Auto merge of #38773 - sanxiyn:rustdoc-ice, r=steveklabnik
Auto merge of #38773 - sanxiyn:rustdoc-ice, r=steveklabnik

Avoid rustdoc ICE when an unstable feature is used

Fix #36159.

aidancully/rust,aidancully/rust,graydon/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,GBGamer/rust,GBGamer/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,GBGamer/rust,GBGamer/rust