Move Rule 7.4 to "No equivalent"

[qryxip]

Semantically there are no equivalent Rust code to char *s = "…";.

On Rust, we cannot even attempt to modify string literals.

let mut s = b"byte string literal";

// error[E0594]: cannot assign to `s[_]` which is behind a `&` reference
//s[0] = b'B';

let s = &s[1..]; // this is OK

// equivalents to `char s[] = {…};`
let s = &mut { *b"byte string literal" };
s[0] = b'B'; // `s` is on the stack. so this is not UB

// equivalents to the wrong semantics of `char *s = "…";`
unsafe {
    // Safety:
    //
    // There is always at most 1 pointer to `S`. I promise.
    static mut S: [u8; 19] = *b"byte string literal";
    let s = &mut S;
    s[0] = b'B'; // also this is not UB as long as we obey the "Shared Xor Mutable" rule
}

[qryxip]

I forgot this.

unsafe {
    // Yes, I know this is instant UB!
    #[allow(mutable_transmutes, clippy::transmute_ptr_to_ptr)]
    let s = std::mem::transmute::<_, &mut [u8]>(&b"text"[..]);

    s[0] = b'T';
}

Maybe should I change to "Enforceable by default"? But char *s = "…"; itself is not UB unlike std::mem::transmute::<_, &mut _> on Rust.