Why is changing hyphenated crate names to underscored names possible and what are the rules for naming under such ambiguous scenarios?

I created a new Cargo project: cargo new --lib hyphen-crate.

src/lib.rs

pub fn add(a: u32, b: u32) -> u32 {
    a + b
}

tests/addition.rs

use hyphen_crate::add;

#[test]
fn addition_test() {
    assert_eq!(5, add(2, 3));
}

Cargo.toml

[package]
name = "hyphen-crate"
version = "0.1.0"
authors = ["xolve"]
edition = "2018"

[dependencies]

I have searched and seen many discussions if hyphens should be allowed in names of crates or packages, but no link mentions a solution.

What I see is that the crate name hyphen-crate is automatically transformed to hyphen_crate and it compiles and tests successfully.