Homebrew's Rust receipt is still at 1.4 :(
https://github.com/Homebrew/homebrew/blob/master/Library/For...
https://github.com/Homebrew/homebrew/blob/master/Library/For...
https://github.com/Homebrew/homebrew/commit/dee292132fb73820...
It is 1.4 as I've brewed rust few minutes ago.
And you get the point, don't need to picky on the specifics.