rustdoc panic while documenting impl trait in argument position

[KodrAus]

The problem

I'm hitting a panic from an assertion when attempting to document a library with a nested impl trait in argument position. In the repro the method a hits the assertion and panics, whereas b passes and documents successfully.

Output

$ cargo doc --no-deps
 Documenting doc-impl-trait-repro v0.1.0 (file:///Users/user/dev/doc-impl-trait-repro)
thread '<unnamed>' panicked at 'assertion failed: cx.impl_trait_bounds.borrow().is_empty()', librustdoc/clean/mod.rs:4050:5
stack backtrace:
   0: std::sys::unix::backtrace::tracing::imp::unwind_backtrace
   1: std::sys_common::backtrace::print
   2: std::panicking::default_hook::{{closure}}
   3: std::panicking::default_hook
   4: std::panicking::rust_panic_with_hook
   5: std::panicking::begin_panic
   6: rustdoc::clean::enter_impl_trait
   7: <rustc::hir::ImplItem as rustdoc::clean::Clean<rustdoc::clean::Item>>::clean
   8: <alloc::vec::Vec<T> as alloc::vec::SpecExtend<T, I>>::from_iter
   9: <rustdoc::doctree::Impl as rustdoc::clean::Clean<alloc::vec::Vec<rustdoc::clean::Item>>>::clean
  10: <alloc::vec::Vec<T> as alloc::vec::SpecExtend<T, I>>::spec_extend
  11: <rustdoc::doctree::Module as rustdoc::clean::Clean<rustdoc::clean::Item>>::clean
  12: <rustdoc::visit_ast::RustdocVisitor<'a, 'tcx, 'rcx> as rustdoc::clean::Clean<rustdoc::clean::Crate>>::clean
  13: rustdoc::core::run_core::{{closure}}
  14: rustc::ty::context::tls::enter_global
  15: rustc::ty::context::TyCtxt::create_and_enter
  16: rustc_driver::driver::phase_3_run_analysis_passes
  17: rustdoc::core::run_core
  18: <scoped_tls::ScopedKey<T>>::set
  19: syntax::with_globals
  20: <std::panic::AssertUnwindSafe<F> as core::ops::function::FnOnce<()>>::call_once
  21: std::panicking::try::do_call
  22: __rust_maybe_catch_panic
  23: rustc_driver::monitor
  24: rustdoc::rust_input
  25: rustdoc::main_args
  26: <scoped_tls::ScopedKey<T>>::set
  27: syntax::with_globals
  28: std::panicking::try::do_call
  29: __rust_maybe_catch_panic
  30: <F as alloc::boxed::FnBox<A>>::call_box
  31: std::sys_common::thread::start_thread
  32: std::sys::unix::thread::Thread::new::thread_start
  33: _pthread_body
  34: _pthread_start

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 1.27.0-nightly (79252ff4e 2018-04-29) running on x86_64-apple-darwin

error: Could not document `doc-impl-trait-repro`.

Caused by:
  process didn't exit successfully: `rustdoc --crate-name doc_impl_trait_repro src/lib.rs -o /Users/user/dev/doc-impl-trait-repro/target/doc -L dependency=/Users/user/dev/doc-impl-trait-repro/target/debug/deps` (exit code: 101)

Repro

use std::io::Read;

struct D<R>(R);

struct A;
impl A {
    // thread '<unnamed>' panicked at 'assertion failed: cx.impl_trait_bounds.borrow().is_empty()', librustdoc/clean/mod.rs:4050:5
    fn a(&self, d: impl IntoIterator<Item = D<impl Read>>) -> Result<(), ()> { unimplemented!() }

    // ok
    fn b(&self, d: impl IntoIterator<Item = D<()>>) -> Result<(), ()> { unimplemented!() }
}

Meta

rustc 1.27.0-nightly (79252ff4e 2018-04-29)
binary: rustc
commit-hash: 79252ff4e25d82f9fe856cb66f127b79cdace163
commit-date: 2018-04-29
host: x86_64-apple-darwin
release: 1.27.0-nightly
LLVM version: 6.0

[pietroalbini]

This would also be nice to backport into beta.