quadratic blowup on -Zunpretty=hir-tree injected 2021-10-19

[pnkfelix]

Spawned from https://rust-lang.zulipchat.com/#narrow/stream/131828-t-compiler/topic/regression.3A.20hir-tree.20became.20an.20order.20of.20magnitude.20larger/near/282616475

git clone https://github.com/diesel-rs/diesel.git
cd diesel/diesel

# This takes 2 minutes and requires 1.7 GB RAM, and the output is 1.7 GB
time cargo +nightly-2021-10-18 rustc --lib -- -Z unpretty=hir-tree > /tmp/18.txt

# This takes 20 minutes and requires 12.0 GB RAM, and the output is 12.0 GB
time cargo +nightly-2021-10-19 rustc --lib -- -Z unpretty=hir-tree > /tmp/19.txt

[pnkfelix]

@cjgillot reports on the zulip thread that this is due to PR #89124