let_chains: note re. back-compat wrt. expr beginning.
let_chains: note re. back-compat wrt. expr beginning.

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