Auto merge of #4886 - rust-lang:must-use-pub-only, r=phansch
Auto merge of #4886 - rust-lang:must-use-pub-only, r=phansch

Lint only exported must_use_candidates

As promised on #4779, here's the check for publicly visible items for `must_use_candidate`

changelog: none

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