RFC: add lint warning when ignoring return values

[brendanzab]

Currently rustc will allow you to silently ignore return values:

 fn hi() -> int { 1 }

fn main() {
     hi();
}

This has the potential to introduce subtle bugs to code. Perhaps it would be a good idea to have a warning like:

test.rs:4:9: 4:14 warning: unused variable: `x`, #[warn(ignored_return_values)] on by default. Use `let _ = hi();` to silence. 
test.rs:4         hi();
                  ^~~~~

This lint could be disabled using #[allow(ignored_return_values)].

[alexcrichton]

Closing as a dupe of #2974, but certainly an interesting idea! Tricky to get right...

[brendanzab]

Woops, sorry about that!