cargo doc panics with internal compiler error: librustc/traits/structural_impls.rs:178: impossible case reached

[jamesray1]

Duplicate of rust-lang/rust#52873.

To reproduce:

# git clone, pull, or fetch, reset --hard, and clean -d --force upstream/master
git clone https://github.com/libp2p/rust-libp2p.git
cd rust-libp2p/core
cargo doc
# snip
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
note: Run with `RUST_BACKTRACE=1` for a 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 1.30.0 (da5f414c2 2018-10-24) running on x86_64-unknown-linux-gnu

error: Could not document `typenum`.

Caused by:
  process didn't exit successfully: `rustdoc --crate-name typenum /home/jray/.cargo/registry/src/github.com-1ecc6299db9ec823/typenum-1.10.0/src/lib.rs --cap-lints allow --color always -o /home/jray/rlb/target/doc -L dependency=/home/jray/rlb/target/debug/deps` (exit code: 1)
warning: build failed, waiting for other jobs to finish...
error: build failed

FYI:

[jray@jr-dl core]$ uname -r
4.19.6-1-MANJARO # Linux
[jray@jr-dl core]$ rustc --version
rustc 1.30.0 (da5f414c2 2018-10-24)