Repro case for rustdoc crash

Steps to reproduce:

Nightly:

rustup toolchain install nightly-2019-08-22
cargo +nightly-2019-08-22 clean
cargo +nightly-2019-08-22 doc

Stable (1.37.0), with RUSTC_BOOTSTRAP=1:

rustup toolchain install 1.37.0
cargo +1.37.0 clean
RUSTC_BOOTSTRAP=1 cargo +1.37.0 doc

Stable (1.37.0), without RUSTC_BOOTSTRAP=1, does not reproduce:

rustup toolchain install 1.37.0
cargo +1.37.0 clean
cargo +1.37.0 doc