Rollup merge of #38993 - krdln:patch-1, r=steveklabnik
Rollup merge of #38993 - krdln:patch-1, r=steveklabnik

Add `&mut expr` to syntax index

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