rustdoc panics on None at html::render

[Manishearth]

Running make docs with --enable-compiler-docs

rustc: x86_64-apple-darwin/stage2/lib/rustlib/x86_64-apple-darwin/lib/librustc_plugin
rustdoc: doc/rustc_typeck/index.html
rustdoc: doc/rustc_mir/index.html
rustc: x86_64-apple-darwin/stage2/lib/rustlib/x86_64-apple-darwin/lib/librustc_mir
rustc: x86_64-apple-darwin/stage2/lib/rustlib/x86_64-apple-darwin/lib/librustc_typeck
thread '<unnamed>' panicked at 'called `Option::unwrap()` on a `None` value', src/libcore/option.rs:326
stack backtrace:
   1:        0x1091ef59a - std::sys::backtrace::tracing::imp::write::hacf40ae6a3c9a7e3
   2:        0x1091fdd5f - std::panicking::default_hook::_{{closure}}::ha6b33fef3a323878
   3:        0x1091fc3e5 - std::panicking::default_hook::h7c2212d31b24a51e
   4:        0x1091fc9f6 - std::panicking::rust_panic_with_hook::h4841ecb3443c75a3
   5:        0x1091fc844 - std::panicking::begin_panic::h64a3b104b90706b0
   6:        0x1091fc7b2 - std::panicking::begin_panic_fmt::h6609000392f80161
   7:        0x1091fc717 - rust_begin_unwind
   8:        0x10925c740 - core::panicking::panic_fmt::h2a8bbd097a596c45
   9:        0x10925c644 - core::panicking::panic::h44f94ad2f4e3e170
  10:        0x10534385d - rustdoc::html::render::run::h41ebd971ec08ba37
  11:        0x10538d354 - rustdoc::main_args::h6af3626fc80bcdf3
  12:        0x1052490ab - std::panicking::try::do_call::h41e7123dce60db3a
  13:        0x10920493a - __rust_maybe_catch_panic
  14:        0x10526d9b3 - _<F as alloc..boxed..FnBox<A>>::call_box::hbd9296e731429739
  15:        0x1091fb1d4 - std::sys::thread::Thread::new::thread_start::hcaa3a549050fafca
  16:     0x7fff8ee9ac12 - _pthread_body
  17:     0x7fff8ee9ab8f - _pthread_start

cc @alexcrichton

[retep998]

Might be worth having --enable-compiler-docs on some of the buildbots.

[Manishearth]

I've wanted an official rust-internals-docs for a while now 😄