strange coverage

[AlbanMinassian]

src/lib.rs:

pub fn add_two(a: i32) -> i32 {
    internal_adder(a, 2)
}

fn internal_adder(a: i32, b: i32) -> i32 {
    a + b
}

tests/integration_test.rs:

extern crate adder;

#[test]
fn it_adds_two() {
    assert_eq!(4, adder::add_two(2));
}

src/lib => coverage 50% !

"cargo cov" not show "internal_adder" function .. strange
rustc --version : rustc 1.30.0-nightly (1c2e17f4e 2018-09-04)

Ami44