Calling a static trait method on a type doesn't mark it as used.

[michaelsproul]

enum Empty {}
struct Unit;

trait Run {
    fn run() -> u8;
}

impl Run for Empty {
    fn run() -> u8 { 0 }
}

impl Run for Unit {
    fn run() -> u8 { 1 }
}

fn main() {
    println!("{} {}", Empty::run(), Unit::run());
}

$ rustc -V
rustc 1.3.0-nightly (6d71ce536 2015-07-06)

$ rustc test.rs 
test.rs:1:1: 1:14 warning: enum is never used: `Empty`, #[warn(dead_code)] on by default
test.rs:1 enum Empty {}
          ^~~~~~~~~~~~~
test.rs:2:1: 2:13 warning: struct is never used: `Unit`, #[warn(dead_code)] on by default
test.rs:2 struct Unit;

The same doesn't happen for static methods implemented directly on the types.

[apasel422]

Duplicate of #23808.