Auto merge of #46116 - oli-obk:json_ui, r=nikomatsakis
Check //~ERROR comments in ui tests
r? @nikomatsakis
cc #44844 @Phlosioneer @estebank @petrochenkov
this depends on https://github.com/rust-lang/rust/pull/46052 getting merged first (the commits are included in here)
The relevant changes of this PR are c2f0af7 and 979269b