Improve the "must use" lint for `Future`
Improve the "must use" lint for `Future`

Co-Authored-By: Mazdak Farrokhzad <ff865cd2e2cdd09a913011ca3890d90f7014c98c@gmail.com>

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