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With Cargo's default test discovery, why can tests/api.rs call double but not private_helper?

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// src/lib.rs
fn private_helper(x: i32) -> i32 {
    x * 2
}

pub fn double(x: i32) -> i32 {
    private_helper(x)
}

// tests/api.rs (a separate file; the library crate is named my_crate)
#[test]
fn doubles_a_value() {
    assert_eq!(my_crate::double(21), 42);
}