Rollup merge of #45671 - est31:master, r=alexcrichton
Rollup merge of #45671 - est31:master, r=alexcrichton

Tidy: track rustc_const_unstable feature gates as well

This is important for the unstable book stub generation.

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