cargo check complains about main() being unused

[jrmuizel]

running cargo check on a binary crate gives:

warning: function is never used: `main`, #[warn(dead_code)] on by default
   --> main.rs:86:1
    |
86  | fn main() {
    | ^

[alexcrichton]

I believe this is a dupe of rust-lang/rust#38273, but thanks for the report!