::with_anon_task
  81: rustc::traits::select::SelectionContext::evaluate_predicate_recursively
  82: rustc::infer::InferCtxt::probe
  83: <&'a mut I as core::iter::iterator::Iterator>::next
  84: <alloc::vec::Vec<T> as alloc::vec::SpecExtend<T, I>>::from_iter
  85: rustc::traits::select::SelectionContext::candidate_from_obligation_no_cache
  86: rustc::dep_graph::graph::DepGraph::with_anon_task
  87: rustc::traits::select::SelectionContext::candidate_from_obligation
  88: rustc::traits::select::SelectionContext::evaluate_stack
  89: rustc::dep_graph::graph::DepGraph::with_anon_task
  90: rustc::traits::select::SelectionContext::evaluate_predicate_recursively
  91: rustc::infer::InferCtxt::probe
  92: <&'a mut I as core::iter::iterator::Iterator>::next
  93: <alloc::vec::Vec<T> as alloc::vec::SpecExtend<T, I>>::from_iter
  94: rustc::traits::select::SelectionContext::candidate_from_obligation_no_cache
  95: rustc::dep_graph::graph::DepGraph::with_anon_task
  96: rustc::traits::select::SelectionContext::candidate_from_obligation
  97: rustc::traits::select::SelectionContext::evaluate_stack
  98: rustc::dep_graph::graph::DepGraph::with_anon_task
  99: rustc::traits::select::SelectionContext::evaluate_predicate_recursively

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 1.30.0-nightly (f8d34596f 2018-08-30) running on x86_64-unknown-linux-gnu

error: Could not document `typenum`.

Caused by:
  process didn't exit successfully: `rustdoc --crate-name typenum src/lib.rs -o /home/mackendy/src/rust/typenum/target/doc -L dependency=/home/mackendy/src/rust/typenum/target/debug/deps` (exit code: 1)

[ecstatic-morse]

This is a duplicate of #52873.